4.3 MB

4.3 MB
Program
on
Improved
Seismic
Safety
Provisions
Of the National Institute of Building Sciences
1997 Edition
NEHRP RECOMMENDED PROVISIONS
FOR SEISMIC REGULATIONS
FOR NEW BUILDINGS
AND OTHER STRUCTURES
Part 2: Commentary (FEMA 303)
The Building Seismic Safety Council (BSSC) was established in 1979 under the auspices of the National Institute of
Building Sciences as an entirely new type of instrument for dealing with the complex regulatory, technical, social, and
economic issues involved in developing and promulgating building earthquake hazard mitigation regulatory provisions
that are national in scope. By bringing together in the BSSC all of the needed expertise and all relevant public and private interests, it was believed that issues related to the seismic safety of the built environment could be resolved and jurisdictional problems overcome through authoritative guidance and assistance backed by a broad consensus.
The BSSC is an independent, voluntary membership body representing a wide variety of building community interests.
Its fundamental purpose is to enhance public safety by providing a national forum that fosters improved seismic safety
provisions for use by the building community in the planning, design, construction, regulation, and utilization of buildings.
To fulfill its purpose, the BSSC: (1) promotes the development of seismic safety provisions suitable for use throughout
the United States; (2) recommends, encourages, and promotes the adoption of appropriate seismic safety provisions in
voluntary standards and model codes; (3) assesses progress in the implementation of such provisions by federal, state,
and local regulatory and construction agencies; (4) identifies opportunities for improving seismic safety regulations and
practices and encourages public and private organizations to effect such improvements; (5) promotes the development of
training and educational courses and materials for use by design professionals, builders, building regulatory officials,
elected officials, industry representatives, other members of the building community, and the public; (6) advises government bodies on their programs of research, development, and implementation; and (7) periodically reviews and evaluates research findings, practices, and experience and makes recommendations for incorporation into seismic design
practices.
See Appendix E of the Commentary volume for a full description of BSSC activities.
BOARD OF DIRECTION: 1997
Chairman
Eugene Zeller, City of Long Beach, California
Vice Chairman
William W. Stewart, Stewart-Scholberg Architects, Clayton, Missouri (representing the American
Institute of Architects)
Secretary
Mark B. Hogan, National Concrete Masonry Association, Herndon, Virginia
Ex-Officio
James E. Beavers, James E. Beavers Consultants, Oak Ridge, Tennessee
Members
Eugene Cole, Carmichael, California (representing the Structural Engineers Association of
California); S. K. Ghosh, Portland Cement Association, Skokie, Illinois; Nestor Iwankiw, American
Institute of Steel Construction, Chicago, Illinois; Gerald H. Jones, Kansas City, Missouri
(representing the National Institute of Building Sciences); Joseph Nicoletti, URS/John A. Blume
and Associates, San Francisco, California (representing the Earthquake Engineering Research
Institute); Jack Prosek, Turner Construction Company, San Francisco, California (representing the
Associated General Contractors of America); W. Lee Shoemaker, Metal Building Manufacturers
Association, Cleveland, Ohio; John C. Theiss, Theiss Engineers, Inc., St. Louis, Missouri
(representing the American Society of Civil Engineers); Charles Thornton, Thornton-Tomasetti Engineers, New York, New York (representing the Applied Technology Council); David P. Tyree,
American Forest and Paper Association, Colorado Springs, Colorado; David Wismer, Department
of Licenses and Inspections, Philadelphia, Pennsylvania (representing the Building Officials and
Code Administrators International); Richard Wright, National Institute of Standards and
Technology, Gaithersburg, Maryland (representing the Interagency Committee for Seismic Safety in
Construction)
BSSC Staff
James R. Smith, Executive Director; Claret M. Heider, Technical Writer-Editor/Program Manager;
Thomas Hollenbach, Deputy Executive; Lee Lawrence Anderson, Director, Special Projects; Mary
Marshall, Administrative Assistant; Patricia Blasi, Administrative Assistant
BSSC Program on Improved Seismic Safety Provisions
NEHRP RECOMMENDED PROVISIONS
(National Earthquake Hazards Reduction Program)
FOR SEISMIC REGULATIONS
FOR NEW BUILDINGS AND
OTHER STRUCTURES
1997 EDITION
Part 2: COMMENTARY
(FEMA 303)
Prepared by the
Building Seismic Safety Council
for the
Federal Emergency Management Agency
BUILDING SEISMIC SAFETY COUNCIL
Washington, D.C.
1997
NOTICE: Any opinions, findings, conclusions, or recommendations expressed in this publication do
not necessarily reflect the views of the Federal Emergency Management Agency. Additionally,
neither FEMA nor any of its employees make any warranty, expressed or implied, nor assume any
legal liability or responsibility for the accuracy, completeness, or usefulness of any information,
product, or process included in this publication.
This report was prepared under Contract EMW-C-4536 between the Federal Emergency
Management Agency and the National Institute of Building Sciences.
Building Seismic Safety Council activities and products are described at the end of this report. For
further information, contact the Building Seismic Safety Council, 1090 Vermont, Avenue, N.W.,
Suite 700, Washington, D.C. 20005; phone 202-289-7800; fax 202-289-1092; e-mail [email protected]
Copies of this report may be obtained by contacting the FEMA Publication Distribution Facility at
1-800-480-2520.
ii
CONTENTS
NEHRP RECOMMENDED PROVISIONS FOR
SEISMIC REGULATIONS FOR NEW BUILDINGS AND OTHER STRUCTURES
1997 EDITION
PART 2: COMMENTARY
Chapter 1 Commentary, General Provisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Seismic Use Groups and Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Chapter 2 Commentary, Glossary and Notations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Chapter 3 Commentary, Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1
3.2
3.3
3.4
3.5
3.6
Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quality Assurance Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Special Inspections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Testing: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structural Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reporting and Compliance Procedures: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
34
35
36
36
36
Chapter 4 Commentary, Ground Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.1 Determining Maximum Considered Earthquake and Design Earthquake
Ground Motion Accelerations and Response Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2 Seismic Design Category . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Chapter 5 Commentary, Structural Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2
5.3
5.4
5.5
Design Basis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Equivalent Lateral Force Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Modal Analysis Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Soil-structure Interaction Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Chapter 6 Commentary, Architectural, Mechanical, and Electrical Components
Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
iii
6.2 Architectural Component Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6.3 Mechanical and Electrical Component Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
Related Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Chapter 7 Commentary, Foundation Design Requirements . . . . . . . . . . . . . . . . . . . . . . . 153
7.1
7.2
7.3
7.4
7.5
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Strength of Components and Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seismic Design Categories A and B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seismic Design Category C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seismic Design Categories D, E, and F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
153
153
153
153
169
Chapter 8 Commentary, Steel Structure Design Requirements . . . . . . . . . . . . . . . . . . . . 177
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.5
Reference Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seismic Requirements for Steel Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seismic Design Categories A, B, and C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seismic Design Categories D, E, and F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cold-formed Steel Seismic Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Light-framed Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Seismic Requirements for Steel Deck Diaphragms . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Steel Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
177
177
177
177
177
177
178
178
Chapter 9 Commentary, Concrete Structure Design Requirements . . . . . . . . . . . . . . . . . 179
9.1 Reference Document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2 Bolts and Headed Stud Anchors in Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3 Classification of Moment Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4 Seismic Design Category A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5 Seismic Design Category B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.6 Seismic Design Category C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.7 Seismic Design Categories D, E, and F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix to Chapter 9, Reinforced Concrete Structural Systems
Composed of Interconnected Precast Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
179
184
186
186
186
186
186
191
Chapter 10 Commentary, Composite Steel and Concrete
Structure Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
Chapter 11 Commentary, Masonry Structure Design Requirements . . . . . . . . . . . . . . . . 199
11.1
11.2
11.3
11.5
11.6
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Construction Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Strength and Deformation Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Flexure and Axial Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iv
199
199
200
202
204
11.8 Special Requirements for Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.9 Special Requirements for Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.10 Special Requirements for Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.11 Special Requirements for Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix to Chapter 11 Commentary, Alternative Masonry Structure
Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
210
211
212
212
221
Chapter 12 Commentary, Wood Structure Design Requirements . . . . . . . . . . . . . . . . . . 223
12.1 Reference Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2 Design Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.3 Engineered Wood Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.4 Diaphragms and Shear Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.5 Conventional Light-frame Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.6 Seismic Design Category A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.7 Seismic Design Categories B, C, and D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.8 Seismic Design Categories E and F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
223
224
225
231
237
256
256
256
Chapter 13 Commentary, Seismically Isolated Structures . . . . . . . . . . . . . . . . . . . . . . . . 259
13.1
13.2
13.3
13.4
13.5
13.6
13.8
13.9
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Criteria Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Equivalent-lateral-force Design Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Dynamic Lateral Response Procedure: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nonstructural Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detailed System Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Design and Construction Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Required Tests of the Isolation System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
261
262
264
267
268
268
269
269
Chapter 14 Commentary, Nonbuilding Structure Design Requirements . . . . . . . . . . . . . 273
14.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.2 Structural Design Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.3 Nonbuilding Structures Similar to Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.4 Nonbuilding Structures Not Similar to Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix to Chapter 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
273
274
277
277
277
Commentary Appendix A, Development of Maximum Considered Earthquake
Ground Motion Maps 1 Through 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
Commentary Appendix B, Development of the USGS Seismic Maps . . . . . . . . . . . . . . . . . . . 303
The Council: Its Purpose and Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
v
Chapter 1 Commentary
GENERAL PROVISIONS
Chapter 1 sets forth general requirements for applying the analysis and design provisions
contained in Chapters 2 through 14 of the NEHRP Recommended Provisions for Seismic
Regulations for New Buildings and Other Structures. It is similar to what might be incorporated
in a code as administrative regulations.
Chapter 1 is designed to be as compatible as possible with normal code administrative provisions
(especially as exemplified by the three national model codes), but it is written as the guide to use
of the rest of the document, not as a regulatory mechanism. The word "shall" is used in the
Provisions, not as a legal imperative, but simply as the language necessary to ensure fulfillment of
all the steps necessary to technically meet a minimum standard of performance.
It is important to note that the NEHRP Recommended Provisions is intended to serve as a source
document for use by any interested member of the building community. Thus, some users may
alter certain information within the Provisions (e.g., the determination of which use groups are
included within the higher Seismic Use Groups might depend on whether the user concluded that
the generally more-demanding design requirements were necessary). It is strongly emphasized,
however, that such "tailoring" should be carefully considered by highly qualified individuals who
are fully aware of all the implications of any changes on all affected procedures in the analysis and
design sequences of the document.
Further, although the NEHRP Recommended Provisions is national in scope, it presents minimum
criteria. It is neither intended to nor does it justify any reduction in higher standards that have
been locally established, particularly in areas of highest seismicity.
Reference is made throughout the document to decisions and actions that are delegated to an
unspecified “authority having jurisdiction." The document is intended to be applicable to many
different types of jurisdictions and chains of authority, and an attempt has been made to recognize
situations where more than technical decision-making can be presumed. In fact, the document
anticipates the need to establish standards and approval systems to accommodate the use of the
document for development of a regulatory system. A good example of this is in Sec. 1.2.6, "Alternate Materials and Alternate Means and Methods of Construction," where the need for
well-established criteria and systems of testing and approval are recognized even though few such
systems are in place. In some instances, the decision-making mechanism referred to is clearly
most logically the province of a building official or department; in others, may be a law-making
body such as a state legislature, a city council, or some other state or local policy-making body.
The term "authority having jurisdiction" has been used to apply to all of these entities. A good
example of the need for keeping such generality in mind is provided by the California law
concerning the design and construction of schools. That law establishes requirements for
independent special inspection approved and supervised by the Office of the State Architect, a
state-level office that does not exist in many other states.
1
General Provisions
Note that Appendix A to the Commentary volume presents a detailed explanation of the
development of Provisions Maps 1 through 24 and Appendix B describes development of the
U.S. Geological Survey seismic hazard maps on which the Provisions maps are based. An
overview of the Building Seismic Safety Council (BSSC) and its activities appears at the end of
the volume.
1.1 PURPOSE: The goal of the NEHRP Recommended Provisions is to present criteria for the
design and construction of new structures subject to earthquake ground motions in order to
minimize the hazard to life for all structures, to increase the expected performance of structures
having a substantial public hazard due to occupancy or use as compared to ordinary structures,
and to improve the capability of essential facilities to function after an earthquake. To this end,
the Provisions provides the minimum criteria considered prudent for the protection of life safety
in structures subject to earthquakes. The Provisions document has been reviewed extensively and
balloted by the architectural, engineering, and construction communities and, therefore, it is a
proper source for the development of building codes in areas of seismic exposure.
Some design standards go farther than these provisions and attempt to minimize damage as well
as protect building occupants. For example, the California Building Code has added property
protection in relation to the design and construction of hospitals and public schools. The NEHRP
Recommended Provisions document generally considers property damage as it relates to occupant
safety for ordinary structures. For high occupancy and essential facilities, damage limitation
criteria are more strict in order to better provide for the safety of occupants and the continued
functioning of the facility.
Some structural and nonstructural damage can be expected as a result of the "design ground
motions" because the Provisions allow inelastic energy dissipation in the structural system. For
ground motions in excess of the design levels, the intent of the Provisions is for the structure to
have a low likelihood of collapse.
It must be emphasized that absolute safety and no damage even in an earthquake event with a
reasonable probability of occurrence cannot be achieved for most structures. However, a high
degree of life safety, albeit with some structural and nonstructural damage, can be economically
achieved in structures by allowing inelastic energy dissipation in the structure. The objective of
the Provisions therefore is to set forth the minimum requirements to provide reasonable and
prudent life safety . For most structures designed and constructed according to the Provisions, it
is expected that structural damage from even a major earthquake would likely be repairable, but
the damage may not be economically repairable.
Where damage control is desired, the design must provide not only sufficient strength to resist the
specified seismic loads but also the proper stiffness to limit the lateral deflection. Damage to
nonstructural elements may be minimized by proper limitation of deformations; by careful
attention to detail; and by providing proper clearances for exterior cladding, glazing, partitions,
and wall panels. The nonstructural elements can be separated or floated free and allowed to move
independently of the structure. If these elements are tied rigidly to the structure, they should be
protected from deformations that can cause cracking; otherwise, one must expect such damage.
It should be recognized, however, that major earthquake ground motions can cause deformations
much larger than the specified drift limits in the Provisions.
2
1997 Commentary, Chapter 1
Where prescribed wind loading governs the stress or drift design, the resisting system still must
conform to the special requirements for seismic force resisting systems. This is required in order
to resist, in a ductile manner, potential seismic loadings in excess of the prescribed loads.
A proper continuous load path is an obvious design requirement for equilibrium, but experience
has shown that it often is overlooked and that significant damage and collapse can result. The
basis for this design requirement is twofold:
1. To ensure that the design has fully identified the seismic force resisting system and its
appropriate design level and
2. To ensure that the design basis is fully identified for the purpose of future modifications or
changes in the structure.
Detailed requirements for selecting or identifying and designing this load path are given in the
appropriate design and materials chapters.
1.2.1 Scope: The scope statement establishes in general terms the applicability of the Provisions
as a base of reference. Certain structures are exempt and need not comply:
1. Detached one- and two-family dwellings located where SDS is less than 0.4g also are exempt
because they represent exceptionally low risks (see Sec. 1.2).
2. A simple procedure is specified for detached one- and two-story wood frame dwellings in
regions of higher seismicity. Although some control is necessary to ensure the integrity of
such structures, it is felt that the requirements of Sec. 12.5 are adequate to provide the safety
required based on the history of such frame construction--especially low structures--in
earthquakes.
3. Agricultural storage structures are generally exempt from most code requirements because of
the exceptionally low risk to life involved and that is the case of the Provisions.
Existing structures, except for additions thereto and changes of occupancy therein, are not within
the scope of the Provisions. FEMA, however, currently is sponsoring a program on mitigation of
the seismic hazard to existing buildings; for information, write FEMA, Earthquake Programs,
Washington, D.C. 20472.
The Provisions are not written to prevent damage due to earth slides (such as those that occurred
in Anchorage, Alaska), to liquefaction (such as occurred in Niigata, Japan), or to tsunami (such
as occurred in Hilo, Hawaii). It provides for only minimum required resistance to earthquake
ground-shaking, without settlement, slides, subsidence, or faulting in the immediate vicinity of the
structure.
The Provisions are not retroactive and apply only to existing structures when there is an addition,
change of use, or alteration.
1.2.3 Additions: Additions that are structurally independent of an existing structure are
considered to be new structures required to conform with the Provisions. For additions that are
not structurally independent, the intent is that the addition as well as the existing structure be
made to comply with the Provisions except that an increase of up to 5 percent of the mass
3
General Provisions
contributing to seismic forces is permitted in any elements of the existing structure without
bringing the entire structure into conformance with the Provisions.
1.2.4 Change of Use:
It is strongly recommended that changes to an existing structure:
1. Should not reduce the lateral force resistance of the structure,
2. Should provide for the seismic forces required by the Provisions, or
3. Should comply with legally adopted provisions regulating the repair and rehabilitation of
existing structures as related to earthquake resistance.
When a change in use results in a change to a higher Seismic Use Group, the structure must be
made to conform to the Provisions for the new Seismic Use Group.
1.2.6 Alternate Materials and Alternate Means and Methods of Construction: It is not
possible for a design standard to provide criteria for the use of all possible materials and their
combinations and methods of construction either existing or anticipated. While not citing specific
materials or methods of construction currently available that require approval, this section serves
to emphasize the fact that the evaluation and approval of alternate materials and methods require
a recognized and accepted approval system. The requirements for materials and methods of
construction contained within the document represent the judgment of the best use of the
materials and methods based on well-established expertise. It is important that any replacement
or substitute be evaluated with an understanding of all the ramifications of performance, strength,
and durability implied by the Provisions.
It also is recognized that until needed approval standards and agencies are created, authorities
having jurisdiction will have to operate on the basis of the best evidence available to substantiate
any application for alternates. If accepted standards are lacking, it is strongly recommended that
applications be supported by extensive reliable data obtained from tests simulating, as closely as is
practically feasible, the actual load and/or deformation conditions to which the material is
expected to be subjected during the service life of the structure. These conditions, where
applicable, should include several cycles of full reversals of loads and deformations in the inelastic
range.
1.3 SEISMIC USE GROUPS AND FACTORS: The expected performance of buildings under
the various earthquakes that can affect them are controlled by assignment of each building to one
of three Seismic Use Groups. These Seismic Use Groups are categorized based on the occupancy
of the buildings within the group and the relative consequences of severe earthquake induced
damage to these buildings. The Provisions specify progressively more conservative strength, drift
control, system selection and detailing requirements for buildings contained in the three groups, in
order to attain minimum levels of earthquake performance suitable to the individual occupancies.
In previous editions of the Provisions, this categorization of structures, by occupancy, or use, was
termed a Seismic Hazard Exposure Group. The name Seismic Use Group was adopted in the
1997 Provisions as being more representative of what this classification is. Seismic hazard relates
to the severity and frequency of ground motion expected to affect a building. Since buildings
contained in these groups are spread across the various zones of seismicity, from high to low
hazard, the groups do not really relate to hazard. Rather, the groups are used to establish design
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1997 Commentary, Chapter 1
criteria, intended to produce specific types of performance in design earthquake events, based on
the importance of preventing the building from experiencing excessive damage, due to its
occupancy.
Historically, building code occupancy classifications have been based primarily on fire-safety
considerations. It was concluded, however, that these traditional classifications would at least in
part reflect some considerations contrary to good seismic design. Thus, it was decided that a new
approach was needed for defining occupancy exposure to seismic hazards based on a
commonality of conditions proposed for the use of a building facility or space. These conditions
involve evaluation of parameters consisting of, but not limited to the number, age, and condition
of the persons normally expected to be within or without the immediate environs of the building;
the size, height, and area of the building; the spacing of the building relative to public
rights-of-way over which the designer has no control relative to the future number of persons
exposed to risk by the buildings; and the varying degree of built-in or brought-in hazards based on
possible use of the building. Accordingly, early in the development of the preliminary version of
the Provisions occupancy types were regrouped and expanded to cover the range of factors
critical to seismic safety in terms of life loss. The expanded classification types were derived from
the 1973 Uniform Building Code (UBC).
In terms of post-earthquake recovery and redevelopment, certain types of occupancies are vital to
public needs, and these special occupancies were identified and given specific recognition (i.e., in
terms of disaster preparedness, fire and police stations, hospitals, and regional communication
centers identified as critical emergency services should not be included in the same classification
as retail stores, office buildings, and factories as is presently the case in some codes).
Because of vital public needs immediately following a natural disaster, attention also was given to
the preservation of strategic contents in distinct building types (e.g., storage facilities for medical
supplies, critical foodstuffs, and other emergency materials). It was noted that disaster recovery
officials initially considered the identification and protection of critical stocks needed during or
immediately following an earthquake to be of paramount importance. This was not to imply,
however, that all warehouses and storage facilities must be designed for the ultimate protection of
any or all contents. What was indicated was that warehouse facilities should be designed on the
basis of their maximum level of intended function or, to state it another way, medical supply
warehouses being designed under higher standards may house anything while storage facilities of
lesser ratings may not store critical supplies unless brought up to a higher level of seismic
performance. Subsequent discussions with disaster recovery officials revealed that emergency
contingency plans contemplated bringing needed medical and other recovery items including
foodstuffs into a disaster area from outside staging areas and, therefore, no separate category of
warehousing was required for the storage of critical materials. Thus, nine occupancy groups, A
through I, were identified with some individual occupancies and groups bearing little or no
relationship to current code groupings.
The occupancies then were consolidated into five basic groups by making a few compromises.
This consolidation was done in an effort to place those occupancies initially identified into groups
that shared common component performance criteria. The consolidation indicated that these
groups were easily identifiable by use patterns, confirmation of the original occupancy-component-performance criteria rating. The intermediate grouping involved the following: Group
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General Provisions
I--fire, police, hospitals; Group II--public assembly, open air stands, day care, schools, colleges,
retail stores, shopping centers, offices, hotels, apartments, emergency vehicles, power utilities;
Group III--restrained occupants, nurseries (nonambulatory); Group IV--aircraft hangers,
woodworking, factories, repair garages, service stations, storage garages, wholesale, general
warehouse, printing plants, factories, ice plants, dwellings, hazardous flammable storage, less hazardous flammable storage; and Group V--private garages, sheds, barns.
The occupancy grouping used in the 1985 Edition of the Provisions resulted from a logical consolidation of the grouping, consideration of code enforcement problems, and the need to use a
common hazard exposure grouping for all of the design requirements. The grouping and
definitions were modified in the 1988 Edition with further modifications in the 1997 Provisions.
It is felt that this grouping can be augmented as local conditions warrant.
Specific consideration is given to Group III, essential facilities required for post-earthquake
recovery. Also included are structures housing substances deemed to be hazardous to the public
if they are released. Added in the 1991 Edition was a flag to urge consideration of the need for
utility services after an earthquake. It is at the discretion of the authority having jurisdiction
which structures are required for post-earthquake response and recovery. This is emphasized
with the term “designated” before many of the structures listed in Sec. 1.3.1. Using Item 3,
“designated medical facilities having emergency treatment facilities” as an example, the authority
having jurisdiction should inventory medical facilities having emergency treatment facilities within
the jurisdiction and designate those to be required for post-earthquake response and recovery. In
a rural location where there may not be a major hospital, the authority having jurisdiction may
choose to require outpatient surgery clinics to be designated Group III structures. On the other
hand, these same clinics in a major jurisdiction with hospitals nearby probably would not be
designated Group III structures.
Group II structures are those having a large number of occupants and those where the occupants’
ability to exit is restrained. The potential density of public assembly uses in terms of number of
people warrant an extra level of care. The level of protection warranted for schools, day care
centers, and medical facilities is greater than the level of protection warranted for occupancies
where individuals are relatively self-sufficient in responding to an emergency.
Group I contains all uses other than those excepted generally from the requirements in Sec. 1.2.
Those in Group I have lesser life hazard only insofar as there is the probability of lesser numbers
of occupants in the buildings and the buildings are lower and/or smaller.
In buildings with multiple uses, the 1988 Edition of the Provisions required that the building be
assigned the classification of the highest group occupying 15 percent or more of the total building
area. This was changed in the 1991 Edition to require the building to be assigned to the highest
group present. These requirements were further modified to allow different portions of a
structure to be assigned different Seismic Use Group provided the higher group is not negatively
impacted by the lower group. When a lower group impacts a higher group, the higher group must
either be seismically independent of the other or the two must be in one structure designed
seismically to the standards of the higher group. Care must be taken, however, for the case in
which the two uses are seismically independent but are functionally dependent. The fire and lifesafety requirements relating to exiting, occupancy, fire-resistive construction and the like of the
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1997 Commentary, Chapter 1
higher group must not be reduced by interconnection to the lower group. Conversely, one must
also be aware that there are instances, although uncommon, where certain fire and life-safety
requirements for a lower group may be more restrictive than those for the higher group. Such
assignments also must be considered when changes are made in the use of a building even though
existing buildings are not within the scope of the Provisions.
Consideration has been given to reducing the number of groupings by combining Groups I and II
and leaving Group III the same as is stated above; however, the consensus of those involved in
the Provisions development and update efforts to date is that such a merging would not be
responsive to the relative performance desired of structures in these individual groups.
Although the Provisions explicitly require design for only a single level of ground motion, it is
expected that buildings designed and constructed in accordance with these requirements will
generally be able to meet a number of performance criteria, when subjected to earthquake ground
motions of differing severity. The performance criteria discussed here were jointly developed
during the BSSC Guidelines and Commentary for Seismic Rehabilitation of Buildings Project
(ATC, 1995) and the Structural Engineers Association of California Vision 2000 Project
(SEAOC, 1995). In the system established by these projects, building earthquake performance is
defined in terms of several standardized performance levels and reference ground motion levels.
Each performance level is defined by a limiting state in which specified levels of degradation and
damage have occurred to the structural and nonstructural building components. The ground
motion levels are defined in terms of their probability of exceedance.
Four performance levels are commonly described as meaningful for the design of buildings.
Although other terminology has been used in some documents, these may respectively be termed
the operational, immediate occupancy, life safe, and collapse prevention levels. Of these, the
operational level represents the least level of damage to the structure. Structures meeting this
level when responding to an earthquake will experience only negligible damage to their structural
systems and minor damage to nonstructural systems. The building will retain nearly all of its preearthquake strength and stiffness and all mechanical, electrical, plumbing, and other systems
necessary for the normal operation of the building will be functional. If repairs are required, these
can be conducted at the convenience of the occupants. The risk to life safety during an
earthquake in a building meeting this performance level is negligible. Note, that in order for a
building to meet this level, all utilities required for normal operation must be available, either
through standard public service or emergency sources maintained for that purpose. Except for
very low levels of ground motion, it is generally not practical to design buildings to meet this
performance level.
The immediate occupancy level is similar to the operational level although somewhat more
damage to non-structural systems is anticipated. Damage to the structural systems is very slight
and the building retains all of its pre-earthquake strength and nearly all of its stiffness. Nonstructural elements, including ceilings and cladding, but also mechanical and electrical components,
remain secured and do not represent hazards. The building remains safe to occupy, however,
some repair and clean-up is probably required before the building can be restored to normal
service. In particular, it is expected that utilities necessary for normal function of all systems will
not be available, although those necessary for life safety systems would be provided. Some
equipment and systems used in normal function of the building may experience internal damage
7
General Provisions
due to shaking of the building, but most would be expected to operate if the necessary utility
service was available. Similar to the operational level, the risk to life safety during an earthquake
in a building meeting this performance level is negligible. Structural repair may be completed at
the occupant’s convenience, however, significant nonstructural repair and cleanup is probably
required before normal function of the building can be restored.
At the life safe level, significant structural and nonstructural damage has occurred. The building
may have lost a substantial amount of its original lateral stiffness and strength but still retains a
significant margin against collapse. The structure may have permanent lateral offset and some
elements of the lateral force resisting system may exhibit substantial cracking, spalling, yielding
and buckling. Nonstructural elements of the building, while secured and not presenting falling
hazards, are severely damaged and can not function. The building is not safe for continued
occupancy until repairs are instituted as strong ground motion from aftershocks could result in life
threatening damage. Repair of the building is expected to be feasible, however, it may not be
economically attractive to do so. The risk to life during the earthquake, in a building meeting this
performance level is very low.
At the near collapse level a building has sustained nearly complete damage. The lateral force
resisting system has lost most of its original stiffness and strength and little margin remains against
collapse. Substantial degradation of the structural elements has occurred including extensive
cracking and spalling of masonry and concrete elements and buckling and fracture of steel
elements. The structure may have significant permanent lateral offset. Nonstructural elements of
the building have experienced substantial damage and may have become dislodged creating falling
hazards. The building is unsafe for occupancy as even relatively moderate ground motion from
aftershocks could induce collapse. Repair of the building and restoration to service is probably
not practically achievable.
The design ground motion contained in the Provisions is taken as two-thirds of the maximum
considered earthquake ground motion. Such ground motion may have a return period varying
from a few hundred years to a few thousand years, depending on the regional seismicity. It is
expected that buildings designed in accordance with the requirements for Group I would achieve
the life safe or better performance level for these ground motions. Buildings designed in
accordance with the requirements for Group III should be able to achieve the Immediate
Occupancy or better performance level for this ground motion. Buildings designed to the
requirements for Group II would be expected to achieve performance better than the life safe level
but perhaps less than the immediate occupancy level for this ground motion.
While the design ground motion represents a rare earthquake event, it may not be the most severe
event that could ever effect a site. In zones of moderate seismicity, it has been common practice
in the past to consider ground motion with a 98 percent chance of nonexceedance in 50 years, or
an average return period of 2,500 years, as being reasonably representative of the most severe
ground motion ever likely to effect a site. This earthquake has been variously termed a maximum
credible earthquake, maximum capable event and, most recently, a maximum considered event.
The recent terminology is adopted here in recognition that ground motion of this probability level
is not the most severe motion that could ever effect the site, but is considered sufficiently
improbable that more severe ground motions need not practically be considered. In regions near
major active faults, such as coastal California, estimates of ground motion at this probability of
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1997 Commentary, Chapter 1
exceedance can produce structural demands much larger than has typically been recorded in past
earthquakes. Consequently, in these zones, the maximum considered earthquake is now
commonly taken based on conservative estimates of the ground motion from a deterministic
event, representing the largest magnitude event that the nearby faults are believed capable of
producing.
It is expected that buildings designed to the requirements for Group I would be capable of
responding to the maximum considered earthquake at a near collapse or better performance level.
Structures designed to the requirements for Group III should be capable of responding to such
ground motions at the Life Safe level. Structures designed and constructed to the requirements
for Group II structures should be capable of responding to maximum considered earthquake
ground motions with a performance intermediate to the near collapse and life safe levels.
In zones of high seismicity, buildings may experience strong motion earthquakes several times
during their lives. It is also important to consider the performance expected of structures for
these somewhat less severe, but much more frequent, events. For this purpose, earthquake
ground shaking with a 50 percent
probability of nonexceedance in 50 years
may be considered. Sometime termed a
maximum probable event (MPE), such
ground motion would be expected to recur
at a site, one time, every 72 years.
Buildings designed to the requirements for
Group I would be expected to respond to
such ground motion at the Immediate Occupancy level. Buildings designed and
constructed to either the Group II or
Group III requirements would be expected
to perform to the Operational level for
these events. This performance is
summarized in Figure C1.3.
It is important to note that while the
FIGURE C1.3 Expected building performance.
performance indicated in Figure C1.3 is
generally indicative of that expected for
buildings designed in accordance with the Provisions, there can be significant variation in the
performance of individual buildings from these expectations. This variation results from
individual site conditions, quality of construction, structural systems, detailing, overall building
configuration, inaccuracies in our analytical techniques and a number of other complex factors.
As a result of these many factors, and intentional conservatism contained in the Provisions, most
buildings will perform better than indicated in the figure and others will not perform as well.
1.3.5 Group III Structure Access Protection: This section establishes the requirement for
access protection for Seismic Use Group III structures. There is a need for ingress/egress to
those structures that are essential post-earthquake facilities and this shall be considered in the
siting and design of a structure.
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General Provisions
1.4 OCCUPANCY IMPORTANCE FACTOR: The concept of an occupancy importance
factor has been present for many years in the Uniform Building Code; however, it was first
adopted into the Provisions for the 1997 Edition. This is one of several ways the Provisions
attempt to control the seismic performance capability of buildings in different Seismic Use
Groups. Specifically, the occupancy importance factor modifies the R coefficients used to
determine minimum design base shear forces. Structures assigned occupancy importance factors
exceeding 1.0 must be designed for larger base shear forces. As a result, they are expected to
experience lower ductility demands than structures designed with lower occupancy importance
factors and, hence, would be expected to sustain less damage. The Provisions also control
structural vulnerability to damage by specifying more stringent drift limits for structures in some
Seismic Use Groups. Further discussion of these concepts may be found in Commentary Sec.
5.2.1 and 5.2.8.
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Chapter 2 Commentary
GLOSSARY AND NOTATIONS
The definitions that appear in Provisions Chapter 2 are reproduced here to facilitate reference by
the reader. Note, however, that in the Provisions volume, terms defined in Chapter 2 are printed
in italics.
2.1 GLOSSARY:
Active Fault: A fault for which there is an average historic slip rate of 1 mm per year or more
and geologic evidence of seismic activity within Holocene times (past 11,000 years).
Addition: An increase in building area, aggregate floor area, height, or number of stories of a
structure.
Adjusted Resistance: The reference resistance adjusted to include the effects of all applicable
adjustment factors resulting from end use and other modifying factors. Time effect factor (8
8)
adjustments are not included.
Alteration: Any construction or renovation to an existing structure other than an addition.
Appendage: An architectural component such as a canopy, marquee, ornamental balcony, or
statuary.
Approval: The written acceptance by the authority having jurisdiction of documentation that
establishes the qualification of a material, system, component, procedure, or person to fulfill the
requirements of these provisions for the intended use.
Architectural Component Support: Those structural members or assemblies of members,
including braces, frames, struts and attachments, that transmit all loads and forces between
architectural systems, components, or elements and the structure.
Attachments: Means by which components and their supports are secured or connected to the
seismic-force-resisting system of the structure. Such attachments include anchor bolts, welded
connections, and mechanical fasteners.
Base: The level at which the horizontal seismic ground motions are considered to be imparted to
the structure.
Base Shear: Total design lateral force or shear at the base.
Basement. A basement is any story below the lowest story above grade.
Boundary Elements: Diaphragm and shear wall boundary members to which sheathing transfers
forces. Boundary members include chords and drag struts at diaphragm and shear wall
perimeters, interior openings, discontinuities, and re-entrant corners.
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Commentary, Chapter 1
Braced Wall Line: A series of braced wall panels in a single story that meets the requirements of
Sec. 12.5.2.
Braced Wall Panel: A section of wall braced in accordance with Sec. 12.5.2.
Building: Any structure whose use could include shelter of human occupants.
Boundary Members: Portions along wall and diaphragm edges strengthened by longitudinal and
transverse reinforcement and/or structural steel members.
Cantilevered Column System: A seismic-force-resisting system in which lateral forces are
resisted entirely by columns acting as cantilevers from the foundation.
Component: A part or element of an architectural, electrical, mechanical, or structural system.
Component, Equipment: A mechanical or electrical component or element that is part of a
mechanical and/or electrical system within or without a building system.
Component, Flexible: Component, including its attachments, having a fundamental period
greater than 0.06 sec.
Component, Rigid: Component, including its attachments, having a fundamental period less
than or equal to 0.06 sec.
Concrete:
Plain Concrete: Concrete that is either unreinforced or contains less reinforcement than the
minimum amount specified in Ref. 6-1 for reinforced concrete.
Reinforced Concrete: Concrete reinforced with no less than the minimum amount required
by Ref. 6-1, prestressed or nonprestressed, and designed on the assumption that the two
materials act together in resisting forces.
Confined Region: That portion of a reinforced concrete component in which the concrete is
confined by closely spaced special transverse reinforcement restraining the concrete in directions
perpendicular to the applied stress.
Construction Documents: The written, graphic, electronic, and pictorial documents describing
the design, locations, and physical characteristics of the project required to verify compliance with
these Provisions.
Container: A large-scale independent component used as a receptacle or vessel to accommodate
plants, refuse, or similar uses.
Coupling Beam: A beam that is used to connect adjacent concrete wall piers to make them act
together as a unit to resist lateral loads.
Deformability: The ratio of the ultimate deformation to the limit deformation.
High Deformability Element: An element whose deformability is not less than 3.5
when subjected to four fully reversed cycles at the limit deformation.
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General Provisions
Limited Deformability Element: An element that is neither a low deformability or a
high deformability element.
Low Deformability Element: An element whose deformability is 1.5 or less.
Deformation:
Limit Deformation: Two times the initial deformation that occurs at a load equal to
40 percent of the maximum strength.
Ultimate Deformation: The deformation at which failure occurs and which shall be
deemed to occur if the sustainable load reduces to 80 percent or less of the maximum
strength.
Design Earthquake Ground Motion: The earthquake effects that buildings and structures are
specifically proportioned to resist as defined in Sec. 4.1.
Design Earthquake: Earthquake effects that are two-thirds of the corresponding maximum
considered earthquake.
Designated Seismic System: Those architectural, electrical, and mechanical systems and their
components that require design in accordance with Sec. 6.1 and that have a component
importance factor (Ip) greater than 1.
Diaphragm: A horizontal or nearly horizontal system acting to transfer lateral forces to the vertical resisting elements. Diaphragms are classified as either flexible or rigid according to the
requirements of Sec. 5.2.3.1 and 12.3.4.2.
Diaphragm, Blocked: A diaphragm in which all sheathing edges not occurring on a framing
member are supported on and fastened to blocking.
Diaphragm Boundary: A location where shear is transferred into or out of the diaphragm
sheathing. Transfer is either to a boundary element or to another force-resisting element.
Diaphragm Chord: A diaphragm boundary element perpendicular to the applied load that is
assumed to take axial stresses due to the diaphragm moment in a manner analogous to the flanges
of a beam. Also applies to shear walls.
Displacement
Design Displacement: The design earthquake lateral displacement, excluding additional
displacement due to actual and accidental torsion, required for design of the isolation system.
Total Design Displacement: The design earthquake lateral displacement, including additional
displacement due to actual and accidental torsion, required for design of the isolation system
or an element thereof.
Total Maximum Displacement: The maximum considered earthquake lateral displacement,
including additional displacement due to actual and accidental torsion, required for verification
of the stability of the isolation system or elements thereof, design of structure separations, and
vertical load testing of isolator unit prototypes.
13
Commentary, Chapter 1
Displacement Restraint System: A collection of structural elements that limits lateral
displacement of seismically isolated structures due to maximum considered earthquake ground
shaking.
Drag Strut (Collector, Tie, Diaphragm Strut): A diaphragm or shear wall boundary element
parallel to the applied load that collects and transfers diaphragm shear forces to the vertical-forceresisting elements or distributes forces within the diaphragm or shear wall. A drag strut often is
an extension of a boundary element that transfers forces into the diaphragm or shear wall.
Effective Damping: The value of equivalent viscous damping corresponding to energy
dissipated during cyclic response of the isolation system.
Effective Stiffness: The value of the lateral force in the isolation system, or an element thereof,
divided by the corresponding lateral displacement.
Enclosure: An interior space surrounded by walls.
Equipment Support: Those structural members or assemblies of members or manufactured
elements, including braces, frames, legs, lugs, snuggers, hangers or saddles, that transmit gravity
load and operating load between the equipment and the structure.
Essential Facility: A facility or structure required for post-earthquake recovery.
Factored Resistance (8N
8ND): Reference resistance multiplied by the time effect and resistance
factors. This value must be adjusted for other factors such as size effects, moisture conditions,
and other end-use factors.
Flexible Equipment Connections: Those connections between equipment components that
permit rotational and/or translational movement without degradation of performance. Examples
include universal joints, bellows expansion joints, and flexible metal hose.
Frame:
Braced Frame: An essentially vertical truss, or its equivalent, of the concentric or eccentric
type that is provided in a building frame system or dual frame system to resist shear wall.
Concentrically Braced Frame (CBF): A braced frame in which the members are
subjected primarily to axial forces.
Eccentrically Braced Frame (EBF): A diagonally braced frame in which at least one
end of each brace frames into a beam a short distance from a beam-column joint or from
another diagonal brace.
Ordinary Concentrically Braced Frame (OCBF): A steel concentrically braced frame
in which members and connections are designed in accordance with the provisions of Ref.
8-3 without modification.
Special Concentrically Braced Frame (SCBF): A steel or composite steel and concrete
concentrically braced frame in which members and connections are designed for ductile
behavior. Special concentrically braced frames shall conform to Sec. 8.2.1 .
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General Provisions
Moment Frame: A frame provided with restrained connections between the beams and
columns to permit the frame to resist lateral forces through the flexural rigidity and strength of
its members.
Intermediate Moment Frame: A moment frame of reinforced concrete meeting the
detailing requirements of Ref. 9-1, Sec. 21.8, of structural steel meeting the detailing
requirements of Ref. 8-3, Sec. 10, or of composite construction meeting the requirements
of Ref. 10-3, Part II, Sec. 6.4b, 7, 8 and 10.
Ordinary Moment Frame: A moment frame of reinforced concrete conforming to the
requirements of Ref. 9-1 exclusive of Chapter 21, of structural steel meeting the detailing
requirements of Ref. 8-3, Sec. 12, or of composite construction meeting the requirements
of Ref. 10-3, Part II, Sec. 6.4a, 7, 8 and 11.
Special Moment Frame (SMF): A moment frame of reinforced concrete meeting the
detailing requirements of Ref. 9-1, Sec. 21.2 through 21.5, of structural steel meeting the
detailing requirements of Ref. 8-3, Sec. 9, or of composite construction meeting the
requirements of Ref. 10-3, Part II, Sec. 6.4a, 7, 8 and 9.
Frame System:
Building Frame System: A structural system with an essentially complete space frame
system providing support for vertical loads. Seismic-force resistance is provided by shear
walls or braced frames.
Dual Frame System: A structural system with an essentially complete space frame system
providing support for vertical loads. Seismic force resistance is provided by a moment
resisting frame and shear walls or braced frames as prescribed in Sec. 5.2.2.1.
Space Frame System: A structural system composed of interconnected members, other
than bearing walls, that is capable of supporting vertical loads and that also may provide
resistance to shear wall.
Grade Plane. A reference plane representing the average of finished ground level adjoining the
structure at all exterior walls. Where the finished ground level slopes away from the exterior
walls, the reference plane shall be established by the lowest points within the area between the
buildings and the lot line or, where the lot line is more than 6 ft. (1829 mm) from the structure,
between the structure and a point 6 ft. (1829 mm) from the structure.
Hazardous Contents: A material that is highly toxic or potentially explosive and in sufficient
quantity to pose a significant life-safety threat to the general public if an uncontrolled release were
to occur.
High Temperature Energy Source: A fluid, gas, or vapor whose temperature exceeds 220
degrees F (378 K).
Inspection, Special: The observation of the work by the special inspector to determine
compliance with the approved construction documents and these Provisions.
15
Commentary, Chapter 1
Continuous Special Inspection: The full-time observation of the work by an approved
special inspector who is present in the area where work is being performed.
Periodic Special Inspection: The part-time or intermittent observation of the work by an approved special inspector who is present in the area where work has been or is being
performed.
Inspector, Special (who shall be identified as the Owner's Inspector): A person approved by
the authority having jurisdiction as being qualified to perform special inspection required by the
approved quality assurance plan. The quality assurance personnel of a fabricator is permitted to
be approved by the authority having jurisdiction as a special inspector.
Inverted Pendulum Type Structures: Structures that have a large portion of their mass
concentrated near the top and, thus, have essentially one degree of freedom in horizontal
translation. The structures are usually T-shaped with a single column supporting the beams or
framing at the top.
Isolation Interface: The boundary between the upper portion of the structure, which is isolated,
and the lower portion of the structure, which moves rigidly with the ground.
Isolation System: The collection of structural elements that includes all individual isolator units,
all structural elements that transfer force between elements of the isolation system, and all
connections to other structural elements. The isolation system also includes the wind-restraint
system, energy-dissipation devices, and/or the displacement restraint system if such systems and
devices are used to meet the design requirements of Chapter 13.
Isolator Unit: A horizontally flexible and vertically stiff structural element of the isolation system
that permits large lateral deformations under design seismic load. An isolator unit is permitted to
be used either as part of or in addition to the weight-supporting system of the structure.
Joint: That portion of a column bounded by the highest and lowest surfaces of the other
members framing into it.
Load:
Dead Load: The gravity load due to the weight of all permanent structural and nonstructural
components of a building such as walls, floors, roofs, and the operating weight of fixed
service equipment.
Gravity Load (W): The total dead load and applicable portions of other loads as defined in
Sec. 5.3.2.
Live Load: The load superimposed by the use and occupancy of the building not including
the wind load, earthquake load, or dead load; see Sec. 5.3.2.
Maximum Considered Earthquake Ground Motion: The most severe earthquake effects
considered by these Provisions as defined in Sec. 4.1.
Nonbuilding Structure: A structure, other than a building, constructed of a type included in
Chapter 14 and within the limits of Sec. 14.1.1.
16
General Provisions
Occupancy Importance Factor: A factor assigned to each structure according to its Seismic
Use Group as prescribed in Sec. 1.4.
Owner: Any person, agent, firm, or corporation having a legal or equitable interest in the
property.
Partition: A nonstructural interior wall that spans from floor to ceiling, to the floor or roof
structure immediately above, or to subsidiary structural members attached to the structure above.
P-Delta Effect: The secondary effect on shears and moments of structural members induced due
to displacement of the structure.
Quality Assurance Plan: A detailed written procedure that establishes the systems and components subject to special inspection and testing.
Reference Resistance: The resistance (force or moment as appropriate) of a member or
connection computed at the reference end use conditions.
Registered Design Professional: An architect or engineer, registered or licensed to practice
professional architecture or engineering, as defined by the statutory requirements of the
professional registrations laws of the state in which the project is to be constructed.
Roofing Unit: A unit of roofing material weighing more than 1 pound (0.5 kg).
Seismic Design Category: A classification assigned to a structure based on its Seismic Use
Group and the severity of the design earthquake ground motion at the site.
Seismic-Force-Resisting System: That part of the structural system that has been considered in
the design to provide the required resistance to the shear wall prescribed herein.
Seismic Forces: The assumed forces prescribed herein, related to the response of the structure
to earthquake motions, to be used in the design of the structure and its components.
Seismic Response Coefficient: Coefficient Cs as determined from Sec. 5.3.2.1.
Seismic Use Group: A classification assigned to a structure based on its use as defined in Sec.
1.3.
Shallow Anchors: Anchors with embedment length-to-diameter ratios of less than 8.
Shear Panel: A floor, roof, or wall component sheathed to act as a shear wall or diaphragm.
Site Class: A classification assigned to a site based on the types of soils present and their
engineering properties as defined in Sec. 4.1.2.
Site Coefficients: The values of Fa and Fv indicated in Tables 1.4.2.3a and 1.4.2.3b, respectively.
Special Transverse Reinforcement: Reinforcement composed of spirals, closed stirrups, or
hoops and supplementary cross-ties provided to restrain the concrete and qualify the portion of
the component, where used, as a confined region.
Storage Racks: Include industrial pallet racks, movable shelf racks, and stacker racks made of
cold-formed or hot-rolled structural members. Does not include other types of racks such as
17
Commentary, Chapter 1
drive-in and drive-through racks, cantilever racks, portable racks, or racks made of materials
other than steel.
Story: The portion of a structure between the top to top of two successive finished floor surfaces
and, for the topmost story, from the top of the floor finish to the top of the roof structural
element.
Story Above Grade: Any story having its finished floor surface entirely above grade, except that
a story shall be considered as a story above grade where the finished floor surface of the story
immediately above is more that 6
ft (1829 mm ) above the grade
plane, more than 6 ft (1829 mm)
above the finished ground level
for more than 40 percent of the
total structure perimeter, or more
than 12 ft (3658 mm ) above the
finished ground level at any point.
This definition is illustrated in
Figure 2.1.
Story Drift Ratio: The story
drift, as determined in Sec. 5.3.7,
divided by the story height.
FIGURE 2.1 Definition of story above grade.
Story Shear: The summation of design lateral forces at levels above the story under consideration.
Strength:
Design Strength: Nominal strength multiplied by a strength reduction factor, N.
Nominal Strength: Strength of a member or cross section calculated in accordance with the
requirements and assumptions of the strength design methods of these Provisions (or the
referenced standards) before application of any strength reduction factors.
Required Strength: Strength of a member, cross section, or connection required to resist
factored loads or related internal moments and forces in such combinations as stipulated by
these Provisions.
Structure: That which is built or constructed and limited to buildings and nonbuilding structures
as defined herein.
Structural Observations: The visual observations performed by the registered design
professional in responsible charge (or another registered design professional) to determine that the
seismic-force-resisting system is constructed in general conformance with the construction
documents.
Structural-Use Panel: A wood-based panel product that meets the requirements of Ref. 12-10
or 12-11 and is bonded with a waterproof adhesive. Included under this designation are plywood,
oriented strand board, and composite panels.
18
General Provisions
Subdiaphragm: A portion of a diaphragm used to transfer wall anchorage forces to diaphragm
cross ties.
Testing Agency: A company or corporation that provides testing and/or inspection services.
The person in responsible charge of the special inspector(s) and the testing services shall be a
registered design professional.
Tie-Down (Hold-Down): A device used to resist uplift of the chords of shear walls. These
devices are intended to resist load without significant slip between the device and the shear wall
chord or be shown with cyclic testing to not reduce the wall capacity or ductility.
Time Effect Factor: A factor applied to the adjusted resistance to account for effects of duration
of load.
Torsional Force Distribution: The distribution of horizontal shear wall through a rigid
diaphragm when the center of mass of the structure at the level under consideration does not
coincide with the center of rigidity (sometimes referred to as diaphragm rotation).
Toughness: The ability of a material to absorb energy without losing significant strength.
Utility or Service Interface: The connection of the structure's mechanical and electrical
distribution systems to the utility or service company's distribution system.
Veneers: Facings or ornamentation of brick, concrete, stone, tile, or similar materials attached to
a backing.
Wall: A component that has a slope of 60 degrees or greater with the horizontal plane used to
enclose or divide space.
Bearing Wall: An exterior or interior wall providing support for vertical loads.
Cripple Wall: A framed stud wall, less than 8 feet (2400 mm) in height, extending from the
top of the foundation to the underside of the lowest floor framing. Cripple walls can occur in
both engineered structures and conventional construction.
Light-Framed Wall: A wall with wood or steel studs.
Light-Framed Wood Shear Wall: A wall constructed with wood studs and sheathed with
material rated for shear resistance.
Nonbearing Wall: An exterior or interior wall that does not provide support for vertical
loads other than its own weight or as permitted by the building code administered by the
authority having jurisdiction.
Nonstructural Wall: All walls other than bearing walls or shear walls.
Shear Wall (Vertical Diaphragm): A wall designed to resist lateral forces parallel to the
plane of the wall (sometimes referred to as a vertical diaphragm).
Wall System, Bearing: A structural system with bearing walls providing support for all or major
portions of the vertical loads. Shear walls or braced frames provide seismic-force resistance.
19
Commentary, Chapter 1
Wind-Restraint System: The collection of structural elements that provides restraint of the
seismic-isolated structure for wind loads. The wind-restraint system may be either an integral part
of isolator units or a separate device.
2.2 NOTATIONS:
A, B, C, D, E, F
Site classes as defined in Sec. 4.1.2.
Ab
Area (in.2 or mm2) of anchor bolt or stud in Chapters 6 and 11.
A ch
Cross-sectional area (in.2 or mm2) of a component measured to the outside of
the special lateral reinforcement.
An
Net cross-sectional area of masonry (in.2 or mm2) in Chapter 11.
Ao
The area of the load-carrying foundation (ft2 or m2).
Ap
The area of an assumed failure surface taken as a pyramid in Eq. 9.2.4.1-3 or
in Chapter 9.
Ap
Projected area on the masonry surface of a right circular cone for anchor bolt
allowable shear and tension calculations (in.2 or mm2) in Chapter 11.
As
The area of an assumed failure surface taken as a pyramid in Eq. .2.4.1-3 or in
Chapter 9.
As
Cross-sectional area of reinforcement (in.2 or mm2) in Chapters 6 and 11.
A sh
Total cross-sectional area of hoop reinforcement (in.2 or mm2), including
supplementary cross-ties, having a spacing of sh and crossing a section with a
core dimension of hc.
At
The area (in.2 or mm2) of the flat bottom of the truncated pyramid of an
assumed concrete failure surface in Sec. 9.2.4.1 or Eq. 9.2.4.1-3.
Avd
Required area of leg (in.2 or mm2) of diagonal reinforcement.
Ax
The torsional amplification factor.
ab
Length of compressive stress block (in. or mm) in Chapter 11.
ad
The incremental factor related to P-delta effects in Sec. 5.3.6.2.
ap
The component amplification factor as defined in Sec. 6.1.3.
Ba
Nominal axial strength of an anchor bolt (lb or N) in Chapter 11.
BD
Numerical coefficient as set forth in Table 13.3.3.1 for effective damping
equal to $D.
BM
Numerical coefficient as set forth in Table 13.3.3.1 for effective damping
equal to $M.
Bv
Nominal shear strength of an anchor bolt (lb or N) in Chapter 11.
20
General Provisions
b
The shortest plan dimension of the structure, in feet (mm), measured
perpendicular to d.
ba
Factored axial force on an anchor bolt (lb or N) in Chapter 11.
b
The shortest plan dimension of the structure, in feet (mm), measured perpendicular to dp (Sec. 5.6).
bv
Factored shear force on an anchor bolt (lb or N) in Chapter 11.
bw
Web width (in. or mm) in Chapter 11.
Cu
Coefficient for upper limit on calculated period; see Table 5.3.3.
Cd
The deflection amplification factor as given in Table 5.2.2.
Cs
The seismic response coefficient (dimensionless) determined in Sec. 5.3.
C̃s
The seismic response coefficient (dimensionless) determined in Sec. 5.5.2.1
and 5.5.3.1.
Csm
The modal seismic response coefficient (dimensionless) determined in
Sec. 5.4.5.
CT
The building period coefficient in Sec. 5.3.3.1.
Cvx
The vertical distribution factor as determined in Sec. 5.3.4.
c
Distance from the neutral axis of a flexural member to the fiber of maximum
compressive strain (in. or mm).
ceq
Effective energy dissipation device damping coefficient (Eq. 13.3.2.1).
D
Reference resistance in Chapter 12.
D
The effect of dead load in Sec. 5.2.7 and Chapter 13.
D’
Adjusted resistance in Chapter 12.
DD
Design displacement, in inches (mm), at the center of rigidity of the isolation
system in the direction under consideration as prescribed by Eq. 13.3.3.1.
DD’
Design displacement, in inches (mm), at the center of rigidity of the isolation
system in the direction under consideration, as prescribed by Eq. 13.4.2-1.
DM
Maximum displacement, in inches (mm), at the center of rigidity of the
isolation system in the direction under consideration, as prescribed by Eq.
13.3.3.3.
DM'
Maximum displacement, in inches (mm), at the center of rigidity of the
isolation system in the direction under consideration, as prescribed by Eq.
13.4.2-2 .
Dp
Relative seismic displacement that the component must be designed to
accommodate as defined in Sec. 6.1.4.
21
Commentary, Chapter 1
Ds
The total depth of the stratum in Eq. 5.5.2.1.2-4 (ft or m).
DTD
Total design displacement, in inches (mm), of an element of the isolation
system including both translational displacement at the center of rigidity and
the component of torsional displacement in the direction under consideration
as prescribed by Eq. 13.3.3.5-1.
DTM
Total maximum displacement, in inches (mm), of an element of the isolation
system including both translational displacement at the center of rigidity and
the component of torsional displacement in the direction under consideration
as prescribed by Eq. 13.3.3.5-2.
d
Overall depth of member (in. or mm) in Chapters 5 and 11.
d
The longest plan dimension of the structure, in ft (mm), in Chapter 13.
db
Diameter of reinforcement (in. or mm) in Chapter 11.
de
Distance from the anchor axis to the free edge (in. or mm) in Chapter 9.
dp
The longest plan dimension of the structure, in feet (mm).
E
The effect of horizontal and vertical earthquake-induced forces (Sec. 5.2.7
and Chapter 13).
Eloop
Energy dissipated in kip-inches (kN-mm), in an isolator unit during a full cycle
of reversible load over a test displacement range from )+ to ∆-, as measured
by the area enclosed by the loop of the force-deflection curve.
Em
Chord modulus of elasticity of masonry (psi or MPa) in Chapter 11.
Es
Modulus of elasticity of reinforcement (psi or MPa) in Chapter 11.
Ev
Modulus of rigidity of masonry (psi or MPa) in Chapter 11.
e
The actual eccentricity, in feet (mm), measured in plan between the center of
mass of the structure above the isolation interface and the center of rigidity of
the isolation system, plus accidental eccentricity, in feet (mm), taken as 5
percent of the maximum building dimension perpendicular to the direction of
force under consideration.
Fa
Acceleration-based site coefficient (at 0.3 sec period).
F-
Maximum negative force in an isolator unit during a single cycle of prototype
testing at a displacement amplitude of )-.
F+
Positive force in kips (kN) in an isolator unit during a single cycle of
prototype testing at a displacement amplitude of )+.
Fi , Fn , Fx
The portion of the seismic base shear, V, induced at Level I, n, or x, respectively, as determined in Sec. 5.3.4 (kip or kN).
Fp
The seismic design force center of gravity and distributed relative to the
component's weight distribution as determined in Sec. 6.1.3.
22
General Provisions
Fp
The induced seismic force on connections and anchorages as determined in
Sec. 5.2.5.1.
Fu
Specified ultimate tensile strength (psi or MPa) of an anchor (Sec. 9.2.4).
Fv
Velocity-based site coefficient (at 1.0 sec period).
Fx
Total force distributed over the height of the structure above the isolation
interface as prescribed by Eq. 13.3.5.
Fxm
The portion of the seismic base shear, Vm, induced at Level x as determined in
Sec. 5.4.6 (kip or kN).
)
fc
)
Specified compressive strength of concrete used in design.
fm
Specified compressive strength of masonry (psi or MPa) at the age of 28 days
unless a different age is specified, Chapter 11.
fr
Modulus of rupture of masonry (psi or MPa) in Chapter 11.
fNs
Ultimate tensile strength (psi or MPa) of the bolt, stud, or insert leg wires.
For A307 bolts or A108 studs, is permitted to be assumed to be 60,000 psi
(415 MPa).
fy
Specified yield strength of reinforcement (psi or MPa).
fyh
Specified yield stress of the special lateral reinforcement (psi or kPa).
G
(vs²/g = the average shear modulus for the soils beneath the foundation at
large strain levels (psf or Pa).
Go
(vso²/g = = the average shear modulus for the soils beneath the foundation at
small strain levels (psf or Pa).
g
Acceleration of gravity in in./sec2 (mm/s2).
H
Thickness of soil.
h
The height of a shear wall measured as the maximum clear height from the
foundation to the bottom of the floor or roof framing above or the maximum
clear height from the top of the floor or roof framing to the bottom of the
floor or roof framing above.
h̄
The effective height of the building as determined in Sec. 5.5.2 or 5.5.3 (ft or
m).
h
Height of a wood shear panel or diaphragm (ft or mm) in Chapter 12.
h
The roof elevation of a structure in Chapter 6.
h
Height of the member between points of support (in. or mm) in Chapter 11.
hc
The core dimension of a component measured to the outside of the special
lateral reinforcement (in. or mm).
23
Commentary, Chapter 1
hi , hn , hx
The height above the base Level I, n, or x, respectively (ft or m).
hsx
The story height below Level x = hx - hx-1 (ft or m).
I
The occupancy importance factor in Sec. 1.4.
Icr
Moment of inertia of the cracked section (in.4 or mm4) in Chapter 11.
In
Moment of inertia of the net cross-sectional area of a member (in.4 or mm4) in
Chapter 11.
Io
The static moment of inertia of the load-carrying foundation; see Sec. 5.5.2.1
(in4 or mm4).
Ip
The component importance factor as prescribed in Sec. 6.1.5.
I
The building level referred to by the subscript I; I = 1 designates the first level
above the base.
Kp
The stiffness of component or attachment as defined in Sec. 6.3.3.
Ky
The lateral stiffness of the foundation as defined in Sec. 5.5.2.1.1 (lb/in. or
N/m).
K2
The rocking stiffness of the foundation as defined in Sec. 5.5.2.1.1
(ft"lb/degree or N"m/rad).
KL/r
The lateral slenderness of a compression member measured in terms of its
effective buckling length, KL, and the least radius of gyration of the member
cross section, r.
k
The distribution exponent given in Sec. 5.3.4
Kdmax
Maximum effective stiffness, in kips/inch (kN/mm), of the isolation system at
the design displacement in the horizontal direction under consideration as
prescribed by Eq. 13.9.5.1-1.
KDmin
Minimum effective stiffness, in kips/inch (kN/mm), of the isolation system at
the design displacement in the horizontal direction under consideration as
prescribed by Eq. 13.9.5.1-2.
Kmax
Maximum effective stiffness, in kips/inch (kN/mm), of the isolation system at
the maximum displacement in the horizontal direction under consideration as
prescribed by Eq. 13.9.5.1-3.
KMin
Minimum effective stiffness, in kips/inch (kN/mm), of the isolation system at
the maximum displacement in the horizontal direction under consideration, as
prescribed by Eq. 13.9.5.1-4.
keff
Effective stiffness of an isolator unit, as prescribed by Eq. 13.9.3-1.
k̄
The stiffness of the building as determined in Sec. 5.5.2.1.1 (lb/ft or N/m).
24
General Provisions
L
The overall length of the building (ft or m) at the base in the direction being
analyzed.
L
Length of bracing member (in. or mm) in Chapter 8.
L
Length of coupling beam between coupled shear walls in Chapter 11 (in. or
mm).
L
The effect of live load in Chapter 13.
Lo
The overall length of the side of the foundation in the direction being
analyzed, Sec. 5.5.2.1.2 (ft or m).
l
The dimension of a diaphragm perpendicular to the direction of application of
force. For open-front structures, l is the length from the edge of the
diaphragm at the open front to the vertical resisting elements parallel to the
direction of the applied force. For a cantilevered diaphragm, l is the length of
the cantilever.
Rb
Effective embedment length of anchor bolt (in. or mm) in Chapter 11.
Rbe
Anchor bolt edge distance (in. or mm) in Chapter 11.
Rd
Development length (in. or mm) in Chapter 11.
Rdh
Equivalent development length for a standard hook (in. or mm) in Chapter 11.
Rld
Minimum lap splice length (in. or mm) in Chapter 11.
M
Moment on a masonry section due to unfactored loads (in."lb or N"mm) in
Chapter 11.
Ma
Maximum moment in a member at stage deflection is computed (in."lb or
N"mm) in Chapter 11.
Mcr
Cracking moment strength of the masonry (in."lb or N"mm) in the Chapter 11.
Md
Design moment strength (in."lb or N"mm) in Chapter 11.
Mf
The foundation overturning design moment as defined in Sec. 5.3.6 (ft"kip or
kN"m).
Mo, Mo1
The overturning moment at the foundation-soil interface as determined in
Sec. 5.5.2.3 and 5.5.3.2 (ft"lb or N"m).
Mnb
Unfactored ultimate moment capacity at balanced strain conditions (Sec.
7.5.3.4).
Mt
The torsional moment resulting from the location of the building masses,
Sec. 5.3.5.1 (ft"kip or kN"m).
Mta
The accidental torsional moment as determined in Sec. 5.3.5.1 (ft"kip or
kN"m).
25
Commentary, Chapter 1
Mu
Required flexural strength due to factored loads (in."lb or N"mm) in Chapter
11.
M 1, M 2
Nominal moment strength at the ends of the coupling beam (in."lb or N"mm)
in Chapter 11.
Mx
The building overturning design moment at Level x as defined in Sec. 5.3.6 or
Sec. 5.4.10 (ft"kip or kN"m).
m
A subscript denoting the mode of vibration under consideration; i.e., m = 1 for
the fundamental mode.
N
Number of stories, Sec. 5.3.3.1.
N
Standard penetration resistance, ASTM D1536-84.
N̄
Average field standard penetration test for the top 100 ft (30 m); see Sec.
4.1.2.1.
Nch
Average standard penetration for cohesionless soil layers for the top 100 ft
(30 m); see Sec. 4.1.2.1.
Nv
Force acting normal to shear surface (lb or N) in Chapter 11.
n
Designates the level that is uppermost in the main portion of the building.
n
Number of anchors (Sec. 9.2.4).
P
Axial load on a masonry section due to unfactored loads (lb or N) in Chapter
11.
Pc
Design tensile strength governed by concrete failure of anchor bolts (Sec.
9.2.4).
PD
Required axial strength on a column resulting from application of dead load,
D, in Chapter 5 (kip or kN).
PE
Required axial strength on a column resulting from application of the
amplified earthquake load, EN, in Chapter 5 (kip or kN).
PL
Required axial strength on a column resulting from application of live load, L,
in Chapter 5 (kip or kN).
Pn
Nominal axial load strength (lb or N) in Chapter 8.
Pn
The algebraic sum of the shear wall and the minimum gravity loads on the
joint surface acting simultaneously with the shear (lb or N).
Pn
Nominal axial load strength (lb or N) in Chapter 11.
Ps
Design tensile strength governed by steel of anchor bolts in Chapter 9.
Pu
Required axial load (lb or N) in Chapter 11.
Pu
Tensile strength required due to factored loads (lb or N) in Chapter 9.
26
General Provisions
P*u
Required axial strength on a brace (kip or kN) in Chapter 8.
Px
The total unfactored vertical design load at and above Level x (kip or kN).
PI
Plasticity index, ASTM D4318-93.
QE
The effect of horizontal building forces (kip or kN); see Sec. 5.2.6.
QV
The load equivalent to the effect of the horizontal and vertical shear strength
of the vertical segment, Appendix to Chapter 8.
R
The response modification coefficient as given in Table 5.2.2.
RI
Numerical coefficient related to the type of lateral-force-resisting system
above the isolation system as set forth in Table 13.3.4.2 for seismically
isolated structures.
Rp
The component response modification factor as defined in Sec. 6.1.3.
r
A characteristic length of the foundation as defined in Sec. 5.5.2.1 (ft or m).
r
Radius of gyration (in. or mm) in Chapter 11.
ra
The characteristic foundation length defined by Eq. 5.5.2.1.2-2 (ft or m).
rm
The characteristic foundation length as defined by Eq. 5.5.2.1.2-3 (ft or m).
rx
The ratio of the design story shear resisted by the most heavily loaded single
element in the story, in direction x, to the total story shear.
S
Section modulus based on net cross sectional area of a wall (in.3 or mm3) in
Chapter 11.
S1
The mapped maximum considered earthquake, 5% damped, spectral response
acceleration at a period of 1 second as defined in Sec. 4.1.1.
SD1
The design, 5% damped, spectral response acceleration at a period of one
second as defined in Sec. 4.1.2.
SDS
The design, 5% damped, spectral response acceleration at short periods as
defined in Sec. 4.1.2.
SM1
The maximum considered earthquake, 5 percent damped, spectral response
acceleration at a period of 1 second adjusted for Site Class effects as defined
in Sec. 4.1.2.
SMS
The maximum considered earthquake, 5% damped, spectral response
acceleration at short periods adjusted for Site Class effects as defined in Sec.
4.1.2.
SS
The mapped maximum considered earthquake, 5% damped, spectral response
acceleration at short periods as defined in Sec. 4.1.2.
Spr
Probable strength of precast element connectors (Sec. 9A.5.1).
27
Commentary, Chapter 1
s̄ u
Average undrained shear strength in top 100 ft (30.5 m); see Sec. 4.1.2.1,
ASTM D2166-91 or ASTM D2850-87.
sh
Spacing of special lateral reinforcement (in. or mm).
T
The fundamental period (sec) of the building as determined in Sec. 5.3.3 or
the modal period (sec) of the building modified as appropriate to account for
the effective stiffness of the energy dissipation system (Sec. 13.3.2.1).
T̃, T̃1
The effective fundamental period (sec) of the building as determined in
Sec. 5.5.2.1.1 and 5.5.3.1.
Ta
The approximate fundamental period (sec) of the building as determined in
Sec. 5.3.3.1.
TD
Effective period, in seconds (sec), of the seismically isolated structure at the
design displacement in the direction under consideration as prescribed by Eq.
13.3.3.2.
Tp
The fundamental period (sec) of the component and its attachment(s) as
defined in Sec. 6.3.3.
T0
0.2SD1/SDS.
TS
SD1/SDS .
TM
Effective period, in seconds (sec), of the seismically isolated structure at the
maximum displacement in the direction under consideration as prescribed by
Eq. 13.3.3.4.
Tm
The modal period of vibration (sec) of the mth mode of the building as determined in Sec. 5.4.5.
T4
Net tension in steel cable due to dead load, prestress, live load, and seismic
load (Sec. 8.5).
t
Specified wall thickness dimension or least lateral dimension of a column (in.
or mm) in Chapter 11.
tc
Thickness of masonry cover over reinforcing bars measured from the surface
of the masonry to the surface of the reinforcing bars (in. or mm) in Chapter
11.
V
The total design lateral force or shear at the base (kip or kN).
V
Shear on a masonry section due to unfactored loads (lb or N) in Chapter 11.
Vb
The total lateral seismic design force or shear on elements of the isolation
system or elements below the isolation system as prescribed by Eq. 13.3.4.1.
Vm
Shear strength provided by masonry (lb or N) in Chapter 11.
Vn
Nominal shear strength (lb or N) in Chapter 11.
28
General Provisions
Vs
The total lateral seismic design force or shear on elements above the isolation
system as prescribed by Eq. 13.3.4.2.
Vs
Shear strength provided by shear reinforcement (lb or N) in Chapters 6 and
11.
Vt
The design value of the seismic base shear as determined in Sec. 5.4.8 (kip or
N).
Vu
Required shear strength (lb or N) due to factored loads in Chapters 6 and 11.
Vx
The seismic design shear in Story x as determined in Sec. 5.3.5 or Sec. 5.4.8
(kip or kN).
Ṽ¹
The portion of the seismic base shear, Ṽ, contributed by the fundamental
mode, Sec. 5.5.3 (kip or kN).
)V
The reduction in V as determined in Sec. 5.5.2 (kip or kN).
)V¹
The reduction in V¹ as determined in Sec. 5.5.3 (kip or kN).
vs
The average shear wave velocity for the soils beneath the foundation at large
strain levels, Sec. 5.5.2 (ft/s or m/s).
v̄s
Average shear wave velocity in top 100 ft (30 m); see Sec. 4.1.2.1.
vso
The average shear wave velocity for the soils beneath the foundation at small
strain levels, Sec. 5.5.2 (ft/s or m/s).
W
The total gravity load of the building as defined in Sec. 5.3.2 (kip or kN). For
calculation of seismic-isolated building period, W is the total seismic dead load
weight of the building as defined in Sec. 5.5.2 and 5.5.3 (kip or kN).
W̄
The effective gravity load of the building as defined in Sec. 5.5.2 and 5.5.3
(kip or kN).
WD
The energy dissipated per cycle at the story displacement for the design
earthquake (Sec. 13.3.2).
W̄m
The effective modal gravity load determined in accordance with Eq. 5.4.5-1
(kip or kN).
Wp
Component operating weight (lb or N).
w
Width of a wood shear panel or diaphragm in Chapter 9 (ft or mm).
w
Moisture content (in percent), ASTM D2216-92.
w
The dimension of a diaphragm or shear wall in the direction of application of
force.
wi , wx
The portion of the total gravity load, W, located or assigned to Level I or x
(kip or kN).
z
The level under consideration; x = 1 designates the first level above the base.
29
Commentary, Chapter 1
x
Elevation in structure of a component addressed by Chapter 6.
y
Elevation difference between points of attachment in Chapter 6.
y
The distance, in feet (mm), between the center of rigidity of the isolation
system rigidity and the element of interest measured perpendicular to the
direction of seismic loading under consideration Chapter 13).
"
The relative weight density of the structure and the soil as determined in
Sec. 5.5.2.1.
"
Angle between diagonal reinforcement and longitudinal axis of the member
(degree or rad).
$
Ratio of shear demand to shear capacity for the story between Level x and x 1.
$̃
The fraction of critical damping for the coupled structure-foundation system,
determined in Sec. 5.5.2.1.
$D
Effective damping of the isolation system at the design displacement as
prescribed by Eq. 13.9.5.2-1.
$M
Effective damping of the isolation system at the maximum displacement as
prescribed by Eq. 13.9.5.2-2.
$o
The foundation damping factor as specified in Sec. 5.5.2.1.
$eff
Effective damping of the isolation system as prescribed by Eq. 13.9.3-2.
(
Lightweight concrete factor (Sec. 9.2.4.1).
(
The average unit weight of soil (lb/ft3 or kg/m3).
)
The design story drift as determined in Sec. 5.3.7.1 (in. or mm).
)
The displacement of the dissipation device and device supports across the
story (Sec. 13.3.2.1).
)
Suspended ceiling lateral deflection (calculated) in Sec. 6.2.6.4.2 (in. or mm).
)a
The allowable story drift as specified in Sec. 5.2.7 (in. or mm).
)m
The design modal story drift determined in Sec. 5.4.6 (in. or mm)
)p
Relative displacement that the component must be designed to accommodate
as defined in Sec. 6.2.2.2 or 6.3.2.2.
*cr
Deflection based on cracked section properties (in. or mm) in Chapter 11.
)+
Maximum positive displacement of an isolator unit during each cycle of
prototype testing.
)-
Maximum negative displacement of an isolator unit during each cycle of
prototype testing.
30
General Provisions
*max
The maximum displacement at Level x (in. or mm).
*avg
The average of the displacements at the extreme points of the structure at
Level x (in. or mm).
*x
The deflection of Level x at the center of the mass at and above Level x, Eq.
5.3.7.1 (in. or mm).
*xe
The deflection of Level x at the center of the mass at and above Level x determined by an elastic analysis, Sec. 5.3.7.1 (in. or mm).
*xem
The modal deflection of Level x at the center of the mass at and above Level x
determined by an elastic analysis, Sec. 5.4.6 (in. or mm).
*xm, *˜ xm
The modal deflection of Level x at the center of the mass at and above Level x
as determined by Eq. 5.4.6-3 and 5.5.3.2-1 (in. or mm).
*˜ x, *˜ x1
The deflection of Level x at the center of the mass at and above Level x, Eq.
5.5.2.3 and 5.5.3.2-1 (in. or mm).
0mu
Maximum usable compressive strain of masonry (in./in. or mm/mm) in
Chapter 11.
2
The stability coefficient for P-delta effects as determined in Sec. 5.3.6.2.
J
The overturning moment reduction factor (Sec. 5.3.6).
D
A reliability coefficient based on the extent of structural redundance present in
a building as defined in Sec. 5.2.7.
D
Ratio of the area of reinforcement to the net cross sectional area of masonry
in a plane perpendicular to the reinforcement in Chapter 11.
Db
Reinforcement ratio producing balanced strain conditions in Chapter 11.
Dh
Ratio of the area of shear reinforcement to the cross sectional area of masonry
in a plane perpendicular to the reinforcement in Chapter 11.
Ds
Spiral reinforcement ratio for precast prestressed piles in Sec. 7.5.3.4.
Dv
Ratio of vertical or horizontal reinforcement in walls (Ref. 7-2).
Dx
A reliability coefficient based on the extent of structural redundancy present in
the seismic-force-resisting system of a building in the x direction.
8
Time effect factor.
N
The capacity reduction factor.
N
Strength reduction factor in Chapters 6 and 11.
N
Resistance factor for steel in Chapter 8 and wood in Chapter 12.
Nim
The displacement amplitude at the ith level of the building for the fixed base
condition when vibrating in its mth mode, Sec. 5.4.5.
31
Commentary, Chapter 1
S0
Overstrength factor as defined in Table 5.2.2.
S
Factor of safety in Chapter 8.
3ED
Total energy dissipated, in kip-inches (kN-mm), in the isolation system during
a full cycle of response at the design displacement, DD.
3EM
Total energy dissipated, in kip-inches (kN-mm), on the isolation system
during a full cycle of response at the maximum displacement, DM.
3*FD+*max
Sum, for all isolator units, of the maximum absolute value of force, in kips
(kN), at a positive displacement equal to DD.
3*FD+*min
Sum, for all isolator units, of the minimum absolute value of force, in kips
(kN), at a positive displacement equal to DD.
3*FD-*max
Sum, for all isolator units, of the maximum absolute value of force, in kips
(kN), at a negative displacement equal to DD.
3*FD-*min
Sum, for all isolator units, of the minimum absolute value of force, in kips
(kN), at a negative displacement equal to DD.
3*FM+*max
Sum, for all isolator units, of the maximum absolute value of force, in kips
(kN), at a positive displacement equal to DM.
3*FM+*min
Sum, for all isolator units, of the minimum absolute value of force, in kips
(kN), at a positive displacement equal to DM.
3*FM-*max
Sum, for all isolator units, of the maximum absolute value of force, in kips
(kN), at a negative displacement equal to DM.
3*FM-*min
Sum, for all isolator units, of the minimum absolute value of force, in kips
(kN), at a negative displacement equal to DM.
32
Chapter 3 Commentary
QUALITY ASSURANCE
3.1 SCOPE: Earthquake related failures of structures that are directly traceable to poor quality
control during construction are innumerable. The literature containing earthquake damage is
replete with reports indicating that the collapse of structures could have been prevented had
proper quality assurance been exercised. The remarkable performance during earthquakes by
California schools constructed since 1933 is due in great part to the rigorous supervision of the
design and construction by the Office of the State Architect, as required by state law. The
Provisions are written to rely heavily on the concept of quality controls to ensure good
construction.
For structures located in areas of seismic risk, and subject to potential earthquake ground motion,
good quality control and verification are especially important because of the serious consequences
of failure and the unique, more complex, nature of the design and construction of buildings and
structures when required to resist earthquake forces. The weakest components in the seismicforce-resisting systems are those affected by lateral forces. Generally, the failures of structures
can be traced directly to a lack of quality control during design or construction, or both, when
these components or details are slighted.
The registered design professional in responsible charge of the structure specifies the quality
assurance requirements, the prime contractor exercises the control necessary to achieve the
desired quality, and the owner monitors the construction process through special inspections and
testing to protect the health, safety and welfare of the general public in built environment. Thus,
the special inspector is the owner's inspector. It is essential that each party recognize their
responsibilities, understand the procedures, and be capable of carrying them out. Because the
contractor and the specialty subcontractors are performing the work and exercising control on
quality, it is essential that the special inspections be performed by someone not in their direct
employ and also be approved by the authority having jurisdiction. When the owner is also the
contractor, the owner should engage independent agencies to conduct these special inspections
rather than try to qualify his or her own employees.
These Provisions are concerned with those components that affect the performance of structures
during an earthquake or structures that may be adversely affected by earthquake motions as
specified in other sections of the Provisions. The requirements included in Chapter 3 are the
minimum, and it could be the decision of the registered design professionals to include all phases
of construction, throughout the project, under a quality assurance plan. For many structures, the
additional cost to do so would be minimal. The primary method of achieving quality assurance is
through the use of specially qualified inspectors approved by the authority having jurisdiction and
working on behalf of the owner. The number of such inspectors actually employed will vary
widely depending on the size, complexity, and function of the structure under construction. These
Provisions allow the registered design professional or his or her employees to perform these
special inspections, as long as the individuals are approved by the authority having jurisdiction,
and they can demonstrate reasonable competence in the particular category of work they inspect.
33
Commentary, Chapter 3
3.2 QUALITY ASSURANCE PLAN: Introduced in this section is the concept that the quality
assurance plan ) must be prepared by the registered design professional responsible for the design
of each designated seismic system that is subject to quality assurance, whether it be architectural,
electrical, mechanical, or structural in nature. The quality assurance plan may be a very simple
listing of those elements of each system that have been designated as being important enough to
receive special inspections and/or testing. The extent and duration of the inspections shall be set
forth in the quality assurance plan, as well as the specific tests and the frequency of testing that is
required.
Although some registered design professionals have expressed reluctance to accept the
responsibility for the quality assurance plan, because of an assumed increase in potential liability, it
has been demonstrated by the performance of schools in California that have been subjected to
earthquakes, that the improved quality also acts to protect the registered design professional.
Furthermore, the registered design professional is the most qualified person to prepare the quality
assurance plan since the registered design professional is the most familiar with the design of the
structure.
The authority having jurisdiction, however, must approve the quality assurance plan and must
obtain from each contractor a written statement that the contractor understands the requirements
of the quality assurance plan and will exercise the necessary control to obtain conformance. The
exact methods of control are the responsibility of the individual contractors, subject to approval
by the authority having jurisdiction. However, special inspections of the work are required in
specific situations to provide the authority having jurisdiction reasonable assurance that there is
compliance with the approved construction documents.
The exception to the preparation of a quality assurance plan is intended for those structures
constructed of light wood framing and light gauge cold–formed steel framing with a height not
greater than 35 feet above grade that are located in areas of low seismic risk (SDS does not exceed
0.50g) and that satisfy all of the criteria indicated, or those structures constructed of reinforced
masonry not more than 25 feet above grade that are located in areas of low seismic risk (SDS does
not exceed 0.50g), and that satisfy all of the criteria indicated. All special inspection(s) and
testing that are otherwise required by the Provisions are not exempt and must be performed in
accordance with the applicable sections of the Provisions.
The exception will typically include structures for low–rise multifamily dwellings, commercial,
mercantile, and office buildings that are included in Seismic Use Group I. The exception is also
limited to those structures that do not have any of the following irregularities: torsional
irregularity, extreme torsional irregularity, nonparallel systems, stiffness irregularity (soft story),
stiffness irregularity (extreme soft story), or discontinuity in capacity (weak story). Any structure
that does not satisfy all of the criteria included in the exception or is otherwise exempted by the
Provisions is required to have a quality assurance plan prepared by a registered design
professional. It is important to emphasize that this exception is for the preparation of a quality
assurance plan, and is not an exception for the design of the structure in accordance with the
requirements of the Provisions.
The extent of the qualifications of the contractor and subcontractors can vary considerably, hence
the extent of the quality control can vary considerably. The quality assurance plan, therefore, is an
34
Quality Assurance
opportunity to identify those areas of special concern that must be addressed during the
construction process. Those areas include but are not limited to types of testing, frequency of
testing, types of inspections, frequency of inspections, and the extent of the structural
observations to be performed.
3.3 SPECIAL INSPECTIONS: The requirements listed in this section, from foundation
systems through cold formed steel framing, have been included in the national model codes for
many years, and it is a premise of these Provisions that there will be available an adequate supply
of knowledgeable and experienced inspectors to provide the necessary special inspections for the
various structural categories of work. Special training programs may have to be developed and
implemented for the nonstructural categories.
A special inspector is a person approved by the authority having jurisdiction as being qualified to
perform special inspections for the category of work involved. As a guide to the authority having
jurisdiction, it is contemplated that the special inspector is to be one of the following:
1. A person employed and supervised by the registered design professional in responsible charge
for the design of the designated seismic system or the seismic-force-resisting system for which
the special inspector is engaged.
2. A person employed by an approved inspection and/or testing agency who is under the direct
supervision of a registered design professional also employed by the same agency, using
inspectors or technicians qualified by recognized industry organizations as approved by the
authority having jurisdiction.
3. A manufacturer or fabricator of components, equipment, or machinery that has been approved
for manufacturing components that satisfy seismic safety standards and that maintains a
quality assurance plan approved by the authority having jurisdiction. The manufacturer or
fabricator is required to provide evidence of such approval by clearly marked on each
designated seismic system or seismic-force-resisting system component shipped to the
construction site.
The extent and duration of special inspections, types of testing, and the frequency of the testing
must be clearly delineated in the quality assurance plan.
3.3.9 Architectural Components: It is anticipated that the minimum requirements for
architectural components will be complied with when the special inspector is satisfied that the
method of anchoring or fastening and the number, spacing, and types of fasteners actually used
conforms with the approved construction documents for the component installed. It is noted that
such special inspection requirements are only for those components in Seismic Design Categories
D or E.
3.3.10 Mechanical and Electrical Components: It is anticipated that the minimum
requirements for mechanical and electrical components will be complied with when the special
inspector is satisfied that the method of anchoring or fastening and the number, spacing, and types
of fasteners actually used conforms with the approved construction documents for the component
installed. It is noted that such special inspection requirements are for selected electrical, lighting,
piping, and ductwork components in all Seismic Design Categories except A, and for all other
electrical equipment in Seismic Design Categories E and F.
35
Commentary, Chapter 3
3.4 TESTING: The specified testing of the structural materials follows procedures and tests
long established by industry standards. The acceptance criteria for each material to be tested
should be included in the construction documents prepared for the project.
3.4.5 Mechanical and Electrical Equipment: The registered design professional should
consider requirements to demonstrate the seismic performance of mechanical and electrical
components critical to the post–earthquake life safety of the occupants. Any requirements should
be clearly indicated on the construction documents. Any currently accepted technology should be
acceptable to demonstrate compliance with the requirements.
3.5 STRUCTURAL OBSERVATIONS: The requirements included in this section are for the
structural observation of those structures included in Seismic Design Categories D, E, or F when
one or more of the following conditions exists: the structures is included in Seismic Use Group I
or Seismic Use Group II or the structure is more than 75 feet above grade. The intent of
requiring structural observations by a registered design professional for the aforementioned
structures is to assure that the seismic-force-resisting systems and the designated seismic systems
are constructed in general conformance with the construction documents.
3.6 REPORTING AND COMPLIANCE PROCEDURES: The success of a quality assurance
plan depends upon the experience, training, and knowledge of the special inspector and the
accuracy and thoroughness of the reports prepared by the special inspector. It should be
emphasized that both the special inspector and the contractor are required to submit to the
authority having jurisdiction a final certification that the completed work is in conformance with
the approved construction documents. The contractor, having day-to-day knowledge of the
construction of the project, is in the best position to state whether or not all the construction has
been completed in accordance with the approved construction documents. To be fully aware,
however, the contractor must institute a system of reporting within his or her organization that
enables the contractor to effectively practice quality control. The special inspector can only attest
to the work that he or she has personally inspected and, therefore, the special inspector acts more
as an auditor or monitor of the quality control program exercised by the contractor and the testing
conducted by the testing agency.
Continuous inspection does not imply that the special inspector has observed all of the work as it
is being installed, rather it implies that the special inspector has observed all of the critical
conditions of the work to be sufficiently confident that the work was completed in conformance
with the construction documents.
36
Chapter 4 Commentary
GROUND MOTION
4.1 DETERMINING MAXIMUM CONSIDERED EARTHQUAKE AND DESIGN
EARTHQUAKE GROUND MOTION ACCELERATIONS AND RESPONSE SPECTRA:
This section sets alternative procedures for determining ground shaking parameters for use in
the design process. The design requirements generally use response spectra to represent
ground motions in the design process. For the purposes of these Provisions, these spectra are
permitted to be determined using either a generalized procedure in which mapped seismic
response acceleration parameters are referred to or by site-specific procedures. The
generalized procedure in which mapped values are used is described in Sec. 4.1.2. The sitespecific procedure is described in Sec. 4.1.3.
4.1.1 Maximum Considered Earthquake Ground Motions: The Provisions are intended to
provide uniform levels of performance for structures, depending on their occupancy and use
and the risk to society inherent in their failure. Sec. 1.3 of the Provisions establishes a series
of Seismic Use Groups that are used to categorize structures based on the specific Seismic
Design Category. It is the intent of these Provisions that a uniform margin of failure to meet
the seismic design criteria be provided for all structures within a given Seismic Use Group.
In past editions of the Provisions, seismic hazards around the nation were defined at a uniform
10 percent probability of exceedance in 50 years and the design requirements were based on
assigning a structure to a Seismic Hazard Exposure Group and a Seismic Performance
Category. While this approach provided for a uniform likelihood throughout the nation that
the design ground motion would not be exceeded, it did not provide for a uniform margin of
failure for structures designed for that ground motion. The reason for this is that the rate of
change of earthquake ground motion versus likelihood is not constant in different regions of
the United States.
The approach adopted in these Provisions is intended to provide for a uniform margin against
collapse at the design ground motion. In order to accomplish this, ground motion hazards are
defined in terms of maximum considered earthquake ground motions. The maximum
considered earthquake ground motions are based on a set of rules that depend on the
seismicity of an individual region. The design ground motions are based on a lower bound
estimate of the margin against collapse inherent in structures designed to the Provisions. This
lower bound was judged, based on experience, to be about a factor of 1.5 in ground motion.
Consequently, the design earthquake ground motion was selected at a ground shaking level that
is 1/1.5 (2/3) of the maximum considered earthquake ground motion.
For most regions of the nation, the maximum considered earthquake ground motion is defined
with a uniform likelihood of exceedance of 2 percent in 50 years (return period of about 2500
years). While stronger shaking than this could occur, it was judged that it would be
economically impractical to design for such very rare ground motions and the selection of the
37
1997 Commentary, Chapter 4
2 percent in 50 years likelihood as the maximum considered earthquake ground motion would
result in acceptable levels of seismic safety for the nation.
In regions of high seismicity, such as coastal California, the seismic hazard is typically
controlled by large-magnitude events occurring on a limited number of well defined fault
systems. Ground shaking calculated at a 2 percent in 50 years likelihood would be much
larger than that which would be expected based on the characteristic magnitudes of earthquakes
on these known active faults. This is because these major active faults can produce
characteristic earthquakes every few hundred years. For these regions, it is considered more
appropriate to directly determine maximum considered earthquake ground motions based on
the characteristic earthquakes of these defined faults. In order to provide for an appropriate
level of conservatism in the design process, when this approach to calculation of the maximum
considered earthquake ground motion is used, the median estimate of ground motion resulting
for the characteristic event is multiplied by 1.5.
Sec. 4.1.1 of the Provisions defines the maximum considered earthquake ground motion in
terms of the mapped values of the spectral response acceleration at short periods, SS , and at 1
second, S1 , for Site Class B sites. These values may be obtained directly from Maps 1
through 24, respectively. A detailed explanation for the development of Maps 1 through 24
appears as Appendix A of this Commentary volume. The logic by which these maps were
created, as described above and in Appendix A, is also included in the Provisions under Sec
4.1.3, Site-Specific Procedures, so that registered design professionals performing such a
study may use methods consistent with those that served as the basis for developing the maps.
4.1.2 General Procedure for Determining Maximum Considered Earthquake Ground
Motions and Design Spectral Response Accelerations: This section provides the procedure
for obtaining design site spectral response accelerations using the maps provided with the
Provisions. Most buildings and structures will be designed using the equivalent lateral force
technique of Sec. 5.3, and this general procedure to determine the design spectral response
acceleration parameters, SDS and SD1, that are directly used in that procedure. Some structures
will be designed using the modal analysis procedures of Sec. 5.4. This section also provides for
the development of a general response spectrum, which may be used directly in the modal analysis
procedure, from the design spectral response acceleration parameters, SDS and SD1.
Maps 1 and 2 respectively provide two parameters SS and S1, based on a national seismic hazard
study conducted by the U.S. Geological Survey. For most buildings and sites, they provide a
suitably accurate estimate of the maximum considered earthquake ground shaking for design
purposes. For some sites, with special soil conditions or for some buildings with special design
requirements, it may be more appropriate to determine a site specific estimate of the maximum
considered earthquake ground shaking response accelerations. Section 4.1.3 provides guidance
on site-specific procedures.
SS is the mapped value, from Map 1 of the 5% damped maximum considered earthquake spectral
response acceleration, for short period structures founded on Class B, firm rock, sites. The short
period acceleration has been determined at a period 0.2 seconds. This is because it was concluded
that 0.2 seconds was reasonably representative of the shortest effective period of buildings and
38
Ground Motion
structures that are designed by these Provisions, considering the effects of soil compliance,
foundation rocking and other factors typically neglected in structural analysis.
Similarly, S1 is the mapped value from Map 2 of the 5% damped maximum considered earthquake
spectral response acceleration at a period of 1 second on Site Class B. The spectral response
acceleration at periods other than 1 second can typically be derived from the acceleration at 1
second. Consequently, these two response acceleration parameters, SS and S1, are sufficient to
define an entire response spectrum for the period range of importance for most buildings and
structures, for maximum considered earthquake ground shaking on Class B sites.
In order to obtain acceleration response parameters that are appropriate for sites with other
characteristics, it is necessary to modify the SS and S1 values, as indicated in Sec.4.1.2.4. This
modification is performed with the use of two coefficients, Fa and Fv which respectively scale the
SS and S1 values determined for firm rock sites to appropriate values for other site conditions. The
maximum considered earthquake spectral response accelerations adjusted for Site Class effects
are designated respectively, SMS and SM1, for short period and 1 second period response. As
described above, structural design in these Provisions is performed for earthquake demands that
are 2/3 of the maximum considered earthquake response spectra. Two additional parameters, SDS
and SD1 are used to define the acceleration response spectrum for this design level event. These
are taken, respectively as 2/3 of the maximum considered earthquake values SMS and SM1, and
completely define a design response spectrum for sites of any characteristics.
Section 4.1.2.1 provides a categorization of the various classes of site conditions, as they affect
the design response acceleration parameters. Section 4.1.2.2 describes the method by which sites
can be classified according as belonging to one of these Site Classes. Section 4.1.2.3 provides
definitions of some site parameters referenced in the preceding section.
4.1.2.1 Site Class Definitions: It has long been recognized that the effects of local soil
conditions on ground motion characteristics should be considered in building design, and most
countries considering these effects have developed different design criteria for several different
soil conditions. The 1989 Loma Prieta earthquake provided abundant strong motion data that
was used extensively together with other information in developing the 1994 Provisions.
Evidence of the effects of local soil conditions has been observed globally including eastern North
America. An example of the latter is a pocket of high intensity reported on soft soils in
Shawinigan, Quebec, approximately 155 miles (250 km) from the 1925 Charlevoix magnitude 7
earthquake (Milne and Davenport, 1969).
The Applied Technology Council (ATC) study that generated the preliminary version of the
Provisions provided for the use of three Soil Profile Types considered, in the late 1970s, to be
different enough in seismic response to warrant separate site coefficients (S factors) and
experience from the September 1985 Mexico City earthquake prompted the addition of a fourth
Soil Profile Type. These have been revised for the 1994 Provisions to conform to the experiences
of the Mexico City and the 1989 Loma Prieta earthquake in California as well as to other
observations and studies showing the effects of level of shaking, rock stiffness, and soil type,
stiffness and depth on the amplification of ground motions at short and long periods. The
resulting use of higher seismic coefficients in areas of lower shaking and the addition of a "hard
rock" category in the 1994 Provisions better reflect the conditions in some parts of the country
39
1997 Commentary, Chapter 4
and incorporate recent efforts toward a seismic code for New York City (Jacob, 1990 and 1991).
The need for improvement in codifying site effects was discussed at a 1991 National Center for
Earthquake Engineering Research (NCEER) workshop devoted to the subject (Whitman, 1992),
which made several general recommendations. At the urging of Robert V. Whitman, a committee
was formed during that workshop to pursue resolution of pending issues and develop specific
code recommendations. Serving on this committee were M. S. Power (chairman), R. D.
Borcherdt, C. B. Crouse, R. Dobry, I. M. Idriss, W. B. Joyner, G. R. Martin, E. E. Rinne, and R.
B. Seed. The committee collected information, guided related research, discussed the issues, and
organized a November 1992 Site Response Workshop in Los Angeles (Martin, 1994). This
workshop discussed the results of a number of empirical and analytical studies and approved
consensus recommendations that form the basis for the 1994 Provisions.
Amplification of Peak Ground Acceleration: Seed and coworkers (1976a) conducted a statistical
study of peak accelerations developed at locations with different site conditions using 147 records
from each western U.S. earthquake of about magnitude 6.5. Based on these results, judgment and
analysis, they proposed the acceleration relations of Figure C4.1.2-1a that are applicable to any
earthquake magnitude of engineering interest. It must be noted that the data base of that study
did not include any soft clay sites and, thus, the corresponding curve in the figure was based on
the authors' experience and, consequently, was somewhat more speculative.
Idriss (1990a and 1990b), using data from the 1985 Mexico City and 1989 Loma Prieta
earthquakes, recently modified the curve for soft soil sites as shown in Figure C4.1.2-1b. In these
earthquakes, low maximum rock accelerations of 0.05g to 0.10g were amplified by factors of
from about 1.5 to 4 at sites containing soft clay layers ranging in thickness from a few feet to
more than a hundred feet and having depths of rock up to several hundred feet. As shown by the
data and site response calculations included in Figure C4.1.2-1b, the average amplification factor
for soft soil sites tends to decrease as the rock acceleration increases--from 2.5 to 3 at low
accelerations to about 1.0 for a rock acceleration of 0.4g. Since this effect is directly related to
the nonlinear stress-strain behavior in the soil as the acceleration increases, the curve in Figure
C4.1.2-1b can be applied in first approximation to any earthquake magnitude of engineering
interest.
It is clear from Figure C4.1.2-1b that low peak accelerations can be amplified several times at soil
sites, especially those containing soft layers and where the rock is not very deep. On the other
hand, larger peak accelerations can be amplified to a lesser degree and can even be slightly
deamplified at very high rock accelerations. In addition to peak rock acceleration, a number of
factors including soil softness and layering play a role in the degree of amplification. One
important factor is the impedance contrast between soil and underlying rock.
Spectral Shapes: Spectral shapes representative of the different soil conditions discussed above
were selected on the basis of a statistical study of the spectral shapes developed on such soils
close to the seismic source zone in past earthquakes (Seed et al., 1976a and 1976b; Hayashi et al.,
1971).
The mean spectral shapes determined directly from the study by Seed and coworkers (1976b),
based on 104 records from 21 earthquakes in the western part of the United States, Japan and
Turkey, are shown in Figure C4.1.2-2. The ranges of magnitudes and peak accelerations covered
40
Ground Motion
by this data base are 5.0 to 7.8 and 0.04g to 0.43g, respectively. All spectra used to generate the
mean curve for soft to medium clay and sand in Figure C4.1.2-2 correspond to rather low peak
accelerations in the soil (less than 0.10g). The spectral shapes in the figure also were compared
with the studies of spectral shapes conducted by Newmark et al. (1973), Blume et al. (1973), and
Mohraz (1976) and with studies for use in model building regulations. It was considered
appropriate to simplify the form of the curves to a family of three by combining the spectra for
rock and stiff soil conditions leading to the normalized spectral curves shown in Figure C4.1.2-3.
The curves in this figure therefore apply to the three soil conditions in the original version (1985)
of the Provisions.
The three conditions corresponding to the three lines in Figure C4.1.3-3 plus a fourth condition
introduced following the 1985 Mexico City earthquake are described as follows:
1. Soil Profile Type S1--A soil profile with either: (1) rock of any characteristic, either shale-like
or crystalline in nature, that has a shear wave velocity greater than 2,500 ft/s (762 m/s) or (2)
stiff soil conditions where the soil depth is less than 200 ft (61 m) and the soil types overlying
the rock are stable deposits of sands, gravels, or stiff clays.
2. Soil Profile Type S2--A soil profile with deep cohesionless or stiff clay conditions where the
soil depth exceeds 200 ft (61 m) and the soil types overlying rock are stable deposits of sands,
gravels, or stiff clays.
3. Soil Profile Type S3--A soil profile containing 20 to 40 ft (6 to 12 m) in thickness of soft- to
medium-stiff clays with or without intervening layers of cohesionless soils.
4. Soil Profile Type S4--A soil profile characterized by a shear wave velocity of less than 500
ft/sec (152 m/s) containing more than 40 ft (12 m) of soft clays or silts.
The post-Loma Prieta studies (Martin, 1994) have resulted in considerable modification of these
profile types resulting in the Soil Profile Types in the 1994 Provisions, A through F.
Response of Soft Sites to Low Rock Accelerations: Earthquake records on soft to medium clay
sites subjected to low acceleration levels indicate that the soil/rock amplification factors for longperiod spectral accelerations can be significantly larger than those in Figures C4.1.2-1 and C4.1.22 (Seed et al., 1974). Furthermore, the largest amplification often occurs at the natural period of
the soil deposit. In Mexico City in 1985, the maximum rock acceleration was amplified four times
by a soft clay deposit that would have been classified as S4 whereas the spectral amplitudes were
about 15 to 20 times larger than on rock at a period near 2 sec. In other parts of the valley where
the clay is thicker, the spectral amplitudes at periods ranging between 3 and 4 sec also were
amplified about 15 times, but the damage was less due to the low rock motion intensity at these
very long periods (Seed et al., 1988). Inspection of the records obtained at some soft clay sites
during the 1989 Loma Prieta earthquake indicates a maximum amplification of long-period
spectral amplitudes of the order of three to six times.
Figure C4.1.2-4 shows a comparison of average response spectra measured on rock and soft soil
sites in San Francisco and Oakland during this magnitude 7.1 earthquake. A preliminary study of
the Loma Prieta records at one 285-ft (87 m) soil deposit on rock containing a 55-ft (17 m) soft
to medium stiff clay layer (Treasure Island) seems to suggest that the largest soil/rock
41
1997 Commentary, Chapter 4
amplification of response spectra occurred at the natural period of the soil deposit, similarly to
Mexico City (Seed et al., 1990).
FIGURE C4.1.2-1 Relationships between maximum acceleration on rock and other local
site conditions: (top) Seed et al., 1976a, and (bottom) Idriss, 1990a and 1990b.
42
Ground Motion
FIGURE C4.1.2-2 Average acceleration spectra for different site conditions (Seed et al.,
1976a and 1976b).
FIGURE C4.1.2-3 Normalized response spectra, damping = 0.05.
Some relevant theoretical and experimental findings are reviewed briefly below to clarify the role
of key site parameters in determining the magnitude of the soil/rock amplification of spectral
ordinates at long periods for sites containing soft layers. These parameters are the thickness of
the soft soil, the shear wave velocity of the soft soil, the soil/rock impedance ratio (IR), the
layering and properties of the stiffer soil between soft layer and rock, and the modulus and
damping properties of the soft soil. The basic assumptions used are those typically used in onedimensional site response analyses and, thus, the conclusions drawn are restricted to sites where
these conditions are fulfilled (i.e., flat sites with horizontal layering of significant extension and far
from rock outcrops and with a clear soil-rock interface at a depth not exceeding several hundred
feet).
43
1997 Commentary, Chapter 4
FIGURE C4.4.2-4 Average spectra recorded during 1989 Loma Prieta earthquake at
rock sites and soft soil sites (Housner, 1990).
The uniform layer on elastic rock sketched in Figure C4.1.2-5 is subjected to a vertically
propagating shear wave representing the earthquake. The soil layer is assumed to behave linearly
and it has a thickness h, total (saturated) unit weight gs, shear wave velocity vs, and internal
damping ratio bs. The rock has total unit weight gr, shear wave velocity vr, and zero damping.
FIGURE C4.1.2-5 Uniform soil layer on elastic rock subjected to vertical shear waves.
44
Ground Motion
Due to the soil-rock interaction effect, the motion at the soil-rock interface C is different
(typically less) from that at the rock outcrop B. Only if the rock is rigid (vs = ¥) are the motions
at C and B equal. Of interest here is the
ratio between the motions on top of the
soil (point A) and on the rock outcrop
(point B).
When the acceleration at B is a harmonic
motion of frequency f (cps) and
amplitude aB, the acceleration at A is also
harmonic of the same frequency and
amplitude aA. The amplification ratio
aA/aB is a function of the ratio of
frequencies f/(vs/4h), of the soil damping
FIGURE C4.1.2-6 Amplification ratio soil/rock for h = 100 ft
bs, and of the rock/soil impedance ratio
(30.5 m), Vs = 1.88 cps, and IR = 6.7 (Roesset, 1977).
which is equal to grvr/gsvs. Figure
C4.1.2-6 presents aA/aB calculated for a
layer with h = 100 ft (30.5 m), vs/4h = 1.88 cps, and IR = 6.7 (Roesset, 1977). The maximum
amplification occurs essentially at the natural frequency of the layer, fsoil = Vs/4h, and is
approximately equal to:
aA
aB
'
max
I
1
IR
%
B
2
$s
(C4.1.2-1)
That is, the maximum soil/rock amplification for steady-state harmonic motion in this simple
model depends on two factors--bs and IR. When IR = ¥ (rigid rock), the only way the system can
dissipate energy is in the soil and (aA/aB)max = 2/pbs can be very large. For example, if IR = ¥ and
bs = 0.04, (aA/aB)max = 16. If IR decreases, the amplification (aA/aB)max also decreases. For
example, if IR = 15 and bs = 0.04, the amplification is cut in half, (aA/aB)max = 8.
Another way of expressing the contribution of the impedance ratio IR in Eq. C4.1.2-1 is as an
"additional equivalent soil damping" with a total damping btot in the system at its natural
frequency:
$tot . $s %
2
BIR
(C4.1.2-2)
Eq. C4.1.2-2 is very important since the maximum amplification (aA/aB)max is always inversely
proportional to btot, not only for the case of the uniform layer but also for other soil profiles on
rock. btot always includes an internal damping contribution (bs) and a second term reflecting the
rock-soil impedance contrast IR although the specific definition of IR and the numerical factor 2/p
generally will change depending on the profile. When a soft layer lies on top of a significant
45
1997 Commentary, Chapter 4
thickness of stiffer soil followed by rock, Eq. C4.1.2-2 is still qualitatively valid, but the
calculations are more complicated. In that case, the impedance contrast must consider the whole
soil profile and, thus, both soft and stiff soils play a role in determining btot and (aA/aB)max. Also,
the maximum amplification may occur at the natural frequency of the soft layer, of the whole
profile, or at some other frequency.
Two-Factor Approach and the 1992 Site Response Workshop: The recommendations developed
during the NCEER/SEAOC/BSSC Site Response Workshop mentioned above were summarized
by Rinne and Dobry (1992) and are reprinted as Appendix F of this commentary to provide the
reader with a better understanding of the thinking behind the current Provisions. Some additional
background information taken mostly from the proceedings of that workshop (Martin, 1994) is
included below.
As discussed above, soil sites generally amplify more the rock spectral accelerations at long
periods than at short periods and, for a severe level of shaking (SS >> 1.0g; S1 >> 0.4g), the shortperiod amplification or deamplification is small; this was the basis for the use in the previous
versions of the Provisions. However, the evidence that short-period accelerations including the
peak acceleration can be amplified several times, especially at soft sites subjected to low levels of
shaking, suggested the replacement of the normalized spectrum approach by the two-factor
approach sketched in Figure C4.1.2-7. In this approach, adopted in the 1994 Provisions, the
short-period plateau, represented by SMS, is multiplied by a short-period site coefficient Fa and the
long period curve represented by SM!/T is multiplied by a long-period site coefficient Fv. Both Fa
and Fv depend on the site conditions and on the level of shaking, defined respectively by the
values of SS and S! .
FIGURE C4.1.2-7 Two-factor approach to local site response.
Strong-motion recordings, as obtained from the Loma Prieta earthquake of October 17, 1989,
provide important quantitative measures of the in situ response of a variety of geologic deposits
to damaging levels of shaking. Average amplification factors derived from these data with respect
to "firm to hard rock" for short-period (0.1-0.5 sec), intermediate-period (0.5-1.5 sec), midperiod (0.4-2.0 sec), and long-period (1.5-5.0 sec) bands show that a short- and mid-period factor
46
Ground Motion
are sufficient to characterize the response of the local site conditions (Borcherdt, 1994). This
important result is consistent with the two-factor approach summarized in Figure C4.1.2-7.
Empirical regression curves fit to these amplification data as a function of mean shear wave
velocity at the site are shown in Figure C4.1.2-8.
These curves provide empirical estimates of the site coefficients Fa and Fv as a function of mean
shear wave velocity for input ground motion levels near 0.1g (Borcherdt and Glassmoyer, 1993).
The empirical amplification factors predicted by these curves are in good agreement with those
derived independently based on numerical modeling of the Loma Prieta strong-motion data (Seed
et al., 1992) and those derived from parametric studies of several hundred soil profiles (Dobry et
al., 1994b). These empirical relations are consistent with theory in that they imply that the
average amplification at a site increases as the rock/soil impedance ratio (IR) increases, similar to
the trend described by Eq. C4.1.2-1. They also are consistent with observed correlations between
amplification and shear velocity for soft clays in Mexico City (Ordaz and Arciniegas, 1992).
These short- and mid-period amplification factors implied by the Loma Prieta strong-motion data
and related calculations for the same earthquake by Joyner et al. (1994) as well as modeling
results at the 0.1g level provided the basis for the consensus values provided in Tables 4.1.2a and
4.1.2b. Values at higher levels were initially determined from modeling results for soft clays
derived by Seed (1994) with values for intermediate soil conditions derived by linear
extrapolation. A rigorous framework for extrapolation of the Loma Prieta results consistent with
the results in Tables C4.1.2a and C4.1.2b is given in the following paragraph.
FIGURE C4.1.2-8 Short-period Fa and mid-period Fv amplification factors with respect to "firm to hard" rock
plotted as a continuous function of mean shear wave velocity using the regression equations derived from the
strong-motion recordings of the Loma Prieta earthquake. The 95 percent confidence intervals for the ordinate to
the true population regression line and the amplification factors for the simplified site classes also are shown
(Borcherdt, 1994).
47
1997 Commentary, Chapter 4
Extrapolation of amplification estimates at the 0.1g level as derived from the Loma Prieta earthquake must necessarily be based on laboratory and theoretical modeling considerations because
few or no strong-motion recordings have been obtained at higher levels of motion, especially on
soft soil deposits. Resulting estimates should be consistent with other relations between large
rock and soil motions and local site conditions as summarized in Figure C4.1.2-1. The form of
the regression curve in Figure C4.1.2-8 suggests a simple and well defined procedure for
extrapolation. It shows that the functional relationship between the logarithms of amplification
and mean shear velocity is a straight line (Borcherdt, 1993). Consequently, as the amplification
factor for "firm to hard" rock is necessarily unity, the extrapolation problem is determined by
specification of the amplification factors at successively higher levels of motion for the soft-soil
site class. For input ground motion levels near 0.1g, Borcherdt (1993) began with amplification
levels specified by the empirical regression curves (Figure C4.1.2-8) for the Loma Prieta strongmotion data. Higher levels of motion were inferred from laboratory and numerical modeling
results (Seed et al., 1992; Dobry et al., 1994a). The resulting short-period (Fa) and mid-period
(Fv) site coefficients as a function of mean shear velocity (v--labeled s elsewhere in this
Commentary and in the Provisions) and input ground motion level (Ia) specified with respect to
"firm to hard" rock are given in Figure C4.1.2-9 and plotted with logarithmic scales. These
expressions state that the average amplification at a site is equal to the "rock-soil" impedance ratio
raised to an exponent (ma or mv). These exponents are defined as the slope of the straight line
determined by the logarithms of the amplification factors and the shear velocities for the soft-soil
and the "firm to hard" rock site classes at the specified input ground motion level (Borcherdt,
1993). The equations in Figure C4.1.2-9 provide a framework to illustrate a simple procedure for
derivation of amplification factors that are in general agreement with the consensus values
included in Tables 1.4.2.3a and 1.4.2.3b of the Provisions. However, the numbers in these tables
of the Provisions are not necessarily identical to the equations' predictions due to other
considerations discussed during the consensus process.
Extensive site response studies using both equivalent linear and nonlinear programs were conducted by several groups as listed by Rinne and Dobry (1992). The main objectives of these
studies were to generalize the experience of well documented earthquakes such as Loma Prieta
and Mexico City to a variety of site conditions and earthquake types and levels of shaking. Some
results obtained by Dobry et al. (1994a) are reproduced in Figures C4.1.2-10 to C4.1.2-12.
Figure C4.1.2-10 presents values of peak amplification at long periods for soft sites (labeled
RRSmax in the figure) calculated using the equivalent linear approach as a function of the plasticity
index (PI) of the soil, rock wave velocity vr, and for weak and strong shaking. The effect of PI is
due to the fact that soils with higher PI exhibit less stress-strain nonlinearity and a lower damping
bs (Vucetic and Dobry, 1991). For SSAa = 0.25g, S! = 0.1g, vr = 4,000 ft/sec (1220 m/s) and PI =
50, roughly representative of Bay area soft sites in the Loma Prieta earthquake, RRSmax = 4.4,
which coincides with the upper part of the range backfigured by Borcherdt from the records.
Note the reduction of this value of RRSmax from 4.4 to about 3.3 when SS = 1.0g, S! = 0.4g due to
soil nonlinearity. Evidence such as this is used in the 1994 Provisions to extrapolate values of Fa
and Fv at low levels of shaking--based on both analysis and observations--to high levels of shaking
for which no observations on soft sites currently are available.
48
Ground Motion
FIGURE C4.1.2-9 (a) short-period Fa and (b) mid-period Fv amplification factors with respect to "firm to hard"
rock (SC-Ib) plotted with logarithmic scales as a continuous function of mean shear wave velocity using the
indicated equations for specified levels of input ground motion. The equations correspond to straight lines
determined by the points defined as the logarithms of the amplification factors and shear velocities for the "softsoil" and "firm to hard" rock site classes. The amplification factors for the "soft-soil" site class are based on strong
motion recordings at the 0.1g level and on numerical modeling and expert opinion results for higher levels of
motion. The exponents ma and mv are given by the slope of the indicated straight lines. Amplification factors with
respect to SC-Ib for the simplified site classes are shown for the corresponding mean shear wave velocity interval
for input ground motion levels near 0.1g (Borcherdt, 1993).
49
1997 Commentary, Chapter 4
FIGURE C4.1.2-10 Summary of uniform layer analyses using simple SHAKE (Dobry et al., 1994a).
Specific equivalent linear runs using the SHAKE program corresponding to the same situation are
included in Figure C4.1.2-11 while Figure C4.1.2-12 summarizes and compares them with
calculations by Joyner et al. (1994) from the Loma Prieta records on soft sites similar to the work
by Borcherdt mentioned above.
FIGURE C4.1.2-11 Summary of uniform layer analyses using SHAKE program, h $ 50
ft (15.2 m) (Dobry et al., 1994a).
50
Ground Motion
Another important observation from analytical results such as shown in Figure C4.1.2-11 is that
the values of RRSmax are about 20 percent higher for soft sites on "hard rock"--characterized by vr
= 7,500 ft/sec (2290 m/s)--than for soft sites on "regular rock" corresponding to vr = 4,000 ft/sec
(1220 m/s). This is again the impedance ratio effect previously discussed. Separate studies
indicate that earthquake motions on outcrops of "hard rock" tend to be smaller than on outcrops
of "regular rock" by 10 to 40 percent at both short and long periods (except at very small periods
under about 0.2 sec where the reverse may be true); see Su et al. (1992) and Silva (1992). On the
basis of these studies and observations, the 1994 Provisions incorporate the difference between
"regular" rock (B) and "hard" rock of s > 5,000 ft/sec (1520 m/s) by defining a new "hard rock"
site category (A) and assigning to it site factors Fa = Fv = 0.8.
FIGURE 4.1.2-12 Comparison between RRS SHAKE program results and those
obtained by Joyner et al. (1994) for the 1989 Loma Prieta event (Dobry et al., 1994a).
Use of Geotechnical Parameters Instead of vs: Based on the studies and observations discussed
above, the site categories in the 1994 Provisions are defined in terms of the average shear wave
velocity in the top 100 ft (30.5 m) of the profile, vs. If the shear wave velocities are available for
the site, they should be used.
However, in recognition of the fact that in many cases the shear wave velocities are not available,
alternative definitions of the site categories also are included in the 1994 Provisions. They use the
standard penetration resistance for cohesionless soil layers and the undrained shear strength for
cohesive soil layers. These alternative definitions are rather conservative since the correlation
between site amplification and these geotechnical parameters is more uncertain than that with vs.
That is, there will be cases when the values of Fa and Fv will be smaller if the site category is
based on vs rather than on the geotechnical parameters. Also, the reader must not interpret the
site category definitions as implying any specific numerical correlation between shear wave
velocity on the one hand and standard penetration or shear strength on the other.
4.1.2.5 General Procedure Design Response Spectrum: This section provides a general
method for obtaining a 5% damped response spectrum from the site design acceleration response
51
1997 Commentary, Chapter 4
parameters SaS and Sa1. This spectrum is based on that proposed by Newmark and Hall, as a series
of three curves representing in the short period, a region of constant spectral response
acceleration; in the long period a range of constant spectral response velocity; and in the very long
period, a range of constant response spectral displacement. Response acceleration at any period
in the long period range can be related to the constant response velocity by the equation:
S a ' TSv '
2B
S
T v
(C4.1.2.5-1)
where T is the circular frequency of motion, T is the period and Sv is the constant spectral
response velocity. The site design spectral response acceleration at 1 second, Sa1, therefore is
simply related to the constant spectral velocity for the spectrum by the relation:
S a1 ' 2BSv
(C4.1.2.5-2)
and the spectral response acceleration at any period in the constant velocity range can be obtained
from the relationship:
Sa '
Sa1
(C4.1.2.5-3)
T
The constant displacement domain of the response spectrum is not included on the generalized
response spectrum because relatively few structures have a period long enough to fall into this
range. Response accelerations in the constant displacement domain can be related to the constant
displacement by a 1/T2 relationship. Section 5.4 of the Provisions, which provides the
requirements for modal analysis also provides instructions for obtaining response accelerations in
the very long period range.
4.2 SEISMIC DESIGN CATEGORY: This section establishes the five design categories that
are the keys for establishing design requirements for any building based on its use (Seismic Use
Group) and on the level of expected seismic ground motion. Once the Seismic Design Category
(A, B, C, D, E, or F) for the building is established, many other requirements such as detailing,
quality assurance, systems and height limitations, specialized requirements, and change of use are
related to it.
In previous editions of the Provisions, these categories were termed Seismic Performance
Categories. While the desired performance of the building, under the design earthquake, was one
consideration used to determine which category a building should be assigned to, it was not the
only factor. The seismic hazard at the site was actually the principle parameter that affected a
building’s category. The name was changed to Seismic Design Category to represent the uses of
these categories, which is to determine the specific design requirements.
The earlier editions of the Provisions utilized the peak velocity related acceleration, Av, to
determine a building’s Seismic Performance Category. However, this coefficient does not
adequately represent the damage potential of earthquakes on sites with soil conditions other than
rock. Consequently, the 1997 Provisions adopted the use of response spectral acceleration
52
Ground Motion
parameters SDS and SD1, which include site soil effects for this purpose. Instead of a single table,
as was present in previous editions of the Provisions, two tables are now provided, relating
respectively to short period and long period structures.
Seismic Design Category A represents structures in regions where anticipated ground motions are
minor, even for very long return periods. For such structures, the Provisions require only that a
complete lateral-force-resisting system be provided and that all elements of the structure be tied
together. A nominal design force of 1 percent of the weight of the structure is used to proportion
the lateral system.
It is not considered necessary to specify seismic-resistant design on the basis of a maximum
considered earthquake ground motion for Seismic Design Category A structures because the
ground motion computed for the areas where these structures are located is determined more by
the rarity of the event with respect to the chosen level of probability than by the level of motion
that would occur if a small but close earthquake actually did occur. However, it is desirable to
provide some protection against both earthquakes and many other types of unanticipated loadings.
Thus, the requirements for Seismic Design Category A provide a nominal amount of structural
integrity that will improve the performance of buildings in the event of a possible but rare
earthquake even though it is possible that the ground motions could be large enough to cause
serious damage or even collapse. The result of design to Seismic Design Category A
requirements is that fewer building would collapse in the vicinity of such an earthquake.
The integrity is provided by a combination of requirements. First, a complete load path for lateral
forces must be identified. Then it must be designed for a lateral force equal to a 1 percent
acceleration on the mass. The minimum connection forces specified for Seismic Design Category
A also must be satisfied.
The 1 percent value has been used in other countries as a minimum value for structural integrity.
For many structures, design for the wind loadings specified in the local buildings codes normally
will control the lateral force design when compared to the minimum integrity force on the
structure. However, many low-rise, heavy structures or structures with significant dead loads
resulting from heavy equipment may be controlled by the nominal 1 percent acceleration. Also,
minimum connection forces may exceed structural forces due to wind in some structures.
Seismic Design Category B includes Seismic Use Group I and II structures is regions of seismicity
where only moderately destructive ground shaking is anticipated. In addition to the requirements
for Seismic Design Category A, structures in Seismic Design Category B must be designed for
forces determined using Maps 1 through 24.
Seismic Design Category C includes Seismic Use Group III structures in regions where
moderately destructive ground shaking may occur as well as Seismic Use Group I and II
structures in regions with somewhat more severe ground shaking potential. In Seismic Design
Category C, the use of some structural systems is limited and some nonstructural components
must be specifically design for seismic resistance.
Seismic Design Category D includes structures of Seismic Use Group I, II, and III located in
regions expected to experience destructive ground shaking but not located very near major active
faults. In Seismic Design Category D, severe limits are placed on the use of some structural
53
1997 Commentary, Chapter 4
systems and irregular structures must be subjected to dynamic analysis techniques as part of the
design process.
Seismic Design Category E includes Seismic Use Group I and II structures in regions located
very close to major active faults and Seismic Design Category F includes Seismic Use Group III
structures in these locations. Very severe limitations on systems, irregularities, and design
methods are specified for Seismic Design Categories E and F. For the purpose of determining if a
structure is located in a region that is very close to a major active fault, the Provisions use a
trigger of a mapped maximum considered earthquake spectral response acceleration at 1 second
periods, S1, of 0.75g or more regardless of the structure’s fundamental period. The mapped short
period acceleration, SS, was not used for this purpose because short period response accelerations
do not tend to be affected by near-source conditions as strongly as do response accelerations at
longer periods.
Local or regional jurisdictions enforcing building regulations need to consider the effect of the
maps, typical soil conditions, and Seismic Design Categories on the practices in their jurisdictional
areas. For reasons of uniformity of practice or reduction of potential errors, adopting ordinances
could stipulate particular values of ground motion, particular Site Classes, or particular Seismic
Design Categories for all or part of the area of their jurisdiction. For example:
1. An area with an historical practice of high seismic zone detailing might mandate a minimum
Seismic Design Category of D regardless of ground motion or Site Class.
2. A jurisdiction with low variation in ground motion across the area might stipulate particular
values of the ground motion rather than requiring use of the maps.
3. An area with unusual soils might require use of a particular Site Class unless a geotechnical
investigation proves a better Site Class.
4.2.2 Site Limitation for Seismic Design Category E and F Structures: The forces that
result on a structure located astride the trace of a fault rupture that propagates to the surface are
extremely large and it is not possibly to reliably design a structure to resist such forces.
Consequently, the requirements of this section limit the construction of buildings in Seismic
Design Categories E and F on sites subject this hazard. Similarly, the effects of landsliding,
liquefaction, and lateral spreading can be highly damaging to a building. However, the effects of
these site phenomena can more readily be mitigated through the incorporation of appropriate
design measures than can direct ground fault rupture. Consequently, construction on sites with
these hazards is permitted, if appropriate mitigation measures are included in the design.
REFERENCES
Borcherdt, R. D. 1994. "Simplified Site Classes and Empirical Amplification Factors for SiteDependent Code Provisions." In Proceedings of the NCEER/SEAOC/BSSC Workshop on Site
Response During Earthquakes and Seismic Code Provisions, University of Southern California,
Los Angeles, November 18-20, edited by G. M. Martin.
Borcherdt, R. D. 1993. "General Methodology for Estimating Free-Field Site Specific Response
Spectra." In Minutes of the Building Seismic Safety Council meeting, April 5-6, 1993, San
Francisco, California.
54
Ground Motion
Borcherdt, R. D., and G. Glassmoyer. 1993. "Influence of Local Geology and Weak Ground
Motions in the San Francisco Bay Region, California, and Their Implications for Site-Specific
Code Provisions." In The Loma Prieta Earthquake of October 17, 1989 - Strong Ground
Motion, USGS Professional Paper 1551-A, edited by R. D. Borcherdt.
Dobry, R., G. M. Martin, E. Parra, and A. Bhattacharyya. 1994a. "Development of SiteDependent Ratios of Elastic Response Spectra (RRS) and Site Categories for Building Seismic
Codes." In Proceedings of the NCEER/SEAOC/BSSC Workshop on Site Response During
Earthquakes and Seismic Code Provisions, University of Southern California, Los Angeles,
November 18-20, edited by G. M. Martin.
Dobry, R., G. M. Martin, E. Parra, and A. Bhattacharyya. 1994b. Study of Ratios of Response
Spectra Soil/Rock and of Site Categories for Seismic Codes. Buffalo, New York: National
Center for Earthquake Engineering Research.
Hayashi, S., Tsuchida, and E. Kurata. 1971. "Average Response Spectra for Various Subsoil
Conditions." Paper presented at the Third Joint Meeting of the U.S.-Japan Panel on Wind and
Seismic Effects, UNJR, Tokyo, May 10-12.
Housner, G. W. 1990. Competing Against Time: Report to Governor George Deukmejian from
the Governor's Board of Inquiry on the 1989 Loma Prieta Earthquake.
Idriss, I. M. 1990a. "Response of Soft Soil Sites During Earthquakes." In Proceedings of the
Symposium to Honor Professor H. B. Seed, Berkeley.
Idriss, I. M. 1990b. "Influence of Local Site Conditions on Earthquake Ground Motions." In
Proceedings of the 4th U.S. National Conference on Earthquake Engineering, Palm Springs, Vol.
1, pp. 55-57.
Jacob, K. 1990. "Seismic Hazards and the Effects of Soils on Ground Motions for the Greater
New York City Metropolitan Region." In Geotechnical Aspects of Seismic Design in the N.Y.C.
Metropolitan Areas; Risk Assessment, Code Requirements and Design Techniques, ASCE
Metropolitan Section, page 24, New York, New York, November 13-14.
Jacob, K. 1991. "Seismic Zonation and Site Response: Are Building-Code Soil Factors
Adequate to Account for Variability of Soil Conditions Across the US?" In Proceedings of the
4th International Conference on Seismic Zonation, Vol. II, pp. 695-702. Oakland, California:
Earthquake Engineering Research Institute.
Joyner, W. B., T. E. Fumal, and G. Glassmoyer. 1994. "Empirical Spectral Response Ratios for
Strong Motion Data from the 1989 Loma Prieta, California, Earthquake." In Proceedings of the
NCEER/SEAOC/BSSC Workshop on Site Response During Earthquakes and Seismic Code
Provisions, University of Southern California, Los Angeles, November 18-20, edited by G. M.
Martin.
Martin, G. M., Editor. 1994. Proceedings of the NCEER/SEAOC/BSSC Workshop on Site
Response During Earthquakes and Seismic Code Provisions, University of Southern California,
Los Angeles, November 18-20.
55
1997 Commentary, Chapter 4
Milne, W. G., and A. G. Davenport. 1969. "Distribution of Earthquake Risk in Canada."
Bulletin of the Seismological Society of America 59(2):754-779.
Mohraz, B. 1976. "A Study of Earthquake Response Spectra for Different Geological
Conditions." Bulletin of the Seismological Society of America 66 (3):915-935.
Newmark, N. M. 1973. A Study of Vertical and Horizontal Spectra, Report WASH-1255.
Washington, D.C.: U.S. Atomic Energy Commission, Directorate of Licensing.
Ordaz, M., and Arciniegas. 1992. Personal communication to Ricardo Dobry.
Rinne, E., and R. Dobry. 1992. Preliminary Site Recommendations, Memorandum to Roland
Sharpe, Chairman TS 2, Building Seismic Safety Council, December 11.
Roesset, J. M. 1977. "Soil Amplification of Earthquakes." In Numerical Methods in Geotechnical Engineering, edited by C. S. Desai and J. T. Christian, Chapter 19, pp. 639-682. New
York: McGraw-Hill.
Seed, R. B. 1994. In Proceedings of the NCEER/SEAOC/BSSC Workshop on Site Response
During Earthquakes and Seismic Code Provisions, University of Southern California, Los
Angeles, November 18-20, edited by G. M. Martin.
Seed, R. B., S. E. Dickenson, G. A. Rau, R. K. White, and C. M. Mok. 1992. "Observations
Regarding Seismic Response Analyses for Soft and Deep Clay Sites."
Seed, R., S. E. Dickenson, M. F. Riemer, J. D. Bray, N. Sitar, J. K. Mitchell, I. M. Idriss, R. E.
Kayen, A. Kropp, L. F. Harder Jr., and M. S. Power. 1990. Preliminary Report on the Principal
Geotechnical Aspects of the October 17, 1989, Loma Prieta Earthquake. Report UCB/EERC90/05. Berkeley, California: EERC.
Seed, H. B., R. Murarka, J. Lysmer, and I. M. Idriss. 1976a. "Relationships Between Maximum
Acceleration, Maximum Velocity, Distance from Source and Local Site Conditions for
Moderately Strong Earthquakes." Bulletin of the Seismological Society of America 66 (4):1323-1342.
Seed, H. B., M. P. Romo, J. I. Sun, A. Jaime, and J. Lysmer. 1988. "The Mexico Earthquake of
September 19, 1985--Relationships Between Soil Conditions and Earthquake Ground Motions."
Earthquake Spectra 4(4):687-729.
Seed, H. B., C. Ugas, and J. Lysmer. 1976b. "Site Dependent Spectra for Earthquake-Resistant
Design." Bulletin of the Seismological Society of America 66 (1):221-244.
Seed, H. B., C. Ugas, and J. Lysmer. 1974. Site-Dependent Spectra for Earthquake-Resistant
Design, Report EERC 74-12. Berkeley, California: EERC.
Silva, W. 1992. Personal communication to Ricardo Dobry.
Structural Engineers Association of California. 1970. Report of the Ad Hoc Committee on Cost
of Design for Earthquakes. Washington, D.C.: Office of Science and Technology, Task Force
on Earthquake Hazard Reduction.
56
Ground Motion
Su, F., K. Aki, T. Teng, Y. Zeng, S. Koyanagi, and K. Mayeda. 1992. "The Relation Between
Site Amplification Factor and Surficial Geology in Central California." Bulletin of the
Seismological Society of America 82(2):580-602.
Vucetic, M., and R. Dobry. 1991. "Effect of Soil Plasticity on Cyclic Response." Journal of
Geotechnical Engineering, ASCE, 117(1):89-107.
Whitman, R., Editor. 1992. Proceedings of the Site Effects Workshop, October 24-24, 1991,
Report NCEER-92-0006. Buffalo, New York: National Center for Earthquake Engineering
Research.
57
Chapter 5 Commentary
STRUCTURAL DESIGN CRITERIA
5.1 REFERENCE DOCUMENT: ASCE 7 is referenced for the combination of earthquake
loadings with other loads as well as for the computation of other loads; it is not referenced for the
computation of earthquake loads.
5.2 DESIGN BASIS: Structural design for acceptable seismic resistance includes:
1. The selection of vertical and lateral-force-resisting systems that are appropriate to the
anticipated intensity of ground shaking;
2. Layout of these systems such that they provide a continuous, regular and redundant load path
capable of ensuring that the structures act as integral units in responding to ground shaking;
and
3. Proportioning the various members and connections such that adequate lateral and vertical
strength and stiffness is present to limit damage in a design earthquake to acceptable levels.
In the Provisions, the proportioning of structures’ elements (sizing of individual members,
connections, and supports) is typically based on the distribution of internal forces computed based
on linear elastic response spectrum analyses using response spectra that are representative of,
but substantially reduced from the anticipated design ground motions. As a result, under the
severe levels of ground shaking anticipated for many regions of the nation, the internal forces and
deformations produced in most structures will substantially exceed the point at which elements of
the structures start to yield and buckle and behave in an inelastic manner. This approach can be
taken because historical precedent, and the observation of the behavior of structures that have
been subjected to earthquakes in the past demonstrates that if suitable structural systems are
selected, and structures are detailed with appropriate levels of ductility, regularity, and continuity,
it is possible to perform an elastic design of structures for reduced forces and still achieve
acceptable performance. Therefore, these procedures adopt the approach of proportioning
structures such that under prescribed design lateral forces that are significantly reduced, by the
response modification coefficient R, from those that would actually be produced by a design
earthquake they will not deform beyond a point of significant yield. The elastic deformations
calculated under these reduced design forces are then amplified, by the deflection amplification
factor Cd to estimate the expected deformations likely to be experienced in response to the design
ground motion. (The deflection amplification is specified in Sec. 5.3.7.) Considering the intended
structural performance and acceptable deformation levels, Sec. 5.2.8 prescribes the story drift
limits for the expected (i.e. amplified) deformations. These procedures differ from those in earlier
codes and design provisions wherein the drift limits were treated as a serviceability check.
The term "significant yield" is not the point where first yield occurs in any member but, rather, is
defined as that level causing complete plastification of at least the most critical region of the structure (e.g., formation of a first plastic hinge in the structure). A structural steel frame comprised of
compact members is assumed to reach this point when a “plastic hinge” develops in the most
highly stressed member of the structure. A concrete frame reaches this significant yield when at
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1997 Commentary, Chapter 5
least one of the sections of its most highly stressed component reaches its strength as set forth in
Chapter 9. For other structural materials that do not have their sectional yielding capacities as
easily defined, modifiers to working stress values are provided. These requirements contemplate
that the design includes a seismic force resisting system with redundant characteristics wherein
significant structural overstrength above the level of significant yield can be obtained by plastification at other points in the structure prior to the formation of a complete mechanism. For
example, Figure C5.2-1 shows the lateral load-deflection curve for a typical structure. Significant
yield is the level where plastification occurs at the most heavily loaded element in the structure,
shown as the lowest yield hinge on the load-deflection diagram. With increased loading, causing
the formation of additional plastic hinges, the capacity increases (following the solid curve ) until
a maximum is reached. The overstrength capacity obtained by this continued inelastic action
provides the reserve strength necessary for the structure to resist the extreme motions of the
actual seismic forces that may be generated by the design ground motion.
It should be noted that the structural overstrength described above results from the development
of sequential plastic hinging in a properly designed, redundant structure. Several other sources
will further increase structural overstrength. First, material overstrength (i.e. actual material
strengths higher than the nominal material
strengths specified in the design) may
increase the structural overstrength
Elastic response of structure
V
significantly. For example, a recent survey
V
shows that the mean yield strength of A36
steel is about 30 to 40 percent higher than
succesive
the minimum specified strength, nominally
yield hinges
used in design calculations. Second,
Fully yielded strength
V
member design strengths usually
incorporate a strength reduction (or
resistance) factor, N, to ensure a low
Design force level
V
probability of failure under design loading.
D
D
D
C
Third, designers themselves introduce
Design drift
additional overstrength by selecting
Lateral Deformation (Drift), D
sections or specifying reinforcing patterns
FIGURE C5.2-1 Inelastic force-deformation curve.
that exceed those required by the
computations. Similar situations occur
when minimum requirements of the Provisions, for example, minimum reinforcement ratios,
control the design. Finally, the design of many flexible structural systems, such as moment
resisting frames, are often controlled by the drift rather than strength limitations of the Provisions,
with sections selected to control lateral deformations rather than provide the specified strength.
The results is that structures typically have a much higher lateral resistance than specified as a
minimum by the Provisions and first actual significant yielding of structures may occur at lateral
load levels that are 30 to 100 percent higher than the prescribed design seismic forces. If
provided with adequate ductile detailing, redundancy and regularity, full yielding of structures
may occur at load levels that are two to four times the prescribed design force levels.
E
Y
S
S
d
E
Figure C5.2-1 indicates the significance of design parameters contained in the Provisions
including the response modification coefficient, R, the deflection amplification factor, Cd, and the
59
Structural Design Criteria
structural overstrength coefficient S0. The values of the response modification coefficient, R,
structural overstrength coefficient, S0, and the deflection amplification factor, Cd, provided in
Table 5.2.2, as well as the criteria for story drift including P-delta effects have been established
considering the characteristics of typical properly designed structures. If excessive “optimization”
of a structural design is performed, with lateral resistance provided by only a few elements, the
successive yield hinge behavior depicted in Figure C5.2-1 will not be able to form and the values
of the design parameters contained in the Provisions may not be adequate to provide the intended
seismic performance.
The response modification coefficient, R, essentially represents the ratio of the forces that would
develop under the specified ground motion if the structure had an entirely linearly elastic response
to the prescribed design forces (see Figure C5.2-1). The structure is to be designed so that the
level of significant yield exceeds the prescribed design force. The ratio R, expressed by the
equation:
R '
VE
(C5.2.1-1)
VS
is always larger then 1.0; thus, all structures are designed for forces smaller than those the design
ground motion would produce in a completely linear-elastic responding structure. This reduction
is possible for a number of reasons. As the structure begins to yield and deform inelastically, the
effective period of response of the structure tends to lengthen, which for many structures, results
in a reduction in strength demand. Furthermore, the inelastic action results in a significant amount
of energy dissipation, also known as hysteretic damping, in addition to the viscous damping. The
combined effect, which is also known as the ductility reduction, explains why a properly designed
structure with a fully yielded strength (Vy, in Figure C.5.2-1) that is significantly lower than the
elastic seismic force demand (VE in Figure C.5.2.1) can be capable of providing satisfactory
performance under the design ground motion excitations. Defining a system ductility reduction
factor Rd as the ratio between VE and VY (Newmark and Hall, 1981):
Rd '
VE
VY
(C5.2.1-2)
then it is clear from Figure C5.2-1 that the response modification coefficient, R, is the product of
the ductility reduction factor and structural overstrength factor (Uang, 1991):
R ' Rd S0
(C5.2.1-3)
The energy dissipation resulting from hysteretic behavior can be measured as the area enclosed by
the force-deformation curve of the structure as it experiences several cycles of excitation. Some
structures have far more energy dissipation capacity than do others. The extent of energy
dissipation capacity available is largely dependent on the amount of stiffness and strength
degradation the structure undergoes as it experiences repeated cycles of inelastic deformation.
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1997 Commentary, Chapter 5
Figure C5.2-2 indicates representative load-deformation curves for two simple substructures, such
as a beam-column assembly in a frame. Hysteretic curve (a) in the figure is representative of the
behavior of substructures that have been detailed for ductile behavior. The substructure can
maintain nearly all of its strength and stiffness over a number of large cycles of inelastic
deformation. The resulting force-deformation “loops” are quite wide and open, resulting in a
large amount of energy dissipation capacity. Hysteretic curve (b) represents the behavior of a
substructure that has not been detailed for ductile behavior. It rapidly looses stiffness under
inelastic deformation and the resulting hysteretic loops are quite pinched. The energy dissipation
capacity of such a substructure is much lower than that for the substructure (a). Structural
systems with large energy dissipation capacity have larger Rd values, and hence are assigned
higher R values, resulting in design for lower forces, than systems with relatively limited energy
dissipation capacity.
FIGURE C5.2-2 Typical hysteretic curves.
Some contemporary building codes, including those adopted in Canada and Europe have
attempted to directly quantify the relative contribution of overstrength and inelastic behavior to
the permissible reduction in design strength. Recently, the Structural Engineers Association of
California proposed such an approach for incorporation into the 1997 Uniform Building Code.
That proposal incorporated two R factor components, termed Ro and Rd to represent the
reduction due to structural overstrength and inelastic behavior, respectively. The design forces
are then determined by forming a composite R, equal to the product of the two components (See
Eq. C5.2.1-3). A similar approach was considered for adoption into the 1997 NEHRP
Provisions. However, this approach was not taken for several reasons. While it was
acknowledged that both structural overstrength and inelastic behavior are important contributors
to the R coefficients, and can be quantified for individual structures, it was felt that there was
insufficient research available at the current time to support implementation in the Provisions. In
addition, there was concern that there can be significant variation between structures in the
relative contribution of overstrength and inelastic behavior and that, therefore, this would prevent
accurate quantification on a system by system basis. Finally, it was felt that this would introduce
additional complexity into the Provisions. While it was decided not to introduce the split R value
concept into the Provisions in the 1997 update cycle, this should be considered in the future as
additional research on the inelastic behavior of structures becomes available, and as the
sophistication of design offices improves to the point that quantification of structural overstrength
can be done as a routine part of the design process. As a first step in this direction, however, the
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Structural Design Criteria
factor S0 was added to Table 5.2.2, to replace the previous 2R/5 factor used for evaluation of
brittle structural behavior modes in previous editions of the Provisions.
The R values, contained in the current Provisions, are largely based on engineering judgment of
the performance of the various materials and systems in past earthquakes. The values of R must
be chosen and used with careful judgment. For example, lower values must be used for structures
possessing a low degree of redundancy wherein all the plastic hinges required for the formation of
a mechanism may be formed essentially simultaneously and at a force level close to the specified
design strength. This situation can result in considerably more detrimental P-delta effects. Since
it is difficult for individual designers to judge the extent to which R factors should be adjusted,
based on the inherent redundancy of their designs, a new coefficient D, that is calculated based on
percent of the total lateral force resisted by any individual element has been introduced into the
Provisions in Sec. 5.2.4. Additional discussion of this issue is contained in that section.
In a departure from previous editions of the Provisions, the 1997 edition introduces an
importance factor I into the base shear equation, that varies for different types of occupancies.
This importance factor has the effect of adjusting the permissible response modification factor, R,
based on the desired seismic performance for the structure. It recognizes that as structures
experience greater levels of inelastic behavior, they also experience more damage. Thus,
introducing the importance factor, I, allows for a reduction of the R value to an effective value R/I
as a partial control on the amount of damage experienced by the structure under a design
earthquake. Strength alone is not sufficient to obtain enhanced seismic performance. Therefore,
the improved performance characteristics desired for more critical occupancies are also obtained
through application of the design and detailing requirements set forth in Sec. 5.2.6 for each Seismic Design Category and the more stringent drift limits in Table 5.2.8. These factors, in addition
to strength, are extremely important to obtaining the seismic performance desired for buildings in
some Seismic Use Groups.
Sec. 5.2.1 in effect calls for the seismic design to be complete and in accordance with the
principles of structural mechanics. The loads must be transferred rationally from their point of
origin to the final points of resistance. This should be obvious but it often is overlooked by those
inexperienced in earthquake engineering.
5.2.2 Basic Seismic-Force-Resisting Systems: For purposes of these seismic analyses and
design requirements, building framing systems are grouped in the structural system categories
shown in Table 5.2.2. These categories are similar to those contained for many years in the
requirements of the Uniform Building Code; however, a further breakdown is included for the
various types of vertical components in the seismic-force-resisting system. In selecting a
structural system, the designer is cautioned to consider carefully the interrelationship between
continuity, toughness (including minimizing brittle behavior), and redundancy in the structural
framing system as is subsequently discussed in this commentary.
Specification of R factors requires considerable judgment based on knowledge of actual
earthquake performance as well as research studies; yet, they have a major effect on building
costs. The factors in Table 5.2.2 continue to be reviewed in light of recent research results. In
the selection of the R values for the various systems, consideration has been given to the general
observed performance of each of the system types during past earthquakes, the general toughness
(ability to dissipate energy without serious degradation) of the system, and the general amount of
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1997 Commentary, Chapter 5
damping present in the system when undergoing inelastic response. The designer is cautioned to
be especially careful in detailing the more brittle types of systems (low Cd values).
A bearing wall system refers to that structural support system wherein major load-carrying
columns are omitted and the walls and/or partitions are of sufficient strength to carry the gravity
loads for some portion of the building (including live loads, floors, roofs, and the weight of the
walls themselves). The walls and partitions supply, in plane, lateral stiffness and stability to resist
wind and earthquake loadings as well as any other lateral loads. In some cases, vertical trusses
are employed to augment lateral stiffness. In general, this system has comparably lower values of
R than the other systems due to the frequent lack of redundancy for the vertical and horizontal
load support. The category designated "light frame walls with shear panels" is intended to cover
wood or steel stud wall systems with finishes other than masonry veneers.
A building frame system is a system in which the gravity loads are carried primarily by a frame
supported on columns rather than by bearing walls. Some minor portions of the gravity load may
be carried on bearing walls but the amount so carried should not represent more than a few
percent of the building area. Lateral resistance is provided by nonbearing structural walls or
braced frames. The light frame walls with shear panels are intended only for use with wood and
steel building frames. Although there is no requirement to provide lateral resistance in this
framing system, it is strongly recommended that some moment resistance be incorporated at the
joints. In a structural steel frame, this could be in the form of top and bottom clip angles or tees
at the beam- or girder-to-column connections. In reinforced concrete, continuity and full
anchorage of longitudinal steel and stirrups over the length of beams and girders framing into
columns would be a good design practice. With this type of interconnection, the frame becomes
capable of providing a nominal secondary line of resistance even though the components of the
seismic-force-resisting system are designed to carry all the seismic force.
A moment resisting space frame system is a system having an essentially complete space frame as
in the building frame system. However, in this system, the primary lateral resistance is provided
by moment resisting frames composed of columns with interacting beams or girders. Moment
resisting frames may be either ordinary, intermediate, or special moment frames as indicated in
Table 5.2.2 and limited by the Seismic Design Categories.
Special moment frames must meet all the design and detail requirements of Chapter 8, 9, or 11.
The ductility requirements for these frame systems are appropriate for all structures anticipated to
experience large inelastic demands. For this reason, they are required in zones of high seismicity
with large anticipated ground shaking accelerations. In zones of lower seismicity, the inherent
overstrength in typical structural designs is such that the anticipated inelastic demands are
somewhat reduced, and less ductile systems may be safely employed. Intermediate moment
frames of concrete must meet the requirements of Sec. 9.3.2. For buildings in which these
special design and detailing requirements are not used, lower R values are specified indicating that
ordinary framing systems do not possess as much toughness and that less reduction from the
elastic response can be tolerated. Note that Sec. 5.2.2 (Table 5.2.2) requires moment frames in
Categories D and E or F greater than 160 ft and 100 ft in height, respectively, to be special
moment frames.
Requirements for composite steel-concrete systems were newly introduced in the 1994 Edition.
The R, S0, and Cd values for the composite systems in Table 5.2.2 are similar to those for
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Structural Design Criteria
comparable systems of structural steel and reinforced concrete. The values shown in Table 5.2.2
are only allowed when the design and detailing requirements for composite structures in Chapter
10 are followed.
Inverted pendulum structures are singled out for special consideration because of their unique
characteristics. These structures have little redundancy and overstrength and concentrate inelastic
behavior at their bases. As a result, they have substantially less energy dissipation capacity than
other systems. A number of buildings incorporating this system experienced very severe damage,
and in some cases, collapse, in the 1994 Northridge earthquake.
5.2.2.1 Dual System: A dual system consists of a three-dimensional space frame made up of
columns and beams that provide primary support for the gravity loads. Primary lateral resistance
is supplied by structural nonbearing walls or bracing; the frame is provided with a redundant
lateral-force-resisting system that is a moment frame complying with the requirements of Chapters
8 and 9. The moment frame is required to be capable of resisting at least 25 percent (judgmentally selected) of the specified seismic force. Normally the moment frame would be a part of the
basic space frame. The walls or bracing acting together with the moment frame must be capable
of resisting all of the design seismic force. The following analyses are required for dual systems:
1.
The frame and shear walls or braced frames must resist the prescribed lateral seismic force in
accordance with their relative rigidities considering fully the interaction of the walls or braced
frames and the moment frames as a single system. This analysis must be made in accordance
with the principles of structural mechanics considering the relative rigidities of the elements
and torsion in the system. Deformations imposed upon members of the moment frame by
their interaction with the shear walls or braced frames must be considered in this analysis.
2.
The moment frame must be designed to have a capacity to resist at least 25 percent of the
total required lateral seismic force including torsional effects.
5.2.2.2 Combinations of Framing Systems: For those cases where combinations of structural
systems are employed, the designer must use judgment in selecting appropriate R, S0, and Cd
values. The intent of Sec. 5.2.2.2.1 is to prohibit support of one system by another possessing
characteristics that result in a lower base shear factor. The entire system should be designed for
the higher seismic shear as the provision stipulates. The exception is included to permit the use of
such systems as a braced frame penthouse on a moment frame building in which the mass of the
penthouse does not represent a significant portion of the total building and, thus, would not
materially affect the overall response to earthquake motions.
Sec. 5.2.2.2.2 pertains to details and is included to help ensure that the more ductile details
inherent with the design for the higher R value system will be employed throughout. The intent is
that details common to both systems be designed to remain functional throughout the response in
order to preserve the integrity of the seismic-force-resisting system.
5.2.2.3 - 5.2.2.6 Seismic Design Categories : General framing system requirements for the
building Seismic Design Categories are given in these sections. The corresponding design and
detailing requirements are given in Sec. 5.2.6 and Chapters 8 through 14. Any type of building
framing system permitted by the Provisions may be used for Categories A, B, and C except
frames limited to Category A or Categories A and B only by the requirements of Chapters 9 and
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1997 Commentary, Chapter 5
12. Limitations regarding the use of different structural systems are given for Categories D, E
and F.
5.2.2.4 Seismic Design Categories D and E: Sec. 5.2.2.4 covers Categories D and E, which
compares roughly to California design practice for normal buildings other than hospitals.
According to the requirements of Chapters 8 and 9, all moment-resisting frames of steel or
concrete must be special moment frames. Note that present SEAOC and UBC recommendations
have similar requirements for concrete frames; however, ordinary moment frames of structural
steel may be used for heights up to 160 ft (49 m). In keeping with the philosophy of present
codes for zones of high seismic risk, these requirements continue limitations on the use of certain
types of structures over 160 ft (49 m) in height but with some changes. Although it is agreed that
the lack of reliable data on the behavior of high-rise buildings whose structural systems involve
shear walls and/or braced frames makes it convenient at present to establish some limits, the
values of 160 ft (49 m) and 240 ft (73 m) introduced in these requirements are arbitrary.
Considerable disagreement exists regarding the adequacy of these values, and it is intended that
these limitations be the subject of further study.
These requirements require that buildings in Category D over 160 ft (49 m) in height have one of
the following seismic-force-resisting systems:
1. A moment resisting frame system with special moment frames capable of resisting the total
prescribed seismic force. This requirement is the same as present SEAOC and UBC
recommendations.
2. A dual system as defined in the Glossary, wherein the prescribed forces are resisted by the
entire system and the special moment frame is designed to resist at least 25 percent of the
prescribed seismic force. This requirement is also similar to SEAOC and UBC recommendations. The purpose of the 25 percent frame is to provide a secondary defense system with
higher degrees of redundancy and ductility in order to improve the ability of the building to
support the service loads (or at least the effect of gravity loads) after strong earthquake shaking. It should be noted that SEAOC and UBC requirements prior to 1987 required that shear
walls or braced frames be able to resist the total required seismic lateral forces independently
of the special moment frame. These provisions require only that the true interaction behavior
of the frame-shear wall (or braced frame) system be considered (see Table 5.2.2). If the
analysis of the interacting behavior is based only on the seismic lateral force vertical distribution recommended in the equivalent lateral force procedure of Sec. 5.3, the interpretation
of the results of this analysis for designing the shear walls or braced frame should recognize
the effects of higher modes of vibration. The internal forces that can be developed in the
shear walls in the upper stories can be more severe than those obtained from such analysis.
3. The use of a shear wall (or braced frame) system of cast-in-place concrete or structural steel
up to a height of 240 ft (73 m) is permitted only if braced frames or shear walls in any plane
do not resist more than 50 percent of the seismic design force including torsional effects and
the configuration of the lateral-force-resisting system is such that torsional effects result in less
than a 20 percent contribution to the strength demand on the walls or frames. The intent is
that each of these shear walls or braced frames be in a different plane and that the four or
more planes required be spaced adequately throughout the plan or on the perimeter of the
65
Structural Design Criteria
building in such a way that the premature failure of one of the single walls or frames will not
lead to excessive inelastic torsion.
FIGURE C5.2.2.4-1 Arrangement of shear walls and FIGURE C5.2.2.4-2 Arrangement of shear walls and
braced frames--not recommended. Note that the heavy braced frames-- recommended. Note that the heavy
lines indicate shear walls and/or braced frames.
lines indicate shear walls and/or braced frames.
Although a structural system with lateral force resistance concentrated in the interior core (Figure
C5.2.2.4-1) is acceptable according to the Provisions, it is highly recommended that use of such a
system be avoided, particularly for taller buildings. The intent is to replace it by the system with
lateral force resistance distributed across the entire building (Figure C5.2.2.4-2). The latter
system is believed to be more suitable in view of the lack of reliable data regarding the behavior of
tall buildings having structural systems based on central cores formed by coupling shear walls or
slender braced frames.
5.2.2.4.2 Interaction Effects: This section relates to the interaction of elements of the seismicforce-resisting system with elements that are not part of this system. A classic example of such
interaction is the behavior of infill masonry walls used as architectural elements in a building
provided with a seismic-force-resisting system composed of moment resisting frames. Although
the masonry walls are not intended to resist seismic forces, at low levels of deformation they will
be substantially more rigid than the moment resisting frames and will participate in lateral force
resistance. A common effect of such walls is that they can create shear-critical conditions in the
columns they infill against by reducing the effective flexural height of these columns to the height
of the openings in the walls. If these walls are not uniformly distributed throughout the structure,
or not effectively isolated from participation in lateral force resistance they can also create
torsional irregularities and soft story irregularities in structures that would otherwise have regular
configuration.
Infill walls are not the only elements not included in seismic-force-resisting systems that can affect
a structure’s seismic behavior. For example, in parking garage structures, the ramps between
levels can act as effective bracing elements and resist a large portion of the seismic induced forces.
They can induce large thrusts in the diaphragms where they connect, as well as large vertical
forces on the adjacent columns and beams. In addition, if not symmetrically placed in the
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1997 Commentary, Chapter 5
structure they can induce torisional irregularities. This section requires consideration of these
potential effects.
5.2.2.4.3 Deformational Compatibility: The purpose of this section is to require that the
seismic-force-resisting system provide adequate deformation control to protect elements of the
structure that are not part of the seismic-force-resisting system. In regions of high seismicity, it is
relatively common to apply ductile detailing requirements to elements which are intended to resist
seismic forces but to neglect such practices in nonstructural elements or elements intended to only
resist gravity forces. The fact that many elements of the structure are not intended to resist
seismic forces and are not detailed for such resistance does not prevent them from actually
participating in this resistance and becoming severely damaged as a result.
The 1994 Northridge earthquake provided several examples where this was a cause of failure. In a
preliminary reconnaissance report of that earthquake (EERI, 1994) it was stated: “Of much
significance is the observation that six of the seven partial collapses (in modern precast concrete
parking structures) seem to have been precipitated by damage to the gravity load system.
Possibly, the combination of large lateral deformation and vertical load caused crushing in poorly
confined columns that were not detailed to be part of the lateral load resisting system.” The
report also noted that: “Punching shear failures were observed in some structures at slab-tocolumn connections such as at the Four Seasons building in Sherman Oaks. The primary lateral
load resisting system was a perimeter ductile frame that performed quite well. However, the
interior slab-column system was incapable of undergoing the same lateral deflections and
experienced punching failures.”
In response to a preponderance of evidence, SEAOC successfully submitted a change to the
Uniform Building Code in 1994 to clarify and strengthen the existing requirements intended to
require deformation compatibility. The statement in support of that code change included the
following reasons: “Deformation compatibility requirements have largely been ignored by the
design community. In the 1994 Northridge earthquake, deformation-induced damage to elements
which were not part of the lateral-force-resisting system resulted in structural collapse. Damage to
elements of the lateral-framing system, whose behavior was affected by adjoining rigid elements,
was also observed. This has demonstrated a need for stronger and clearer requirements. The
proposed changes attempt to emphasize the need for specific design and detailing of elements not
part of the lateral system to accommodate expected seismic deformation….”
The new language in the 1997 Provisions is largely based on SEAOC's successful 1995 change to
the Uniform Building Code. Rather than implicitly relying on designers to assume appropriate
levels of stiffness, the new language in Sec. 5.2.2.4.3 explicitly requires that the "stiffening effects
of adjoining rigid structural and nonstructural elements shall be considered and a rational value of
member and restraint stiffness shall be used" for the design of components that are not part of the
lateral-force-resisting system. This will keep designers from neglecting the potentially adverse
stiffening effects that such components can have on structures. This section also adds a
requirement to address shears that can be induced in structural components that are not part of
the lateral-force-resisting system since sudden shear failures have been catastrophic in past
earthquakes.
The exception in Sec. 5.2.4.3 is intended to encourage the use of intermediate or special detailing
in beams and columns that are not part of the lateral-force-resisting system. In return for better
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detailing, such beams and columns are permitted to be designed to resist moments and shears
from unamplified deflections. This reflects observations and experimental evidence that welldetailed components can accommodate large drifts by responding inelastically without losing
significant vertical load carrying capacity.
5.2.2.5 Seismic Design Category F: Sec. 5.2.2.5 covers Category F, which is restricted to
essential facilities on sites located within a few kilometers of major active faults. Because of the
necessity for reducing risk (particularly in terms of protecting life safety or maintaining function
by minimizing damage to nonstructural building elements, contents, equipment, and utilities), the
height limitations for Category F are reduced. Again, the limits--100 ft (30 m) and 160 ft (49
m)--are arbitrary and require further study. The developers of these requirements believe that, at
present, it is advisable to establish these limits, but the importance of having more stringent requirements for detailing the seismic-force-resisting system as well as the nonstructural components of the building must be stressed. Such requirements are specified in Sec. 5.2.6 and Chapters 8 through 12.
5.2.3 Structure Configuration: The configuration of a structure can significantly affect its
performance during a strong earthquake that produces the ground motion contemplated in the
Provisions. Configuration can be divided into two aspects, plan configuration and vertical
configuration. The Provisions were basically derived for buildings having regular configurations.
Past earthquakes have repeatedly shown that buildings having irregular configurations suffer
greater damage than buildings having regular configurations. This situation prevails even with
good design and construction. There are several reasons for this poor behavior of irregular
structures. In a regular structure, inelastic demands produced by strong ground shaking tend to
be well distributed throughout the structure, resulting in a dispersion of energy dissipation and
damage. However, in irregular structures, inelastic behavior can concentrate in the zone of
irregularity. resulting in rapid failure of structural elements in these areas. In addition, some
irregularities introduce unanticipated stresses into the structure which designers frequently
overlook when detailing the structural system. Finally, the elastic analysis methods typically
employed in the design of structures often can not predict the distribution of earthquake demands
in an irregular structure very well, leading to inadequate design in the zones of irregularity. For
these reasons, these requirements are designed to encourage that buildings be designed to have
regular configurations and to prohibit gross irregularity in buildings located on sites close to major
active faults, where very strong ground motion and extreme inelastic demands can be experienced.
5.2.3.2 Plan Irregularity: Sec. 5.2.3.2 indicates, by reference to Table 5.2.3.2, when a building
must be designated as having a plan irregularity for the purposes of the Provisions. A building
may have a symmetrical geometric shape without re-entrant corners or wings but still be classified
as irregular in plan because of distribution of mass or vertical seismic resisting elements. Torsional effects in earthquakes can occur even when the static centers of mass and resistance
coincide. For example, ground motion waves acting with a skew with respect to the building axis
can cause torsion. Cracking or yielding in a nonsymmetrical fashion also can cause torsion.
These effects also can magnify the torsion due to eccentricity between the static centers. For this
reason, buildings having an eccentricity between the static center of mass and the static center of
resistance in excess of 10 percent of the building dimension perpendicular to the direction of the
seismic force should be classified as irregular. The vertical resisting components may be arranged
so that the static centers of mass and resistance are within the limitations given above and still be
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1997 Commentary, Chapter 5
unsymmetrically arranged so that the prescribed torsional forces would be unequally distributed to
the various components. In the 1997 Provisions, torsional irregularities have been subdivided
into two categories, with a category of extreme irregularity having been created. Extreme
torsional irregularities are prohibited for structures located very close to major active faults and
should be avoided, when possible, in all structures.
There is a second type of distribution of vertical resisting components that, while not being
classified as irregular, does not perform well in strong earthquakes. This arrangement is termed a
core-type building with the vertical components of the seismic-force-resisting system concentrated
near the center of the building. Better performance has been observed when the vertical
components are distributed near the perimeter of the building. In recognition of the problems
leading to torsional instability, a torsional amplification factor is introduced in Section 5.3.5.2.
A building having a regular configuration can be square, rectangular, or circular. A square or
rectangular building with minor re-entrant corners would still be considered regular but large
re-entrant corners creating a crucifix form would be classified as an irregular configuration. The
response of the wings of this type of building is generally different from the response of the
building as a whole, and this produces higher local forces than would be determined by application of the Provisions without modification. Other plan configurations such as H-shapes that
have a geometrical symmetry also would be classified as irregular because of the response of the
wings.
Significant differences in stiffness between portions of a diaphragm at a level are classified as
irregularities since they may cause a change in the distribution of seismic forces to the vertical
components and create torsional forces not accounted for in the normal distribution considered
for a regular building. Examples of plan irregularities are illustrated in Figure C5.2.3.2.
Where there are discontinuities in the lateral force resistance path, the structure can no longer be
considered to be "regular." The most critical of the discontinuities to be considered is the out-ofplane offset of vertical elements of the seismic force resisting elements. Such offsets impose
vertical and lateral load effects on horizontal elements that are, at the least, difficult to provide for
adequately.
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Structural Design Criteria
FIGURE C5.2.3.2 Building plan irregularities.
Where vertical elements of the lateral-force-resisting system are not parallel to or symmetric with
major orthogonal axes, the static lateral force procedures of the Provisions cannot be applied as
given and, thus, the structure must be considered to be "irregular."
5.2.3.3 Vertical Irregularity: Sec. 5.2.3.3 indicates, by reference to Table 5.2.3.3, when a
structure must be considered to have a vertical irregularity. Vertical configuration irregularities
affect the responses at the various levels and induce loads at these levels that are significantly
different from the distribution assumed in the equivalent lateral force procedure given in Sec. 5.3.
A moment resisting frame building might be classified as having a vertical irregularity if one story
were much taller than the adjoining stories and the resulting decrease in stiffness that would
normally occur was not, or could not be, compensated for. Examples of vertical irregularities are
illustrated in Figure C5.2.3.3.
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1997 Commentary, Chapter 5
FIGURE C5.2.3.3 Building elevation irregularities.
A building would be classified as irregular if the ratio of mass to stiffness in adjoining stories
differs significantly. This might occur when a heavy mass, such as a swimming pool, is placed at
one level. Note that the exception in the Provisions provides a comparative stiffness ratio
between stories to exempt structures from being designated as having a vertical irregularity of the
types specified.
One type of vertical irregularity is created by unsymmetrical geometry with respect to the vertical
axis of the building. The building may have a geometry that is symmetrical about the vertical axis
and still be classified as irregular because of significant horizontal offsets in the vertical elements
of the lateral-force-resisting system at one or more levels. An offset is considered to be
significant if the ratio of the larger dimension to the smaller dimension is more than 130 percent.
The building also would be considered irregular if the smaller dimension were below the larger
dimension, thereby creating an inverted pyramid effect.
Weak story irregularities occur whenever the strength of a story to resist lateral demands is
significantly less than that of the story above. This is because buildings with this configuration
tend to develop all of their inelastic behavior at the weak story. This can result in a significant
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Structural Design Criteria
change in the deformation pattern of the building, with most earthquake induced displacement
occurring within the weak story. This can result in extensive damage within the weak story and
even instability and collapse. Note that an exception has been provided in Sec. 5.2.6.2.4 when
there is considerable overstrength of the "weak" story.
In the 1997 Provisions, the soft story irregularity has been subdivided into two categories with an
extreme soft story category being created. Like weak stories, soft stories can lead to instability
and collapse. Buildings with extreme soft stories are now prohibited on sites located very close to
major active faults.
5.2.4 Redundancy: The 1997 Provisions introduces specific requirements intended to quantify
the importance of redundancy. Many parts of the Provisions, particularly the response
modification coefficients, R, were originally developed assuming that structures possess varying
levels of redundancy that heretofore were undefined. Commentary Sec. 5.2.1 recommends that
lower R values be used for non-redundant systems, but does not provide guidance on how to
select and justify appropriate reductions. As a result, many non-redundant structures have been
designed in the past using values of R that were intended for use in designing structures with
higher levels of redundancy. For example, current R values for special moment resisting frames
were initially established in the 1970s based on the then widespread use of complete or nearly
complete frame systems in which all beam-column connections were designed to participate in the
lateral-force-resisting system. High R values were justified by the large number of potential
hinges that could form in such redundant systems, and the beneficial effects of progressive yield
hinge formation described in Sec. C5.2.1. However, in recent years, economic pressures have
encouraged the now prevalent use of much less redundant special moment frames with relatively
few bays of moment resisting framing supporting large floor and roof areas. Similar observations
have been made of other types of construction as well. Modern concrete and masonry shear wall
buildings, for example, have many fewer walls than were once commonly provided in such
buildings.
In order to quantify the effects of redundancy, the 1997 Provisions introduce the concept of a
reliability factor, D, that is applied to the design earthquake loads in the basic load combination
equations of Sec. 5.6, for structures in Seismic Design Categories D, E, and F. The value of the
reliability factor D varies from 1 to 1.5. In effect this reduces the R values for less redundant
structures and should provide greater economic incentive for the design of structures with well
distributed lateral-force-resisting systems. The formulation for the equation from which D is
derived is similar to that developed by SEAOC for inclusion in the 1997 edition of the Uniform
Building Code. It bases the value of D on the floor area of the building and the parameter “r”
which relates to the amount of the building’s design lateral force carried by any single element.
There are many other considerations than just floor area and element/story shear ratios that should
be considered in quantifying redundancy. Conceptually, the element demand/capacity ratios,
types of mechanisms which may form, the individual characteristics of building systems and
materials, building height, number of stories, irregularity, torsional resistance, chord and collector
length, diaphragm spans, the number of lines of resistance, and the number of elements per line
are all important and will intrinsically influence the level of redundancy in systems and their
reliability.
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1997 Commentary, Chapter 5
The SEAOC proposed code change to the 1997 UBC recommends addressing redundancy in
irregular buildings by evaluating the ratio of element shear to design story shear, “r” only in the
lower one-third height. However, many failures of buildings have occurred at and above midheights. Therefore, the Provisions base the D factor on the worst “r” for the least redundant
story, which should then be applied throughout the height of the building.
The Applied Technology Council, in its as-yet-unpublished final draft ATC 19 report suggests
that future redundancy factors be based on reliability theory. For example, if the number of hinges
in a moment frame required to achieve a minimally redundant system were established, a
redundancy factor for less redundant systems could be based on the relationship of the number of
hinges actually provided to those required for minimally redundant systems. ATC suggests that
similar relationships could be developed for shear wall systems using reliability theory. However,
much work yet remains to be completed before such approaches will be ready for adoption into
the Provisions.
The Provisions limit special moment resisting frames to configurations that provide maximum D
values of 1.25 and 1.1, respectively, in Seismic Design Categories D, and E or F, to compensate
for the strength based factor in what are typically drift controlled systems. Other seismic-forceresisting systems that are not typically drift controlled may be proportioned to exceed the
maximum D factor of 1.5; however, it is not recommended that this be done.
5.2.5 ANALYSIS PROCEDURES: Many of the standard procedures for the analysis of forces and
deformations in structures subjected to earthquake ground motion, including the two procedures
specified in the Provisions, are listed below in order of increasing rigor and expected accuracy:
1. Equivalent lateral force procedure (Sec. 5.3).
2. Modal analysis procedure (response spectrum analysis) (Sec. 5.4).
3. Inelastic static procedure, involving incremental application of a pattern of lateral forces and
adjustment of the structural model to account for progressive yielding under load application
(push-over analysis).
4. Inelastic response history analysis involving step-by-step integration of the coupled equations of
motion.
Each procedure becomes more rigorous if effects of soil-structure interaction are considered, either as
presented in Sec. 5.5 or through a more complete analysis of this interaction as appropriate. Every
procedure improves in rigor if combined with use of results from experimental research (not described
in these Provisions).
The equivalent lateral force (ELF) procedure specified in Sec. 5.3 is similar in its basic concept to
SEAOC recommendations in 1968, 1973, and 1974, but several improved features have been
incorporated. A significant revision to this procedure, that more closely adopts the direct consideration
of ground motion response spectra, has been adopted in the 1997 Provisions in parallel with a similar
concept developed by SEAOC.
The modal superposition method is a general procedure for linear analysis of the dynamic response of
structures. In various forms, modal analysis has been widely used in the earthquake-resistant design of
special structures such as very tall buildings, offshore drilling platforms, dams, and nuclear power
plants, for a number of years; however, it use is also becoming more common for ordinary structures as
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Structural Design Criteria
well. In previous editions of the Provisions, the modal analysis procedure specified in Sec. 5.4 was
simplified from the general case by restricting consideration to lateral motion in a single plane. Only
one degree of freedom was required per floor for this type of analysis. In recent years, with the advent
of high speed, desktop computers, and the proliferation of relatively inexpensive, user-friendly
structural analysis software capable of performing three dimensional modal analyses, such
simplifications have become unecessary. Consequently, the 1997 Provisions adopted the more general
approach describing a three-dimensional modal analysis of the structure. When modal analysis is
specified by the Provisions, a three-dimensional analysis generally is required except in the case of
highly regular structures or structures with flexible diaphragms.
The ELF procedure of Sec. 5.3 and the modal analysis procedure of Sec. 5.4 are both based on the
approximation that the effects of yielding can be adequately accounted for by linear analysis of the
seismic-force-resisting system for the design spectrum, which is the elastic acceleration response
spectrum reduced by the response modification factor, R. The effects of the horizontal component of
ground motion perpendicular to the direction under consideration in the analysis, the vertical
component of ground motion, and torsional motions of the structure are all considered in the same
simplified approaches in the two procedures. The main difference between the two procedures lies in
the distribution of the seismic lateral forces over the building. In the modal analysis procedure, the
distribution is based on properties of the natural vibration modes, which are determined from the mass
and stiffness distribution. In the ELF procedure, the distribution is based on simplified formulas that
are appropriate for regular structures as specified in Sec. 5.3.4. Otherwise, the two procedures are
subject to the same limitations.
The simplifications inherent in the ELF procedure result in approximations that are likely to be
inadequate if the lateral motions in two orthogonal directions and the torsional motion are strongly
coupled. Such would be the case if the building were irregular in its plan configuration (see Sec.
5.2.3.2) or if it had a regular plan but its lower natural frequencies were nearly equal and the centers of
mass and resistance were nearly coincident. The modal analysis method introduced in the 1997
Provisions includes a general model that is more appropriate for the analysis of such structures. It
requires at least three degrees of freedom per floor--two translational and one torsional motion.
The methods of modal analysis can be generalized further to model the effect of diaphragm flexibility,
soil-structure interaction, etc. In the most general form, the idealization would take the form of a large
number of mass points, each with six degrees of freedom (three translation and three rotational)
connected by generalized stiffness elements.
The ELF procedure (Sec. 5.3) and the modal analysis procedure are all likely to err systematically on
the unsafe side if story strengths are distributed irregularly over height. This feature is likely to lead to
concentration of ductility demand in a few stories of the building. The inelastic static (or so-called
pushover) procedure is a method to more accurately account for irregular strength distribution.
However, it also has limitations and is not particularly applicable to tall structures or structures with
relatively long fundamental periods of vibration.
The actual strength properties of the various components of a structure can be explicitly considered
only by a nonlinear analysis of dynamic response by direct integration of the coupled equations of
motion. This method has been used extensively in earthquake research studies of inelastic structural
response. If the two lateral motions and the torsional motion are expected to be essentially uncoupled,
it would be sufficient to include only one degree of freedom per floor, the motion in the direction along
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1997 Commentary, Chapter 5
which the structure is being analyzed; otherwise at least three degrees of freedom per floor, two
translational motions and one torsional, should be included. It should be recognized that the results of
a nonlinear response history analysis of such mathematical structural models are only as good as are the
models chosen to represent the structure vibrating at amplitudes of motion large enough to cause
significant yielding during strong ground motions. Furthermore, reliable results can be achieved only
by calculating the response to several ground motions--recorded accelerograms and/or simulated motions--and examining the statistics of response.
It is possible with presently available computer programs to perform two- and three-dimensional
inelastic analyses of reasonably simple structures. The intent of such analyses could be to estimate the
sequence in which components become inelastic and to indicate those components requiring strength
adjustments so as to remain within the required ductility limits. It should be emphasized that with the
present state of the art in analysis, there is no one method that can be applied to all types of structures.
Further, the reliability of the analytical results are sensitive to:
1. The number and appropriateness of the input motion records,
2. The practical limitations of mathematical modeling including interacting effects of innelastic
elements,
3. The nonlinear solution algorithms, and
4. The assumed member hysteretic behavior.
Because of these sensitivities and limitations, the maximum base shear produced in an inelastic analysis
should not be less than that required by Sec. 5.4.
The least rigorous analytical procedure that may be used in determining the design seismic forces and
deformations in structure s depends on the Seismic Design Category and the structural characteristics
(in particular, regularity). Regularity is discussed in Sec. 5.2.3.
Neither regular nor irregular buildings in Seismic Design Category A are required to be analyzed as a
whole for seismic forces, but certain minimum requirements are given in Sec. 5.2.5.1. In addition,
there is a requirement that Seismic Design Category A structure should be evaluated for a total lateral
force equal to a nominal percentage of their effective weight. The purpose of this provision is to
assure that a complete lateral-force-resisting system is provided for all structures.
For the higher Seismic Design Categories, the ELF procedure is the minimum level of analysis except
that a more rigorous procedure is required for some Category D, E and F structures as identified in
Table 5.2.5.3. The modal analysis procedure adequately addresses vertical irregularities of stiffness,
mass, or geometry, as limited by the Provisions. Other irregularities must be carefully considered.
The basis for the ELF procedure and its limitations were discussed above. It is adequate for most
regular structures; however, the designer may wish to employ a more rigorous procedure (see list of
procedures at beginning of this section for those regular structures where it may be inadequate). The
ELF procedure is likely to be inadequate in the following cases:
1. Structures with irregular mass and stiffness properties in which case the simple equations for
vertical distribution of lateral forces (Eq. 5.3.4-1 and 5.3.4-2) may lead to erroneous results;
2. Structures (regular or irregular) in which the lateral motions in two orthogonal directions and the
torsional motion are strongly coupled; and
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Structural Design Criteria
3. Structures with irregular distribution of story strengths leading to possible concentration of
ductility demand in a few stories of the building.
In such cases, a more rigorous procedure that considers the dynamic behavior of the structure should
be employed.
Structures with certain types of vertical irregularities may be analyzed as regular structures in
accordance with the requirements of Sec. 5.3. These structures are generally referred to as setback
structures. The following procedure may be used:
1. The base and tower portions of a building having a setback vertical configuration may be analyzed
as indicated in (2) below if:
a. The base portion and the tower portion, considered as separate structures , can be classified as
regular and
b. The stiffness of the top story of the base is at least five times that of the first story of the tower.
When these conditions are not met, the building must be analyzed in accordance with Sec. 5.4.
2. The base and tower portions may be analyzed as separate structures in accordance with the
following:
a. The tower may be analyzed in accordance with the procedures in Sec. 5.3 with the base taken
at the top of the base portion.
b. The base portion then must be analyzed in accordance with the procedures in Sec. 5.3 using
the height of the base portion of hn and with the gravity load and seismic base shear forces of
the tower portion acting at the top level of the base portion.
The design requirements in Sec. 5.4 include a simplified version of modal analysis that accounts for
irregularity in mass and stiffness distribution over the height of the building. It would be adequate, in
general, to use the ELF procedure for structures whose floor masses and cross-sectional areas and
moments of inertia of structural members do not differ by more than 30 percent in adjacent floors and
in adjacent stories.
For other structures, the following procedure should be used to determine whether the modal analysis
procedures of Sec. 5.4 should be used:
1. Compute the story shears using the ELF procedure specified in Sec. 5.3.
2. On this basis, approximately dimension the structural members, and then compute the lateral
displacements of the floor.
3. Replace h in Eq. 5.3.4-2 with these displacements, and recompute the lateral forces to obtain the
revised story shears.
4. If at any story the recomputed story shear differs from the corresponding value as obtained from
the procedures of Sec. 5.3 by more than 30 percent, the building should be analyzed using the procedure of Sec. 5.4. If the difference is less than this value, the building may be designed using the
story shear obtained in the application of the present criterion and the procedures of Sec. 5.4 are
not required.
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1997 Commentary, Chapter 5
Application of this procedure to these structures requires far less computational effort than the use of
the modal analysis procedure of Sec. 5.4. In the majority of the structures, use of this procedure will
determine that modal analysis need not be used and will also furnish a set of story shears that
practically always lie much closer to the results of modal analysis than the results of the ELF
procedure.
This procedure is equivalent to a single cycle of Newmark's method for calculation of the fundamental
mode of vibration. It will detect both unusual shapes of the fundamental mode and excessively high
influence of higher modes. Numerical studies have demonstrated that this procedure for determining
whether modal analysis must be used will, in general, detect cases that truly should be analyzed
dynamically; however, it generally will not indicate the need for dynamic analysis when such an analysis
would not greatly improve accuracy.
Section 5.2.5.3 of the Provisions requires "special consideration of dynamic characteristics" when:
1. The building is assigned to Seismic Design Category D, E or F and
2. The building has one or more of the plan structural irregularities listed in Table 5.2.3.2 and/or
3. The building has a vertical structural irregularity of Type 4 and/or 5 listed in Table 5.2.3.3.
When special dynamic analysis is required and irregularities of the plan type exist, three-dimensional
modal analysis must be employed
5.2.6 DESIGN, DETAILING REQUIREMENTS, AND STRUCTURAL COMPONENT
LOAD EFFECTS: The design and detailing requirements for components of the seismic-forceresisting system are stated in this section. The combination of load effects is specified in Sec. 5.2.7.
The requirements of this section are spelled out in considerable detail. The major reasons for this are
presented below.
The provision of detailed design ground motions and requirements for analysis of the structure do not
by themselves make a building earthquake resistant. Additional design requirements are necessary to
provide a consistent degree of earthquake resistance in buildings. The more severe the expected
seismic ground motions, the more stringent these additional design requirements should be. Not all of
the necessary design requirements are expressed in codes, and although experienced seismic design
engineers account for them, engineers lacking experience in the design and construction of earthquakeresistant structures often overlook them. Considerable uncertainties exist regarding:
1. The actual dynamic characteristics of future earthquake motions expected at a building site;
2. The soil-structure-foundation interaction;
3. The actual response of buildings when subjected to seismic motions at their foundations; and
4. The mechanical characteristics of the different structural materials, particularly when they undergo
significant cyclic straining in the inelastic range that can lead to severe reversals of strains.
It should be noted that the overall inelastic response of a structure is very sensitive to the inelastic
behavior of its critical regions, and this behavior is influenced, in turn, by the detailing of these regions.
Although it is possible to counteract the consequences of these uncertainties by increasing the level of
design forces, it is considered more feasible to provide a building system with the largest energy dissipation consistent with the maximum tolerable deformations of nonstructural components and
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Structural Design Criteria
equipment. This energy dissipation capacity, which is usually denoted simplistically as "ductility," is
extremely sensitive to the detailing. Therefore, in order to achieve such a large energy dissipation
capacity, it is essential that stringent design requirements be used for detailing the structural as well as
the nonstructural components and their connections or separations. Furthermore, it is necessary to
have good quality control of materials and competent inspection. The importance of these factors has
been clearly demonstrated by the building damage observed after both moderate and severe
earthquakes.
It should be kept in mind that a building's response to seismic ground motion most often does not
reflect the designer's or analyst's original conception or modeling of the structure on paper. What is
reflected is the manner in which the building was constructed in the field. These requirements
emphasize the importance of detailing and recognize that the detailing requirements should be related
to the expected earthquake intensities and the importance of the building's function and/or the density
and type of occupancy. The greater the expected intensity of earthquake ground-shaking and the more
important the building function or the greater the number of occupants in the building, the more stringent the design and detailing requirements should be. In defining these requirements, the Provisions
uses the concept of Seismic Design Categories (Tables 4.2.1a and 4.2.1b ), which relate to the design
ground motion severities, given by the spectral response acceleration coefficients SDS and SD1 (Sec.
4.1.1 ) and the Seismic Use Group (Sec. 1.3).
5.2.6.1 Seismic Design Category A: Because of the very low seismicity associated with sites with
SDS less than 0.25g and SD1 less than 0.10g , it is considered appropriate for Category A buildings to
require only a complete lateral-force-resisting system. good quality of construction materials and
adequate ties and anchorage as specified in this section. Category A buildings will be constructed in a
large portion of the United States that is generally subject to strong winds but low earthquake risk.
Those promulgating construction regulations for these areas may wish to consider many of the
low-level seismic requirements as being suitable to reduce the windstorm risk. Since the Provisions
considers only earthquakes, no other requirements are prescribed for Category A buildings. Only a
complete lateral-force-resisting system, ties, and wall anchorage are required by these Provisions.
5.2.6.1.1 Component Load Effects: This section specifies that the direction of the applied seismic
force be that which produces the most critical load effect on the building. In past codes, it was
necessary only to independently consider loads on the main axes of the building. For beams and
girders, this gives maximum design stresses. However, if earthquake forces affect the building in a
direction other than the main axes, corner columns can be subjected to higher stresses, which may
partially explain the vulnerability of such columns in past earthquakes.
5.2.6.1.2 Connections: The analysis of a structure and the provision of a design ground motion
alone do not make a structure earthquake resistant; additional design requirements are necessary to
provide adequate earthquake resistance in buildings. Experienced seismic designers normally fill these
requirements, but because some were not formally specified, they often are overlooked by
inexperienced engineers.
Probably the most important single attribute of an earthquake-resistant building is that it is tied
together to act as a unit. This attribute not only is important in earthquake-resistant design, but also is
indispensable in resisting high winds, floods, explosion, progressive failure, and even such ordinary
hazards as foundation settlement. Sec. 5.2.6.1.2 requires that all parts of the building (or unit if there
are separation joints) be so tied together that any part of the structure is tied to the rest to resist a force
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1997 Commentary, Chapter 5
of SDS/7.5 (with a minimum of 5 percent g) times the weight of the smaller. In addition, beams must be
tied to their supports or columns and columns to footings for a minimum of 5 percent of the dead and
live load reaction.
Certain connections of buildings with plan irregularities must be designed for higher forces than
calculated due to the simplifying assumptions used in the analysis by Sec. 5.2, 5.3, and 5.4 (see Sec.
5.2.6.4.3 ).
5.2.6.1.3 Anchorage of Concrete or Masonry Walls: One of the major hazards from buildings
during an earthquake is the pulling away of heavy masonry or concrete walls from floors or roofs.
Although requirements for the anchorage to prevent this separation are common in highly seismic
areas, they have been minimal or nonexistent in most other parts of the country. This section requires
that anchorage be provided in any locality to the extent of 400SDS pounds per linear foot (plf) or 5,840
times SDS Newtons per meter (N/m). This requirement alone may not provide complete earthquake-resistant design, but observations of earthquake damage indicate that it can greatly increase the earthquake resistance of buildings and reduce hazards in those localities where earthquakes may occur but
are rarely damaging.
5.2.6.2 Seismic Design Category B: Category B and Category C buildings will be constructed in
the largest portion of the United States. Earthquake-resistant requirements are increased appreciably
over Category A requirements, but they still are quite simple compared to present requirements in
areas of high seismicity.
The Category B requirements specifically recognize the need to design diaphragms, provide collector
bars, and provide reinforcing around openings. There requirements may seem elementary and obvious
but, because they are not specifically covered in many codes, some engineers totally neglect them.
5.2.6.2.4 Nonredundant Systems: Design consideration should be given to potentially adverse effects where there is a lack of redundancy. Because of the many unknowns and uncertainties in the
magnitude and characteristics of earthquake loading, in the materials and systems of construction for
resisting earthquake loadings and in the methods of analysis, good earthquake engineering practice has
been to provide as much redundancy as possible in the seismic-force-resisting system of buildings.
Redundancy plays an important role in determining the ability of the building to resist earthquake
forces. In a structural system without redundant components, every component must remain operative
to preserve the integrity of the building structure. On the other hand, in a highly redundant system, one
or more redundant components may fail and still leave a structural system that retains its integrity and
can continue to resist lateral forces, albeit with diminished effectiveness.
Redundancy often is accomplished by making all joints of the vertical load-carrying frame moment
resisting and incorporating them into the seismic-force-resisting system. These multiple points of
resistance can prevent a catastrophic collapse due to distress or failure of a member or joint. (The
overstrength characteristics of this type of frame were discussed in the commentary on Sec. 5.2.1.)
The designer should be particularly aware of the proper selection of R when using only one or two
one-bay rigid frames in one direction for resisting seismic loads. A single one-bay frame or a pair of
such frames provides little redundancy so the designer may wish to consider a modified (smaller) R to
account for a lack of redundancy. As more one-bay frames are added to the system, however, overall
system redundancy increases. The increase in redundancy is a function of frame placement and total
number of frames.
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Structural Design Criteria
Redundant characteristics also can be obtained by providing several different types of seismic-forceresisting systems in a building. The backup system can prevent catastrophic effects if distress occurs in
the primary system.
In summary, it is good practice to incorporate redundancy into the seismic-force-resisting system and
not to rely on any system wherein distress in any member may cause progressive or catastrophic
collapse.
5.2.6.2.5 Collector Elements: Many buildings have shear walls or other bracing elements that are
not uniformly spaced around the diaphragms. Such conditions require that collector or drag members
be provided. A simple illustration is shown in Figure C5.2.6.2.5.
Consider a building as shown in the plan with four short shear walls at the corners arranged as shown.
For north-south earthquake forces, the diaphragm shears on Line AB are uniformly distributed
between A and B if the chord reinforcing is assumed to act on Lines BC and AD. However, wall A is
quite short so reinforcing steel is required to collect these shears and transfer them to the wall. If Wall
A is a quarter of the length of AB, the steel must carry, as a minimum, three-fourths of the total shear
on Line AB. The same principle is true for the other walls. In Figure C5.2.6.2.5 reinforcing is
required to collect the shears or drag the forces from the diaphragm into the shear wall. Similar
collector elements are needed in most shear walls and some frames.
5.2.6.2.6 Diaphragms: Diaphragms are deep beams or trusses that distribute the lateral loads from
their origin to the components where they are resisted. As such, they are subject to shears, bending
moments, direct stresses (truss member, collector elements), and deformations. The deformations
must be minimized in some cases because they could overstress the walls to which they are connected.
The amount of deflection permitted in the diaphragm must be related to the ability of the walls (normal
to the direction being analyzed) to deflect without failure.
A detail commonly overlooked by many engineers is the requirement to tie the diaphragm together so
that it acts as a unit. Wall anchorages tend to tear off the edges of the diaphragm; thus, the ties must
be extended into the diaphragm so as to develop adequate anchorage. During the San Fernando
earthquake, seismic forces from the walls caused separations in roof diaphragms 20 or more ft (6 m)
from the edge in several industrial buildings.
When openings occur in shear walls, diaphragms, etc., it is not adequate to only provide temperature
trim bars. The chord stresses must be provided for and the chords anchored to develop the chord
stresses by embedment. The embedment must be sufficient to take the reactions without overstressing
the material in any respect. Since the design basis depends on an elastic analysis, the internal force
system should be compatible with both static and the elastic deformations.
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1997 Commentary, Chapter 5
FIGURE C5.2.6.2.5 Collector element used to (a) transfer shears and (b)
transfer drag forces from diaphragm to shear wall.
5.2.6.2.7 Bearing Walls: A minimum anchorage of bearing walls to diaphragms or other resisting
elements is specified. To ensure that the walls and supporting framing system interact properly, it is
required that the interconnection of dependent wall elements and connections to the framing system
have sufficient ductility or rotational capacity, or strength, to stay as a unit. Large shrinkage or
settlement cracks can significantly affect the desired interaction.
5.2.6.2.8 Inverted Pendulum-Type Structures: Inverted pendulum-type structures have a large
portion of their mass concentrated near the top and, thus, have essentially one degree of freedom in
horizontal translation. Often the structures are T-shaped with a single column supporting a beam or
slab at the top. For such a structure, the lateral motion is accompanied by rotation of the horizontal
element of the T due to rotation at the top of the column, resulting in vertical accelerations acting in
opposite directions on the overhangs of the structure. Dynamic response amplifies this rotation; hence,
a bending moment would be induced at the top of the column even though the procedures of Sec. 5.3.2
and 5.3.5 would not so indicate. A simple provision to compensate for this is specified in this section.
The bending moments due to the lateral force are first calculated for the base of the column according
to the requirements of Sec. 5.3.2 and 5.3.5. One-half of the calculated bending moment at the base is
applied at the top and the moments along the column are varied from 1.5 M at the base to 0.5 M at the
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Structural Design Criteria
top. The addition of one-half the moment calculated at the base in accordance with Sec. 5.3.2 and
5.3.5 is based on analyses of inverted pendulums covering a wide range of practical conditions.
5.2.6.2.9 Anchorage of Nonstructural Systems: Anchorage of nonstructural systems and
components of buildings is required when prescribed in Chapter 6.
5.2.6.3 Seismic Design Category C: The material requirements in Chapters 8 through 12 for
Category C are somewhat more restrictive than those for Categories A and B. Also, a nominal interconnection between pile caps and caissons is required.
5.2.6.4 Seismic Design Category D: Category D requirements compare roughly to present design
practice in California seismic areas for buildings other than schools and hospitals. All moment resisting
frames of concrete or steel must meet ductility requirements. Interaction effects between structural and
nonstructural elements must be investigated. Foundation interaction requirements are increased.
Sec. 5.2.5.4.1 requires for Category D buildings that the effects from seismic loads applied in one
direction be combined with those from the other direction. This may affect more than just the
columns. The second order effect that is referenced is explained more fully in Sec. 5.3.7.
5.2.6.4.1 Orthogonal Effects: Earthquake forces act in both principal directions of the building
simultaneously, but the earthquake effects in the two principal directions are unlikely to reach their
maximum simultaneously. This section provides a reasonable and adequate method for combining
them. It requires that structural elements be designed for 100 percent of the effects of seismic forces in
one principal direction combined with 30 percent of the effects of seismic forces in the orthogonal
direction.
The following combinations of effects of gravity loads, effects of seismic forces in the x-direction, and
effects of seismic forces in the y-direction (orthogonal to x-direction) thus pertain:
gravity ± 100% of x-direction ± 30% of y-direction
gravity ± 30% of x-direction ± 100% of y-direction
The combination and signs (plus or minus) requiring the greater member strength are used for each
member. Orthogonal effects are slight on beams, girders, slabs, and other horizontal elements that are
essentially one-directional in their behavior, but they may be significant in columns or other vertical
members that participate in resisting earthquake forces in both principal directions of the building. For
two-way slabs, orthogonal effects at slab-to-column connections can be neglected provided the
moment transferred in the minor direction does not exceed 30 percent of that transferred in the
orthogonal direction and there is adequate reinforcement within lines one and one-half times the slab
thickness either side of the column to transfer all the minor direction moment.
5.2.7 COMBINATION OF LOAD EFFECTS: The load combination statements in the Provisions
combine the effects of structural response to horizontal and vertical ground accelerations. They do not
show how to combine the effect of earthquake loading with the effects of other loads. For those
combinations, the user is referred to ASCE 7 (Ref. 5-1). The pertinent combinations are:
1.2D + 1.0E + 0.5L + 0.2S
0.9D + 1.0E
(Additive)
(Counteracting)
where D, E, L, and S are, respectively, the dead, earthquake, live, and snow loads.
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1997 Commentary, Chapter 5
The design basis expressed in Sec. 5.2.1 reflects the fact that the specified earthquake loads are at the
design level without amplification by load factors; thus, for sufficiently redundant structures, a load
factor of 1.0 is assigned to the earthquake load effects in Eq. 5.2.7-1 and 5.2.7-2.
In Eq. 5.2.7-1 and 5.2.7-2 , a factor of 0.2SDS was placed on the dead load to account for the effects of
vertical acceleration. The 0.2SDS factor on dead load is not intended to represent the total vertical
response. The concurrent maximum response of vertical accelerations and horizontal accelerations,
direct and orthogonal, is unlikely and, therefore, the direct addition of responses was not considered
appropriate.
The D factor was introduced into Eq. 5.2.7-1 and 5.2.7-2 in the 1997 Provisions. This factor,
determined in accordance with Sec. 5.2.4, relates to the redundancy inherent in the lateral-forceresisting system and is, in essence, a reliability factor, penalizing designs which are likely to be
unreliable due to concentration of the structure’s resistance to lateral forces in a relatively few
elements.
There is very little research that speaks directly to the merits of redundancy in buildings for seismic
resistance. The SAC joint venture recently studied the relationships between damage to welded steel
moment frame connections and redundancy (Bonowitz, et al, 1995). While this study found no
specific correlation between damage and the number of bays of moment resisting framing per moment
frame, it did find increased rates of damage in connections that resisted larger floor areas. This study
included modern low-, mid- and high-rise steel buildings.
Another study (Wood, 1991) that addresses the potential effects of redundancy evaluated the
performance of 165 Chilean concrete buildings ranging from 6 to 23 stories in height. These concrete
shear wall buildings with non-ductile details and no boundary elements experienced moderately strong
shaking (MMI VII to VIII) with a strong shaking duration of over 60 seconds, yet performed well.
One plausible explanation for this generally good performance was the substantial amount of wall area
(2 to 4 percent of the floor area) commonly used in Chile. However, Wood’s study found no
correlation between damage rates and higher redundancy in buildings with wall areas greater than 2
percent.
The special load combination of Sect. 5.2.7.1 is intended to address those situations where failure of an
isolated, individual, brittle element can result in the loss of a complete lateral-force-resisting system or
in instability and collapse. This section has evolved over several editions. In the 1991 Edition, a 2R/5
factor was introduced to better represent the behavior of elements sensitive to overstrength in the
remainder of the seismic resisting system or in specific other structural components. The particular
number was selected to correlate with the 3Rw/8 factor that had been introduced in Structural
Engineers Association of California (SEAOC) recommendations and the Uniform Building Code. This
is a somewhat arbitrary factor that attempts to quantify the maximum force that can be delivered to
sensitive elements based on historic observation that the real force that could develop in a structure
may be 3 to 4 times the design levels. In the 1997 Provisions, an attempt has been made to determine
this force more rationally through the assignment of the S0 factor in Table 5.2.2, dependent on the
individual system.
The special load combinations of Eq. 5.2.6-3 and 5.2.6-4 were first introduced in the 1991 Edition of
the Provisions, for the design of elements that could fail in an undesirable manner when subjected to
demands that are significantly larger than those used to proportion them. It recognizes the fact that the
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Structural Design Criteria
actual response (forces and deformations) developed by a structure subjected to the design earthquake
ground motion will be substantially larger than that predicted by the design forces. Through the use of
the So coefficient, this special equation provides an estimate of the maximum forces actually likely to
be experienced by an element.
When originally introduced in the 1991 Provisions, the overstrength factor So was represented by the
factor 2R/5. That particular value was selected to correlate with the 3Rw/8 factor that had been
previously introduced in Structural Engineers Association of California (SEAOC) recommendations
and the Uniform Building Code in 1988. Typically, both of these factors resulted in a three to four
fold amplification in the design force levels, based on the historic judgment that the real forces
experienced by a structure in a major earthquake are probably on the order of 3 to 4 times the design
force levels.
In recent years, a number of researchers have investigated the factors that permit structures designed
for reduced forces to survive design earthquakes. Although these studies have principally been
focused on the development of more reliable response modification coefficients, R, they have identified
the importance of structural overstrength, and identified a number of sources of such overstrength.
This has made it possible to replace the single 2R/5 factor formerly contained in the Provisions with a
more system-specific estimate, represented by the So coefficient.
It is recognized, that no single value, whether obtained by formula related to the R factor or otherwise
obtained will provide a completely accurate estimate for the overstrength of all structures with a given
seismic-force-resisting system. However, most structures designed with a given lateral-force-resisting
system, will fall within a range of overstrength values. Since the purpose of the Ω0 factor in Eq. 5.2.73 and 5.2.7-4 is to estimate the maximum force that can be delivered to a component that is sensitive to
overstress, the values of this factor tabulated in Table 5.2.2 are intended to be representative of the
larger values in this range for each system.
Figure C5.2.7 and the following discussion explore some of the factors that contribute to structural
overstrength. The figure shows a plot of lateral structural strength vs. displacement for an elasticperfectly-plastic structure. In addition, it shows a similar plot for a more representative real structure,
that posses significantly more strength than the design strength. This real strength is represented by the
lateral force Fn. Essentially, the Ω0 coefficient is intended to be a somewhat conservative estimate of
the ratio of Fn to the design strength FE/R. As shown in the figure, there are three basic components to
the overstrength. These are the design overstrength (SD), the material overstrength (SM) and the
system overstrength (SS). Each of these is discussed separately.The design overstrength (SD) is the
most difficult of the three to estimate. It is the difference between the lateral base shear force at which
the first significant yield of the structure will occur (point 1 in the figure) and the minimum specified
force given by FE/R. To some extent, this is system dependent. Systems that are strength controlled,
such as most braced frames and shear wall structures, will typically have a relatively low value of
design overstrength, as most designers will seek to optimize their designs and provide a strength that is
close to the minimum specified by the Provisions. For such structures, this portion of the overstrength
coefficient could be as low as 1.0.
84
Elastic response force
Material Overstrength
Fn
i
Design drift
Lateral Force
System Overstrength
El
as
tic
Design Overstrength
3
2
F2
Inadequate
Strength
FE/R
1
Design force
CdδE/R
δE/R
Elastic response drift
FE
Re
sp
on
se
1997 Commentary, Chapter 5
n
δE
Lateral Displacement (Drift)
FIGURE C5.2.7 Factors affecting overstrength.
Drift controlled systems such as moment frames, however, will have substantially larger design o
verstrengths since it will be necessary to oversize the sections of such structures in order to keep the
lateral drifts within prescribed limits. In a recent study of a number of special moment resisting steel frames conducted by the SAC Joint Venture design overstrengths on the order of a factor of two to
three were found to exist (Analytical Investigation of Buildings Affected by the 1994 Northridge Earthquake, Volumes 1
and 2, SAC 95-04A and B. SAC Joint Venture, Sacramento, CA, 1995). Design overstrength is also potentially
regionally dependent. The SAC study was conducted for frames in Seismic Design Category D and E,
which represent the most severe design conditions. For structures in Seismic Design Categories A, B
and C, seismic force resistance would play a less significant role in the sizing of frame elements to
control drifts, and consequently, design overstrengths for these systems would be somewhat lower. It
seems reasonable to assume that this portion of the design overstrength for special moment frame
structures is on the order of 2.0.
Architectural design considerations have the potential to play a significant role in design overstrength.
Some architectural designs will incorporate many more and larger lateral force resisting elements than
are required to meet the strength and drift limitations of the code. An example of this are warehouse
type structures, wherein the massive perimeter walls of the structure can provide very large lateral
strength. However, even in such structures, there is typically some limiting element, such as the
diaphragm, that prevents the design overstrength from becoming uncontrollably large. Thus, although
the warehouse structure may have very large lateral resistance in its shear walls, typically the roof
diaphragm will have a lateral force resisting capacity comparable to that specified as a minimum by the
Provisions.
Finally, the structural designer can affect the design overstrength. While some designers seek to
optimize their structures with regard to the limitations contained in the Provisions, others will seek to
85
Structural Design Criteria
intentionally provide greater strength and drift control than required. Typically design overstrength
intentionally introduced by the designer will be on the order of 10 percent of the minimum required
strength, but it may range as high as 50 to 100 percent in some cases. A factor of 1.2 should probably
be presumed for this portion of the design overstrength to include the effects of both architectural and
structural design overstrength. Designers who intentionally provide greater design overstrength should
keep in mind that the Ω0 factors used in their designs should be adjusted accordingly.
Material overstrength (SM) results from the fact that the design values used to proportion the elements
of a structure are specified by the Provisions to be conservative lower bound estimates of the actual
probable strengths of the structural materials and their effective strengths in the as-constructed
structure. It is represented in the figure by the ratio of F2/F1, where F2 and F1 are respectively the
lateral force at points 2 and 1 on the curve. All structural materials have considerable variation in the
strengths that can be obtained in given samples of the material from a specific grade. The design
requirements typically base proportioning requirements on minimum specified values that are further
reduced through strength reduction (N) factors. The actual expected strength of the as-constructed
structure is significantly higher than this design value and should be calculated using the mean strength
of the material, based on statistical data, by removal of the N factor from the design equation, and by
providing an allowance for strain hardening, where significant yielding is expected to occur. Code
requirements for reinforced masonry, concrete and steel have historically used a factor of 1.25 to
account for the ratio of mean to specified strength and the effects of strain hardening. Considering a
typical capacity reduction factor on the order of 0.9, this would indicate that the material overstrength
for systems constructed of these materials would be on the order of 1.25/0.9, or 1.4.
System overstrength (SS) is the ratio of the ultimate lateral force the structure is capable of resisting, Fn
in the figure, to the actual force at which first significant yield occurs, F2 in the figure. It is dependent
on the amount of redundancy contained in the structure as well as the extent to which the designer has
optimized the various elements that participate in lateral force resistance. For structures, with a single
lateral force resisting element, such as a braced frame structure with a single bay of bracing, the system
overstrength (SS) factor would be 1.0, since once the brace in the frame yields, the system becomes
fully yielded. For structures that have a number of elements participating in lateral seismic force
resistance, whether or not actually intended to do so, the system overstrength will be significantly
larger than this, unless the designer has intentionally optimized the structure such that a complete
sidesway mechanism develops at the level of lateral drift at which the first actual yield occurs.
Structural optimization is most likely to occur in structures where the actual lateral force resistance is
dominated by the design of elements intended to participate as part of the lateral-force-resisting
system, and where the design of those elements is dominated by seismic loads, as opposed to gravity
loads. This would include concentric braced frames and eccentric braced frames in all Seismic Design
Categories and Special Moment Frames in Seismic Design Categories D and E. For such structures,
the system overstrength may be taken on the order of 1.1. For dual system structures, the system
overstrength is set by the Provisions at an approximate minimum value of 1.25. For structures where
the number of elements that actually resist lateral forces is based on other than seismic design
considerations, the system overstrength may be somewhat larger. In light framed residential
construction, for example, the number of walls is controlled by architectural rather than seismic design
consideration. Such structures may have a system overstrength on the order of 1.5. Moment frames,
the design of which is dominated by gravity load considerations can easily have a system overstrength
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1997 Commentary, Chapter 5
of 2.0 or more. This affect is somewhat balanced by the fact that such frames will have a lower design
overstrength related to the requirement to increase section sizes to obtain drift control. Table C5.2.7-1
presents some possible ranges of values for the various components of overstrength for various
structural systems as well as the overall range of values that may occur for typical structures.
Table C5.2.7-1 Typical Range of Overstrength for Various Systems
Design
Overstrength
SD
Material
Overstrength
SM
System
Overstrength
SS
S0
Special Moment Frames Steel &
Concrete
1.5-2.5
1.2-1.6
1.0-1.5
2-3.5
Intermediate Moment Frames
Steel & Concrete
1.0-2.0
1.2-1.6
1.0-2.0
2-3.5
Ordinary Moment Frames Steel &
Concrete
1.0-1.5
1.2-1.6
1.5-2.5
2-3.5
Masonry Wall Frames
1.0-2.0
1.2-1.6
1.0-1.5
2-2.5
Braced Frames
1.5-2.0
1.2-1.6
1.0-1.5
1.5-2
Reinforced Bearing Wall
1.0-1.5
1.2-1.6
1.0-1.5
1.5-2.5
Reinforced Infill Wall
1.0-1.5
1.2-1.6
1.0-1.5
1.5-2.5
Unreinforced Bearing Wall
1.0-2.0
0.8-2.0
1.0-2.0
2-3
Unreinforced Infill Wall
1.0-2.0
0.8-2.0
1.0-2.0
2-3
Dual System Bracing & Frame
1.1-1.75
1.2-1.6
1.0-1.5
1.5-2.5
Light Bearing Wall Systems
1.0-0.5
1.2-2.0
1.0-2.0
2.5-3.5
Structural System
In recognition of the fact that it is difficult to accurately estimate the amount of overstrength a
structure will have, based solely on the type of seismic-force-resisting system that is present, in lieu of
using the values of the overstrength coefficient S0 provided in Table 5.2.2, designers are encouraged to
base the maximum forces used in Eqs. 5.2.7.3 and 5.2.7.4 on the results of a suitable nonlinear analysis
of the structure. Such analyses should use the actual expected, rather than specified values, of material
and section properties. Appropriate forms of such analyses could include a plastic mechanism analysis,
a static pushover analysis or a nonlinear time history analysis. If a plastic mechanism analysis is
utilized, the maximum seismic force that ever could be produced in the structure, regardless of the
ground motion experienced is, estimated. If static pushover or nonlinear time history analyses are
utilized, the forces utilized for design as the maximum force, should probably be that determined
forMaximum Considered Earthquake level ground shaking demands.
While overstrength can be quite beneficial in permitting structures to resist actual seismic demands that
are larger than those for which they have been specifically designed, it is not always beneficial. Some
elements incorporated in structures behave in a brittle manner and can fail in an abrupt manner if
substantially overloaded. The existence of structural overstrength results in a condition where such
overloads are likely to occur, unless they are specifically accounted for in the design process. This is
the purpose of Eq. 5.2.7-3 and 5.2.7-4.
One case where structural overstrength should specifically be considered is in the design of column
elements beneath discontinuous braced frames and shear walls, such as occurs at vertical in-plane and
out-of-plane irregularities. Overstrength in the braced frames and shear walls could cause buckling
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Structural Design Criteria
failure of such columns with resulting structural collapse. Columns subjected to tensile loading in
which splices are made using partial penetration groove welds, a type of joint subject to brittle fracture
when overloaded, are another example of a case where these special load combinations should be used.
Other design situations that warrant the use of these equations are noted throughout the Provisions.
Although the Provisions note the most common cases in which structural overstrength can lead to an
undesirable failure mode, it is not possible for them to note all such conditions. Therefore, designers
using the Provisions should be alert for conditions where the isolated independent failure of any
element can lead to a condition of instability or collapse and should use the special load combinations
of Eq. 5.6.2-3 and 5.2.7-4 for the design of these elements. Other conditions which may warrant such
a design approach, although not specifically noted in the Provisions, include the design of transfer
structures beneath discontinuous lateral force resisting elements; and the design of diaphragm force
collectors to shear walls and braced frames, when these are the only method of transferring force to
these elements at a diaphragm level.
5.2.8 DEFLECTION AND DRIFT LIMITS: This section provides procedures for the limitation of
story drift. The term "drift" has two connotations:
1. "Story drift" is the maximum lateral displacement within a story (i.e., the displacement of one floor
relative to the floor below caused by the effects of seismic loads).
2. The lateral displacement or deflection due to design forces is the absolute displacement of any
point in the structure relative to the base. This is not "story drift" and is not to be used for drift
control or stability considerations since it may give a false impression of the effects in critical
stories. However, it is important when considering seismic separation requirements.
There are many reasons for controlling drift; one is to control member inelastic strain. Although use of
drift limitations is an imprecise and highly variable way of controlling strain, this is balanced by the
current state of knowledge of what the strain limitations should be.
Stability considerations dictate that flexibility be controlled. The stability of members under elastic and
inelastic deformation caused by earthquakes is a direct function of both axial loading and bending of
members. A stability problem is resolved by limiting the drift on the vertical load carrying elements and
the resulting secondary moment from this axial load and deflection (frequently called the P-delta effect). Under small lateral deformations, secondary stresses are normally within tolerable limits.
However, larger deformations with heavy vertical loads can lead to significant secondary moments
from the P-delta effects in the design. The drift limits indirectly provide upper bounds for these effects.
Buildings subjected to earthquakes need drift control to restrict damage to partitions, shaft and stair
enclosures, glass, and other fragile nonstructural elements and, more importantly, to minimize
differential movement demands on the seismic safety elements. Since general damage control for
economic reasons is not a goal of this document and since the state of the art is not well developed in
this area, the drift limits have been established without regard to considerations such as present worth
of future repairs versus additional structural costs to limit drift. These are matters for building owners
and designers to examine. To the extent that life might be excessively threatened, general nonstructural
damage to nonstructural and seismic safety elements is a drift limit consideration.
The design story drift limits of Table 5.2.8. reflect consensus judgment taking into account the goals of
drift control outlined above. In terms of life safety and damage control objectives, the drift limits
should yield a substantial, though not absolute, measure of safety for well detailed and constructed
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1997 Commentary, Chapter 5
brittle elements and provide tolerable limits wherein the seismic safety elements can successfully
perform, provided they are designed and constructed in accordance with these Provisions.
To provide a higher performance standard, the drift limit for the essential facilities of Seismic Use
Group III is more stringent than the limit for Groups I and II except for masonry shear wall buildings.
The drift limits for low-rise structures are relaxed somewhat provided the interior walls, partitions,
ceilings, and exterior wall systems have been designed to accommodate story drifts. The type of steel
building envisioned by the exception to the table would be similar to a prefabricated steel structure with
metal skin. When the more liberal drift limits are used, it is recommended that special requirements be
provided for the seismic safety elements to accommodate the drift.
It should be emphasized that the drift limits, Da, of Table 5.2.8. are story drifts and, therefore, are
applicable to each story (i.e., they must not be exceeded in any story even though the drift in other
stories may be well below the limit.) The limit, Da is to be compared to the design story drift as
determined by Sec. 5.3.8.1.
Stress or strength limitations imposed by design level forces occasionally may provide adequate drift
control. However, it is expected that the design of moment resisting frames, especially steel building
frames, and the design of tall, narrow shear wall or braced frame buildings will be governed at least in
part by drift considerations. In areas having large design spectral response accelerations, SDS and SD1, it
is expected that seismic drift considerations will predominate for buildings of medium height. In areas
having a low design spectral response accelerations and for very tall buildings in areas with large design
spectral response accelerations , wind considerations generally will control, at least in the lower stories.
Due to probable first mode drift contributions, the Sec. 5.3 ELF procedure may be too conservative
for drift design of very tall moment-frame buildings. It is suggested for these buildings, where the first
mode would be responding in the constant displacement region of a response spectra (where
displacements would be essentially independent of stiffness), that the modal analysis procedure of Sec.
5.4 be used for design even when not required by Sec. 5.2.4.
Building separations and seismic joints are separations between two adjoining buildings or parts of the
same building, with or without frangible closures, for the purpose of permitting the adjoining buildings
or parts to respond independently to earthquake ground motion. Unless all portions of the structure
have been designed and constructed to act as a unit, they must be separated by seismic joints. For
irregular structures that cannot be expected to act reliably as a unit, seismic joints should be utilized to
separate the building into units whose independent response to earthquake ground motion can be predicted.
Although the Provisions do not give precise formulations for the separations, it is required that the
distance be "sufficient to avoid damaging contact under total deflection" in order to avoid interference
and possible destructive hammering between buildings. It is recommended that the distance be equal
to the total of the lateral deflections of the two units assumed deflecting toward each other (this
involves increasing separations with height). If the effects of hammering can be shown not to be
detrimental, these distances can be reduced. For very rigid shear wall structures with rigid diaphragms
whose lateral deflections cannot be reasonably estimated, it is suggested that older code requirements
for structural separations of at least 1 in. (25 mm) plus 1/2 in. (13 mm) for each 10 ft (3 m) of height
above 20 ft (6 m) be followed.
5.3 EQUIVALENT LATERAL FORCE PROCEDURE:
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Structural Design Criteria
5.3.1 General: This section discusses the equivalent lateral force (ELF) procedure for seismic
analysis of structures.
5.3.2 Seismic Base Shear: The heart of the ELF procedure is Eq. 5.3.2.-1 for base shear, which
gives the total seismic design force, V, in terms of two factors: a seismic response coefficient, Cs, and
the total gravity load of the building, W. The seismic response coefficient Cs, is obtained from Eq.
5.3.2..1-1 and 5.3.2..1-2 based on the design spectral response accelerations, SDS and SD1. These
acceleration parameters and the derivation of the response spectrum is discussed more fully in the
Commentary for Chapter 4.
The gravity load W is the total weight of the building and that part of the service load that might
reasonably be expected to be attached to the building at the time of an earthquake. It includes
permanent and movable partitions and permanent equipment such as mechanical and electrical
equipment, piping, and ceilings. The normal human live load is taken to be negligibly small in its
contribution to the seismic lateral forces. Buildings designed for storage or warehouse usage should
have at least 25 percent of the design floor live load included in the weight, W. Snow loads up to 30
psf (1400 Pa) are not considered. Freshly fallen snow would have little effect on the lateral force in an
earthquake; however, ice loading would be more or less firmly attached to the roof of the building and
would contribute significantly to the inertia force. For this reason, the effective snow load is taken as
the full snow load for those regions where the snow load exceeds 30 psf with the proviso that the local
authority having jurisdiction may allow the snow load to be reduced up to 80 percent. The question of
how much snow load should be included in W is really a question of how much ice buildup or snow
entrapment can be expected for the roof configuration or site topography, and this is a question best
left to the discretion of the local authority having jurisdiction.
The base shear formula and the various factors contained therein were arrived at as explained below.
Elastic Acceleration Response Spectra: See the Commentary for Chapter 4 for a full discussion of the
shape of the spectra accounting for dynamic response amplification and the effect of site response.
Elastic Design Spectra: The elastic acceleration response spectra for earthquake motions has a
descending branch for longer values of T, the period of vibration of the system, that varies roughly as
1/T. In previous editions of the Provisions, the actual response spectra that varied in a 1/T
relationship were replaced with design spectra that varied in a 1/T2/3 relationship. This was intentionally
done to provide added conservatism in the design of tall structures. In the development of the 1997
Provisions, a special task force, known as the Seismic Design Procedures Group (SDPG), was
convened to develop a method for using new seismic hazard maps, developed by the USGS in the
Provisions. Whereas older seismic hazard maps provided an effective peak ground acceleration
coefficient Ca and an effective peak velocity related acceleration coefficient Cv, the new maps directly
provide parameters that correspond to points on the response spectrum. It was the recommendation of
the SDPG that the true shape of the response spectrum, represented by a 1/T relationship, be
maintained in the base shear equation. In order to maintain the added conservatism for tall and high
occupancy structures, formerly provided by the design spectra which utilized a 1/T2/3 relationship, the
1997 Provisions adopted an occupancy importance factor I into the base shear equation. This I factor,
which has a value of 1.25 for Seismic Use Group II structures and 1.5 for Seismic Use Group III
structures has the effect of raising the design spectrum for taller, high occupancy structures, to levels
comparable to those for which they were designed in pervious editions of the Provisions.
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1997 Commentary, Chapter 5
Response Modification Factor: The factor R in the denominator of Eq. 5.3.2.1-1 is an empirical response reduction factor intended to account for both damping and the ductility inherent in the
structural system at displacements great enough to surpass initial yield and approach the ultimate load
displacement of the structural system. Thus, for a lightly damped building structure of brittle material
that would be unable to tolerate any appreciable deformation beyond the elastic range, the factor R
would be close to 1 (i.e., no reduction from the linear elastic response would be allowed). At the other
extreme, a heavily damped building structure with a very ductile structural system would be able to
withstand deformations considerably in excess of initial yield and would, therefore, justify the
assignment of a larger response reduction factor R. Table 5.2.2 in the Provisions stipulates R coefficients for different types of building systems using several different structural materials. The coefficient
R ranges in value from a minimum of 1-1/4 for an unreinforced masonry bearing wall system to a
maximum of 8 for a special moment frame system. The basis for the R factor values specified in Table
5.2.2 is explained in the Sec. 5.2.1.
The effective value of R used in the base shear equation is adjusted by the occupancy importance
factor I. The I value, which ranges from 1 to 1.5, has the effect of reducing the amount of ductility the
structure will be called on to provide at a given level of ground shaking. However, it must be
recognized that added strength, by itself, is not adequate to provide for superior seismic performance in
buildings with critical occupancies. Good connections and construction details, quality assurance
procedures, and limitations on building deformation or drift are also important to significantly improve
the capability for maintenance of function and safety in critical facilities and those with a high-density
occupancy. Consequently, the reduction in the damage potential of critical facilities (Group III) is also
handled by using more conservative drift controls (Sec. 5.2.8.) and by providing special design and
detailing requirements (Sec. 5.2.6) and materials limitations (Chapters 8 through 12).
5.3.3 Period Determination: In the denominator of Eq. 5.3.2.1-2, T is the fundamental period of
vibration of the building. It is preferable that this be determined using modal analysis methods and the
principals of structural mechanics. However, methods of structural mechanics cannot be employed to
calculate the vibration period before a building has been designed.Consequently, this section provides
an approximate method that can be used to estimate building period, with minimal information available on the building design. It is based on the use of simple formulas that involve only a general description of the building type (e.g., steel moment frame, concrete moment frame, shear wall system, braced
frame) and overall dimensions (e.g., height and plan length) to estimate the vibration period in order to
calculate an initial base shear and proceed with a preliminary design. It is advisable that this base shear
and the corresponding value of T be conservative. Even for final design, use of a large value for T is
unconservative. Thus, the value of T used in design should be smaller than the true period of the
building. Equations 5.3.3.1-1 and 5.3.3.1-2 for the approximate period Ta are therefore intended to
provide conservative estimates of the fundamental period of vibration. An upper bound is placed on
the value of T calculated using more exact methods, based on Ta and the factor Cu. The coefficient Cu
accommodates the probable fact that buildings in areas with lower lateral force requirements probably
will be more flexible. Furthermore, it results in less dramatic changes from present practice in lower
risk areas. It is generally accepted that the empirical equations for Ta are tailored to fit the type of construction common in areas with high lateral force requirements. It is unlikely that buildings in lower
risk seismic areas would be designed to produce as high a drift level as allowed in the Provisions due to
stability problems (P-delta) and wind requirements. For buildings whose design are actually
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Structural Design Criteria
"controlled" by wind, the use of a large T will not really result in a lower design force; thus, use of this
approach in high-wind regions should not result in unsafe design.
FIGURE C5.3.3-1 Periods computed from accelerograph records during the 1971 San Fernando earthquake-steel frames. The equation TR = 0.035hn 3/4 is intended to be a conservative estimate. The mean value estimate is TR
= 0.049hn3/4. The identification numbers, names, and addresses of the structures considered are as follows: (1) KB
Valley Center, 15910 Ventura; (2) Jet Propulsion Lab Administration Building 180; (3) 6464 Sunset Boulevard; (4)
1900 Avenue of the Stars, Century City; (5) 1901 Avenue of the Stars, Century City; (6) 1880 Century Park East,
Century City; (7) 1888 Century Park East Office Tower, Century City; (8) Mutual Benefit Life Plaza, 5900 Wilshire
Boulevard; (9) Department of Water and Power, 111 North Hope Street; (10) Union Bank Building, 445 South
Figueroa; (11) Kajima International, 250 East First Street; (12) Bunker Hill Tower, 800 West First Street; (13) 3407
West Sixth Street; (14) Occidental Building, 1150 South Hill Street; (15) Crocker Citizens Bank Building, 611 West
Sixth Street; (16) Sears Headquarters, 900 South Fremont, Alhambra; (17) 5260 Century Boulevard.
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1997 Commentary, Chapter 5
Taking the seismic base shear to vary as 1/T and assuming that the lateral forces are distributed linearly
over the height and the deflections are controlled by drift limitations, a simple analysis of the vibration
period by Rayleigh's method (Clough and Penzien, 1975; Newmark and Rosenblueth, 1971; Thomson,
1965; Wiegel, 1970) leads to the conclusion that the vibration period of moment resisting frame structures varies roughly as hn3/4 where hn equals the total height of the building as defined elsewhere.
Equation 5.3.3.1-1 is therefore appropriate and the values of the coefficient CT have been established to
produce values for Ta generally lower than the true fundamental vibration period of moment frame
buildings. This is apparent in Figures C5.3.3-1 and C5.3.3-2. In these figures, Eq. 5.3.3.1-1 is
compared with fundamental vibration periods computed from accelerograph records from
FIGURE C5.3.2-2 Periods computed from accelerograph records
during the 1971 San Fernando earthquake--reinforced concrete frames.
The equation TR = 0.030hn3/4 is intended to be a conservative estimate. The
mean value estimate is TR = 0.035hn 3/4. The identification numbers, names,
and addresses of the structures considered are as follows: (1) Holiday Inn,
8244 Orion Street; (2) Valley Presbyterian Hospital, 15107 Vanowen
Boulevard; (3) Bank of California, 15250 Ventura Boulevard; (4) Hilton
Hotel, 15433 Ventura Boulevard; (5) Sheraton-Universal, 3838 Lankership
Boulevard; (6) Muir Medical center, 7080 Hollywood Boulevard; (7)
Holiday Inn, 1760 North Orchid; (8) 1800 Century Park East, Century
City; (9) Wilshire Christian Towers, 616 South Normandie Avenue; (10)
Wilshire Square One, 3345 Wilshire Boulevard; (11) 533 South Fremont;
(12) Mohn Olympic, 1625 Olympic Boulevard; (13) 120 Robertson; (14)
Holiday Inn, 1640 Marengo. Incomplete study data have suggested that
Buildings 1, 3, 4, 7, 8, 9, 10, 11, 13, and 14 may not act as true frames; these
numbers are marked with an asterisk.
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Structural Design Criteria
upper stories of several buildings during the 1971 San Fernando earthquake. The optional use of T =
0.1N (Eq. 5.3.3.1-2) is an approximation for low to moderate height frames that has been long in use.
As an exception to Eq. 5.3.3.1-1, these requirements allow the calculated fundamental period of
vibration, T, of the seismic-force-resisting system to be used in calculating the base shear. However,
the period, T, used may not exceed CuTa with Ta determined from Eq. 5.3.3.1-1.
FIGURE C5.3.3-3 Periods computed from accelerograph records during the 1971 San Fernando earthquake-reinforced concrete shear wall buildings. The equation TR = 0.5hn/%
% D is intended to be a conservative estimate
for all buildings other than steel frames and reinforced concrete frames. The mean value estimate is TR =
0.07hn % D. The identification numbers, names, and addresses of the buildings considered are as follows: (1)
Certified Life, 14724 Ventura Boulevard; (2) Kaiser Foundation hospital, 4867 Sunset Boulevard; (3) Millikan
Library, Cal Tech, Pasadena; (4) 1888 Century Park East, Century City; (5) 3470 Wilshire Boulevard; (6) Los
Angeles Athletic Club Parking Structure, 646 South Olive; (7) Parking Structure, 808 South Olive; (8) USC
Medical Center, 2011 Zonal; (9) Airport Marina Hotel, 8639 Lincoln, Marina Del Ray.
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1997 Commentary, Chapter 5
For exceptionally stiff or light buildings, the calculated T for the seismic-force-resisting system may be
significantly shorter than Ta calculated by Eq. 5.3.3.1-1. For such buildings, it is recommended that the
period value T be used in lieu of Ta for calculating the seismic response coefficient, Cs.
Although the approximate methods of Section 3.3.3. can be used to determine a period for the design
of structures, the fundamental period of vibration of the seismic-force-resisting system should be
calculated according to established methods of mechanics. Computer programs are available for such
calculations. One method of calculating the period, probably as convenient as any, is the use of the
following formula based on Rayleigh's method (Clough and Penzien, 1975; Newmark and Rosenblueth, 1971; Thomson, 1965; Wiegel, 1970):
j wi*i
n
T ' 2B
2
i'1
(C5.3.3)
gj Fi*i
n
i'1
where:
Fi =
the seismic lateral force at Level i,
wi =
the gravity load assigned in Level i,
di =
the static lateral displacement at Level i due to the forces Fi computed on a linear elastic
basis, and
g
is the acceleration of gravity.
=
The calculated period increases with an increase in flexibility of the structure because the δ term in the
Rayleigh formula appears to the second power in the numerator but to only the first power in the
denominator. Thus, if one ignores the contribution of nonstructural elements to the stiffness of the
structure in calculating the deflections δ, the deflections are exaggerated and the calculated period is
lengthened, leading to a decrease in the seismic response coefficient Cs and, therefore, a decrease in the
design force. Nonstructural elements do not know that they are nonstructural. They participate in the
behavior of the structure even though the designer may not rely on them for contributing any strength
or stiffness to the structure. To ignore them in calculating the period is to err on the unconservative
side. The limitation of CuTa is imposed as a safeguard.
5.3.4 Vertical Distribution of Seismic Forces: The distribution of lateral forces over the height of a
structure is generally quite complex because these forces are the result of superposition of a number of
natural modes of vibration. The relative contributions of these vibration modes to the total forces
depends on a number of factors including the shape of the earthquake response spectrum, the natural
periods of vibration of the structure, and the shapes of vibration modes that, in turn, depend on the
mass and stiffness over the height (see Sec. 5.2.3). The basis of this method is discussed below. In
structures having only minor irregularity of mass or stiffness over the height, the accuracy of the lateral
force distribution as given by Eq. 5.3.4-2 is much improved by the procedure described in the last
portion of Sec. 5.2.4 of this commentary. The lateral force at each level, x, due to response in the first
(fundamental) natural mode of vibration is:
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Structural Design Criteria
fx1 ' V1
wx Nx1
j wiNi1
(C5.3.4)
n
i ' 1
where:
V1 = the contribution of this mode to the base shear,
wi = the weight lumped at the ith level, and
Ni = the amplitude of the first mode at the ith level.
This is the same as Eq. 5.4.6-2 in Sec. 5.4 of the Provisions, but it is specialized for the first mode. If
V1 is replaced by the total base shear, V, this equation becomes identical to Eq. 5.3.4-2 with k = 1 if the
first mode shape is a straight line and with k = 2 if the first mode shape is a parabola with its vertex at
the base.
It is well known that the influence of modes of vibration higher than the fundamental mode is small in
the earthquake response of short period structures and that, in regular structures, the fundamental
vibration mode departs little from a straight line. This, along with the matters discussed above, provides the basis for Eq. 5.3.4-2 with k = 1 for structures having a fundamental vibration period of 0.5
seconds or less.
It has been demonstrated that although the earthquake response of long period structures is primarily
due to the fundamental natural mode of vibration, the influence of higher modes of vibration can be
significant and, in regular structures, the fundamental vibration mode lies approximately between a
straight line and a parabola with the vertex at the base. Thus, Eq. 5.3.4-2 with k = 2 is appropriate for
structures having a fundamental period of vibration of 2.5 seconds or longer. Linear variation of k
between 1 at a 0.5 second period and 2 at a 2.5 seconds period provides the simplest possible transition
between the two extreme values.
5.3.5 Horizontal Shear Distribution: The story shear in any story is the sum of the lateral forces
acting at all levels above that story. Story x is the story immediately below Level x (` C5.3.5).
Reasonable and consistent assumptions regarding the stiffness of concrete and masonry elements may
be used for analysis in distributing the shear force to such elements connected by a horizontal diaphragm. Similarly, the stiffness of moment or braced frames will establish the distribution of the story
shear to the vertical resisting elements in that story.
5.3.5.1 Torsion: The torsional moment to be considered in the design of elements in a story consists
of two parts:
1. Mt, the moment due to eccentricity between centers of mass and resistance for that story, is to be
computed as the story shear times the eccentricity perpendicular to the direction of applied earthquake forces.
2. Mta, commonly referred to as "accidental torsion," is to be computed as the story shear times the
"accidental eccentricity," equal to 5 percent of the dimension of the structure, in the story under
consideration perpendicular to the direction of the applied earthquake forces.
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1997 Commentary, Chapter 5
Computation of Mta in this manner is equivalent to the procedure in Sec. 5.3.5 which implies that the
dimension of the structure is the dimension in the story where the torsional moment is being computed
and that all the masses above that story should be assumed to be displaced in the same direction at one
time (e.g., first, all of them to the left and, then, to the right).
Dynamic analyses assuming linear behavior indicate that the torsional moment due to eccentricity
between centers of mass and resistance may significantly exceed Mt (Newmark and Rosenblueth,
1971). However, such dynamic magnification is not included in the Provisions, partly because its
significance is not well understood for structures designed to deform well beyond the range of linear
behavior.
FIGURE C5.3.5 Description of story and level. The shear at
Story x (Vx) is the sum of all the lateral forces at and above Story
x (Fx through Fn).
The torsional moment Mt calculated in accordance with this provision would be zero in those stories
where centers of mass and resistance coincide. However, during vibration of the structure, torsional
moments would be induced in such stories due to eccentricities between centers of mass and resistance
in other stories. To account for such effects, it is recommended that the torsional moment in any story
be not smaller than the following two values (Newmark and Rosenblueth, 1971):
1. The story shear times one-half of the maximum of the computed eccentricities in all stories below
the one being analyzed and
2. One-half of the maximum of the computed torsional moments for all stories above.
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Structural Design Criteria
Accidental torsion is intended to cover the effects of several factors that have not been explicitly
considered in the Provisions. These factors include the rotational component of ground motion about
a vertical axis; unforeseeable differences between computed and actual values of stiffness, yield
strengths, and dead-load masses; and unforeseeable unfavorable distributions of dead- and live-load
masses.
There are indications that the 5 percent accidental eccentricity may be too small in some structures
since they may develop torsional dynamic instability. Some examples are the upper stories of tall
structures having little or no nominal eccentricity, those structures where the calculations of relative
stiffnesses of various elements are particularly uncertain (e.g., those that depend largely on masonry
walls for lateral force resistance or those that depend on vertical elements made of different materials),
and nominally symmetrical structures that utilize core elements alone for seismic resistance or that
behave essentially like elastic nonlinear systems (e.g., some prestressed concrete frames). The
amplification factor for torsionally irregular structures (Eq. 5.3.5.1) was introduced in the 1988 Edition
as an attempt to account for some of these problems in a controlled and rational way.
The way in which the story shears and the effects of torsional moments are distributed to the vertical
elements of the seismic-force-resisting system depends on the stiffness of the diaphragms relative to
vertical elements of the system.
Where the diaphragm stiffness in its own plane is sufficiently high relative to the stiffness of the vertical
components of the system, the diaphragm may be assumed to be indefinitely rigid for purposes of this
section. Then, in accordance with compatibility and equilibrium requirements, the shear in any story is
to be distributed among the vertical components in proportion to their contributions to the lateral
stiffness of the story while the story torsional moment produces additional shears in these components
that are proportional to their contributions to the torsional stiffness of the story about its center of
resistance. This contribution of any component is the product of its lateral stiffness and the square of
its distance to the center of resistance of the story. Alternatively, the story shears and torsional
moments may be distributed on the basis of a three-dimensional analysis of the structure, consistent
with the assumption of linear behavior.
Where the diaphragm in its own plane is very flexible relative to the vertical components, each vertical
component acts almost independently of the rest. The story shear should be distributed to the vertical
components considering these to be rigid supports. Analysis of the diaphragm acting as a continuous
horizontal beam or truss on rigid supports leads to the distribution of shears. Because the properties of
the beam or truss may not be accurately computed, the shears in vertical elements should not be taken
to be less than those based on "tributary areas." Accidental torsion may be accounted for by adjusting
the position of the horizontal force with respect to the supporting vertical elements.
There are some common situations where it is obvious that the diaphragm can be assumed to be either
rigid or very flexible in its own plane for purposes of distributing story shear and considering torsional
moments. For example, a solid monolithic reinforced concrete slab, square or nearly square in plan, in
a structure with slender moment resisting frames may be regarded as rigid. A large plywood
diaphragm with widely spaced and long, low masonry walls may be regarded as very flexible. In
intermediate situations, the design forces should be based on an analysis that explicitly considers diaphragm deformations and satisfies equilibrium and compatibility requirements. Alternatively, the
design forces should be the envelope of the two sets of forces resulting from both extreme assumptions
regarding the diaphragms--rigid or very flexible.
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1997 Commentary, Chapter 5
Where the horizontal diaphragm is not continuous, the story shear can be distributed to the vertical
components based on their tributary areas.
5.3.6 Overturning: This section requires that the structure be designed to resist overturning
moments statically consistent with the design story shears, except for reduction factor J in Eq. 5.3.6.
There are several reasons for reducing the statically computed overturning moments:
1. The distribution of design story shears over height computed from the lateral forces of Sec. 5.3.2 is
intended to provide an envelope since the shears in all stories do not attain their maximum
simultaneously. Thus, the overturning moments computed statically from the envelope of story
shears will be overestimated.
2. It is intended that the design shear envelope, which is based on the simple distribution of forces
specified in Sec. 5.3.4, be conservative. If the shear in a specific story is close to the exact value,
the shears in almost all other stories are almost necessarily overestimated. Hence, the overturning
moments statically consistent with the design story shears will be overestimated.
3. Under the action of overturning moments, one edge of the foundation may lift from the ground for
short durations of time. Such behavior leads to substantial reduction in the seismic forces and,
consequently, in the overturning moments.
The overturning moments computed statically from the envelope of story shears may be reduced by no
more than 20 percent. This value is similar to those obtained from results of dynamic analysis taking
into account the first two reasons presented above. No reduction is permitted in the uppermost 10
stories primarily because the statically computed overturning moment in these stories may err on the
unsafe side (Newmark and Rosenblueth, 1971). In any case, there is hardly any benefit in reducing the
overturning moments in the stories near the top of structures because design of vertical elements in
these stories is rarely governed by overturning moments. For the eleventh to the twentieth stories from
the top, linear variation of J provides the simplest transition between the minimum and maximum
values of 0.8 and 1.0.
In the design of the foundation, the overturning moment may be calculated at the foundation-soil
interface using Eq. 5.3.6 with J = 0.75 for structures of all heights. This is appropriate because a slight
uplifting of one edge of the foundation during vibration leads to reduction in the overturning moment
and because such behavior does not normally cause structural distress.
Formerly, many building codes and design recommendations allowed more drastic reduction in overturning moments relative to their value statically consistent with the design story shears. These
reductions appeared to be excessive in light of the damage to structures during the 1967 Caracas
earthquake where a number of column failures were due primarily to effects of overturning moment.
In later versions of the SEAOC recommendations (1973), no reduction was allowed. The moderate
reduction permitted in Sec. 5.3.6, which is consistent with results of dynamic analyses (Newmark and
Rosenblueth, 1971), is more appropriate because use of the full statically determined overturning
moment cannot be justified in light of the reasons mentioned above.
5.3.7 Drift Determination and P-delta Effects: This section defines the design story drift as the
difference of the deflections, *x, at the top and bottom of the story under consideration. The
deflections, *x, are determined by multiplying the deflections, *xe (determined from an elastic analysis),
by the deflection amplification factor, Cd, given in Table 5.2.2. The elastic analysis is to be made for
the seismic-force-resisting system using the prescribed seismic design forces and considering the
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Structural Design Criteria
structure to be fixed at the base. Stiffnesses other than those of the seismic-force-resisting system
should not be included since they may not be reliable at higher inelastic strain levels.
The deflections are to be determined by combining the effects of joint rotation of members, shear
deformations between floors, the axial deformations of the overall lateral resisting elements, and the
shear and flexural deformations of shear walls and braced frames. The deflections are determined
initially on the basis of the distribution of lateral forces stipulated in Sec. 5.3.4. For frame structures,
the axial deformations from bending effects, although contributing to the overall structural distortion,
may or may not affect the story-to-story drift; however, they are to be considered. Centerline
dimensions between the frame elements often are used for analysis, but clear span dimensions with
consideration of joint panel zone deformation also may be used.
For determining compliance with the story drift limitation of Sec. 5.2.7, the deflections, *x, may be
calculated as indicated above for the seismic-force-resisting system and design forces corresponding to
the fundamental period of the structure, T (calculated without the limit T # CuTa specified in Sec.
5.3.3), may be used. The same model of the seismic-force-resisting system used in determining the
deflections must be used for determining T. The waiver does not pertain to the calculation of drifts for
determining P-delta effects on member forces, overturning moments, etc. If the P-delta effects
determined in Sec. 5.3.7.2 are significant, the design story drift must be increased by the resulting
incremental factor.
The P-delta effects in a given story are due to the eccentricity of the gravity load above that story. If
the story drift due to the lateral forces prescribed in Sec. 5.3.4 were ), the bending moments in the
story would be augmented by an amount equal to ) times the gravity load above the story. The ratio
of the P-delta moment to the lateral force story moment is designated as a stability coefficient, 2, in Eq.
5.3.7.2-1. If the stability coefficient 2 is less than 0.10 for every story, the P-delta effects on story
shears and moments and member forces may be ignored. If, however, the stability coefficient 2
exceeds 0.10 for any story, the P-delta effects on story drifts, shears, member forces, etc., for the
whole structure must be determined by a rational analysis.
An acceptable P-delta analysis, based upon elastic stability theory, is as follows:
1. Compute for each story the P-delta amplification factor, ad = 2/(1 - 2). ad takes into account the
multiplier effect due to the initial story drift leading to another increment of drift that would lead to
yet another increment, etc. Thus, both the effective shear in the story and the computed
eccentricity would be augmented by a factor 1 + 2 + 2 2 + 2 3 ..., which is 1/(1 - 2) or (1 + ad).
2. Multiply the story shear, Vx, in each story by the factor (1 + ad) for that story and recompute the
story shears, overturning moments, and other seismic force effects corresponding to these
augmented story shears.
This procedure is applicable to planar structures and, with some extension, to three-dimensional
structures. Methods exist for incorporating two- and three-dimensional P-delta effects into computer
analyses that do not explicitly include such effects (Rutenburg, 1985). Many programs explicitly
include P-delta effects. A mathematical description of the method employed by several popular
programs is given by Wilson and Habibullah (1987).
The P-delta procedure cited above effectively checks the static stability of a structure based on its
initial stiffness. Since the inception of this procedure with ATC 3-06, however, there has been some
debate regarding its accuracy. This debate stems from the intuitive notion that the structure's secant
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1997 Commentary, Chapter 5
stiffness would more accurately represent inelastic P-delta effects. Given the additional uncertainty of
the effect of dynamic response on P-delta behavior and the (apparent) observation that instabilityrelated failures rarely occur in real structures, the P-delta requirements remained as originally written
until revised for the 1991 Edition.
There was increasing evidence that the use of inelastic stiffness in determining theoretical P-delta
response is unconservative. Given a study carried out by Bernal (1987), it was argued that P-delta
amplifiers should be based on secant stiffness and that, in other words, the Cd term in Eq. 5.3.7.2-1
should be deleted. However, since Bernal's study was based on the inelastic response of single-degreeof-freedom elastic-perfectly plastic systems, significant uncertainties existed regarding the
extrapolation of the concepts to the complex hysteretic behavior of multi-degree-of-freedom systems.
Another problem with accepting a P-delta procedure based on secant stiffness was that design forces
would be greatly increased. For example, consider an ordinary moment frame of steel with a Cd of 4.0
and an elastic stability coefficient 2 of 0.15. The amplifier for this structure would be 1.0/0.85 = 1.18
according to the 1988 Edition of the Provisions. If the P-delta effects were based on secant stiffness,
however, the stability coefficient would increase to 0.60 and the amplifier would become 1.0/0.4 =
2.50. (Note that the 0.9 in the numerator of the amplifier equation in the 1988 Edition was dropped
for this comparison.) This example illustrates that there could be an extreme impact on the
requirements if a change was implemented that incorporated P-delta amplifiers based on static secant
stiffness response.
There was, however, some justification for retaining the P-delta amplifier as based on elastic stiffness.
This justification was the apparent lack of stability-related failures. The reasons for the lack of
observed failures included:
1. Many structures display strength well above the strength implied by code-level design forces (see
Figure C5.1.1). This overstrength likely protects structures from stability-related failures.
2. The likelihood of a stability failure decreases with increased intensity of expected ground-shaking.
This is due to the fact that the stiffness of most structures designed for extreme ground motion is
significantly greater than the stiffness of the same structure designed for lower intensity shaking or
for wind. Since damaging low-intensity earthquakes are somewhat rare, there would be little
observable damage.
Due to the lack of stability-related failures, therefore, the requirements of the 1988 Edition of the
Provisions regarding P-delta amplifiers remain in the 1991 and 1994 Editions with the exception that
the 0.90 factor in the numerator of the amplifier has been deleted. This factor originally was used to
create a transition from cases where P-delta effects need not be considered (2 # 0.10, amplifier = 1.0)
to cases where such effects need be considered (2 > 1.0, amplifier > 1.0).
However, the 1991 Edition introduced a requirement that the computed stability coefficient, 2, not
exceed 0.25 or 0.5/$Cd, where $Cd is an adjusted ductility demand that takes into account the fact that
the seismic strength demand may be somewhat less than the code strength supplied. The adjusted
ductility demand is not intended to incorporate overstrength beyond that computed by the means
available in Chapters 8 through 14 of the Provisions.
The purpose of this requirement is to protect structures from the possibility of stability failures
triggered by post-earthquake residual deformation. The danger of such failures is real and may not be
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Structural Design Criteria
eliminated by apparently available overstrength. This is particularly true of structures designed in
regions of lower seismicity.
The computation of 2max, which, in turn, is based on $Cd, requires the computation of story strength
supply and story strength demand. Story strength demand is simply the seismic design shear for the
story under consideration. The story strength supply may be computed as the shear in the story that
occurs simultaneously with the attainment of the development of first significant yield of the overall
structure. To compute first significant yield, the structure should be loaded with a seismic force
pattern similar to that used to compute seismic story strength demand. A simple and conservative
procedure is to compute the ratio of demand to strength for each member of the seismic-force-resisting
system in a particular story and then use the largest such ratio as $. For a structure otherwise in
conformance with the Provisions, $ = 1.0 is obviously conservative.
The principal reason for inclusion of $ is to allow for a more equitable analysis of those structures in
which substantial extra strength is provided, whether as a result of added stiffness for drift control,
from code-required wind resistance, or simply a feature of other aspects of the design. $ = story shear
demand/story shear capacity is conservatively 1.0 for any design that meets the remainder of the
Provisions. Some structures inherently possess more strength than required, but instability is not
typically a concern for such structures. For many flexible structures, the proportions of the structural
members are controlled by the drift requirements rather than the strength requirements; consequently, $
is less than 1.0 because the members provided are larger and stronger than required. This has the
effect of reducing the inelastic component of total seismic drift and, thus, $ is placed as a factor on Cd.
Accurate evaluation of $ would require consideration of all pertinent load combinations to find the
maximum value of seismic load effect demand to seismic load effect capacity in each and every
member. A conservative simplification is to divide the total demand with seismic included by the total
capacity; this covers all load combinations in which dead and live effects add to seismic. If a member is
controlled by a load combination where dead load counteracts seismic, to be correctly computed, the
ratio $ must be based only on the seismic component, not the total; note that the vertical load P in the
P-delta computation would be less in such a circumstance and, therefore, 2 would be less. The
importance of the counteracting load combination does have to be considered, but it rarely controls
instability.
5.4 MODAL ANALYSIS PROCEDURE:
5.4.1 General, and 5.4.2 Modeling: Modal analysis (Newmark and Rosenblueth, 1971; Clough and
Penzien, 1975; Thomson, 1965; Wiegel, 1970) is applicable for calculating the linear response of
complex, multi-degree-of-freedom structures and is based on the fact that the response is the
superposition of the responses of individual natural modes of vibration, each mode responding with its
own particular pattern of deformation (the mode shape), with its own frequency (the modal
frequency), and with its own modal damping. The response of the structure, therefore. can be
modeled by the response of a number of single-degree-of-freedom oscillators with properties chosen to
be representative of the mode and the degree to which the mode is excited by the earthquake motion.
For certain types of damping, this representation is mathematically exact and, for structures, numerous
full-scale tests and analyses of earthquake response of structures have shown that the use of modal
analysis, with viscously damped single-degree-of-freedom oscillators describing the response of the
structural modes, is an accurate approximation for analysis of linear response.
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1997 Commentary, Chapter 5
Modal analysis is useful in design. The Equivalent Lateral Force procedure of Sec. 5.3 is simply a first
mode application of this technique, that assumes all of the structure’s mass is active in the first mode..
The purpose of modal analysis is to obtain the maximum response of the structure in each of its
important modes, which are then summed in an appropriate manner. This maximum modal response
can be expressed in several ways. For the Provisions, it was decided that the modal forces and their
distributions over the structure should be given primary emphasis to highlight the similarity to the
equivalent static methods traditionally used in building codes (the SEAOC recommendations and the
UBC) and the ELF procedure in Sec. 5.3. Thus, the coefficient Csm in Eq. 5.4.5-1 and the distribution
equations, Eq. 5.4.6-1 and 5.4.6-2, are the counterparts of Eq. 5.3.4-1 and 5.3.4-2. This correspondence helps clarify the fact that the simplified modal analysis contained in Sec. 5.4 is simply an
attempt to specify the equivalent lateral forces on a structure in a way that directly reflects the individual dynamic characteristics of the structure. Once the story shears and other response variables for
each of the important modes are determined and combined to produce design values, the design values
are used in basically the same manner as the equivalent lateral forces given in Sec. 5.3.
5.4.3 MODES: This section defines the number of modes to be used in the analysis. For many
structures, including low-rise structures and structures of moderate height, three modes of vibration in
each direction are nearly always sufficient to determine design values of the earthquake response of the
structure. For high-rise structures, however, more than three modes may be required to adequately
determine the forces for design. This section provides a simple rule that the combined participating
mass of all modes considered in the analysis should be equal to or greater than 90 percent of the
effective total mass in each of two orthogonal horizontal directions.
5.4.4 Periods: Natural periods of vibration are required for each of the modes used in the subsequent
calculations. These are needed to determine the modal coefficients Csm from Eq. 5.4.5-3. Because the
periods of the modes contemplated in these requirements are those associated with moderately large,
but still essentially linear, structural response, the period calculations should include only those
elements that are effective at these amplitudes. Such periods may be longer than those obtained from a
small-amplitude test of the structure when completed or the response to small earthquake motions
because of the stiffening effects of nonstructural and architectural components of the structure at small
amplitudes. During response to strong ground-shaking, however, measured responses of structures
have shown that the periods lengthen, indicating the loss of the stiffness contributed by those
components.
There exists a wide variety of methods for calculation of natural periods and associated mode shapes,
and no one particular method is required by the Provisions. It is essential, however, that the method
used be one based on generally accepted principles of mechanics such as those given in well known
textbooks on structural dynamics and vibrations (Clough and Penzien, 1975; Newmark and Rosenblueth, 1971; Thomson, 1965; Wiegel, 1970). Although it is expected that in many cases computer
programs, whose accuracy and reliability are documented and widely recognized, will be used to
calculate the required natural periods and associated mode shapes, their use is not required.
5.4.5 Modal Base Shear: A central feature of modal analysis is that the earthquake response is
considered as a combination of the independent responses of the structure vibrating in each of its
important modes. As the structure vibrates back and forth in a particular mode at the associated
period, it experiences maximum values of base shear, interstory drifts, floor displacements, base
(overturning) moments, etc. In this section, the base shear in the mth mode is specified as the product
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Structural Design Criteria
of the modal seismic coefficient Csm and the effective weight Wm for the mode. The coefficient Csm is
determined for each mode from Eq. 5.4.5-3 using the associated period of the mode, Tm, in addition to
the factors Cv and R, which are discussed elsewhere in the Commentary. An exception to this procedure occurs for higher modes of those structures that have periods shorter than 0.3 second and that
are founded on soils of Site Class D, E, or F. For such modes, Eq. 5.4.5-4 is used. Equation 5.4.5-4
gives values ranging from SDS/2.5R for very short periods to SDS/R for Tm = 0.3. Comparing these
values to the limiting values of Cs of SDS/R for soils with Soil Profile Type D as specified following Eq.
5.4.5-3, it is seen that the use of Eq. 5.4.5-4, when applicable, reduces the modal base shear. This is an
approximation introduced in consideration of the conservatism embodied in using the spectral shape
specified by Eq. 5.4.5-3 and its limiting values. The spectral shape so defined is a conservative
approximation to average spectra that are known to first ascend, level off, and then decay as period
increases. Equation 5.4.5-3 and its limiting values conservatively replace the ascending portion for
small periods by a level portion. For soils with Soil Profile Type A, B and C, the ascending portion of
the spectra is completed by the time the period reaches a small value near 0.1 or 0.2 second. On the
other hand, for soft soils the ascent may not be completed until a larger period is reached. Equation
5.4.5-4 is then a replacement for the spectral shape for soils with Soil Profile Type D, E and F and
short periods that is more consistent with spectra for measured accelerations. It was introduced
because it was judged unnecessarily conservative to use Eq. 5.4.5-3 for modal analysis in the case of
soils with Soil Profile Types D, E, and F. The effective modal gravity load given in Eq. 5.4.5-2 can be
interpreted as specifying the portion of the weight of the structure that participates in the vibration of
each mode. It is noted that Eq. 5.4.5-2 gives values of Wm that are independent of how the modes are
normalized.
The final equation of this section, Eq. 5.4.5-5, is to be used if a modal period exceeds 4 seconds. It can
be seen that Eq. 5.4.5-5 and 5.4.5-3 coincide at Tm = 4 seconds so that the effect of using Eq. 5.4.5-5 is
to provide a more rapid decrease in Csm as a function of the known characteristics of earthquake
response spectra at intermediate and long periods. At intermediate periods, the average velocity
spectrum of strong earthquake motions from large (magnitude 6.5 and larger) earthquakes is approximately constant, which implies that Csm should decrease as 1/Tm. For very long periods, the average
displacement spectrum of strong earthquake motions becomes constant which implies that Csm, a form
of acceleration spectrum, should decay as 1/Tm2. The period at which the displacement response
spectrum becomes constant depends on the size of the earthquake, being larger for great earthquakes,
and a representative period of 4 seconds was chosen to make the transition.
5.4.6 Modal Forces, Deflections, and Drifts: This section specifies the forces and displacements
associated with each of the important modes of response.
Modal forces at each level are given by Eq. 5.4.6-1 and 5.4.6-2 and are expressed in terms of the
gravity load assigned to the floor, the mode shape, and the modal base shear Vm. In applying the forces
Fxm to the structure, the direction of the forces is controlled by the algebraic sign of fxm. Hence, the
modal forces for the fundamental mode will all act in the same direction, but modal forces for the
second and higher modes will change direction as one moves up the structure. The form of Eq. 5.4.6-1
is somewhat different from that usually employed in standard references and shows clearly the relation
between the modal forces and the modal base shear. It therefore is a convenient form for calculation
and highlights the similarity to Eq. 5.3.4-1 in the ELF procedure.
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1997 Commentary, Chapter 5
The modal deflections at each level are specified by Eq. 5.4.6-3. These are the displacements caused
by the modal forces Fxm considered as static forces and are representative of the maximum amplitudes
of modal response for the essentially elastic motions envisioned within the concept of the seismic
response modification coefficient R. This is also a logical point to calculate the modal drifts, which are
required in Sec. 5.4.8. If the mode under consideration dominates the earthquake response, the modal
deflection under the strongest motion contemplated by the Provisions can be estimated by multiplying
by the deflection amplification factor Cd. It should be noted also that dxm is proportional to fxm (this can
be shown with algebraic substitution for Fxm in Eq. 5.4.6-4) and will therefore change direction up and
down the structure for the higher modes.
5.4.7 Modal Story Shears and Moments: This section merely specifies that the forces of Eq. 5.4.61 should be used to calculate the shears and moments for each mode under consideration. In essence,
the forces from Eq. 5.4.6-1 are applied to each mass, and linear static methods are used to calculate
story shears and story overturning moments. The base shear that results from the calculation should
check with Eq. 5.4.5-1.
5.4.8 Design Values: This section specifies the manner in which the values of story shear, moment,
and drift quantities and the deflection at each level are to be combined. The method used, in which the
design value is the square root of the sum of the squares of the modal quantities, was selected for its
simplicity and its wide familiarity (Clough and Penzien, 1975; Newmark and Rosenblueth, 1971;
Wiegel, 1970). In general, it gives satisfactory results, but it is not always a conservative predictor of
the earthquake response inasmuch as more adverse combinations of modal quantities than are given by
this method of combination can occur. The most common instance where combination by use of the
square root of the sum of the squares is unconservative occurs when two modes have very nearly the
same natural period. In this case, the responses are highly correlated and the designer should consider
combining the modal quantities more conservatively (Newmark and Rosenblueth, 1971). In the 1991
Edition of the Provisions the option of combining these quantities by the complete quadratic
combination (CQC) technique was introduced. This method provides somewhat better results than the
square root of the sum of squares method for the case of closely spaced modes.
This section also limits the reduction of base shear that can be achieved by modal analysis compared to
use of the ELF procedure. Some reduction, where it occurs, is thought justified because the modal
analysis gives a somewhat more accurate representation of the earthquake response. Some limit to any
such possible reduction that may occur from the calculation of longer natural periods is necessary
because the actual periods of vibration may not be as long, even at moderately large amplitudes of
motion, due to the stiffening effects of elements not a part of the seismic resisting system and of
nonstructural and architectural components. The limit is imposed by comparison to the ELF
procedure with a 20 percent increase in the factor Cu.
5.4.9 Horizontal Shear Distribution and Torsion: This section requires that the design story shears
calculated in Sec. 5.4.8 and the torsional moments prescribed in Sec. 5.3.5 be distributed to the vertical
elements of the seismic resisting system as specified in Sec. 5.3.5 and as elaborated on in the
corresponding section of this commentary.
5.4.10 Foundation Overturning: Because story moments are calculated mode by mode (properly
recognizing that the direction of forces Fxm is controlled by the algebraic sign of fxm) and then combined
to obtain the design values of story moments, there is no reason for reducing these design moments.
This is in contrast with reductions permitted in overturning moments calculated from equivalent lateral
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Structural Design Criteria
forces in the analysis procedures of Sec. 5.3 (see Sec. 5.3.6 of this commentary). However, in the
design of the foundation, the overturning moment calculated at the foundation-soil interface may be
reduced by 10 percent for the reasons mentioned in Sec. 5.3.6 of this commentary.
5.4.11 P-delta Effects: Sec. 5.3.7 of this commentary applies to this section. In addition, to obtain
the story drifts when using the modal analysis procedure of Sec. 5.4, the story drift for each mode
should be independently determined in each story (Sec. 5.4.6). The story drift should not be determined from the differential combined lateral structural deflections since this latter procedure will tend
to mask the higher mode effects in longer period structures.
5.5 SOIL-STRUCTURE INTERACTION EFFECTS:
5.5.1 General:
Statement of the Problem: Fundamental to the design requirements presented in Sec. 5.3 and 5.4 is the
assumption that the motion experienced by the base of a structure during an earthquake is the same as
the free-field ground motion, a term that refers to the motion that would occur at the level of the
foundation if no structure was present. Strictly speaking, this assumption is true only for structures
supported on essentially rigid ground. For structures supported on soft soil, the foundation motion
generally is different from the free-field motion and may include an important rocking component in
addition to a lateral or translational component. The rocking component may be particularly significant
for tall structures.
A flexibly supported structure also differs from a rigidly supported structure in that a substantial part of
its vibrational energy may be dissipated into the supporting medium by radiation of waves and by
hysteretic action in the soil. The importance of the latter factor increases with increasing intensity of
ground-shaking. There is, of course, no counterpart of this effect of energy dissipation in a rigidly
supported structure.
The effects of soil-structure interaction accounted for in Sec. 5.5 represent the difference in the
response of the structure computed by assuming the motion of the foundation to be the same as the
free-field ground motion and considering the modified or actual motion of the foundation. This
difference depends on the characteristics of the free-field ground motion as well as on the properties of
the structure and the supporting medium.
The interaction effects accounted for in Sec. 5.5 should not be confused with "site effects," which refer
to the fact that the characteristics of the free-field ground motion induced by a dynamic event at a given
site are functions of the properties and geological features of the subsurface soil and rock. The
interaction effects, on the other hand, refer to the fact that the dynamic response of a structure built on
that site depends, in addition, on the interrelationship of the structural characteristics and the properties
of the local underlying soil deposits. The site effects are reflected in the values of the seismic
coefficients employed in Sec. 5.3 and 5.4 and are accounted for only implicitly in Sec. 5.5.
Possible Approaches to the Problem: Two different approaches may be used to assess the effects of
soil-structure interaction. The first involves modifying the stipulated free-field design ground motion
and evaluating the response of the given structure to the modified motion of the foundation whereas
the second involves modifying the dynamic properties of the structure and evaluating the response of
the modified structure to the prescribed free-field ground motion (Veletsos, 1977). When properly
implemented, both approaches lead to equivalent results. However, the second approach, involving
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1997 Commentary, Chapter 5
the use of the free-field ground motion, is more convenient for design purposes and provides the basis
of the requirements presented in the Sec. 5.5.
Characteristics of Interaction: The interaction effects in the approach used here are expressed by an
increase in the fundamental natural period of the structure and a change (usually an increase) in its
effective damping. The increase in
period results from the flexibility of
the foundation soil whereas the
change in damping results mainly
from the effects of energy
dissipation in the soil due to
radiation and material damping.
These statements can be clarified by
comparing the responses of rigidly
and elastically supported systems
subjected to a harmonic excitation of
the base. Consider a linear structure
of weight W, lateral stiffness k, and
coefficient of viscous damping c
(shown in Figure C5.5.1-1) and
assume that it is supported by a
foundation of weight Wo at the
surface of a homogeneous, elastic
halfspace.
FIGURE C5.5.1-1 Simple system investigated.
The foundation mat is idealized as a
rigid circular plate of negligible
thickness bonded to the supporting medium, and the columns of the structure are considered to be
weightless and axially inextensible. Both the foundation weight and the weight of the structure are
assumed to be uniformly distributed over circular areas of radius r. The base excitation is specified by
the free-field motion of the ground surface. This is taken as a horizontally directed, simple harmonic
motion with a period To and an acceleration amplitude am.
The configuration of this system, which has three degrees of freedom when flexibly supported and a
single degree of freedom when fixed at the base, is specified by the lateral displacement and rotation of
the foundation, y and 2, and by the displacement relative to the base of the top of the structure, u. The
system may be viewed either as the direct model of a one-story structural frame or, more generally, as a
model of a multistory, multimode structure that responds as a single-degree-of-freedom system in its
fixed-base condition. In the latter case, h must be interpreted as the distance from the base to the centroid of the inertia forces associated with the fundamental mode of vibration of the fixed-base structure
and W, k, and c must be interpreted as its generalized or effective weight, stiffness, and damping
coefficient, respectively. The relevant expressions for these quantities are given below.
The solid lines in Figures C5.5.1-2 and 5.5.1-3 represent response spectra for the steady-state
amplitude of the total shear in the columns of the system considered in Figure C5.5.1-1. Two different
values of h/r and several different values of the relative flexibility parameter for the soil and the struc-
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Structural Design Criteria
ture, No, are considered. The latter parameter is defined by the equation *o '
h
in which h is the
vs T
height of the structure as previously indicated, vs is the velocity of shear wave propagation in the
halfspace, and T is the fixed-base natural period of the structure. A value of N = 0 corresponds to a
rigidly supported structure.
The results in Figures C5.5.1-2 and 5.5.1-3 are displayed in a dimensionless form, with the abscissa
representing the ratio of the period of the excitation, To, to the fixed-base natural period of the system,
T, and the ordinate representing the ratio of
the amplitude of the actual base shear, V, to
the amplitude of the base shear induced in an
infinitely stiff, rigidly supported structure.
The latter quantity is given by the product
mam, in which m = W/g, g is the acceleration
of gravity, and am is the acceleration
amplitude of the free-field ground motion.
The inclined scales on the left represent the
deformation amplitude of the superstructure,
u, normalized with respect to the
displacement amplitude of the free-field
2
a m T0
ground motion d m '
.
4B2
The damping of the structure in its fixed-base condition, $, is considered to be 2 percent of the critical value, and the additional
parameters needed to characterize completely these solutions are identified in Veletsos and Meek (1974), from which these
figures have been reproduced.
Comparison of the results presented in these
figures reveals that the effects of soil-strucFIGURE C5.5.1-2 Response spectra for systems with h/r = 1
ture interaction are most strikingly reflected
(Veletsos and Meek, 1974).
in a shift of the peak of the response
spectrum to the right and a change in the
magnitude of the peak. These changes, which are particularly prominent for taller structures and more
flexible soils (increasing values of No), can conveniently be expressed by an increase in the natural
period of the system over its fixed-base value and by a change in its damping factor.
Also shown in these figures in dotted lines are response spectra for single-degree-of- freedom (SDF)
oscillators, the natural period and damping of which have been adjusted so that the absolute maximum
(resonant) value of the base shear and the associated period are in each case identical to those of the
actual interacting systems. The base motion for the replacement oscillator is considered to be the same
as the free-field ground motion. With the properties of the replacement SDF oscillator determined in
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1997 Commentary, Chapter 5
this manner, it is important to note that the response spectra for the actual and the replacement systems
are in excellent agreement over wide ranges of the exciting period on both sides of the resonant peak.
In the context of Fourier analysis, an earthquake motion may be viewed as the result of superposition
of harmonic motions of different periods and amplitudes. Inasmuch as the components of the
excitation with periods close to the resonant period are likely to be the dominant contributors to the
response, the maximum responses of the
actual system and of the replacement
oscillator can be expected to be in satisfactory agreement for earthquake ground
motions as well. This expectation has been
confirmed by the results of comprehensive
comparative studies (Veletsos, 1977; Veletsos and Meek, 1974; Veletsos and Nair,
1975).
It follows that, to the degree of
approximation involved in the representation
of the actual system by the replacement SDF
oscillator, the effects of interaction on
maximum response may be expressed by an
increase in the fundamental natural period of
the fixed-base system and by a change in its
damping value. In the following sections, the
natural period of replacement oscillator is denoted by T̃ and the associated damping
factor, by $̃. These quantities will also be referred to as the effective natural period and
the effective damping factor of the interacting
system. The relationships between T̃ and T
and between $̃ and $ are considered in Sec.
5.5.2.1.1 and 5.5.2.1.2.
Basis of Provisions and Assumptions:
Current knowledge of the effects of
soil-structure interactions is derived mainly
from studies of systems of the type referred
to above in which the foundation is idealized as a rigid mat. For foundations of this type, both surface-supported and embedded structures resting on uniform as well as layered soil deposits have been
investigated (Bielak, 1975; Chopra and Gutierrez, 1974; Jennings and Bielak, 1973; Liu and Fagel,
1971; Parmelee et al., 1969; Roesset et al., 1973; Veletsos, 1977; Veletsos and Meek, 1974; Veletsos
and Nair, 1975). However, only a small amount of information is available concerning the interaction
effects for structures supported on spread footings or pile foundations (Blaney et al., n.d.; Novak,
1974; Rainer, 1975b). The requirements presented in Sec. 5.5 for the latter cases represent the best
interpretation and judgment of the developers of the requirements regarding the current state of
knowledge.
FIGURE C5.5.1-3 Response spectra for systems with h/r
= 5 (Veletsos and Meek, 1974).
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Structural Design Criteria
Fundamental to these requirements is the assumption that the structure and the underlying soil are
bonded and remain so throughout the period of ground-shaking. It is further assumed that there is no
soil instability or large foundation settlements. The design of the foundation in a manner to ensure
satisfactory soil performance (e.g., to avoid soil instability and settlement associated with the compaction and liquefaction of loose granular soils), is beyond the scope of Sec. 5.5. Finally, no account is
taken of the interaction effects among neighboring structures.
Nature of Interaction Effects: Depending on the characteristics of the structure and the ground
motion under consideration, soil-structure interaction may increase, decrease, or have no effect on the
magnitudes of the maximum forces induced in the structure itself (Bielak, 1975; Jennings and Bielak,
1973; Veletsos, 1977; Veletsos and Meek, 1974; Veletsos and Nair, 1975). However, for the
conditions stipulated in the development of the requirements for rigidly supported structures presented
in Sec. 5.3 and 5.4, soil-structure interaction will reduce the design values of the base shear and
moment from the levels applicable to a rigid-base condition. These forces therefore can be evaluated
conservatively without the adjustments recommended in Sec. 5.5.
Because of the influence of foundation rocking, however, the horizontal displacements relative to the
base of the elastically supported structure may be larger than those of the corresponding fixed-base
structure, and this may increase both the required spacing between structures and the secondary design
forces associated with the P-delta effects. Such increases generally are small.
Scope: Two procedures are used to incorporate effects of the soil-structure interaction. The first is an
extension of the equivalent lateral force procedure presented in Sec. 5.3 and involves the use of
equivalent lateral static forces. The second is an extension of the simplified modal analysis procedure
presented in Sec. 5.4. In the latter approach, the earthquake-induced effects are expressed as a linear
combination of terms, the number of which is equal to the number of stories involved. Other more
complex procedures also may be used, and these are outlined briefly at the end of this commentary on
Sec. 5.5. However, it is believed that the more involved procedures are justified only for unusual
structures of extreme importance and only when the results of the specified simpler approaches have
revealed that the interaction effects are indeed of definite consequence in the design.
5.5.2 Equivalent Lateral Force Procedure: This procedure is similar to that used in the older
SEAOC recommendations except that it incorporates several improvements (see Sec. 5.3 of this
commentary). In effect, the procedure considers the response of the structure in its fundamental mode
of vibration and accounts for the contributions of the higher modes implicitly through the choice of the
effective weight of the structure and the vertical distribution of the lateral forces. The effects of
soil-structure interaction are accounted for on the assumption that they influence only the contribution
of the fundamental mode of vibration. For structures, this assumption has been found to be adequate
(Bielak, 1976; Jennings and Bielak, 1973; Veletsos, 1977).
5.5.2.1 Base Shear: With the effects of soil-structure interaction neglected, the base shear is defined
by Eq. 5.3.2, V = CsW, in which W is the total dead weight of the structure and of applicable portions
of the design live load (as specified in Sec. 5.3.2) and Cs is the dimensionless seismic response
coefficient (as defined by Eq. 5.3.2.1-1). This term depends on the seismic zone under consideration,
the properties of the site, and the characteristics of the structure itself. The latter characteristics
include the fixed-base fundamental natural period of the structure, T; the associated damping factor, $;
and the degree of permissible inelastic deformation. The damping factor does not appear explicitly in
Eq. 5.3.2.1-1 because a constant value of $ = 0.05 has been used for all structures for which the
110
1997 Commentary, Chapter 5
interaction effects are negligible. The degree of permissible inelastic action is reflected in the choice of
the reduction factor, R. It is convenient to rewrite Eq. 5.3.2.1 in the form:
V ' C s ( T,$ ) W % Cs (T,$ ) [W & W ]
(C5.5.2.1-1)
where W̄ represents the generalized or effective weight of the structure when vibrating in its
fundamental natural mode. The terms in parentheses are used to emphasize the fact that Cs depends
upon both T and $. The relationship between W̄ and W is given below. The first term on the right side
of Eq. C5.5.2.1-1 approximates the contribution of the fundamental mode of vibration whereas the
second term approximates the contributions of the higher natural modes. Inasmuch as soil-structure
interaction may be considered to affect only the contribution of the fundamental mode and inasmuch as
this effect can be expressed by changes in the fundamental natural period and the associated damping
of the system, the base shear for the interacting system, V̄, may be stated in a form analogous to Eq.
C5.5.2.1-1:
V ' Cs ( T,$ )W % C s ( T,$ ) [W & W ]
(C5.5.2.1-2)
The value of Cs in the first part of this equation should be evaluated for the natural period and damping
of the elastically supported system, T̃ and $̃, respectively, and the value of Cs in the second term part
should be evaluated for the corresponding quantities of the rigidly supported system, T and $.
Before proceeding with the evaluation of the coefficients Cs in Eq. C5.5.2.1-2, it is desirable to rewrite
this formula in the same form as Eq. 5.5.2.1-1. Making use of Eq. 5.3.2.1 and rearranging terms, the
following expression for the reduction in the base shear is obtained:
) V ' Cs ( T,$ ) & Cs ( T̃,$̃ ) W
(C5.5.2.1-3)
Within the ranges of natural period and damping that are of interest in studies of structural response,
the values of Cs corresponding to two different damping values but the same natural period (e.g., T̃),
are related approximately as follows:
˜ $̃ ) ' C ( T̃,$ ) $
Cs ( T,
s
$̃
0.4
(C5.5.2.1-4)
This expression, which appears to have been first proposed in Arias and Husid (1962), is in good
agreement with the results of recent studies of earthquake response spectra for systems having
different damping values (Newmark et al., 1973).
Substitution of Eq. C5.5.2.1-4 in Eq. C5.5.2.1-3 leads to:
) V ' Cs ( T,$ ) & C s ( T̃,$ )
$
$̃
0.4
W
111
(C5.5.2.1-5)
Structural Design Criteria
where both values of Cs are now for the damping factor of the rigidly supported system and may be
evaluated from Eq. 5.3.2. If the terms corresponding to the periods T and T̃ are denoted more simply
as Cs and C̄s, respectively, and if the damping factor $ is taken as 0.05, Eq. C5.5.2.1-5 reduces to Eq.
5.5.2.1-2.
Note that C̄s in Eq. 5.5.2.1-2 is smaller than or equal to Cs because Eq. 5.3.2 is a nonincreasing
function of the natural period and T̃ is greater than or equal to T. Furthermore, since the minimum
value of $̃ is taken as $̃ = $ = 0.05 (see statement following Eq. 5.5.2.1.2-1), the shear reduction )V is
a non-negative quantity. It follows that the design value of the base shear for the elastically supported
structure cannot be greater than that for the associated rigid -base structure.
The effective weight of the structure, W̄, is defined by Eq. 5.4.5-2 (Sec. 5.4), in which Nim should be
interpreted as the displacement amplitude of the ith floor when the structure is vibrating in its fixed-base
fundamental natural mode. It should be clear that the ratio W̄/W depends on the detailed
characteristics of the structure. A constant value of W̄ = 0.7 W is recommended in the interest of
simplicity and because it is a good approximation for typical structures. As an example, it is noted that
for a tall structure for which the weight is uniformly distributed along the height and for which the
fundamental natural mode increases linearly from the base to the top, the exact value of W̄ = 0.75 W.
Naturally, when the full weight of the structure is concentrated at a single level, W̄ should be taken
equal to W.
The maximum permissible reduction in base shear due to the effects of soil-structure interaction is set
at 30 percent of the value calculated for a rigid-base condition. It is expected, however, that this limit
will control only infrequently and that the calculated reduction, in most cases, will be less.
5.5.2.1.1 Effective Building Period: Equation 5.5.2.1.1-1 for the effective natural period of the
elastically supported structure, T̃, is determined from analyses in which the superstructure is presumed
to respond in its fixed-base fundamental mode and the foundation weight is considered to be negligible
in comparison to the weight of the superstructure (Jennings and Bielak, 1973; Veletsos and Meek,
1974). The first term under the radical represents the period of the fixed-base structure. The first
portion of the second term represents the contribution to T̃ of the translational flexibility of the
foundation, and the last portion represents the contribution of the corresponding rocking flexibility.
The quantities k̄ and h̄ represent, respectively, the effective stiffness and effective height of the
structure, and Ky and K2 represent the translational and rocking stiffnesses of the foundation.
Equation 5.5.2.1.1-2 for the structural stiffness, k̄, is deduced from the well known expression for the
natural period of the fixed-base system:
T ' 2B
1
g
W
(C5.5.2.1.1-1)
k
The effective height, h̄, is defined by Eq. 5.5.3.1-2, in which Nil has the same meaning as the quantity
Nim in Eq. 5.4.5-2 (Sec. 5.4) when m = 1. In the interest of simplicity and consistency with the
approximation used in the definition of W̄, however, a constant value of h̄ = 0.7hn is recommended
where hn is the total height of the structure. This value represents a good approximation for typical
structures. As an example, it is noted that for tall structures for which the fundamental natural mode
112
1997 Commentary, Chapter 5
increases linearly with height, the exact value of h̄ is 2/3hn. Naturally, when the gravity load of the
structure is effectively concentrated at a single level, hn must be taken as equal to the distance from the
base to the level of weight concentration.
Foundation stiffnesses depend on the geometry of the foundation-soil contact area, the properties of
the soil beneath the foundation, and the characteristics of the foundation motion. Most of the available
information on this subject is derived from analytical studies of the response of harmonically excited
rigid circular foundations, and it is desirable to begin with a brief review of these results.
For circular mat foundations supported at the surface of a homogeneous halfspace, stiffnesses Ky and
K2 are given by:
8"y
Ky '
(C5.5.2.1.1-2)
(2 & v) Gr
and
K2 '
8"2
3(1 & v)
Gr 3
(C5.5.2.1.1-3)
where r is the radius of the foundation; G is the shear modulus of the halfspace; < is its Poisson's ratio;
and ay and a2 are dimensionless coefficients that depend on the period of the excitation, the dimensions
of the foundation, and the properties of the supporting medium (Luco, 1974; Veletsos and Verbic,
1974; Veletsos and Wei, 1971). The shear modulus is related to the shear wave velocity, vs, by the
formula:
2
G '
(vs
(C5.5.2.1.1-4)
g
in which ( is the unit weight of the material. The values of G, vs, and < should be interpreted as
average values for the region of the soil that is affected by the forces acting on the foundation and
should correspond to the conditions developed during the design earthquake. The evaluation of these
quantities is considered further in subsequent sections. For statically loaded foundations, the stiffness
coefficients ay and a2 are unity, and Eq. C5.5.2.1.1-2 and 5.5.2.1.1-3 reduce to:
Ky '
8Gr
2 & v
(C5.5.2.1.1-5)
8Gr 3
3(1 & v)
(C5.5.2.1.1-6)
and
K2 '
113
Structural Design Criteria
Studies of the interaction effects in structure-soil systems have shown that, within the ranges of
parameters of interest for structures subjected to earthquakes, the results are insensitive to the
period-dependency of "y and "2 and that it is sufficiently accurate for practical purposes to use the
static stiffnesses, defined by Eq. C5.5.2.1.1-5 and C5.5.2.1.1-6.
Foundation embedment has the effect of increasing the stiffnesses Ky and K2. For embedded
foundations for which there is positive contact between the side walls and the surrounding soil, Ky and
K2 may be determined from the following approximate formulas:
Ky •
8Gr
2 & <
2
3
1 %
d
r
(C5.5.2.1.1-7)
and
8Gr 3
3 & <
K2 •
d
r
1 % 2
(C5.5.2.1.1-8)
in which d is the depth of embedment. These formulas are based on finite element solutions (Blaney et
al., n.d.).
Both analyses and available test data (Erden, 1974) indicate that the effects of foundation embedment
are sensitive to the condition of the backfill and that judgment must be exercised in using Eq.
C5.5.2.1.1-7 and C5.5.2.1.1-8. For example, if a structure is embedded in such a way that there is no
positive contact between the soil and the walls of the structure, or when any existing contact cannot
reasonably be expected to remain effective during the stipulated design ground motion, stiffnesses Ky
and K2 should be determined from the formulas for surface-supported foundations. More generally,
the quantity d in Eq. C5.5.2.1.1-7 and C5.5.2.1.1-8 should be interpreted as the effective depth of
foundation embedment for the conditions that would prevail during the design earthquake.
The formulas for Ky and K2 presented above are strictly valid only for foundations supported on
reasonably uniform soil deposits. When the foundation rests on a stratum of soft soil underlain by a
much stiffer, rock-like deposit with an abrupt increase in stiffness, Ky and K2 may be determined from
the two generalized formulas in which G is the shear modulus of the soft soil and Ds is the total depth
of the stratum. First, using Eq. C5.5.2.1.1-7:
Ky •
8Gr
1 %
2 & <
2
3
d
r
1 %
1
2
r
Ds
1 %
5
4
1 %
1
6
r
Ds
1 % 0.7
d
Ds
(C5.5.2.1.1-9)
d
Ds
(C5.5.2.1.1-10)
Second, using Eq. C5.5.2.1.1-8:
K2 •
8Gr 3
d
1 % 2
3(1 & <)
r
These formulas are based on analyses of a stratum supported on a rigid base (Elsabee et al.,1977;
Kausel and Roesset, 1975).
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1997 Commentary, Chapter 5
The information for circular foundations presented above may be applied to mat foundations of
arbitrary shapes provided the following changes are made:
1. The radius r in the expressions for Ky in Eq. 5.5.2.1.1-5 is replaced by the quantity:
Ao
ra '
(C5.5.2.1.1-11)
B
which represents the radius of a disk that has the area, Ao, of the actual foundation.
2. The radius r in the expressions for K2 in Eq. 5.5.2.1.1-6 is replaced by the quantity:
4
Io
rm '
(C5.5.2.1.1-12)
B
which represents the radius of a disk that has the moment of inertia, Io, of the actual foundation.
For footing foundations, stiffnesses Ky and K2 are computed by summing the contributions of the
individual footings. If it is assumed that the foundation behaves as a rigid body and that the individual
footings are widely spaced so that they act as independent units, the following formulas are obtained:
Ky ' 'kyi
(C5.5.2.1.1-13)
K2 ' ' kxi y i % ' k2i
(C5.5.2.1.1-14)
and
2
The quantity kyi represents the horizontal stiffness of the ith footing; kxi and k2i represent, respectively,
the corresponding vertical and rocking stiffnesses; and yi represents the normal distance from the
centroid of the ith footing to the rocking axis of the foundation. The summations are considered to
extend over all footings. The contribution to K2 of the rocking stiffnesses of the individual footings, k2i,
generally is small and may be neglected.
The stiffnesses kyi, kxi, and k2i are defined by the formulas:
kyi '
kxi '
8Gi rai
1 % 2/3di
2 & <
rai
4Girai
1 % 0.4d i
1 & <
ra
(C5.5.2.1.1-15)
(C5.5.2.1.1-16)
115
Structural Design Criteria
and
3
k2i '
8G irmi
1 % 2di
2(1 & <)
rmi
(C5.5.2.1.1-17)
in which di is the depth of effective embedment for the ith footing; Gi is the shear modulus of the soil
beneath the ith footing; rai = %&
A&&&
B is the radius of a circular footing that has the area of the ith footo i /&
4
ing, Aoi; and rmi equals %&
A&&&
B = the radius of a circular footing, the moment of inertia of which
o i /&
about a horizontal centroidal axis is equal to that of the ith footing, Ioi, in the direction in which the
response is being evaluated.
For surface-supported footings and for embedded footings for which the side wall contact with the soil
cannot be considered to be effective during the stipulated design ground motion, di in these formulas
should be taken as zero. Furthermore, the values of Gi should be consistent with the stress levels
expected under the footings and should be evaluated with due regard for the effects of the dead loads
involved. This matter is considered further in subsequent sections. For closely spaced footings,
consideration of the coupling effects among footings will reduce the computed value of the overall
foundation stiffness. This reduction will, in turn, increase the fundamental natural period of the system,
T̃, and decrease the value of )V, the amount by which the base shear is reduced due to soil-structure
interaction. It follows that the use of Eq. C5.5.2.1.1-13 and 5.5.2.1.1-14 will err on the conservative
side in this case. The degree of conservatism involved, however, will partly be compensated by the
presence of a basement slab that, even when it is not tied to the structural frame, will increase the
overall stiffness of the foundation.
The values of Ky and K2 for pile foundations can be computed in a manner analogous to that described
in the preceding section by evaluating the horizontal, vertical, and rocking stiffnesses of the individual
piles, kyi, kxi and k2i, and by combining these stiffnesses in accordance with Eq. C5.5.2.1.1-13 and
5.5.2.1.1-14.
The individual pile stiffnesses may be determined from field tests or analytically by treating each pile as
a beam on an elastic subgrade. Numerous formulas are available in the literature (Nair et al., 1969)
that express these stiffnesses in terms of the modulus of the subgrade reaction and the properties of the
pile itself. Although they differ in appearance, these formulas lead to practically similar results. These
stiffnesses typically are expressed in terms of the stiffness of an equivalent freestanding cantilever, the
physical properties and cross-sectional dimensions of which are the same as those of the actual pile but
the length of which is adjusted appropriately. The effective lengths of the equivalent cantilevers for
horizontal motion and for rocking or bending motion are slightly different but are often assumed to be
equal. On the other hand, the effective length in vertical motion is generally considerably greater. For
further details, the reader is referred to Nair et al. (1969).
The soil properties of interest are the shear modulus, G, or the associated shear wave velocity, vs; the
unit weight, (; and Poisson's ratio, <. These quantities are likely to vary from point to point of a construction site, and it is necessary to use average values for the soil region that is affected by the forces
acting on the foundation. The depth of significant influence is a function of the dimensions of the
foundation base and of the direction of the motion involved. The effective depth may be considered to
116
1997 Commentary, Chapter 5
extend to about 4ra below the foundation base for horizontal and vertical motions and to about 1.5rm
for rocking motion. For mat foundations, the effective depth is related to the total plan dimensions of
the mat whereas for structures supported on widely spaced spread footings, it is related to the dimensions of the individual footings. For closely spaced footings, the effective depth may be determined by
superposition of the "pressure bulbs" induced by the forces acting on the individual footings.
Since the stress-strain relations for soils are nonlinear, the values of G and vs also are functions of the
strain levels involved. In the formulas presented above, G should be interpreted as the secant shear
modulus corresponding to the significant strain level in the affected region of the foundation soil. The
approximate relationship of this modulus to the modulus Go corresponding to small amplitude strains
(of the order of 10-3 percent or less) is given in Table 5.5.2.1.1. The backgrounds of this relationship
and of the corresponding relationship for vs/vso are identified below.
The low amplitude value of the shear modulus, Go, can most conveniently be determined from the
associated value of the shear wave velocity, vso, by use of Eq. C5.5.2.1.1-4. The latter value may be
determined approximately from empirical relations or more accurately by means of field tests or
laboratory tests.
The quantities Go and vso depend on a large number of factors (Hardin and Black, 1968; Hardin and
Drnevich, 1975; Richart et al., n.d.), the most important of which are the void ratio, e, and the average
confining pressure, F̄o. The value of the latter pressure at a given depth beneath a particular
foundation may be expressed as the sum of two terms as follows:
Fo ' Fos % Fob
(C5.5.2.1.1-18)
in which F̄os represents the contribution of the weight of the soil and F̄ob represents the contribution of
the superimposed weight of the structure and foundation. The first term is defined by the formula:
Fos '
1 % 2K o
3
( )x
(C5.5.2.1.1-19)
in which x is the depth of the soil below the ground surface, (N is the average effective unit weight of
the soil to the depth under consideration, and Ko is the coefficient of horizontal earth pressure at rest.
For sands and gravel, Ko has a value of 0.5 to 0.6 whereas for soft clays, Ko • 1.0. The pressures F̄ob
developed by the weight of the structure can be estimated from the theory of elasticity (Poulos and
Davis, 1974). In contrast to F̄os which increases linearly with depth, the pressures F̄ob decrease with
depth. As already noted, the value of vso should correspond to the average value of F̄o in the region of
the soil that is affected by the forces acting on the foundation.
For clean sands and gravels having e < 0.80, the low-amplitude shear wave velocity can be calculated
approximately from the formula:
vso ' c1 ( 2.17 & e) ( F )0.25
(C5.5.2.1.1-20)
in which c1 equals 78.2 when F̄ is in lb/ft2 and vso is in ft/sec; c1 equals 160.4 when F̄ is in kg/cm2 and vso
is in m/sec; and c1 equals 51.0 when F̄ is in kN/m2 and vso is in m/sec.
117
Structural Design Criteria
For angular-grained cohesionless soils (e > 0.6), the following empirical equation may be used:
vso ' c2 ( 2.97 & e ) (F )0.25
(C5.5.2.1.1-21)
in which c2 equals 53.2 when F̄ is in lb/ft2 and vso is in ft/sec; c2 equals 109.7 when F̄ is in kg/cm2 and vso
is in m/sec; and c2 equals 34.9 when F̄ is in kN/m2 and vso is in m/sec.
Equation C5.5.2.1.1-21 also may be used to obtain a first-order estimate of vso for normally
consolidated cohesive soils. A crude estimate of the shear modulus, Go, for such soils may also be
obtained from the relationship:
Go ' 1,000Su
(C5.5.2.1.1-22)
in which Su is the shearing strength of the soil as developed in an unconfined compression test. The
coefficient 1,000 represents a typical value, which varied from 250 to about 2,500 for tests on different
soils (Hara et al., 1974; Hardin and Drnevich, 1975).
These empirical relations may be used to obtain preliminary, order-of-magnitude estimates. For more
accurate evaluations, field and/or laboratory determinations may be required. Field evaluations of the
variations of vso throughout the construction site can be carried out by standard seismic refraction
methods or by the cross-hole method. The cross-hole method (Ballard and McLean, 1975; Stokoe and
Woods, 1972) provides information from undisturbed soils below the proposed location of a particular
foundation. The method permits evaluation of vso in layered soils and is not affected by the presence of
water in the soil. The low-amplitude procedure is relatively inexpensive and easy to use. The
disadvantage of this method is that vso is determined only for the stress conditions existing at the time of
the test (usually F̄so). The effect of the changes in the stress conditions caused by construction must be
considered by use of Eq. C5.5.2.1.1-19 and Eq. C5.5.2.1.1-20 to C5.5.2.1.1.21 to adjust the field
measurement of vso to correspond to the prototype situations. The influence of large-amplitude
shearing strains may be evaluated from laboratory tests or approximated through the use of Table
5.5.2.1.1. This matter is considered further in the next two sections.
Laboratory tests to evaluate vso are usually carried out with resonant column devices (Richart et al.,
n.d.). Such tests may be used to assess the effects of changes in confining pressures, shearing strain
amplitudes, stress histories, temperature, and other variables. Consequently, they can easily simulate
variations in prototype loading conditions. They are particularly useful in establishing the effects of
changes in confining pressures. In fact, Eq. C5.5.5.1.1-20 and C5.5.5.1.1-21 were developed from the
results of such tests.
An increase in the shearing strain amplitude is associated with a reduction in the secant shear modulus,
G, and the corresponding value of vs. Extensive laboratory tests (see, for example, Anderson and
Richart, 1976; Hardin and Drnevich, 1972; Kuribayashi et al., 1974) have established the magnitudes of
the reductions in vs for both sands and clays as the shearing strain amplitude increases.
The results of such tests form the basis for the information presented in Table 5.5.5.1.1. For each
severity of anticipated ground-shaking, represented by the effective peak acceleration coefficients Aa
and Av, a representative value of shearing strain amplitude was developed. A conservative value of
vs/vso that is appropriate to that strain amplitude then was established. It should be emphasized that the
118
1997 Commentary, Chapter 5
values in Table 5.5.5.1.1 are first order approximations. More precise evaluations would require
laboratory tests on undisturbed samples from the site and studies of wave propagation for the site to
determine the magnitude of the soil strains induced.
It is satisfactory to assume Poisson's ratio for soils as: < = 0.33 for clean sands and gravels, < = 0.40
for stiff clays and cohesive soils, and < = 0.45 for soft clays. The use of an average value of < = 0.4
also will be adequate for practical purposes.
Regarding an alternative approach, note that Eq. 5.5.5.1.1-3 for the period T̃ of structures supported
on mat foundations was deduced from Eq. 5.5.5.1.1-1 by making use of Eq. C5.5.5.1.1-5 and
C5.5.5.1.1-6, with Poisson's ratio taken as < = 0.4 and with the radius r interpreted as ra in Eq.
C5.5.5.1.1-5 and as rm in Eq. C5.5.5.1.1-6. For a nearly square foundation, for which ra • rm • r, Eq.
5.5.5.1.1-3 reduces to:
T̃ ' T
1 % 25"
rh
1 %
2
vs T 2
1.12h
r
2
(C5.5.2.1.1-23)
The value of the relative weight parameter, ", is likely to be in the neighborhood of 0.15 for typical
structures.
5.5.5.1.2 Effective Damping: Equation 5.5.5.1.2-1 for the overall damping factor of the elastically
supported structure, $̃, was determined from analyses of the harmonic response at resonance of simple
systems of the type considered in Figures C5.5.1-2 and 5.5.1-3. The result is an expression of the form
(Bielak, 1975; Veletsos and Nair, 1975):
$̃ ' $o %
8
T̃
T
3
(C5.5.2.1.2-1)
in which $o represents the contribution of the foundation damping, considered in greater detail in the
following paragraphs, and the second term represents the contribution of the structural damping. The
latter damping is assumed to be of the viscous type. Equation C5.5.5.1.2-1 corresponds to the value of
$ = 0.05 used in the development of the response spectra for rigidly supported systems employed in
Sec. 5.3.
The foundation damping factor, $o, incorporates the effects of energy dissipation in the soil due to the
following sources: the radiation of waves away from the foundation, known as radiation or geometric
damping, and the hysteretic or inelastic action in the soil, also known as soil material damping. This
factor depends on the geometry of the foundation-soil contact area and on the properties of the
structure and the underlying soil deposits.
For mat foundations of circular plan that are supported at the surface of reasonably uniform soils
deposits, the three most important parameters which affect the value of $o are: the ratio T̃/T of the
fundamental natural periods of the elastically supported and the fixed-base structures, the ratio h̄/r of
the effective height of the structure to the radius of the foundation, and the damping capacity of the
119
Structural Design Criteria
soil. The latter capacity is measured by the dimensionless ratio )Ws/Ws, in which )Ws is the area of the
hysteresis loop in the stress-strain diagram for a soil specimen undergoing harmonic shearing
deformation and Ws is the strain energy stored in a linearly elastic material subjected to the same
maximum stress and strain (i.e., the area of the triangle in the stress-strain diagram between the origin
and the point of the maximum induced stress and strain). This ratio is a function of the magnitude of
the imposed peak strain, increasing with increasing intensity of excitation or level of strain.
The variation of $o with T̃/T and h̄/r is given in Figure 5.5.5.1.2 for two levels of excitation. The
dashed lines, which are recommended for values of the effective ground acceleration coefficient, Av,
equal to or less than 0.10, correspond to a value of )Ws/Ws • 0.3, whereas the solid lines, which are
recommended for Av values equal to or greater than 0.20, correspond to a value of )Ws/Ws • 1. (Note
that for the purpose of these evaluations Aa may be taken as 0.4 SDS and Av may be taken as SD1.)
These curves are based on the results of extensive parametric studies (Veletsos, 1977; Veletsos and
Meek, 1974; Veletsos and Nair, 1975) and represent average values. For the ranges of parameters that
are of interest in practice, however, the dispersion of the results is small.
For mat foundations of arbitrary shape, the quantity r in Figure 5.5.5.1.2 should be interpreted as a
characteristic length that is related to the length of the foundation, Lo, in the direction in which the
structure is being analyzed. For short, squatty structures for which h̄/Lo F 0.5, the overall damping of
the structure-foundation system is dominated by the translational action of the foundation, and it is reasonable to interpret r as ra, the radius of a disk that has the same area as that of the actual foundation
(see Eq. 5.5.5.1.1-5). On the other hand, for structures with h̄/Lo $ 1, the interaction effects are dominated by the rocking motion of the foundation, and it is reasonable to define r as the radius rm of a disk
whose static moment of inertia about a horizontal centroidal axis is the same as that of the actual
foundation normal to the direction in which the structure is being analyzed (see Eq. 5.5.5.1.1-6).
Subject to the qualifications noted in the following section, the curves in Figure 5.5.5.1.2 also may be
used for embedded mat foundations and for foundations involving spread footings or piles. In the
latter cases, the quantities Ao and Io in the expressions for the characteristic foundation length, r, should
be interpreted as the area and the moment of inertia of the load-carrying foundation.
In the evaluation of the overall damping of the structure-foundation system, no distinction has been
made between surface-supported foundations and embedded foundations. Since the effect of
embedment is to increase the damping capacity of the foundation (Bielak, 1975; Novak, 1974; Novak
and Beredugo, 1972) and since such an increase is associated with a reduction in the magnitude of the
forces induced in the structure, the use of the recommended requirements for embedded structures will
err on the conservative side.
There is one additional source of conservatism in the application of the recommended requirements to
structures with embedded foundations. It results from the assumption that the free-field ground
motion at the foundation level is independent of the depth of foundation embedment. Actually, there is
evidence to the effect that the severity of the free-field excitation decreases with depth (Seed et al.,
1977). This reduction is ignored both in Sec. 5.5 and in the requirements for rigidly supported
structures presented in Sec. 5.3 and 5.4.
Equations 5.5.5.1.2-1 and C5.5.5.1.2-1, in combination with the information presented in Figure
5.5.5.1.2, may lead to damping factors for the structure-soil system, $̃, that are smaller than the
structural damping factor, $. However, since the representative value of $ = 0.05 used in the develop-
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1997 Commentary, Chapter 5
ment of the design requirements for rigidly supported structures is based on the results of tests on
actual structures, it reflects the damping of the full structure-soil system, not merely of the component
contributed by the superstructure. Thus, the value of $̃ determined from Eq. 5.5.5.1.2-1 should never
be taken less than $, and a low bound of $̃ = $ = 0.05 has been imposed. The use of values of $̃ > $ is
justified by the fact that the experimental values correspond to extremely small amplitude motions and
do not reflect the effects of the higher soil damping capacities corresponding to the large soil strain
levels associated with the design ground motions. The effects of the higher soil damping capacities are
appropriately reflected in the values of $o presented in Figure 5.5.5.1.5.
There are, however, some exceptions. For foundations involving a soft soil stratum of reasonably
uniform properties underlain by a much stiffer, rock-like material with an abrupt increase in stiffness,
the radiation damping effects are practically negligible when the natural period of vibration of the
stratum in shear,
Ts '
4Ds
(C5.5.2.1.2-2)
vs
is smaller than the natural period of the flexibly supported structure, T̃. The quantity Ds in this formula
represents the depth of the stratum. It follows that the values of $o presented in Figure 5.5.5.1.2 are
applicable only when:
Ts
'
4Ds
$ 1
(C5.5.2.1.2-3)
< 1
(C5.5.2.1.2-4)
vsT̃
T̃
For
Ts
T̃
'
4Ds
vsT̃
the effective value of the foundation damping factor, $N,o is less than $o, and it is approximated by the
second degree parabola defined by Eq. 5.5.5.1.2-4.
For Ts/T̃ = 1, Eq. 5.5.5.1.2-4 leads to $No = $o whereas for Ts/T̃ = 0, it leads to $No = 0, a value that
clearly does not provide for the effects of material soil damping. It may be expected, therefore, that the
computed values of $No corresponding to small values of Ts/T̃ will be conservative. The conservatism
involved, however, is partly compensated by the requirement that $̃ be no less than $̃ = $ = 0.05.
5.5.5.2 Vertical Distribution of Seismic Forces and 5.5.5.3 Other Effects: The vertical
distributions of the equivalent lateral forces for flexibly and rigidly supported structures are generally
different. However, the differences are inconsequential for practical purposes, and it is recommended
that the same distribution be used in both cases, changing only the magnitude of the forces to
correspond to the appropriate base shear. A greater degree of refinement in this step would be
inconsistent with the approximations embodied in the requirements for rigidly supported structures.
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Structural Design Criteria
With the vertical distribution of the lateral forces established, the overturning moments and the
torsional effects about a vertical axis are computed as for rigidly supported structures. The above
procedure is applicable to planar structures and, with some extension, to three-dimensional structures.
Methods exist for incorporating two- and three-dimensional P-delta effects into computer analyses that
do not explicable include such effects (Rutenburg, 1985). Many programs explicitly include P-delta
effects. A mathematical description of the method employed by several popular programs is given by
Wilson and Habibullah (1987).
The P-delta procedure cited above effectively checks the static stability of a structure based on its
initial stiffness. Since the inception of this procedure in the ATC 3-06 document, however, there has
been some debate regarding its accuracy. This debate reflects the intuitive notion that a structure's
secant stiffness would more accurately represent inelastic P-delta effects. Due to the additional
uncertainty of the effect of dynamic response on P-delta behavior and on the (apparent) observation
that instability-related failures rarely occur in real structures, the P-delta requirements as originally
written have remained unchanged until now.
There is increasing evidence, however, that the use of inelastic stiffness in determining theoretical Pdelta response is unconservative. Based on a study carried out by Bernal (1987), it can be argued that
P-delta amplifiers should be based on secant stiffness. In other words, the Cd term in Eq. 5.3.7.2-1 of
the Provisions should be deleted. Since Bernal's study was based on the inelastic dynamic response of
single-degree-of-freedom elastic-perfectly plastic systems, significant uncertainties exist in the
extrapolation of the concepts to the complex hysteretic behavior of multi-degree-of-freedom systems.
Another problem with accepting a P-delta procedure based on secant stiffness is that current design
forces would be greatly increased. For example, consider an ordinary moment frame of steel with a Cd
of 4.0 and an elastic stability coefficient, 2, of 0.15. The amplifier for this structure would be 1.0/0.85
= 1.18 according to the current requirements. If the P-delta effects were based on secant stiffness,
however, the stability coefficient would increase to 0.60 and the amplifier would become 1.0/0.4 =
5.50. (Note that the 0.9 in the numerator of the amplifier equation in the 1988 Edition of the
Provisions has been dropped for this comparison.) From this example, it can be seen that there could
be an extreme impact on the requirements if a change was implemented that incorporated P-delta
amplifiers based on static secant stiffness response.
Nevertheless, there must be some justification for retaining the P-delta amplifier as based on elastic
stiffness. This justification is the apparent lack of stability-related failures. The reasons for the lack of
observed failures are, at a minimum, twofold:
1. Many structures display an overstrength well above the strength implied by code-level design
forces (see Figure 5.5.1). This overstrength likely protects structures from stability-related
failures.
5. The likelihood of a stability failure decreases with the increased intensity of expected groundshaking. This is due to the fact that the stiffness of most structures designed for extreme ground
motion is significantly greater than the stiffness of the same structure deigned for lower intensity
shaking or for wind. Since damaging low-intensity earthquakes are somewhat rare, there would be
little observable damage.
Due to the lack of stability-related failures, therefore, the 1991 Edition of the Provisions regarding Pdelta amplifiers has remained unchanged from the 1988 Edition with the exception that the 0.90 factor
122
1997 Commentary, Chapter 5
in the numerator of the amplifier has been deleted. This factor originally was used to create a
transition from cases where P-delta effects need not be considered (2 > 1.0, amplifier > 1.0).
Aside from the amplifier, however, the 1991 Edition of the Provisions added a new requirement that
the computed stability coefficient, 2, not exceed 0.25 or 0.5/$Cd where $Cd is an adjusted ductility
demand that takes into account the fact that the seismic strength demand may be somewhat less than
the code strength supplied. The adjusted ductility demand is not intended to incorporate overstrength
beyond that computed by the means available in Chapters 8 though 14 of the Provisions.
The purpose of this new provision is to protect structures from the possibility of stability-related
failures triggered by post-earthquake residual deformation. The danger of such failures is real and may
not be eliminated by apparently available overstrength. This is particularly true of structures designed
in for regions of lower seismicity.
The computation of 2max, which in turn is based on $Cd, requires the computation of story strength
supply and story strength demand. Story strength demand is simply the seismic design shear for the
story under consideration. The story strength supply may be computed as the shear in the story that
occurs simultaneously with the attainment of the development of first significant yield of the overall
structure. To compute first significant yield, the structure should be loaded with a seismic force
pattern similar to that used to compute seismic story strength demand. A simple and conservative
procedure is to compute the ratio of demand to strength for each member of the seismic-force-resisting
system in a particular story and then use the largest such ratio as $. For a structure otherwise in
conformance with the Provisions, $ = 1.0 is obviously conservative.
The principal reason for inclusion of $ is to allow for a more equitable analysis of those structures in
which substantial extra strength is provided, whether as a result of adding stiffness for drift control, of
code-required wind resistance, or simply of a feature of other aspects of the design.
5.5.3 Modal Analysis Procedure: Studies of the dynamic response of elastically supported multi-degree-of-freedom systems (Bielak, 1976; Chopra and Gutierrez, 1974; Veletsos, 1977) reveal that,
within the ranges of parameters that are of interest in the design of structures subjected to earthquakes,
soil-structure interaction affects substantially only the response component contributed by the
fundamental mode of vibration of the superstructure. In this section, the interaction effects are
considered only in evaluating the contribution of the fundamental structural mode. The contributions
of the higher modes are computed as if the structure were fixed at the base, and the maximum value of
a response quantity is determined, as for rigidly supported structures, by taking the square root of the
sum of the squares of the maximum modal contributions.
The interaction effects associated with the response in the fundamental structural mode are determined
in a manner analogous to that used in the analysis of the equivalent lateral force method, except that the
effective weight and effective height of the structure are computed so as to correspond exactly to those
of the fundamental natural mode of the fixed-base structure. More specifically, W̄ is computed from:
W ' W1 '
'wiNil 2
'wiNil
(C5.5.3)
2
123
Structural Design Criteria
which is the same as Eq. 5.4.5-2, and h̄ is computed from Eq. 5.5.3.1-5. The quantity Nil in these
formulas represents the displacement amplitude of the ith floor level when the structure is vibrating in its
fixed-base fundamental natural mode. The structural stiffness, k̄, is obtained from Eq. 5.5.5.1.1-2 by
taking W̄ = W̄1 and using for T the fundamental natural period of the fixed-base structure, Tl. The
fundamental natural period of the interacting system, T̃,l is then computed from Eq. 5.5.5.1.1-1 (or Eq.
5.5.5.1.2-4 when applicable) by taking T = Tl. The effective damping in the first mode, $, is
determined from Eq. 5.5.5.1.2-1 (and Eq. 5.5.5.1.2-4 when applicable) in combination with the
information given in Figure 5.5.5.1.5. The quantity h̄ in the latter figure is computed from Eq. 5.5.3.15.
With the values of T̃l and $̃l established, the reduction in the base shear for the first mode, )V,l is
computed from Eq. 5.5.5.1-5. The quantities Cs and C̃s in this formula should be interpreted as the
seismic coefficients corresponding to the periods Tl and T̃,l respectively; $̃ should be taken equal to $̃;l
and W̄ should be determined from Eq. C5.5.3.
The sections on lateral forces, shears, overturning moments, and displacements follow directly from
what has already been noted in this and the preceding sections and need no elaboration. It may only be
pointed out that the first term within the brackets on the right side of Eq. 5.5.3.2-1 represents the
contribution of the foundation rotation.
5.5.3.3 Design Values: The design values of the modified shears, moments, deflections, and story
drifts should be determined as for structures without interaction by taking the square root of the sum of
the squares of the respective modal contributions. In the design of the foundation, the overturning
moment at the foundation-soil interface determined in this manner may be reduced by 10 percent as for
structures without interaction.
The effects of torsion about a vertical axis should be evaluated in accordance with the requirements of
Sec. 5.3.5 and the P-delta effects should be evaluated in accordance with the requirements of Sec.
5.3.7.2, using the story shears and drifts determined in Sec. 5.5.3.5.
Other Methods of Considering the Effects of Soil Structure Interaction: The procedures
proposed in the preceding sections for incorporating the effects of soil-structure interaction provide
sufficient flexibility and accuracy for practical applications. Only for unusual structures of major
importance, and only when the requirements indicate that the interaction effects are of definite
consequence in design, would the use of more elaborate procedures be justified. Some of the possible
refinements, listed in order of more or less increasing complexity, are:
1. Improve the estimates of the static stiffnesses of the foundation, Ky and K2, and of the foundation
damping factor, $o, by considering in a more precise manner the foundation type involved, the
effects of foundation embedment, variations of soil properties with depth, and hysteretic action in
the soil. Solutions may be obtained in some cases with analytical or semi-analytical formulations
and in others by application of finite difference or finite element techniques (Blaney et al., 1974;
Luco, 1974; Novak, 1974; Veletsos and Verbic, 1973). It should be noted, however, that these
solutions involve approximations of their own that may offset, at least in part, the apparent
increase in accuracy.
2. Improve the estimates of the average properties of the foundation soils for the stipulated design
ground motion. This would require both laboratory tests on undisturbed samples from the site and
studies of wave propagation for the site. The laboratory tests are needed to establish the actual
124
1997 Commentary, Chapter 5
variations with shearing strain amplitude of the shear modulus and damping capacity of the soil,
whereas the wave propagation studies are needed to establish realistic values for the predominant
soil strains induced by the design ground motion.
3. Incorporate the effects of interaction for the higher modes of vibration of the structure, either
approximately by application of the procedures recommended in Bielak (1976), Roesset et
al. (1973), and Tsai (1974) or by more precise analyses of the structure-soil system. The latter
analyses may be implemented either in the time domain by application of the impulse response
functions presented in Veletsos and Verbic (1974). However, the frequency domain analysis is
limited to systems that respond within the elastic range while the approach involving the use of the
impulse response functions is limited, at present, to soil deposits that can adequately be represented
as a uniform elastic halfspace. The effects of yielding in the structure and/or supporting medium
can be considered only approximately in this approach by representing the supporting medium by a
series of springs and dashpots whose properties are independent of the frequency of the motion and
by integrating numerically the governing equations of motion (Parmelee et al., 1969).
4. Analyze the structure-soil system by finite element method (Seed et al., 1974 and 1977; Vaish and
Chopra, 1974), taking due account of the nonlinear effects in both the structure and the supporting
medium.
It should be emphasized that, while these more elaborate procedures may be appropriate in special
cases for design verification, they involve their own approximations and do not eliminate the uncertainties that are inherent in the modeling of the structure-foundation-soil system and in the specification
of the design ground motion and of the properties of the structure and soil.
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Anderson, D. G., and F. E. Richart, Jr. 1976. "Effects of Straining on Shear Modulus of Clays."
Journal of the ASCE Geotechnical Engineering Division 102 (679):975-987.
Arias, A., and R. Husid. 1965. "Influence of Damping on Earthquake Response of Structures."
Revista del IDIEM 1 (3). In Spanish.
Ballard, R. F., and F. G. McLean. 1975. "Seismic Field Methods for In Situ Moduli." In Proceedings
of the Conference on In Situ Measurement of Soil Properties, Vol. I, pp. 121-150. North Carolina
State University.
Bernal, D. 1987. "Amplification Factors for Inelastic Dynamic P-delta Effects in Earthquake
Analysis." Earthquake Engineering and Structural Dynamics, 15:835-881.
Bielak, J. 1976. "Modal Analysis for Building-Soil Interaction." Journal of the ASCE Engineering
Mechanics Division 102 (EM5):771-786.
Bielak, J. 1975. "Dynamic Behavior of Structures with Embedded Foundations." In Earthquake
Engineering and Structural Dynamics, Vol. 3, pp. 259-274.
Blaney, G. W., I. Kausel, and J. M. Roesset. "Dynamic Stiffness of Piles." In Proceedings of the
Second International Conference on Numerical Methods in Geomechanics, Vol. 2, pp. 1001-1015.
Blacksburg: Virginia Polytechnic Institute and State University.
Bonowitz, D., Youssef, N., and Gross, J. L., “A survey of steel moment-resisting frames buildings
affected by the 1994 northridge earthquake,” Report no. NISTIR 5625, NIST, Gaithersbur, MD, 1995.
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Structural Design Criteria
Chopra, A. K., and J. A. Gutierrez. 1974. "Earthquake Analysis of Multistory Buildings Including
Foundation Interaction." Journal of Earthquake Engineering and Structural Dynamics 3:65-67.
Clough, R. W. and J. Penzien. 1975. Dynamics of Structures. New York: McGraw-Hill.
Earthquake Engineering Research Institute. 1994. Preliminary Reconnaisance Report: Northridge
Earthquake, EERI 94-01.
Elsabee, F., I. Kausel, and J. M. Roesset. 1977. "Dynamic Stiffness of Embedded Foundations." In
Proceedings of the ASCE Second Annual Engineering Mechanics Division Specialty Conference,
pp. 40-43.
Erden, S. M. 1974. Influence of Shape and Embedment on Dynamic Foundation Response. Thesis
submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University
of Massachusetts, Amherst.
Hara, A., T. Ohta, M. Niwa, S. Tanaka, and T. Banno. 1974. "Shear Modulus and Shear Strength of
Cohesive Soils." Journal of the Japanese Society of Soil Mechanics and Foundation Engineering 14
(3):1-24.
Hardin, B. O., and Black. 1968. ???
Hardin, B. O., and V. P. Drnevich. 1975. "Shear Modulus and Damping in Soils: Design Equations
and Curves." Journal of the ASCE Soil Mechanics and Foundation Division 98 (SM7):667-695.
Jennings, P. C., and J. Bielak. 1973. "Dynamics of Building-Soil Interaction." Bulletin of Seismology
Society of America 63 (1):9-48.
Kausel, E., and J. M. Roesset. 1975. "Dynamic Stiffness of Circular Foundations." Journal of the
ASCE Engineering Mechanics Division 101 (EM6):771-785.
Kuribayashi, E., T. Iwasaki, and F. Tatsuoka. 1974. "Effects of Stress Conditions on Dynamic
Properties of Sands." Bulletin of the International Institute of Seismology and Earthquake
Engineering 12:117-130.
Liu, S. C., and L. W. Fagel. 1971. "Earthquake Interaction by Fast Fourier Transform." Journal of
the ASCE Engineering Mechanics Division 97 (EM4):1223-1237.
Luco, J. E. 1974. "Impedance Functions for a Rigid Foundation on a Layered Medium." Nuclear
Engineering and Design (31):204-217.
Nair, K., H. Gray, and N. C. Donovan. 1969. Analysis of Pile Group Behavior. ASTM Special
Technical Publication 44.
Newmark, N. M., J. A. Blume, and K. K. Kapur. 1973. "Seismic Design Spectra for Nuclear Power
Plants." Journal of the ASCE Power Division 99 (PO 2):873-889.
Newmark, N. M., and E. Rosenblueth. 1971. Fundamentals of Earthquake Engineering. New
York: Prentice-Hall.
Novak, M. 1974. "Effect of Soil on Structural Response to Wind and Earthquake." In Earthquake
Engineering and Structural Dynamics, Vol. 3, pp. 79-96.
Novak, M., and Y. O. Beredugo. 1975. "Vertical Vibrations of Embedded Footings." Journal of the
ASCE Soil Mechanics and Foundations Division 98 (SM12):1291-1310.
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1997 Commentary, Chapter 5
Novak, M. 1974. "Dynamic Stiffness and Damping of Piles." Canadian Geotechnical Journal 11:574-598.
Parmelee, R. A., D. S. Perelman, and S. L. Lee. 1969. "Seismic Response of Multistory Structures on
Flexible Foundations." Bulletin of the Seismological Society of America 29:1061-1070.
Poulos, H. G. and E. H. Davis. 1974. Elastic Solutions for Soil and Rock Mechanics. New
York: Wiley and Sons.
Rainer, J. H. 1975. "Simplified Analysis of Dynamic Structure-Ground Interaction." Canadian
Journal of Civil Engineering 2 (3):345-356.
Rainer, J. H. 1975. "Damping in Dynamic Structure-Foundation Interaction." Canadian Geotechnical Journal 12:13-25.
Richart, F. E., J. R. Hall, Jr., and R. D. Woods. Vibrations of Soils and Foundations. Englewood
Cliffs, New Jersey: Prentice-Hall.
Roesset et al. 1973.
Rutenburg, A. 1985. "Simplified P-delta Analysis for Asymmetric Structures." Journal of the
Structural Division, ASCE, 108(ST9).
SAC Joint Venture. 1995. Analytical Investigation of Buildings Affected by the 1994 Northridge
Earthquake, Volumes 1 and 2, SAC 95-04A and B. SAC Joint Venture, Sacramento, CA, 1995.
Seed, H. B., J. Lysmer, and R. Hwang. 1974. "Soil-Structure Interaction Analysis for Seismic
Response." Journal of the ASCE Geotechnical Engineering Division 101 (GT5):439-457.
Seed, H. B., R. V. Whitman, and J. Lysmer. 1977. "Soil-Structure Interaction Effects in the Design of
Nuclear Power Plants." In Structural and Geotechnical Mechanics, A Volume Honoring
N. M. Newmark, edited by W. J. Hall. Englewood Cliffs, New Jersey: Prentice-Hall.
Stokoe, K. H., II, and R. D. Woods. 1975. "In Situ Shear Wave Velocity by Cross-Hole Method."
Journal of the ASCE Soil Mechanics and Foundations Division 98 (SM5):443-460.
Structural Engineers Association of California. 1968, 1973, 1974, 1976, and 1987. Recommended
Lateral Force Requirements and Commentary. San Francisco: SEAOC.
Thomson, W. T. 1965. Vibration Theory and Application. New York: Prentice-Hall.
Tsai, N. C. 1974. "Modal Damping for Soil-Structure Interaction." Journal of the ASCE
Engineering Mechanics Division 100 (EM2):323-341.
Uang, C-M., “Establishing R (or Rw) and Cd Factors for Building Seismic Provisions,” Journal of
Structural Engineering, vol 117, no. 1, pp. 19-28, ASCE, 1991.
Vaish, A. K., and A. K. Chopra. 1974. "Earthquake Finite Element Analysis of Structure-Foundation
Systems." Journal of the ASCE Engineering Mechanics Division 100 (EM6):1011-1016.
Veletsos, A. S., and V. V. Nair. 1975. "Seismic Interaction of Structures on Hysteretic Foundations."
Journal of the ASCE Structural Division 101 (ST1):109-129.
Veletsos, A. S. 1977. "Dynamics of Structure-Foundation Systems." In Structural and Geotechnical
Mechanics, A Volume Honoring N. M. Newmark, edited by W. J. Hall, pp. 333-361. Englewood
Cliffs, New Jersey: Prentice-Hall.
127
Structural Design Criteria
Veletsos, A. S., and J. W. Meek. 1974. "Dynamic Behavior of Building Foundation Systems."
Earthquake Engineering and Structural Dynamics 3 (2):121-138.
Veletsos, A. S., and B. Verbic. 1973. "Vibration of Viscoelastic Foundations." Earthquake
Engineering and Structural Dynamics 2 (1): 87-105.
Veletsos, A. S., and Y. T. Wei. 1971. "Lateral and Rocking Vibration of Footings." Journal of the
ASCE Soil Mechanics and Foundations Division 97 (SM9):1227-1248.
Veletsos, A. S., and B. Verbic. 1974. "Basic Response Functions for Elastic Foundations." Journal
of the ASCE Engineering Mechanics Division 100 (EM2):189-205.
Wiegel, R. L., Ed. 1970. Earthquake Engineering. New York: Prentice-Hall.
Wilson, E. L., and A. Habibullah. 1987. "Static and Dynamic Analysis of Multi-story Buildings
Including P-delta Effects." Earthquake Spectra 3(2).
Wood, S. L., “Performance of Reinforced Concrete Buildings During the 1985 Chile Earthquake,”
EERI Spectra, Nov. 1991
128
Chapter 6 Commentary
ARCHITECTURAL, MECHANICAL, AND
ELECTRICAL COMPONENTS DESIGN REQUIREMENTS
6.1 GENERAL: The general requirements establish minimum design levels for architectural,
mechanical, electrical, and other nonstructural systems and components (hereinafter referred to as
"components") recognizing occupancy use, occupant load, need for operational continuity, and the
interrelation of structural and architectural, mechanical, electrical, and other nonstructural components.
Several exemptions are made to the Provisions:
1. All components in Seismic Design Category A are exempted because of the lower seismic input
for these items
2. All mechanical and electrical components in Seismic Design Categories B and C are exempted if
they have an importance factor (Ip) equal to 1.00 because of the low acceleration and the
classification that they do not contain hazardous substances and are not required to function to
maintain life-safety.
3. All components in all Seismic Design Categories, weighing less than 400 pounds (1780 N), and
are mounted 4 ft (1.22 m) or less above the floor are exempted if they have an importance factor
(Ip) equal to 1.00, because they do not contain hazardous substances, are not required to function
to maintain life safety, and are not considered to be mounted high enough to be a life-safety hazard
if they fell.
The seismic force on any component shall be applied at the center of gravity of the component and
shall be assumed to act in any horizontal direction. Vertical forces on architectural components are
specified in Sec. 6.1.3. Vertical forces on mechanical and electrical components are specified in Sec.
6.3.2.
In the design and evaluation of support structures and the attachment of thearchitectural component,
flexibility should be considered. Components that are subjected to seismic relative displacements (i.e.,
components that are connected to both the floor and ceiling level above) should be designed with
adequate flexibility to accommodate imposed displacements. In the design and evaluation of
equipment support structures and attachments, flexibility will reduce the fundamental frequency of the
supported equipment and increase the amplitude of its induced relative motion. This lowering of the
fundamental frequency of the supported component often will bring it into the range of the fundamental
frequency of the supporting building or into the high energy range of the input motion. In evaluating
the flexibility/stiffness of the component attachment, the load path in the components should be
considered especially in the region near the anchor points.
Although the components included in Tables 6.2.2 and 6.3.2 are listed separately, significant
interrelationships exist among them and should not be overlooked. For example, exterior,
nonstructural, spandrel walls may shatter and fall on the streets or walks below seriously hampering
accessibility and egress functions. Further, the rupture of one component could lead to the failure of
131
1997 Commentary, Chapter 6
another that is dependent on the first. Accordingly, the collapse of a single component ultimately may
lead to the failure of an entire system. Widespread collapse of suspended ceilings and light fixtures in a
building may render an important space or major exit stairway unusable.
Consideration also was given to the design requirements for these components to determine how well
they are conceived for their intended functions. Potential beneficial and/or detrimental interactions with
the structure were examined. The interrelationship between components and their attachments were
surveyed. Attention was given to the performance relative to each other of architectural, mechanical,
and electrical components; building products and finish materials; and systems within and without the
building structure. It should be noted that the modification of one component in Table 6.2.2 or 6.3.2
could affect another and, in some cases, such a modification could help reduce the risk associated with
the interrelated unit. For example, landscaping barriers around the exterior of certain buildings could
decrease the risk due to falling debris although this should not be interpreted to mean that all buildings
must have such barriers.
The design of components that are in contact with or in close proximity to structural or other
nonstructural components must be given special study to avoid damage or failure when seismic motion
occurs. An example is where an important element, such as a motor generator unit for a hospital, is
adjacent to a nonload-bearing partition. The failure of the partition might jeopardize the motor
generator unit and, therefore, the wall should be designed for a performance level sufficient to ensure
its stability.
Where nonstructural wall components may affect or stiffen the structural system because of their close
proximity, care must be exercised in selecting the wall materials and in designing the intersection details
to ensure the desired performance of each component.
6.1.2 COMPONENT FORCE TRANSFER FACTOR: It is required that components be
attached to the building structure and that all the required attachments be fully detailed in the design
documents. These details should take into account the force levels and anticipated deformations
expected or designed into the structure.
If an architectural component were to fail during an earthquake, the mode of failure probably would be
related to faulty design of the component, interrelationship with another component that fails,
interaction with the structural framing, deficiencies in its type of mounting, or inadequacy of its
attachments or anchorage. The last is perhaps the most critical when considering seismic safety.
Building components designed without any intended structural function--such as infill walls--may
interact with the structural framing and be forced to act structurally as a result of excessive building
deformation. The build up of stress at the connecting surfaces or joints may exceed the limits of the
materials. Spatial tolerances between such components thus become a governing factor. These
requirements therefore emphasize the ductility and strength of the attachments for exterior wall
elements and the interrelationship of elements.
Traditionally, mechanical equipment that does not include rotating or reciprocating components (e.g.,
tanks, heat exchangers) is anchored directly to the building structure. Mechanical and electrical
equipment containing rotating or reciprocating components often is isolated from the structure by
vibration isolators (rubber-in-shear, springs, air cushions). Heavy mechanical equipment (e.g., large
boilers) often is not restrained at all, and electrical equipment other than generators, which are normally
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Architectural, Mechanical, and Electrical Components and Systems Design Requirements
isolated to dampen vibrations, usually is rigidly anchored (e.g., switchgear, motor control centers).
The installation of unattached mechanical and electrical equipment should be virtually eliminated for
buildings covered by the Provisions.
Friction produced solely by the effects of gravity cannot be counted on to resist seismic forces as
equipment and fixtures often tend to "walk" due to rocking when subjected to earthquake motions.
This often is accentuated by the vertical ground motions. Because frictional resistance cannot be relied
upon, positive restraint must be provided for each component.
6.1.3 SEISMIC FORCES: The design seismic force is dependent upon the weight of the system or
component, the component amplification factor, the component acceleration at point of attachment to
the structure, the component importance factor, and the component response modification factor.
The seismic design force equations presented originated with a study and workshop sponsored by the
National Center for Earthquake Engineering Research (NCEER) with funding from the National
Science Foundation (NSF) (Bachman et al., 1993). The participants examined recorded acceleration
data in response to strong earthquake motions. The objective was to develop a "supportable" design
force equation that considered actual earthquake data as well as component location in the structure,
component anchorage ductility, component importance, component safety hazard upon separation
from the structure, structural response, site conditions, and seismic zone. Additional studies have
further revised the equation to its present form (Drake and Bachman, 1994 and 1995). In addition, the
term Ca has been replaced by the quantity 0.4SDS to conform with changes in Chapter 4. BSSC
Technical Subcommittee 8 believes that Eq. 6.1.3-1 through 6.1.3-3 achieve the objectives without
unduly burdening the practitioner with complicated formulations.
The component amplification factor (ap) represents the dynamic amplification of the component
relative to the fundamental period of the structure (T). It is recognized that at the time the
components are designed or selected, the structural fundamental period is not always defined or readily
available. It is also recognized that the component fundamental period (Tp) is usually only accurately
obtained by expensive shake-table or pull-back tests. A listing is provided of ap values based on the
expectation that the component will usually behave in either a rigid or flexible manner. In general, if
the fundamental period of the component is less than 0.06 sec, no dynamic amplification is expected. It
is not the intention of the Provisions to preclude more accurate determination of the component
amplification factor when reasonably accurate values of both the structural and component
fundamental periods are available. Figure C 6.1.3-1 is from the NCEER work and is an acceptable
formulation for ap as a function of Tp/T. Minor adjustments from the 1994 Provisions have been made
in the tabulated ap values to be consistent with the 1997 Uniform Building Code.
The component response modification factor (Rp) represents the energy absorption capability of the
component's structure and attachments. Conceptually, the Rp value considers both the overstrength
and deformability of the component’s structure and attachments. In the absence of
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current research, it is believed these separate considerations can be adequately combined into a single
factor. The engineering community is encouraged to address the issue and conduct research into the
component response modification factor that will advance the state of the art. These values are
judgmentally determined utilizing the collective wisdom and experience of the responsible committee.
In general, the following benchmark values were used:
Rp =1.25, low deformability element
Rp = 2.5, limited deformability element
Rp = 3.5, high deformability element
a
p
NCEER STUDY
2.5
1991 NEHRP
2.0
(Reference)
1.0
0.5
0.6
0.7
1.0
1.4
2.0
Tp/T
FIGURE C6.1.3-1 NCEER formulation for ap as function of structural and component
periods.
Minor adjustments from the 1994 Provisions have been made in the tabulated Rp values to correlate
with Fp values determined in accordance with the 1997 Uniform Building Code. Researchers have
proposed a procedure for validating values for Rp with respect to documented earthquake performance
(Bachman and Drake, 1996).
Eq. 6.1.3-1represents a trapezoidal distribution of floor accelerations within the structure, linearly
varying from the acceleration at the ground ( 0.4SDS) to the acceleration at the roof (1.2SDS). The
ground acceleration ( 0.4SDS) is intended to be the same acceleration used as design input for the
structure itself and will include site effects.
Examination of recorded in-structure acceleration data in response to large California earthquakes
reveals that a reasonable maximum value for the roof acceleration is four times the input ground
acceleration to the structure. Earlier work (Drake and Bachman, 1996, 1995 and 1996) indicated that
the maximum amplification factor of four seems suitable (Figure C6.1.3-1). However, a close
examination of recently recorded strong motion data at sites with peak ground accelerations in excess
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of 0.1g indicates that an amplification factor of three is more appropriate (Figure C 6.1.3-2). In the
lower portions of the structure (the lowest 20 percent of the structure), both the amplification factors
of three and four do not bound the mean plus one standard deviation accelerations. However, the
minimum design force in Eq. 6.1.3-3 provides a lower bound in this region.
FIGURE C6.1.3-1 Revised NEHRP equation vs. (Mean + 1F
F) acceleration records -- all
sites.
FIGURE C6.1.3-2 Revised NEHRP equation vs (mean + 1F
F) acceleration records -- sites
with Ag $ 0.1g.
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1997 Commentary, Chapter 6
Examination of the same data indicates that the in-structure accelerations do not decrease with larger
building periods as might be expected from reviewing typical response spectra. One reason for
invalidating the traditional response spectra shape might be that structures with longer fundamental
periods may have designs governed by drift requirements. These structures would be stiffer with more
elastic capacity and also may have lower damping at higher acceleration responses. Also, site soil
amplifications are greater at longer periods than at shorter periods. As a result of these studies, the
structural period effect introduced into the 1994 Provisions for components has been removed from
the 1997 Provisions.
A lower limit for Fp is set to assure a minimal seismic design force. The minimum value for Fp
determined by setting the quantity apAp/Rp equal to0.7Ca which is equivalent to the minimum used in
current practice. In addition, the Ca term was converted to 0.4SDS to be consistent with changes to
Chapter 1. The resultant multiplication of 0.7 times 0.4 equals 0.28 was rounded to 0.3 for simplicity.
To meet the need for a simpler formulation, a conservative maximum value for Fp also was set. Eq.
6.1.3-2is the maximum value for Fp determined by setting the quantity apAp/Rp equal to 4.0. In
addition, the term Ca was converted to 0.4 SDS to be consistent with changes to Chapter 4. Eq. 6.1.32also serves as a reasonable "cutoff" equation to assure that the multiplication of the individual factors
does not yield an unreasonably high design force.
To clarify the application of vertical seismic design forces in combination with horizontal design forces
and service loads, a cross-reference was provided to Sec. 2.2.6. The value for Fp calculated in
accordance with Chapter 6 should be substituted for the value of QE in Sec. 2.2.6.
6.1.4 SEISMIC RELATIVE DISPLACEMENTS: The seismic relative displacement equations
were developed as part of the NCEER/NSF study and workshop described above. It was recognized
that displacement equations were needed to support the design of cladding, stairwells, windows, piping
systems, sprinkler components, and other components that are connected to the structure(s) at multiple
levels or points of connection.
Two equations are given for each situation. Eq. 6.1.4-1 and Eq. 6.1.4-3 yield "real" structural
displacements as determined by elastic analysis, with no structural response modification factor (R)
included. Recognizing that elastic displacements are not always defined or available at the time the
component is designed or procured, default Eq. 6.1.4-2 and Eq. 6.1.4-4 also are provided that allow
the use of structure drift limitations. Use of these default equations must balance the need for a timely
component design/procurement with the possible conservatism of their use. It is the intention that the
lesser of the paired equations be acceptable for use.
The designer also should consider other situations where seismic relative displacements could impose
unacceptable stresses on a component or system. One such example would be a component
connecting two pieces of equipment mounted in the same building at the same elevation, where each
piece of equipment has it's own displacements relative to the mounting location. In this case, the
designer must accommodate the total of the separate seismic displacements relative to the equipment
mounting location.
For some items such as ductile piping, relative seismic displacements between support points generally
are of more significance than forces. Piping made of ductile materials such as steel or copper can
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accommodate relative displacements by local yielding but with strain accumulations well below failure
levels. However, components made of less ductile materials can only accommodate relative
displacement effects by use of flexible connections or avoiding local yielding. It is further the intent of
the Provisions to consider the effects of seismic support relative displacements and displacements
caused by seismic force on mechanical and electrical component assemblies such as piping systems,
cable and conduit systems, and other linear systems, most typically, and the equipment to which they
attach. Impact of components should also be avoided although ductile materials have been shown to
be capable of accommodating fairly significant impact loads. With protective coverings, ductile
mechanical and electrical components and many more fragile components can be expected to survive
all but the most severe impact loads.
6.1.5 COMPONENT IMPORTANCE FACTOR: The component importance factor (Ip)
represents the greater of the life-safety importance of the component and the hazard exposure
importance of the structure. This factor indirectly accounts for the functionality of the component or
structure by requiring design for a higher force level. Use of higher Ip requirements together with
application of the requirements in Sec. 6.3.13 and 6.3.14 should providea better, more functional
component. While this approach will provide a higher degree of confidence in the probable seismic
performance of a component, itmay not be sufficient for all components. For example, individual
ceiling tiles may still fall from the ceiling grid. Seismic qualification approaches presently in use by the
Department of Energy (DOE) and the Nuclear Regulatory Commission (NRC) should be considered
by the registered design professional and/or the owner when unacceptable consequences of failure are
anticipated.
Components that may fall from the structure are among the most hazardous building elements in an
earthquake. These components may not be integral with the structural system and may cantilever
horizontally or vertically from their supports. Critical issues affecting these components include their
weight, their attachment to the structure, and their location (over an entry or exit, public walkway,
atrium, or lower adjacent structure). Examples of items that may pose a falling hazard include
parapets, cornices, canopies, marquees, and precast concrete cladding panels. In addition, mechanical
and electrical components may pose a falling hazard, for example, a rooftop tank or cooling tower,
which if separated from the structure, will fall to the ground.
Special consideration should be given components that could block means of egress or exitways apply
to items that, if they fall during an earthquake, could block the means of egress for the occupants of the
structure. The term "means of egress" has been defined the same way throughout the country, since
egress requirements have been included in building codes because of fire hazard. The requirements for
exitways include intervening aisles, doors, doorways, gates, corridors, exterior exit balconies, ramps,
stairways, pressurized enclosures, horizontal exits, exit passage ways, exit courts, and yards. Example
items that should be included when considering egress include walls around stairs, corridors, veneers,
cornices, canopies, and other ornaments above building exits. In addition, heavy partition systems
vulnerable to failure by collapse, ceilings, soffits, light fixtures, or other objects that could fall or
obstruct a required exit. door or component (rescue window or fire escape) could be considered major
obstructions. Examples of the components that do not pose a significant falling hazard include fabric
awnings and canopies and architectural, mechanical, and electrical components which, if separated
from the structure, will fall in areas that are not accessible (in an atrium or light well not accessible to
the public for instance).
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Sec. 1.3.1 requires that Group III structures shall, in so far as practical, be provided with the capacity
to function after an earthquake. To facilitate this, all nonstructural components and equipment in
structures in Seismic Use Group III, and in Seismic Design Category C or higher, should be designed
with an Ip equal to 1.5. All components and equipment are included because damage to vulnerable
unbraced systems or equipment may disrupt operations following an earthquake, even if they are not
"life-safety" items. Nonessential items can be considered "black boxes." There is no need for
component analysis as discussed in Sec. 6.3.13 and 6.3.14, since operation of these secondary items is
not critical to the post-earthquake operability of the structure.
Until recently, storage racks were primarily installed in low-occupancy ware houses. With the recent
proliferation of warehouse-type retail stores, it has been judged necessary to address the relatively
greater seismic risk that storage racks may pose to the general public, compared to more conventional
retail environments. Under normal operating conditions, retail stores have a far higher occupancy load
than an ordinary warehouse of a reasonable size. Failure of a storage rack system in the retail
environment is much more likely to cause personal injury than a similar failure in a storage warehouse.
Therefore, to provide an appropriate level of additional safety in areas open to the public,Sec 6.1.5
now requires that storage racks in occupancies open to the general public should be designed with an Ip
value equal to 1.50. Storage rack contents, while beyond the scope of the Provisions pose a
potentially serious threat to life should they fall from the shelves in an earthquake. Restraints should be
provided to prevent the contents of rack shelving open to the general public from falling in strong
ground shaking.
6.1.5 COMPONENT ANCHORAGE: In general, it is not recommended that anchors be relied
upon for energy dissipation. Inasmuch as the anchor represents the transfer of load from a relatively
deformable material (e.g., steel) to a low deformability material (e.g., concrete, masonry), the boundary
conditions for ensuring deformable, energy-absorbing behavior in the anchor itself are at best difficult
to achieve. On the other hand, the concept of providing a fuse, or deformable link, in the load path to
the anchor is encouraged. This approach allows the designer to provide the necessary level of ductility
and overstrength in the connection while at the same time protecting the anchor from overload and
eliminates the need for balancing of steel strength and deformability in the anchor with variable edge
distances and anchor spacings.
Allowable loads for anchors should not be increased for earthquake loading. Possible reductions in
allowable loads for particular anchor types to account for loss of stiffness and strength should be
determined through appropriate dynamic testing.
Anchors that are used to support towers, masts, and equipment often are provided with double nuts to
allow for leveling during installation. Where baseplate grout is provided, it should not be relied upon
to carry loads since it can shrink and crack or is often omitted altogether. In this case, the anchors are
loaded in tension, compression, shear, and flexure and should be designed as such.
Prying forces on anchors, which result from a lack of rotational stiffness in the connected part, can be
critical for anchor design and must be considered explicitly.
For anchorages that are not provided with a mechanism to transfer compression loads, the design for
overturning must reflect the actual stiffness of the baseplate, equipment, housing, etc., in determining
the location of the compression centroid and the distribution of uplift loads to the anchors.
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Architectural, Mechanical, and Electrical Components and Systems Design Requirements
Possible reductions in allowable loads for particular anchor types to account for loss of stiffness and
strength should be determined through appropriate dynamic testing.
While the requirements do not prohibit the use or single anchor connections, it is considered necessary
to use at least two anchors in any load-carrying device whose failure might lead to collapse.
Tests have shown that there are consistent shear ductility variations between bolts anchored to drilled
or punched plates with nuts and connections using welded, headed studs. Recommendations for
design are not presently available but should be considered in critical connections subject to dynamic or
seismic loading.
It is important to relate the anchorage demands defined by Chapter 6 with the material capacities
defined in the other chapters.
6.1.6.5: Generally, powder driven fasteners in concrete tend to exhibit variations in load capacity that
are somewhat larger than post-drilled anchors and do not provide the same levels of reliability even
though some installation methods allow for the same reliability as post-drilled expansion anchors. As
such, their qualification under a simulated seismic test program should be demonstrated prior to use.
Such fasteners, when properly installed in steel, are reliable, showing high capacities with very low
variability.
6.1.7 CONSTRUCTION DOCUMENTS: It is deemed important by the committee that there be a
clearly defined basis for each quality assurance activity specified in Chapter 3. As result construction
documents are required for all components requiring special inspection or testing in Chapter 3.
It is also deemed important by the committee that there be some reasonable level of assurance that the
construction and installation of components be consistent with the basis of the supporting seismic
design. Of particular concern are systems involving multiple trades and suppliers. In these cases, it is
important that a registered design professional prepare construction documents for the use by the
multiple trades and suppliers to follow in the course of construction.
6.2 ARCHITECTURAL COMPONENT DESIGN:
6.2.1 GENERAL: The primary focus of the Provisions is on the design of attachments, connections,
and supports for architectural components.
"Attachments" are means by which components are secured or restrained to the seismic force resisting
system of the structure. Such attachments and restraints may include anchor bolting, welded
connections, and fasteners.
"Architectural component supports" are those members or assemblies of members, including braces,
frames, struts and attachments, that transmit all loads and forces between the component and the
building structure. Architectural component supports also transmit lateral forces and/or provide
structural stability for the component to which they connect.
The requirements are intended to reduce the threat of life safety hazards posed by components and
elements from the standpoint of stability and integrity. There are several circumstances where such
components may pose a threat.
1. Where loss of integrity and/or connection failure under seismic motion poses a direct hazard in that
the components may fall on building occupants.
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2. Where loss of integrity and/or connection failure may result in a hazard for people outside of a
building in which components such as exterior cladding and glazing may fall on them.
3. Where failure or upset of interior components may impede access to a required exit.
The requirements are intended to apply to all of the circumstances listed above. Although the safety
hazard posed by exterior cladding is obvious, judgment may be needed in assessing the extent to which
the requirements should be applied to other hazards.
Property loss through damage to architectural components is not specifically addressed in the
Provisions. Function and operation of a building also may be affected by damage to architectural
components if it is necessary to cease operations while repairs are undertaken. In general,
requirements to improve life-safety also will reduce property loss and loss of building function.
In general, functional loss is more likely to be affected by loss of mechanical or electrical components.
Architectural damage, unless very severe, usually can be accommodated on a temporary basis. Very
severe architectural damage results from excessive structural response that often also results in
significant structural damage and building evacuation.
6.2.2 ARCHITECTURAL COMPONENT FORCES: Components that could be damaged or
could damage other components and are fastened to multiple locations of a structure should be
designed to accommodate seismic relative displacements. Examples of components that should be
designed to accommodate seismic relative displacements include glazing, partitions, stairs, and veneer.
Certain types of veneer elements, such as aluminum or vinyl siding and trim, possess high
deformability. These systems are generally light and can undergo large deformations without
separating from the structure. However, care must be taken when designing these elements to ensure
that the low deformability components that may be part of the curtain wall system, such as glazing
panels, have been detailed to accommodate the expected deformations without failure.
6.2.3 ARCHITECTURAL COMPONENT DEFORMATION: Specific requirements for
cladding are provided. Glazing, both exterior and interior, and partitions must be capable of
accommodating story drift without causing a life-safety hazard. Design judgment must be used with
respect to the assessment of life-safety hazard and the likelihood of life-threatening damage. Special
detailing to accommodate drift for typical replaceable gypsum board or demountable partitions is not
likely to be cost-effective, and damage to these components has a low life-safety hazard. Nonstructural
fire-resistant enclosures and fire-rated partitions may require some special detailing to ensure that they
retain their integrity. Special detailing should provide isolation from the adjacent or enclosing structure
for deformation equivalent to the calculated drift (relative displacement). In-plane differential
movement between structure and wall is permitted. Provision also must be made for out-of-plane
restraint. These requirements are particularly important in relation to the larger drifts experienced in
steel or concrete moment frame structures. The problem is less likely to be encountered in stiff shear
wall structures.
Differential vertical movement between horizontal cantilevers in adjacent stories (i.e., cantilevered floor
slabs) has occurred in past earthquakes. The possibility of such effects should be considered in design
of exterior walls.
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6.2.4 EXTERIOR NONSTRUCTURAL WALL ELEMENTS AND CONNECTIONS: The
Provisions requires that nonbearing wall panels that are attached to or enclose the structure shall be
designed to resist the (inertial) forces and shall accommodate movements of the structure resulting
from lateral forces or temperature change. The force requirements often overshadow the importance
of allowing thermal movement and may therefore require special detailing in order to prevent moisture
penetration and allow thermal movements.
Connections should be designed such that, if they were to yield, they would do so in a high
deformation manner without loss of load-carrying capacity. Between points of connection, panels
should be separated from the building structure to avoid contact under seismic action.
The Provisions document requires allowance for story drift. This required allowance can be 2 in. (51
mm) or more from one floor to the next and may present a greater challenge to the registered design
professional than requirements for the forces. In practice, separations between panels are usually
limited to about 3/4 in. (19 mm), with the intent of limiting contact, and hence panel alignment
disruption and/or damage under all but extreme building response, and providing for practical joint
detailing with acceptable appearance. The Provisions calls for a minimum separation of 1/2 in. (13
mm). The design should respect the manufacturing and construction tolerances of the materials used
to achieve this dimension.
If wind loads govern, connectors and panels should allow for not less than two times the story drift
caused by wind loads determined using a return period appropriate to the site location.
The Provisions requirements are in anticipation of frame yielding to absorb energy. The isolation can
be achieved by using slots, but the use of long rods that flex is preferable because this approach is not
dependent on installation precision to achieve the desired action. The rods must be designed to carry
tension and compression in addition to induced flexural stresses. For floor-to-floor wall panels, the
panel usually is rigidly fixed to and moves with the floor structure nearest the panel bottom. In this
condition, the upper attachments become isolation connections to prevent building movement forces
from being transmitted to the panels. and thus the panel translates with the load supporting structure.
The panel also can be supported at the top with the isolation connection at the bottom.
When determining the length of slot or displacement demand for the connection, the cumulative effect
of tolerances in the supporting frame and cladding panel must be considered.
The Provisions requires that fasteners be designed for approximately 4 times the required panel force
and that the connecting member be ductile. This is intended to ensure that the energy absorption takes
place in the connecting member and not at the connection itself and that the more brittle fasteners
remain essentially elastic under seismic loading. The factor of 4 has been incorporated into the ap and
Rp factors in consideration of installation and material variability.
To minimize the effects of thermal movements and shrinkage on architectural cladding panels, the
connection system generally is statically determinant. As a result, cladding panel support systems often
lack redundancy and failure of a single connection can have catastrophic consequences.
6.2.5 OUT-OF-PLANE BENDING: Most walls are subject to out-of-plane forces when a building
is subjected to an earthquake. These forces and the bending they induce must be considered in the
design of wall panels, nonstructural walls, and partitions. This is particularly important for systems
composed of brittle materials and/or low flexural strength materials. The conventional limits based
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upon deflections as a proportion of the span may be used with the applied force as derived in Sec.
6.2.2.
Judgment must be used in assessing the deflection capability of the component. The intent is that a
heavy material (such as concrete block) or an applied finish (such as brittle heavy stone or tile) should
not fail in a hazardous manner as a result of out-of-plane forces. Deflection in itself is not a hazard. A
steel-stud partition might suffer considerable deflection without creating a hazard; but if the same
partition supports a marble facing, a hazard might exist and special detailing may be necessary.
6.2.6 SUSPENDED CEILINGS: Suspended ceiling systems usually are fabricated using a wide
range of building materials with individual components having different material characteristics. Some
systems are homogeneous whereas others incorporate suspension systems with acoustic tile or lay-in
panels. Seismic performance during recent large California earthquakes has raised two concerns:
a. The support of the individual panels at walls and expansion joints and
b. The interaction with fire sprinkler systems.
Thealternate methods provided have been developed in a cooperative effort by registered design
professionals, the ceiling industry, and the fire sprinkler industry in an attempt to address these
concerns. It is hoped that further research and investigation will result in further improvements in
future editions of the Provisions.
Consideration shall be given to the placement of seismic bracing and the relation of light fixtures and
other loads placed into the ceiling diaphragm and the independent bracing of partitions in order to
effectively maintain the performance characteristics of the ceiling system. The ceiling system may
require bracing and allowance for theinteraction of components.
Dynamic testing of suspended ceiling systems constructed according to the requirements of current
industry seismic standards (UBC Standard 25-2) performed by ANCO Engineers, Inc. (1983) has
demonstrated that the splayed wire even with the vertical compression strut may not adequately limit
lateral motion of the ceiling system due to the flexibility introduced by the straightening of the wire end
loops. In addition, splay wires usually are installed slack to prevent unleveling of the ceiling grid and to
avoid above-ceiling utilities. Not infrequently, bracing wires are omitted because of obstructions.
Testing also has shown that system performance without splayed wires or struts was good if adequate
width of closure angles and penetration clearance was provided.
The lateral seismic restraint for a nonrigidly braced suspended ceiling is primarily provided by the
ceiling coming in contact with the perimeter wall. The wall provides a large contact surface to restrain
the ceiling. The key to good seismic performance is that the width of the closure angle around the
perimeter is adequate to accommodate ceiling motion and that penetrations, such as columns and
piping, have adequate clearance to avoid concentrating restraining loads on the ceiling system. The
behavior of an unbraced ceiling system is similar to that of a pendulum; therefore, the lateral
displacement is approximately proportional to the level of velocity-controlled ground motion and the
square root of the suspension length. Therefore, a new section has been added that permits exemption
from force calculations if certain displacement criteria are met. The default displacement limit has been
determined based on anticipated damping and energy absorption of the suspended ceiling system
assuming minimal significant impact with the perimeter wall.
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6.2.7 ACCESS FLOORS: Performance of computer access floors during past earthquakes and
during cyclic load tests indicate that typical raised access floor systems may behave in a brittle manner
and exhibit little reserve capacity beyond initial yielding or failure of critical connections. Recent
testing indicates that individual panels may "pop out" of the supporting grid during seismic motions.
Consideration should be given to mechanically fastening the individual panels to the supporting
pedestals or stringers in egress pathways.
It is acceptable practice for systems with floor stringers to calculate the seismic force Fp for the entire
access floor system within a partitioned space and then distribute the total force to the individual braces
or pedestals. Stringerless systems need to be evaluated very carefully to ensure a viable seismic load
path.
Overturning effects for the design of individual pedestals is a concern. Each pedestal usually is
specified to carry an ultimate design vertical load greatly in excess of the Wp used in determining the
seismic force Fp. It is nonconservative to use the design vertical load simultaneously with the design
seismic force when considering anchor bolts, pedestal bending, and pedestal welds to base plate. The
maximum concurrent vertical load when considering overturning effects is therefore limited to the Wp
used in determining Fp. "Slip on" heads are not mechanically fastened to the pedestal shaft and provide
doubtful capacity to transfer overturning moments from the floor panels or stringers to the pedestal.
To preclude brittle failure behavior, each element in the seismic load path must demonstrate the
capacity for elastic or inelastic energy absorption. Buckling failure modes also must be prevented.
Lesser seismic force requirements are deemed appropriate for access floors designed to preclude brittle
and buckling failure modes.
6.2.8 PARTITIONS: Partitions are sometimes designed to run only from floor to a suspended
ceiling which provides doubtful lateral support. Partitions subject to these requirements must have
independent lateral support bracing from the top of the partition to the building structure or to a
substructure attached to the building structure.
6.2.9 STEEL STORAGE RACKS: Storage racks are considered nonbuilding structures and are
covered in Provisions Chapter 14. See Commentary Sec. 14.3.3.
6.3 MECHANICAL AND ELECTRICAL COMPONENT DESIGN:
6.3.1 GENERAL: The primary focus of these requirements is on the design of attachments and
equipment supports for mechanical and electrical components.
The requirements are intended to reduce the hazard to life posed by the loss of component structural
stability or integrity. The requirements should increase the reliability of component operation but do
not directly address the assurance of functionality.
The design of mechanical and electrical components must consider two levels of earthquake safety.
For the first safety level, failure of the mechanical or electrical component itself poses no significant
hazard. In this case, the only hazard posed by the component is if the support and the means by which
the component and its supports are attached to the building or the ground fails and the component
could slide, topple, fall, or otherwise move in a manner that creates a hazard for persons nearby. In the
first category, the intent of these requirements is only to design the support and the means by which the
component is attached to the structure, defined in the Glossary as "equipment supports" and
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"attachments." For the second safety level, failure of the mechanical or electrical equipment itself poses
a significant hazard. In this case, failure could either be to a containment having hazardous contents or
contents required after the earthquake or failure could be functional to a component required to remain
operable after an earthquake. In this second category, the intent of these requirements is to provide
guidance for the design of the component as well as the means by which the component is supported
and attached to the structure. The requirements should increase the survivability of this second
category of component but the assurance of functionality may require additional considerations.
Examples of this second category include fire protection piping or an uninterruptible power supply in a
hospital. Another example involves the rupture of a vessel or piping that contains sufficient quantities
of highly toxic or explosive substances such that a release would be hazardous to the safety of building
occupants or the general public. In assessing whether failure of the mechanical or electrical equipment
itself poses a hazard, certain judgments may be necessary. For example, small flat-bottom tanks
themselves may not need to be designed for earthquake loads; however, numerous seismic failures of
large flat-bottom tanks and the hazard of a large fluid spill suggest that many, if not most, of these
should be. Distinguishing between large and small, in this case, may require an assessment of potential
damage caused by a spill of the fluid contents over and above the guidance offered in Sec. 6.3.9.
It is intended that the requirements provide guidance for the design of components for both conditions
in the second category. This is primarily accomplished by increasing the design forces with an
importance factor, Ip. However, this only affects structural integrity and stability directly. Function and
operability of mechanical and electrical components may only indirectly be affected by increasing
design forces. For complex components, testing or experience may be the only reasonable way to
improve the assurance of function and operability. On the basis of past earthquake experience, it may
be concluded that if structural integrity and stability are maintained, function and operability after an
earthquake will be reasonably provided for most types of equipment components. On the other hand,
mechanical joints in containment components (tanks, vessels, piping, etc.) may not remain leaktight in
an earthquake even if after the earthquake leaktightness is re-established. Judgment may suggest a
more conservative design related in some manner to the perceived hazard than would otherwise be
provided by these requirements.
It is not intended that all equipment or parts of equipment be designed for seismic forces.
Determination of whether these requirements need to be applied to the design of a specific piece of
equipment or a part of that equipment will sometimes be a difficult task. Damage to or even failure of
a piece or part of a component is not a concern of these requirements so long as a hazard to life does
not exist. Therefore, the restraint or containment of a falling, breaking, or toppling component or its
parts by the use of bumpers, braces, guys, wedges, shims, tethers, or gapped restraints often may be an
acceptable approach to satisfying these requirements even though the component itself may suffer
damage. Judgment will be required if the intent of these requirements is to be fulfilled. The following
example may be helpful: Since the threat to life is a key consideration, it should be clear that a
nonessential air handler package unit that is less than 4 ft (1.2 m) tall bolted to a mechanical room floor
is not a threat to life as long as it is prevented from significant motions by having adequate anchorage.
Therefore, earthquake design of the air handler itself need not be performed. However, most engineers
would agree that a 10-ft (3.0 m) tall tank on 6-ft (1.8 m) angles used as legs mounted on the roof near
a building exit does pose a hazard. It is the intent of these requirements that the tank legs, the
connections between the roof and the legs, the connections between the legs and the tank, and possibly
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even the tank itself be designed to resist earthquake forces. Alternatively, restraint of the tank by guys
or bracing could be acceptable.
It is not the intent of the Provisions to require the seismic design of shafts, buckets, cranks, pistons,
plungers, impellers, rotors, stators, bearings, switches, gears, nonpressure retaining casings and
castings, or similar items. When the potential for a hazard to life exists, it is expected that design
efforts will focus on equipment supports including base plates, anchorages, support lugs, legs, feet,
saddles, skirts, hangers, braces, or ties.
Many mechanical and electrical components consist of complex assemblies of mechanical and/or
electrical parts that typically are manufactured in an industrial process that produces similar or identical
items. Such equipment may include manufacturer's catalog items and often are designed by empirical
(trial-and-error) means for functional and transportation loadings. A characteristic of such equipment
is that it may be inherently rugged. Rugged, as used herein, refers to an ampleness of construction that
renders such equipment the ability to survive strong motions without significant loss of function. By
examining such equipment, an experienced design professional usually should be able to confirm such
ruggedness. The results of equipment ruggedness assessment then will determine the need for an
appropriate method and extent of the seismic design or qualification efforts.
It also is recognized that a number of professional and industrial organizations have developed
nationally recognized codes and standards for the design and construction of specific mechanical and
electrical components. In addition to providing design guidance for normal and upset operating
conditions and various environmental conditions, some have developed earthquake design guidance in
the context of the overall mechanical or electrical design. It is the intent of these requirements that
such codes and standards having earthquake design guidance be used as it is to be expected that the
developers have a greater familiarity with the expected failure modes of the components for which their
design and construction rules are developed. In addition, even if such codes and standards do not have
earthquake design guidance, it is generally regarded that construction of mechanical and electrical
equipment to nationally recognized codes and standards such as those approved by the American
National Standards Institute provide adequate strength (with a safety margin often greater than that
provided by structural codes) to accommodate all normal and upset operating loads. In this case, it
could also be assumed that the component has sufficient strength (especially if constructed of ductile
materials) to not break up or break away from its supports in such a way as to provide a life-safety
hazard. Earthquake damage surveys confirm this.
Specific guidance for selected components or conditions is provided in Sec. 6.3.6 through 6.3.16.
6.3.2 MECHANICAL AND ELECTRICAL COMPONENT FORCES AND DISPLACEMENTS: Components that could be damaged or could damage other components and are fastened to
multiple locations of a structure should be designed to accommodate seismic relative displacements.
Examples of components that should be designed to accommodate seismic relative displacements
include bus ducts, cable trays, conduit, elevator guide rails, and piping systems.
The restriction on Rp values in the footnote to Table 6.3.2 is because of the concern for low
deformationfailure modes in the component anchorage. Anchorages that could be reasonably
expected to fail in a low deformationmanner should be designed using Rp = 1.5. Chemical anchors and
cast-in-place anchor bolts with an embedment length-to-diameter ratio of 8 or less should be
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considered to be "shallow" anchors. Vibration isolator bumper restraints or snubbers should be
designed for the impact load imparted when it is engaged.
See also Commentary Sec. 6.3.1 for a discussionof deformability.
6.3.3 MECHANICAL AND ELECTRICAL COMPONENT PERIOD: Determination of the
fundamental period of an item of mechanical or electrical equipment using analytical or in-situ testing
methods can become very involved and can produce nonconservative results (i.e., underestimated
fundamental periods) if not properly performed.
When using analytical methods, it is absolutely essential to define in detail the flexibility of the elements
of the equipment base, load path, and attachment to determine Kp. This base flexibility typically
dominates equipment component flexibility and thus fundamental period.
When using test methods, it is necessary to ensure that the dominant mode of vibration of concern for
seismic evaluation is excited and captured by the testing. This dominant mode of vibration typically
cannot be discovered in equipment in-situ tests that measure only ambient vibrations. In order for the
highest fundamental period dominant mode of vibration to be excited by in-situ tests, relatively
significant input levels of motion are required (i.e., the flexibility of the base and attachment needs to be
exercised).
Many types of mechanical equipment components have fundamental periods below 0.06 sec and may
be considered to be rigid. Examples include horizontal pumps, engine generators, motor generators,
air compressors, and motor driven centrifugal blowers. Other types of mechanical equipment also are
very stiff but may have fundamental periods up to approximately 0.125 sec. Examples of these
mechanical equipment items include vertical immersion and deep well pumps, belt driven and vane axial
fans, heaters, air handlers, chillers, boilers, heat exchangers, filters, and evaporators. These
fundamental period estimates do not apply when the equipment is on vibration-isolator supports.
Electrical equipment cabinets can have fundamental periods of approximately 0.06 to 0.3 sec
depending upon weight, stiffness of the enclosure assembly, flexibility of the enclosure base, and load
path through to the attachment points. Tall and narrow motor control centers and switchboards lie in
the upper end of this period range. Low and medium-voltage switchgear, transformers, battery
chargers, inverters, instrumentation cabinets, and instrumentation racks usually have fundamental
periods ranging from 0.1 to 0.2 sec. Braced battery racks, stiffened vertical control panels,
benchboards, electrical cabinets with top bracing, and wall-mounted panelboards have fundamental
periods ranging from 0.06 to 0.1 sec.
6.3.4 MECHANICAL AND ELECTRICAL COMPONENT ATTACHMENTS: For some
items such as piping, relative seismic displacements between support points generally are of more
significance than inertial forces. Components made of ductile materials such as steel or copper can
accommodate relative displacement effects by inelastically conforming to the supports' conditions.
However, components made of less ductile materials can only accommodate relative displacement
effects by providing flexibility or flexible connections.
Of most concern are distribution systems that are a significant life-safety hazard and are routed
between two separate building structures. Ductile components with bends and elbows at the building
separation point or components that will be subject to bending stresses rather than direct tensile loads
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due to differential support motion, are not so prone to damage and are not so likely to fracture and fall.
This is valid if the supports can accommodate the imposed loads.
6.3.5 COMPONENT SUPPORTS: It is the intent of these requirements to ensure that all
mechanical and electrical component supports, the means by which a component transfers seismic
loads to the structure, be designed to accommodate the force and displacement effects prescribed.
Component supports are differentiated here from component attachments to emphasize that the
supports themselves, the structural members, braces, frames, skirts, legs, saddles, pedestals, cables,
guys, stays, snubbers, and tethers, even if fabricated with and/or by the mechanical or electrical
component manufacturer, should be designed for seismic forces. This is regardless of whether the
mechanical or electrical component itself is designed for seismic loads. The intention is to prevent a
component from sliding, falling, toppling, or otherwise moving such that the component would imperil
life.
6.3.6 COMPONENT CERTIFICATION: It is intended that the certificate only be requested for
components with an importance factor (Ip) greater than 1.00 and only if the component has a doubtful
or uncertain seismic load path. This certificate should not be requested to validate functionality
concerns.
In the context of the Provisions, seismic adequacy of the component is of concern only when the
component is required to remain operational after an earthquake or contains material that can pose a
significant hazard if released. Meeting the requirements of this section shall be considered as an
acceptable demonstration of the seismic adequacy of a component.
6.3.7 UTILITY AND SERVICE LINES AT STRUCTURE INTERFACES: For essential
facilities, auxiliary on-site mechanical and electrical utility sources are recommended. It is
recommended that an appropriate clause be included if existingcodes for the jurisdiction do not
presently provide for it.
Sec. 6.3.7 requires that adequate flexibility be provided for utilities at the interface of adjacent and
independent structures to accommodate anticipated differential displacement. It affects architectural
and mechanical/electrical fittings only where water and energy lines pass through the interface. The
displacements considered must include the Cd factor of Sec. 5.2.2 and should be in accordance with
Provisions Sec. 6.1.4.
Consideration may be necessary for nonessential piping carrying quantities of materials that could, if
the piping is ruptured, damage essential utilities.
Following a review of information from the Northridge and Loma Prieta earthquakes and discussions
with gas company personnel, automatic earthquake shutoff of gas lines at structure entry points is no
longer required. The primary justification for this is the consensus opinion that shutoff devices tend to
cause more problems than they solve. Commercially available shutoff devices tend to be susceptible to
inadvertent shutoff caused by passing vehicles and other non-seismic vibrations. This leads to
disruption of service and often requires that local gas companies reset the device and relight any pilot
lights. In an earthquake, the majority of shutoff devices which actuate will be attached to undamaged
gas lines. This results in a huge relight effort for the local utility at a time when resources are typically
at a premium. If the earthquake occurs during the winter, a greater life hazard may exist from a lack of
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gas supply than from potential gas leaks. In the future, as shutoff devices improve and gas-fired
appliances which use pilots are phased out, it may be justified to require shutoff devices.
This is not meant to discourage individuals and companies from installing shutoff devices. In
particular, individuals and companies who are capable of relighting gas fired equipment should
seriously consider installation of these devices. In addition, gas valves should be closed whenever
leaks are detected.
6.3.9 STORAGE TANKS: Storage tanks are considered nonbuilding structures and are covered in
Provisions Chapter 14. See Commentary Sec. 14.4.3.
6.3.10 HVAC DUCTWORK: Experience in past earthquakes has shown that, in general, HVAC
duct systemsare rugged and perform well in strong shaking motions. Bracing in accordance with the
Sheet Metal and Air Conditioning Contractors National Association (SMACNA)Ref. 6-17, 6-18, and
6-19 has been shown to be effective in limiting damage to duct systems under earthquake loads.
Typical failures have affected system function only and major damage or collapse has been uncommon.
Therefore, industry standard practices should prove adequate for most installations. Expected
earthquake damage should be limited to opening of the duct joints and tears in the ducts. Connection
details that are prone to brittle failures, especially hanger rods subject tolarge amplitude bending stress
cycles, should be avoided.
Some ductwork systems carry hazardous materials or must remain operational during and after an
earthquake. These ductwork system would be designated as having an Ipgreater than 1.0. A detailed
engineering analysis for these systems should be performed.
All equipment (e.g., fans, humidifiers, and heat exchangers) attached to the ducts and weighing more
than 75 lb (334 N) should be braced independently of the duct. Unbraced in-line equipment can
damage the duct by swinging and impacting it during an earthquake. Items (e.g., dampers, louvers,
and air diffusers) attached to the duct should be positively supported by mechanical fasteners (not
friction-type connections) to prevent their falling during an earthquake.
Where it is desirable to limit the deflection of duct systems under seismic load, bracing in accordance
with the SMACNA referenceslisted in Sec. 6.1.1 may be used.
6.3.11 PIPING SYSTEMS: Experience in past earthquakes has shown that, in general, piping
systems are rugged and perform well in strong shaking motions. Numerous standards and guidelines
have been developed covering a wide variety of piping systems and materials. Construction in
accordance with currentrequirements of the referenced national standardshave been shown to be
effective in limiting damage to and avoiding loss of fluid containment in piping systems under
earthquake conditions. It is therefore the intention of the Provisions that nationally recognized
standards be used to design piping systems provided that the force and displacement demand is equal
to or exceeds the requirements of Sec. 6.1.3 and 6.1.4 and provisions are made to mitigate seismic
interaction issues not normally addressed in the national standards.
The following industry standards, while not adopted by ANSI, are in common use and may be
appropriate reference documents for use in the seismic design of piping systems.
SMACNA
Guidelines for the Seismic Restraint of Mechanical Systems
ASHRAE CH 50-95 Seismic Restraint Design
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Piping, as used herein, are assemblies of pipe, tubing, valves, fittings, and other in-line fluid containing
components, excluding their attachments and supports.
6.3.12 BOILERS AND PRESSURE VESSELS: Experience in past earthquakes has shown that, in
general, boilers and pressure vessels are rugged and perform well in strong shaking motions.
Construction in accordance with current requirements of the ASME Boiler and Pressure Vessel Code
(Ref. 6-2) has been shown to be effective in limiting damage to and avoiding loss of fluid containment
in boilers and pressure vessels under earthquake conditions. It is therefore the intention of the
Provisions that nationally recognized codes be used to design boilers and pressure vessels provided
that the seismic force and displacement demand is equal to or exceeds the requirements of Sec. 6.1.3
and 6.1.4. Until such nationally recognized codes incorporate force and displacement requirements
comparable to the requirements of Sec. 6.1.3 and 6.1.4, it is nonetheless the intention to use the design
acceptance criteria and construction practices of those codes.
Boilers and pressure vessels as used herein are fired or unfired containments, including their internal
and external appurtenances and internal assemblies of pipe, tubing, and fittings, and other fluid
containing components, excluding their attachments and supports.
6.3.13 MECHANICAL EQUIPMENT ATTACHMENTS AND SUPPORTS: Past earthquakes
have demonstrated that most mechanical equipment is inherently rugged and performs well provided
that it is properly attached to the structure. This is because the design of mechanical equipment items
for operational and transportation loads typically envelopes loads due to earthquake. For this reason,
the requirements primarily focus on equipment anchorage and attachments. It was felt, however, that
mechanical equipment components required to maintain containment of flammable or hazardous
materials should themselves be designed for seismic forces.
In addition, thereliability of equipment operability after an earthquake can be increased if the following
items are also considered in design:
a. Internal assemblies are attached with a sufficiency that eliminates the potential of impact with other
internal assemblies and the equipment wall; and
b. Operators, motors, generators, and other such components functionally attached mechanical
equipment by means of an operating shaft or mechanism are structurally connected or commonly
supported with sufficient rigidity such that binding of the operating shaft will be avoided.
6.3.14 ELECTRICAL EQUIPMENT ATTACHMENTS AND SUPPORTS: Past earthquakes
have demonstrated that most electrical equipment is inherently rugged and performs well provided that
it is properly attached to the structure. This is because the design of electrical equipment items for
operational and transportation loads typically envelopes loads due to earthquake. For this reason, the
requirements primarily focus on equipment anchorage and attachments. However, reliability of
equipment operability after an earthquake can be increased if the following items also are considered in
design:
a. Internal assemblies are attached with a sufficiency that electrical subassemblies and contacts will
not be subject to differential movement or impact between the assemblies, contacts, and the
equipment enclosure.
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b. Any ceramic or other nonductile components in the seismic load path should be specifically
evaluated.
c. Adjacent electrical cabinets are bolted together and cabinet lineups are prevented from banging into
adjacent structural members.
6.3.15 ALTERNATE SEISMIC QUALIFICATION METHODS: Testing is a well established
alternative method of seismic qualification for small to medium size equipment. Several national
standards, other than IEEE 344 (Ref. 6-12), have testing requirements adaptable for seismic
qualification.
6.3.16 ELEVATOR DESIGN REQUIREMENTS: The ASME Safety Code for Elevators and
Escalators (Ref. 6-1) has adopted many requirements to improve the seismic response of elevators;
however, they do not apply to some regions covered by this chapter. These changes are to extend
force requirements for elevators to be consistent with the Provisions.
6.3.16.2 Elevator Machinery and Controller Supports and Attachments: The ASME Safety
Code for Elevators and Escalators (Ref. 6-1) has no seismic requirements for supports and
attachments for some structures and zones where the Provisions are applicable. Criteria are provided
to extend force requirements for elevators to be consistent with the intent and scope of the Provisions.
6.3.16.3 Seismic Controls: The purpose of the seismic switch as used here is different from that
provided under the ASME Safety Code for Elevators and Escalators (Ref. 6-3), which has
incorporated several requirements to improve the seismic response of elevators (e.g., rope snag point
guard, rope retainer guards, guide rail brackets) that do not apply to some buildings and zones covered
by the Provisions. Building motions that are expected in these uncovered seismic zones are sufficiently
large to impair the operation of elevators. The seismic switch is positioned high in the structure where
structural response will be the most severe. The seismic switch trigger level is set to shut down the
elevator when structural motions are expected to impair elevator operations.
Elevators in which the seismic switch and counterweight derail device have triggered should not be put
back into service without a complete inspection. However, in the case where the loss of use of the
elevator creates a life-safety hazard, an attempt to put the elevator back into service may be attempted.
Operating the elevator prior to inspection may cause severe damage to the elevator or its components.
The building owner should have detailed written procedures in place directing the elevator
operator/maintenance personnel which elevators in the facility are necessary from a post-earthquake life
safety perspective. It is highly recommended that theseprocedures be in-place, with appropriate
personnel training prior to an event strong enough to trip the seismic switch.
Once the elevator seismic switch is reset, it will respond to any call at any floor. It is important that the
detailed procedure include the posting of "out-of-service for testing" signs at each door at each floor,
prior to resetting the switch. Once the testing is completed, and the elevator operator/maintenance
personnel are satisfied that the elevator is safe to operate, the signs can be removed.
6.3.16.4 Retainer Plates: The use of retainer plates is a very low cost provision to improve the
seismic response of elevators.
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RELATED CONCERNS:
Maintenance: Mechanical and electrical devices installed to satisfy the requirements of the Provisions
(e.g., resilient mounting components or certain protecting devices) require maintenance to ensure their
reliability and provide the protection in case of a seismic event for which they are designed.
Specifically, rubber-in-shear mounts or spring mounts (if exposed to weathering) may deteriorate with
time and, thus, periodic testing is required to ensure that their damping action will be available during
an earthquake. Pneumatic mounting devices and electric switchgear must be maintained free of dirt
and corrosion. How a regulatory agency could administer such periodic inspections was not determined and, hence, requirements to cover this situation have not been included.
Tenant Improvements: It is intended that the requirements in Chapter 6 also apply to newly
constructed tenant improvements that are listed in Tables 6.2.2 and 6.3.2 and that are installed at any
time during the life of the structure.
Minimum Standards: Criteria represented in the Provisions represent minimum standards. They are
designed to minimize hazard for occupants and to improve the likelihood of functioning of facilities required by the community to deal with the consequences of a disaster. They are not designed to protect
the owner's investment, and the designer of the facility should review with the owner the possibility of
exceeding these minimum standards so as to limit his economic risk.
The risk is particularly acute in the case of sealed, air-conditioned structures where downtime after a
disaster can be materially affected by the availability of parts and labor. The parts availability may be
significantly worse than normal because of a sudden increase in demand. Skilled labor also may be in
short demand since available labor forces may be diverted to high priority structures requiring repairs.
Architect-Engineer Design Integration: The subject of architect-engineer design integration is being
raised because it is believed that all members of the profession should clearly understand that Chapter 6
is a compromise based on concerns for enforcement and the need to develop a simple, straightforward
approach. It is imperative that from the outset architectural input concerning definition of occupancy
classification and the required level of seismic resistance be properly integrated with the approach of
the structural engineer to seismic safety if the design profession as a whole is to make any meaningful
impact on the public conscience in this issue. Accordingly, considerable effort was spent in this area of
concern. It is hoped that as the design profession gains more knowledge and sophistication in the use
of seismic design, it will collectively be able to develop a more comprehensive approach to earthquake
design requirements.
REFERENCES
ANCO Engineers, Inc. 1983. Seismic Hazard Assessment of Non-Structural Components -- Phase I,
Final Report for National Science Foundation from ANCO Engineers, Inc., Culver City, California,
September.
Bachman, R. E, and R. M. Drake. 1996. A Study To Empirically Validate the Component Response
Modification Factors in the 1994 NEHRP Provisions Design Force Equations for Architectural,
Mechanical, and Electrical Components, letter report to the National Center for Earthquake
Engineering Research, July.
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1997 Commentary, Chapter 6
Bachman, R. E., R. M. Drake, and P. J. Richter. 1993. 1994 Update to 1991 NEHRP Provisions for
Architectural, Mechanical, and Electrical Components and Systems, letter report to the National
Center for Earthquake Engineering Research, February 22.
Drake, R. M., and R. E. Bachman. 1996. “NEHRP Provisions for 1994 for Nonstructural
Components,” ASCE Journal of Architectural Engineering, March.
Drake, R. M., and R. E. Bachman. 1995. “Interpretation of Instrumented Building Seismic Data and
Implications for Building Codes,” in Proceedings of the 1995 SEAoC Annual Convention.
Drake, R. M., and R. E. Bachman. 1994. “1994 NEHRP Provisions for Architectural Mechanical,
and Electrical Components,” in Proceedings, 5th United States National Conference on Earthquake
Engineering.
Housner, G. W., and M. A. Haroun. 1980. "Seismic Design of Liquid Storage Tanks" in ASCE
Convention Proceedings.
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Chapter 7 Commentary
FOUNDATION DESIGN REQUIREMENTS
7.1 GENERAL: The minimum foundation design requirements that might be suitable when any
consideration must be given to earthquake resistance are set forth in Chapter 7. It is difficult to
separate foundation requirements for minimal earthquake resistance from the requirements for
resisting normal vertical loads. In order to have a minimum base from which to start, this chapter
assumes compliance with all basic requirements necessary to provide support for vertical loads
and lateral loads other than earthquake. These basic requirements include, but are not limited to,
provisions for the extent of investigation needed to establish criteria for fills, slope stability,
expansive soils, allowable soil pressures, footings for specialized construction, drainage, settlement control, and pile requirements and capacities. Certain detail requirements and the allowable
stresses to be used are provided in other chapters of the Provisions as are the additional requirements to be used in more seismically active locations.
7.2 STRENGTH OF COMPONENTS AND FOUNDATIONS: The resisting capacities of
the foundations must meet the provisions of Chapter 7.
7.2.1 Structural Materials: The strength of foundation components subjected to seismic forces
alone or in combination with other prescribed loads and their detailing requirements must be as
determined in Chapters 8, 9, 10, 11, or 12.
7.2.2 Soil Capacities: This section requires that the building foundation without seismic forces
applied must be adequate to support the building gravity load. When seismic effects are considered, the soil capacities can be increased considering the short time of loading and the dynamic
properties of the soil.
7.3 SEISMIC DESIGN CATEGORIES A AND B: There are no special seismic provisions
for the design of foundations for buildings assigned to Categories A and B.
7.4 SEISMIC DESIGN CATEGORY C: Extra precautions are required for the seismic design
of foundations for buildings assigned to Category C.
7.4.1 Investigation: Potential site hazards such as fault rupture, liquefaction, ground deformation, and slope instability should be investigated when the size and importance of the project so
warrants. In this section, procedures for evaluating these hazards are reviewed.
Surface Fault Rupture: Fault ruptures during past earthquakes have led to large surface
displacements that are potentially destructive to engineered construction. Displacements, which
range from a fraction of an inch to tens of feet, generally occur along traces of previously active
faults. The sense of displacement ranges from horizontal strike-slip to vertical dip-slip to many
combinations of these components. The following commentary summarizes procedures to follow
or consider when assessing the hazard of surface fault rupture. This commentary is based in large
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part on Appendix C of California Division of Mines and Geology (CDMG) Special Publication
42, 1988 Revision (Hart, 1988).
Assessment of Surface Faulting Hazard: The evaluation of fault hazard at a given site is based
extensively on the concepts of recency and recurrence of faulting along existing faults. The
magnitude, sense, and frequency of fault rupture vary for different faults or even along different
segments of the same fault. Even so, future faulting generally is expected to recur along preexisting faults. The development of a new fault or reactivation of a long inactive fault is relatively
uncommon and generally need not be a concern. For most engineering applications, a sufficient
definition of an active fault is given in CDMG Special Publication 42 (Hart, 1988): "An active
fault has had displacement in Holocene time (last 11,000 years)."
As a practical matter, fault investigations should be conducted by qualified geologists and directed
at the problem of locating faults and evaluating recency of activity, fault length, and the amount
and character of past displacements. Identification and characterization studies should incorporate evaluation of regional fault patterns as well as detailed study of fault features at and in the
near vicinity (within a few hundred yards to a mile) of the site. Detailed studies should include
trenching to accurately locate, document, and date fault features.
Suggested Approach for Assessing Surface Faulting Hazard: The following approach should be
used, or at least considered, in fault hazard assessment. Some of the investigative methods
outlined below should be carried out beyond the site being investigated. However, it is not
expected that all of the following methods would be used in a single investigation:
1. A review should be made of the published and unpublished geologic literature from the region
along with records concerning geologic units, faults, ground-water barriers, etc.
2. A stereoscopic study of aerial photographs and other remotely sensed images should be made
to detect fault-related topography, vegetation and soil contrasts, and other lineaments of
possible fault origin. Predevelopment air photos are essential to the detection of fault
features.
3. A field reconnaissance study generally is required which includes observation and mapping of
geologic and soil units and structures, geomorphic features, springs, and deformation of manmade structures due to fault creep. This study should be detailed within the site with less
detailed reconnaissance of an area within a mile or so of the site.
4. Subsurface investigations usually are needed to evaluate fault features. These investigations
include trenches, pits, or bore holes to permit detailed and direct observation of geologic units
and fault features.
5. The geometry of fault structures may be further defined by geophysical investigations
including seismic refraction, seismic reflection, gravity, magnetic intensity, resistivity, ground
penetrating radar, etc. These indirect methods require a knowledge of specific geologic
conditions for reliable interpretation. Geophysical methods alone never prove the absence of
a fault and they do not identify the recency of activity.
6. More sophisticated and more costly studies may provide valuable data where geological
special conditions exist or where requirements for critical structures demand a more intensive
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Foundation Design Requirements
investigation. These methods might involve repeated geodetic surveys, strain measurements,
or monitoring of microseismicity and radiometric analysis (14C, K-Ar), stratigraphic correlation (fossils, mineralology) soil profile development, paleomagnetism (magnetostratigraphy),
or other age-dating techniques to date the age of faulted or unfaulted units or surfaces.
The following information should be developed to provide documented support for conclusions
relative to location and magnitude of faulting hazards:
1. Maps should be prepared showing the existence (or absence) and location of hazardous faults
on or near the site.
2. The type, amount, and sense of displacement of past surface faulting episodes should be
documented including sense and magnitude of displacement, if possible.
3. From this documentation, estimates can be made, preferably from measurements of past
surface faulting events at the site, using the premise that the general pattern of past activity
will repeat in the future. Estimates also may be made from empirical correlations between
fault displacement and fault length or earthquake magnitude published by Bonilla et al. (1984)
or by Slemmons et al. (1989). Where fault segment length and sense of displacement are
defined, these correlations may provide an estimate of future fault displacement (either the
maximum or the average to be expected).
There are no codified procedures for estimating the amount or probability of future fault
displacements. Estimates may be made, however, by qualified earth scientists. Because
techniques for making these estimates are not standardized, peer review of reports is useful to
verify the adequacy of the methods used and the estimates reports, to aid the evaluation by the
permitting agency, and to facilitate discussion between specialists that could lead to the development of standards.
The following guidelines are given for safe siting of engineered construction in areas crossed by
active faults:
1. Where ordinances have been developed that specify safe setback distances from traces of
active faults or active fault zones, those distances must be complied with and accepted as the
minimum for safe siting of buildings. For example, the general setback requirement in
California is a minimum of 50 feet from a well-defined zone containing the traces of an active
fault. That setback distance is mandated as a minimum for structures near faults unless a sitespecific special geologic investigation shows that a lesser distance could be safety applied
(California Administrative Code, Title 14, Sec. 3603A).
2. In general, safe setback distances may be determined from geologic studies and analyses as
noted above. Setback requirements for a site should be developed by the site engineers and
geologists in consultation with professionals from the building and planning departments of
the jurisdiction involved. Where sufficient geologic data have been developed to accurately
locate the zone containing active fault traces and the zone is not complex, a 50-foot setback
distance may be specified. For complex fault zones, greater setback distances may be
required. Dip-slip faults, with either normal or reverse motion, typically produce multiple
fractures within rather wide and irregular fault zones. These zones generally are confined to
the hanging-wall side of the fault leaving the footwall side little disturbed. Setback require-
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1997Commentary, Chapter 7
ments for such faults may be rather narrow on the footwall side, depending on the quality of
the data available, and larger on the hanging wall side of the zone. Some fault zones may
contain broad deformational features such as pressure ridges and sags rather than clearly
defined fault scarps or shear zones. Nonessential structures may be sited in these zones
provided structural mitigative measures are applied as noted below. Studies by qualified
geologists and engineers are required for such zones to assure that building foundations can
withstand probable ground deformations in such zones.
Mitigation of Surface Faulting Hazards: There is no mitigative technology that can be used to
prevent fault rupture from occurring. Thus, sites with unacceptable faulting hazard must either be
avoided or structures designed to withstand ground deformation or surface fault rupture. In
general practice, it is economically impractical to design a structure to withstand more than a few
inches of fault displacement. Some buildings with strong foundations, however, have successfully
withstood or diverted a few inches of surface fault rupture without damage to the structure
(Youd, 1989). Well reinforced mat foundations and strongly inter-tied footings have been most
effective. In general, less damage has been inflicted by compressional or shear displacement than
by vertical or extensional displacements.
Liquefaction: Liquefaction of saturated granular soils has been a major source of building
damage during past earthquakes. For example, many structures in Niigata, Japan, suffered major
damage as a consequence of liquefaction during the 1964 earthquake. Loss of bearing strength,
differential settlement, and differential horizontal displacement due to lateral spread were the
direct causes of damage. Many structures have been similarly damaged by differential ground
displacements during U.S. earthquakes such as the San Fernando Valley Juvenile Hall during the
1971 San Fernando, California, earthquake and the Marine Sciences Laboratory at Moss Landing,
California, during the 1989 Loma Prieta event. Design to prevent damage due to liquefaction
consists of three parts: evaluation of liquefaction hazard, evaluation of potential ground
displacement, and mitigating the hazard by designing to resist ground displacement, by reducing
the potential for liquefaction, or by choosing an alternative site with less hazard.
Evaluation of Liquefaction Hazard: Liquefaction hazard at a site is commonly expressed in terms
of a factor of safety. This factor is defined as the ratio between the available liquefaction
resistance, expressed in terms of the cyclic stresses required to cause liquefaction, and the cyclic
stresses generated by the design earthquake. Both of these stress parameters are commonly
normalized with respect to the effective overburden stress at the depth in question.
The following possible methods for calculating the factor of safety against liquefaction have been
proposed and used to various extents:
1. Analytical Methods -- These methods typically rely on laboratory test results to determine
either liquefaction resistance or soil properties that can be used to predict the development of
liquefaction. Various equivalent linear and nonlinear computer methods are used with the
laboratory data to evaluate the potential for liquefaction. Because of the considerable
difficulty in obtaining undisturbed samples of liquefiable sediment for laboratory evaluation of
constitutive soil properties, the use of analytical methods, which rely on accurate constitutive
properties, usually are limited to critical projects or to research.
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Foundation Design Requirements
2. Physical Modeling -- These methods typically involve the use of centrifuges or shaking tables
to simulate seismic loading under well defined boundary conditions. Soil used in the model is
reconstituted to represent different density and geometrical conditions. Because of difficulties
in precisely modeling in-situ conditions at liquefiable sites, physical models have seldom been
used in design studies for specific sites. However, physical models are valuable for analyzing
and understanding generalized soil behavior and for evaluating the validity of constitutive
models under well defined boundary conditions.
3. Empirical Procedures -- Because of the difficulties in analytically or physically modeling soil
conditions at liquefiable sites, empirical methods have become a standard procedure for
determining liquefaction susceptibility in engineering practice. Procedures for carrying out a
liquefaction assessment using the empirical method are given by the National Research
Council (1985).
For most empirical methods, the average earthquake-induced cyclic shear stress is estimated from
a simple equation or from dynamic response analyses using computer programs such as SHAKE
and DESRA. The induced cyclic shear stress is estimated from the peak horizontal acceleration
expected at the site using the following simple equation:
J
)
Fo
' 0.65
amax
Fo
g
Fo
)
rd
(C7.4.1-1)
where (amax/g) = peak horizontal acceleration at ground surface expressed as a decimal fraction of
gravity, Fo = the vertical total stress in the
soil at the depth in question, FoN = the vertical effective stress at the same depth, and
rd = deformation-related stress reduction
factor.
The chart reproduced in Figure C7.4.1-1
is used to estimate rd.
To determine liquefaction resistance of
sandy soils, the induced cyclic stress ratio
computed from Eq. C7.4.1-1 is compared
to the cyclic stress ratio required to generate liquefaction in the soil in question for a
given earthquake of magnitude M. The
most common technique for estimating
liquefaction resistance is from an empirical
relationship between cyclic stress ratio
required to cause liquefaction and normalized blow count, (N1)60.
The most commonly used empirical relationship, compiled by Seed et al. (1985),
FIGURE C7.4.1-1 Range of values for rd for different soil
properties (after Seed and Idriss, 1971).
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1997Commentary, Chapter 7
compares (N1)60 from sites where liquefaction did or did not develop during past earthquakes.
Figure C7.4.1-2 shows the most recent (1988) version of this relationship for M = 7-1/2 earthquakes. On that figure, cyclic stress ratios calculated for various sites are plotted against (N1)60.
Solid dots represent sites where liquefaction occurred and open dots represent sites where surface
evidence of liquefaction was not found. Curves were drawn through the data to separate regions
where liquefaction did and did not develop. Curves are given for sediments with various fines
contents.
FIGURE C7.4.1-2 Relationship between stress ratios causing liquefaction and N1 values
for silty sands for M = 7-1/2 earthquakes.
Although the curves drawn by Seed et al. (1985) envelop the plotted data, it is possible that
liquefaction may have occurred beyond the enveloped data and was not detected at ground
surface. Consequently, a factor of safety of 1.2 to 1.5 is appropriate in engineering design. The
factor to be used is based on engineering judgment with appropriate consideration given to type
and importance of structure and potential for ground deformation.
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Foundation Design Requirements
The maximum acceleration, amax, commonly used in liquefaction analysis is that which would
occur at the site in the absence of liquefaction. Thus, the amax used in Eq. C7.4.1-1 is the
estimated rock acceleration corrected for soil site response but with neglect of excess pore-water
pressures that might develop. Alternatives for obtaining amax are: (1) from standard peak
acceleration attenuation curves valid for comparable soil conditions; (2) from standard peak
acceleration attenuation curves for rock, corrected for site amplification or deamplification by
means of standard amplification curves or computerized site response analysis such as described
in the "Chapter 1 Commentary" for Sec. 1.4.2; (3) obtaining first the value of effective peak
acceleration, Aa, for rock depending on the map area where the site is located and then multiplying this value by a factor between 1 and 3 as discussed in the "Chapter 1 Commentary" for Sec.
1.4.2 to determine amax; (4) from probabilistic maps of amax with or without correction for site
amplification or deamplification depending on the rock or soil conditions used to generate the
map.
The magnitude, M, needed to determine a
magnitude scaling factor from Figure
C7.4.1-3 should correspond to the size of
the design or expected earthquake selected for the liquefaction evaluation. If
Alternative 3 or 4 is selected, the
definition of M is not obvious and
additional studies and considerations are
necessary. In all cases, it should be
remembered that the likelihood of
liquefaction at the site (as defined later by
the factor of safety FL in Eq. C7.4.1-3) is
determined jointly by amax and M.
Because of the longer duration of strong
ground-shaking, large distant earthquakes FIGURE C7.4.1-3 Representative relationship between T/T1
and number of cycles required to cause liquefaction (after
may generate liquefaction at a site while
Seed et al., 1983).
smaller nearby earthquakes may not
generate liquefaction even though amax of
the nearer events is larger than that from the more distant events.
The corrected blow count, (N1)60, required for evaluation of soil liquefaction resistance is
commonly determined from measured standard penetration resistance, Nm, but may also be
determined from cone penetration test (CPT) data using standard correlations to estimate Nm
values from the CPT measurements. The corrected blow count is calculated from Nm as follows:
(N1)60 ' Cn
ERm
60
(C7.4.1-2)
Nm
where Cn = a factor that corrects Nm to an effective overburden pressure of 1 tsf and ERm = the
rod energy ratio for the type of hammer and release mechanism used in the measurement of Nm.
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1997Commentary, Chapter 7
The curve plotted in Figure C7.4.1-4 is
typically used to evaluate Cn. Measured
hammer energies or estimates of hammer
energies from tabulations such as those in
Table C7.4.1 are used to define ERm. An
additional correction should be made to
(N1)60 for shallow soil layers where the
length of drilling rod is 10 feet or less. In
those instances, (N1)60 should be reduced
by multiplying by a factor of 0.75 to account for poor hammer-energy transfer in
such short rod lengths.
Because a variety of equipment and procedures are used to conduct standard penetration tests in present practice and because the measured blow count, Nm, is
sensitive to the equipment and procedures
used, the following commentary and guidance with respect to this test is given.
FIGURE C7.4.1-4 Chart for Cn (after Seed et al., 1985).
Special attention must be paid to the determination of normalized blow count,
(N1)60, used in Figure C7.4.1-2. When developing the empirical relation between blow count and
liquefaction resistance, Seed and his colleagues recognized that the blow count from SPT is
greatly influenced by factors such as the method of drilling, the type of hammer, the sampler
design, and the type of mechanism used for lifting and dropping the hammer. The magnitude of
variations is shown by the data in Table C7.4.1.
TABLE C7.4.1 Summary of Rod Energy Ratios for Japanese SPT Procedures (after
Seed et al., 1985)
Study
Mechanical Trip System (Tonbi)
Rope and Pulley
80-90
76
80
76a
-78
63-72
-67
--67
Nishizawa et al.
Decker, Holtz, and Kovacs
Kovacs and Salomone
Tokimatsu and Yoshimi
Yoshimi and Tokimatsu, Yoshimi et al., Oh-Oka
Adopted for this study
a
Equivalent rod energy ratio if rope and pulley method is assumed to have an energy ratio of 67 percent and values
for mechanical trip method are different from this by a factor of 1.13.
In order to reduce variability in the measurement of N, Seed et al. (1983 and 1985) suggest the
following procedures and specifications for the SPT test for liquefaction investigations:
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Foundation Design Requirements
1. The impact should be delivered by a rope and drum system with two turns of the rope around
the rotating drum to lift a hammer weighing 140 lb or, more preferably, a drive system should
be used for which ERm has been measured or can be reliably estimated.
2. Use of a hole drilled with rotary equipment and filled with drilling mud. The hole should be
approximately 4 in. in diameter and drilled with a tricone or baffled drag bit that produces
upward deflection of the drilling fluid to prevent erosion of soil below the cutting edge of the
bit.
3. In holes less than 50 feet deep, A or AW rod should be used; N or NW rod should be used in
deeper holes.
4. The split spoon sampling tube should be equipped with liners or otherwise have a constant
internal diameter of 1-3/8 inch.
5. Application of blows should be at a rate of 30 to 40 blows per minutes. (Some engineers
suggest a slower rate of 20 to 30 blows per minute since it is easier to achieve and control
and gives comparable results.) The blow count, Nm, is determined by counting the blows
required to drive the penetrometer through the depth interval of 6 to 18 in. below the bottom
of the hole.
Failure to follow these standard guidelines introduces large uncertainties into liquefaction
estimates.
The curves in Figure C7.4.1-2 were developed from data for magnitude 7.5 earthquakes and are
only valid for earthquakes of that magnitude. For larger or smaller earthquakes, the cyclic stress
ratios determined from Figure C7.4.1-2 are corrected for magnitude by multiplying the determined cyclic stress ratio by a magnitude scaling factor taken from Figure C7.4.1-3. As the
magnitude increases, the scaling factor decreases. For example, for an (N1)60 of 20, a clean sand
(fines content < 5 percent) and an earthquake magnitude of 7.5, the CSRL determined from
Figure C7.4.1-2 is 0.22. For the same site conditions but for a magnitude 8.0 earthquake, a
CSRL of 0.20 is obtained after applying the magnitude scaling factor of 0.89 determined from
Figure C7.4.1-3.
Soils composed of sands, silts, and gravels are most susceptible to liquefaction while clayey soils
generally are immune to this phenomenon. The curves in Figure C7.4.1-2 are valid for soils
composed primarily of sand. The curves should be used with caution for soils with substantial
amounts of gravel. Verified corrections for gravel content have not been developed; a geotechnical engineer, experienced in liquefaction hazard evaluation, should be consulted when
gravelly soils are encountered. For soils containing more than 35 percent fines, the curve in
Figure C7.4.1-2 for 35 percent fines should be used provided the following criteria developed by
Seed et al. (1983) are met (i.e, the weight of soil particles finer than 0.005 mm is less than 15
percent of the dry weight of a specimen of the soil, the liquid limit of soil is less than 35 percent,
and the moisture content of the in-place soil is greater than 0.9 times the liquid limit.
In summary, the procedure for evaluation of liquefaction resistance for a site is as follows: First,
from a site investigation determine the measured standard penetration resistance, Nm, the percent
fines, the percent clay ( > 0.005 mm), the natural moisture content, and the liquid limit of the
sediment in question. Check the measured parameters against the fines content and moisture
161
1997Commentary, Chapter 7
criteria listed above to assure that the sediment is of a potentially liquefiable type. If so, correct
Nm to (N1)60 using Eq. C7.4.1-2 and use Figure C7.4.1-2 to determine the cyclic stress ratio
required to cause liquefaction for a magnitude 7.5 earthquake. Then correct that value using the
appropriate magnitude scaling factor. That product is the cyclic stress ratio required to cause
liquefaction in the field (CSRL). Next, calculate the cyclic stress ratio (CSRE) that would be
generated by the expected earthquake using Eq. C7.4.1-1. Then compute the factor of safety, FL,
against liquefaction from the equation:
FL '
CSRL
CSRE
(C7.4.1-3)
If FL is greater than one, then liquefaction should not develop. If at any depth in the sediment
profile, FL is equal to or less than one, then there is a liquefaction hazard. As noted above, a
factor of safety of 1.2 to 1.5 is appropriate for building sites with the factor selected depending on
the importance of the structure and the potential for ground displacement at the site.
Evaluation of Potential for Ground Displacements: Liquefaction by itself may or may not be of
engineering significance. Only when liquefaction is accompanied by loss of ground support
and/or ground deformation does this phenomenon become important to structural design. Loss of
bearing capacity, flow failure, lateral spread, ground oscillation, and ground settlement are ground
failure mechanisms that have caused structural damage during past earthquakes. These types of
ground failure are described by the National Research Council (1985). The type of failure and
amount of ground displacement are a function of several parameters including the thickness and
extent of the liquefied layer, the thickness of unliquefied material overlying the liquefied layer, the
ground slope, and the nearness of a free face. Criteria are given by Ishihara (1985) for evaluating
the influence of thickness of layers on surface manifestation of liquefaction effects (ground
fissures and sand boils) for level sites. These criteria may be used for noncritical or nonessential
structures on level sites. Additional analysis should be required for critical or essential structures.
Loss of Bearing Strength: Loss of bearing strength is not likely for light structures with shallow
footings founded on stable, nonliquefiable materials overlying deeply buried liquefiable layers,
particularly if the liquefiable layers are relatively thin. General guidance for how deep or how thin
the layers must be has not yet been developed. A geotechnical engineer, experienced in liquefaction hazard assessment, should be consulted to provide such guidance. Although loss of bearing
strength may not be a hazard for deeply buried liquefiable layers, liquefaction-induced ground
settlements or lateral-spread displacements could still cause damage and should be evaluated.
Ground Settlement: Tokimatsu and Seed (1987) published an empirical procedure for estimating
ground settlement. It is beyond the scope of this commentary to outline that procedure which,
although explicit, has several rather complex steps. For saturated or dry granular soils in a loose
condition, their analysis suggests that the amount of ground settlement could approach 3 to 4
percent of the thickness of the loose soil layer. The Tokimatsu and Seed technique is
recommended for estimating earthquake-induced ground settlement at sites underlain by granular
soils and can be applied whether liquefaction does or does not occur.
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Foundation Design Requirements
Horizontal Ground Displacement: Only primitive analytical and empirical techniques have been
developed to date to estimate ground displacement, and no single technique has been widely
accepted or verified for engineering design. Analytical techniques generally apply Newmark's
analysis of a rigid body sliding on an infinite or circular failure surface with ultimate shear
resistance estimated from the residual strength of the deforming soil. Alternatively, nonlinear
finite element methods have been used to predict deformations. Empirical procedures use
correlations between past ground displacement and site conditions under which those displacements occurred. The liquefaction severity index (LSI) correlation of Youd and Perkins (1987)
provides a conservative upper bound for displacement for most natural soils (Figure C7.4.1-5;
curves noted for various earthquakes are calculated from the equation on the figure). In this
procedure, maximum horizontal displacement of lateral spreads in late Holocene fluvial deposits
are correlated against earthquake magnitude and distance for the seismic source. The data are
from the western United States and the correlation is valid only for that region. Because
maximum displacements at very liquefiable sites were used in the LSI analysis, displacements
predicted by that technique are conservative in that they predict an upper bound displacement for
most natural deposits. Displacements may be greater, however, on uncompacted fill or extremely
loose natural deposits.
The ground motions to be primarily considered in evaluating liquefaction potential
are consistent with the design earthquake
motions used in structural design. The
structural design should be consistent with
liquefaction-induced deformations resulting from those ground motions.
Liquefaction-induced deformations are not
directly proportional to ground motions
and may be more than 50 percent higher
for maximum considered earthquake
ground motions. The liquefaction potential and resulting deformations for ground
motions consistent with the maximum
considered earthquake should also be
evaluated and, while not required in these
Provisions, should be used by the registered design professional in checking for
FIGURE 7.4.1-5 LSI from several western U.S. and Alaskan
earthquakes plotted against horizontal distance from seismic
building damage that may result in collapse. In addition, Seismic Use Group III energy sources (after Youd and Perkins, 1987).
structures should be designed to retain a
significant margin against collapse following liquefaction-induced deformations resulting from
maximum considered earthquake ground motions.
The following further information is given for general guidance for ground conditions and range
of displacements commonly associated with liquefaction-induced ground failures (National
Research Council, 1995; Barlett and Youd, 1995):
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1997Commentary, Chapter 7
1. Flow failures generally develop in loose saturated sands or silts on slopes greater than 3
degrees (5 percent) and may displace large masses of soil tens of meters. Standard limit
equilibrium slope stability analyses may be used to assess flow failure potential with the
residual strength used as the strength parameter in the analyses. The residual strength may be
determined from empirical correlations such as that published by Seed and Harder (1989).
2. Lateral spreads generally develop on gentle slopes between 0.5 and 3 degrees (0.1 and 5
percent) and may induce up to several feet of lateral displacement. Empirical correlations
have been developed by Bartlett and Youd (1995) to estimate lateral ground displacement
due to liquefaction. Analytical procedures using appropriately reduced (residual) strengths of
soils also are available to estimate displacements. These procedures range from simplified
Newmark-type sliding block methods (e.g., Newmark, 1985; Makdisi and Seed, 1978) to
more sophisticated finite element analyses. In general, the empirical correlations are simple to
apply, do not require data beyond the commonly compiled engineering site investigations, and
are usually adequate for routine engineering applications.
3. Ground oscillation occurs on nearly flat surfaces where the slope is too gentle to induce
permanent horizontal displacement. During an earthquake, however, ground oscillation
generates transient vertical or horizontal displacements that may range up to a few feet. For
example, ground oscillation caused the rather chaotic pattern of ground displacements that
offset pavements, thrust sidewalks over curbs, etc., in San Francisco's Marina District
following the 1989 Loma Prieta earthquake.
Mitigation of Liquefaction Hazard: With respect to liquefaction hazard, three mitigative
measures might be considered: design the structure to resist the hazard, stabilize the site to
reduce the hazard, or choose an alternative site. Structural measures that are used to reduce the
hazard include deep foundations, mat foundations, or footings interconnected with ties as
discussed in Sec. 7.4.3. Deep foundations have performed well at level sites of liquefaction where
effects were limited to ground settlement and ground oscillation with no more than a few inches
of lateral displacement. Deep foundations, such as piles, may receive very little soil support
through the liquefied layer and may be subjected to transient lateral displacements across the
layer. Well reinforced mat foundations also have performed well at localities where ground
displacements were less than 1 foot although releveling of the structure has been required in some
instances (Youd, 1989). Strong ties between footings also should provide increased resistance to
damage where differential ground displacements are less than a few inches.
Evaluations of structural performance following two recent Japanese earthquakes, 1993
Hokkaido Nansei-Oki (M = 8.2) and 1995 (Kobe) Hyogo-Ken Nanbu (M = 7.2), indicate that
small structures on shallow foundations performed well in liquefaction areas. Sand boil eruptions
and open ground fissures in these areas indicate minor effects of liquefaction, including ground
oscillation and up to several tenths of a meter of lateral spread displacement. Many small
structures (mostly houses, shops, schools, etc.) were structurally undamaged although a few tilted
slightly. Foundations for these structures consist of reinforced concrete perimeter wall footings
with reinforced concrete interior wall footings tied into the perimeter walls at intersections. These
foundations acted as diaphragms causing the soil to yield beneath the foundation which prevented
fracture of foundations and propagation of differential displacements into the superstructure.
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Foundation Design Requirements
FIGURE C7.4.1-5 Measured displacements plotted against predicted displacements for U.S. and
Japanese case-history data (after Bartlett and Youd, 1995).
Similarly, well reinforced foundations that would not fracture could be used in U.S. practice as a
mitigative measure to reduce structural damage in areas subject to liquefaction but with limited
potential for lateral (< 0.3 m) or vertical (< 0.05 m) ground displacements. Such strengthening
also would serve as an effective mitigation measure against damage from other sources of limited
ground displacement including fault zones, landslides, and cut fill boundaries. Where slab-ongrade or basement slabs are used as foundation elements, these slabs should be reinforced and tied
to the foundation walls to give the structure adequate strength to resist ground displacement.
Although strengthening of foundations, as noted above, would largely mitigate damage to the
structure, utility connections may be adversely affected unless special flexibility is built into these
nonstructural components.
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1997Commentary, Chapter 7
Another possible consequence of liquefaction to structures is increased lateral pressures against
basement walls. A common procedure used in design for such increased pressures is to assume
that the liquefied material acts as a dense fluid having a unit weight of the liquefied soil. The wall
then is designed assuming that hydrostatic pressure for the dense fluid acts along the total
subsurface height of the wall. The procedure applies equivalent horizontal earth pressures that
are greater than typical at-rest earth pressures but less than passive earth pressures. As a final
consideration, to prevent buoyant rise as a consequence of liquefaction, the total weight of the
structure should be greater than the volume of the basement or other cavity times the unit weight
of liquefied soil. (Note that structures with insufficient weight to counterbalance buoyant effects
could differentially rise during an earthquake.)
At sites where expected ground displacements are unacceptably large, ground modification to
lessen the liquefaction or ground failure hazard or selection of an alternative site may be required.
Techniques for ground stabilization to prevent liquefaction of potentially unstable soils include
removal and replacement of soil; compaction of soil in place using vibrations, heavy tamping,
compaction piles, or compaction grouting; buttressing; chemical stabilization with grout; and
installation of drains. Further explanation of these methods is given by the National Research
Council (1985).
Slope Instability: The stability of slopes composed of dense (nonliquefiable) or nonsaturated
sandy soils or nonsensitive clayey soils can be determined using standard procedures.
For initial evaluation, the pseudostatic analysis may be used. (The deformational analysis
described below, however, is now preferred.) In the pseudostatic analysis, inertial forces
generated by earthquake shaking are represented by an equivalent static horizontal force acting on
the slope. The seismic coefficient for this analysis should be the peak acceleration, amax, or Aa.
The factor of safety for a given seismic coefficient can be estimated by using traditional slope
stability calculation methods. A factor of safety greater than one indicates that the slope is stable
for the given lateral force level and further analysis is not required. A factor of safety of less than
one indicates that the slope will yield and slope deformation can be expected and a deformational
analysis should be made using the techniques discussed below.
Deformational analyses yielding estimates of slope displacement are now accepted practice. The
most common analysis uses the concept of a frictional block sliding on a sloping plane or arc. In
this analysis, seismic inertial forces are calculated using a time history of horizontal acceleration as
the input motion. Slope movement occurs when the driving forces (gravitational plus inertial)
exceed the resisting forces. This approach estimates the cumulative displacement of the sliding
mass by integrating increments of movement that occur during periods of time when the driving
forces exceed the resisting forces. Displacement or yield occurs when the earthquake ground
accelerations exceed the acceleration required to initiate slope movement or yield acceleration.
The yield acceleration depends primarily on the strength of the soil and the gradient and height
and other geometric attributes of the slope. See Figure C7.4.1-6 for forces and equations used in
analysis and Figure C7.4.1-7 for a schematic illustration for a calculation of the displacement of a
soil block toward a bluff.
The cumulative permanent displacement will depend on the yield acceleration as well as the
intensity and duration of ground-shaking. As a general guide, a ratio of yield acceleration to
166
Foundation Design Requirements
maximum acceleration of 0.5 will result in slope displacements of the order of a few inches for
typical magnitude 6.5 earthquakes and perhaps several feet of displacement for magnitude 8
earthquakes. Further guidance on slope displacement is given by Makdisi and Seed (1978).
Fda = drifing force due to active soil pressure
Fdi = driving force due to earthquake inertia
Frs = resisting force due to soil shear strength
Fdp = resisting force due to passive soil pressure
Fdi ' Kmax W
where Kmax = maximum seismic coefficient and
W = weight of soil block
Frs ' Su L
where Su = average undrained shear strength of
soil and L = length of soil block
Yield seismic coefficient:
F & Fda
K y & rs
W
FIGURE C7.4.1-7 Schematic illustration for calculating displacement of soil block toward the bluff
(National Research Council, 1985; from Idriss, 1985,
adapted from Goodman and Seed, 1966).
FIGURE C7.4.1-6 Forces and equations used in analysis
of translatory landslides for calculating permanent lateral
displacements from earthquake ground motions (National Research Council, 1985; from Idriss, 1985).
Mitigation of Slope Instability Hazard: With respect to slope instability, three general mitigative
measures might be considered: design the structure to resist the hazard, stabilize the site to
reduce the hazard, or choose an alternative site. Ground displacements generated by slope
instability are similar in destructive character to fault displacements generating similar senses of
movement: compression, shear, extension or vertical. Thus, the general comments on structural
design to prevent damage given under mitigation of fault displacement apply equally to slope
displacement. Techniques to stabilize a site include reducing the driving forces by grading and
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1997Commentary, Chapter 7
drainage of slopes and increasing the resisting forces by subsurface drainage, buttresses, ground
anchors, or chemical treatment.
7.4.2 POLE-TYPE STRUCTURES: The use of pole-type structures is permitted. These
structures are inherently sensitive to earthquake motions. Bending in the poles and the soil
capacity for lateral resistance of the portion of the pole embedded in the ground should be
considered and the design completed accordingly.
7.4.3 FOUNDATION TIES: One of the prerequisites of adequate performance of a building
during an earthquake is the provision of a foundation that acts as a unit and does not permit one
column or wall to move appreciably with respect to another. A common method used to attain
this is to provide ties between footings and pile caps. This is especially necessary where the
surface soils are soft enough to require the use of piles or caissons. Therefore, the pile caps or
caissons are tied together with nominal ties capable of carrying, in tension or compression, a force
equal to Ca/4 times the larger pile cap or column load.
A common practice in some multistory buildings is to have major columns that run the full height
of the building adjacent to smaller columns in the basement that support only the first floor slab.
The coefficient applies to the heaviest column load.
Alternate methods of tying foundations together are permitted (e.g., using a properly reinforced
floor slab that can take both tension and compression). Lateral soil pressure on pile caps is not a
recommended method because the motion is imparted from soil to structure (not inversely as is
commonly assumed), and if the soil is soft enough to require piles, little reliance can be placed on
soft-soil passive pressure to restrain relative displacement under dynamic conditions.
If piles are to support structures in the air or over water (e.g., in a wharf or pier), batter piles may
be required to provide stability or the piles may be required to provide bending capacity for lateral
stability. It is up to the foundation engineer to determine the fluidity or viscosity of the soil and
the point where lateral buckling support to the pile can be provided (i.e., the point where the flow
of the soil around the piles may be negligible).
7.4.4 SPECIAL PILE REQUIREMENTS: Special requirements for concrete or composite
concrete and steel piles are given in this section. The piles must be connected to the pile caps
with dowels.
Although unreinforced concrete piles are common used in certain areas of the country, their brittle
nature when trying to conform to ground deformations makes their use in earthquake-resistant
design undesirable. Nominal longitudinal reinforcing is specified to reduce this hazard. The
reinforcing steel should be extended into the footing to tie the elements together and to assist in
load transfer at the top of pile to the pile cap. Experience has shown that concrete piles tend to
hinge or shatter immediately below the pile cap so tie spacing is reduced in this area to better
contain the concrete. In the case of the metal-cased pile, it is assumed that the metal casing
provides containment and also a nominal amount of longitudinal reinforcement in the lower
portion of the pile.
Bending stresses in piles caused by transfer of seismic motions from ground to structure need not
be considered unless the foundation engineer determines that it is necessary. It has been a
convenient analytical assumption to assume that earthquake forces originate in the building and
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Foundation Design Requirements
are transmitted into and resisted by the ground. Actually the force or motion comes from the
ground--not the structure. This makes the necessity of interconnecting footings more important,
but what is desired is stability--not the introduction of forces.
Possibly the simplest illustration is shown in Figure C7.4.4. Consider a small structure subjected
to an external force such as wind; the piles must resist that force in lateral pressure on the lee side
of the piles. However, if the structure is forced to move during an earthquake, the wave motion is
transmitted through the firmer soils, causing the looser soils at the surface and the building to
move. For most structures, the structure weight is negligible in comparison to the weight of the
surrounding surface soils. If an unloaded pile were placed in the soil, it would be forced to bend
similar to a pile supporting a building.
The primary requirement is stability, and this is best provided by piles that can support their loads
while still conforming to the ground motions and, hence, the need for ductility.
7.5 SEISMIC DESIGN CATEGORIES D , E, AND F: For Category D, E, or F construction,
all the preceding provisions for Categories A, B, and C apply for the foundations, but the earthquake detailing is more severe and demanding. Adequate pile ductility is required and provision
must be made for additional reinforcing to ensure, as a minimum, full ductility in the upper portion
of the pile.
7.5.1 INVESTIGATION: In addition to the potential site hazard discussed in Provisions Sec.
7.4.1, consideration of lateral pressures on earth retaining structures shall be included in investigations for Seismic Design Categories D, E, and F.
Earth Retaining Structures: Increased lateral pressures on retaining structures during earthquakes have long been recognized; however, design procedures have not been prescribed in U.S.
model building codes. Waterfront structures often have performed poorly in major earthquake
due to excess pore water pressure and liquefaction conditions developing in relatively loose,
saturated granular soils. Damage reports for structures away from waterfronts are generally
limited with only a few cases of stability failures or large permanent movements (Whitman, 1991).
Due to the apparent conservatism or overstrength in static design of most walls, the complexity of
nonlinear dynamic soil-structure interaction, and the poor understanding of the behavior of
retaining structures with cohesive or dense granular soils, Whitman (1991) recommends that
“engineers must rely primarily on a sound understanding of fundamental principles and of general
patterns of behavior.”
For many years, recommendations by Seed and Whitman (1970) have been used widely in design
of earth retaining structures for dynamic loads. In this reference, a simplified Mononobe-Okabe
seismic coefficient method of analysis was proposed for design practice. At a 1990 Cornell
University conference, Whitman (1990) indicated that “the Mononobe-Okabe equation for earth
pressure is still used widely for design, although actual conditions during earthquake shaking of
retaining structures are quite different from those assumed in developing the equation.” More
recently, the design approach based on a permissible displacement (Richards and Elms, 1979) has
received wider acceptance. More recent research studies are included in Prakash (1996).
The Mononobe-Okabe method assumes that a wall yields for the backfill to experience an active
limiting condition during ground shaking (Seed and Whitman, 1970; Prakash, 1981). If a wall
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1997Commentary, Chapter 7
FIGURE C7.4.4 Response to earthquake.
moves rigidly with the underlying base (i.e., nonyielding wall), it is reasonable to expect dynamic
pressures larger than the Mononobe-Okabe active case For the nonyielding wall condition using
elastic theory, Wood (1973) provided solutions for material properties being constant with depth.
In a parallel model test investigation of this case, Yong (1985) reported that the measured
dynamic earth thrust exceeded those predicted by the Mononobe-Okabe equation. Conversely,
Chang and coworkers (1990) indicated, based on dynamic earth pressure data recorded on a
partially embedded reactor containment model structure, that the magnitude of the dynamic earth
pressures were consistent with the Mononobe-Okabe equation.
Exterior basement walls of a multistory building experience dynamic earth pressures during
ground shaking through a complex soil-structure interaction (Itoh and Nogami, 1990; Soydemir
and Celebi, 1992) in which the inertia effect of the superstructure could play a more dominant role
than the inertia effect of the soil containing the substructure (i.e., basement walls). In this case
there are similarities between basement walls of high-rise buildings and abutment walls of bridges
as they perform under ground shaking (Lam and Martin, 1986). Ganev and coworkers (1995)
provided dynamic earth pressure data collected at the basement walls of a reinforced concrete
tower subjected to earthquake-induced ground shaking having magnitudes exceeding those
calculated from the Mononobe-Okabe formulation. It appears that rocking due to the inertia of
the superstructure is a source mechanism intimately associated with the dynamic earth pressures
generated against the basement walls.
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Foundation Design Requirements
7.5.2 FOUNDATION TIES: The additional requirement is made that spread footings on soft
soil profiles should be interconnected by ties. The reasoning explained above under Sec. 7.4.3
also applies here.
7.5.3 SPECIAL PILE REQUIREMENTS: Additional pile reinforcing over that specified for
Category C buildings is required. The reasoning explained above under Sec. 7.4.4 applies here.
Special consideration is required in the design of concrete piles subject to significant bending
during earthquake shaking. Bending can become crucial to pile design where portions of the
foundation piles may be supported in soils such as loose granular materials and/or soft soils that
are susceptible to large deformations and/or strength degradation. Severe pile bending problems
may result from various combinations of soil conditions during strong ground shaking.
For example:
1. Soil settlement at the pile-cap interface either from consolidation of soft soil prior to the
earthquake or from soil compaction during the earthquake can create a free-standing short
column adjacent to the pile cap.
2. Large deformations and/or reduction in strength resulting from liquefaction of loose granular
materials can cause bending and/or conditions of free-standing columns.
3. Large deformations in soft soils can cause varying degrees of pile bending. The degree of pile
bending will depend upon thickness and strength of the soft soil layer(s) and/or the properties
of the soft/stiff soil interface(s).
Such conditions can produce shears and/or curvatures in piles that may exceed the bending
capacity of conventionally designed piles and result in severe damage. Analysis techniques to
evaluate pile bending are discussed by Margason and Holloway (1977) and these effects on
concrete piles are further discussed by Shepard (1983). For homogeneous, elastic media and
assuming the pile follows the soil, the free-field curvature (soil strains without a structure present)
can be estimated by dividing the peak ground acceleration by the square of the shear wave
velocity of the soil although considerable judgment is necessary in utilizing this simple relationship
in a layered, inelastic profile with pile-soil interaction effects. Norris (1994) discusses methods to
assess pile-soil interaction with regard to pile foundation behavior.
The designer needs to consider the variation in soil conditions and driven pile lengths in providing
for pile ductility at potential high curvature interfaces. Interaction between the geotechnical and
structural engineers is essential.
It is prudent to design piles to remain functional during and following earthquakes in view of the
fact that it is difficult to repair foundation damage. The desired foundation performance can be
accomplished by proper selection and detailing of the pile foundation system. Such design should
accommodate bending from both reaction to the building's inertial loads and those induced by the
motions of the soils themselves. Examples of designs of concrete piles include:
1. Use of a heavy spiral reinforcement and
2. Use of exterior steel liners to confine the concrete in the zones with large curvatures or shear
stresses.
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1997Commentary, Chapter 7
These provide proper confinement to ensure adequate ductility and maintenance of functionality
of the confined core of the pile during and after the earthquake.
REFERENCES
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Generated by Liquefaction-Induced Lateral Spread, Technical Report NCEER-92-0021.
Buffalo, New York: National Center for Earthquake Engineering Research.
Bartlett, S. F., and T. L. Youd. 1995. “Empirical Prediction of Liquefaction-Induced Lateral
Spread.” Journal of Geotechnical Engineering ASCE 121(4): 316-329.
Bertero, V. V, and H. A. Brauer, G. C. Fotinos, G. C. Gerwick, Jr., Y. T. Lin, and H. B. Seed.
1974. "Aseismic Design of Prestressed Concrete Piling." In Proceedings of the 7th Congress of
the Federation Internationale de la Precontrainte, New York, May 25.
Bieganousky, W. A., and E. F. Marcuson, III. 1976. Liquefaction Potential of Dams and
Foundations: Report 1--Laboratory Standard Penetration Test on Reid Bedford Model and
Ottawa Sands, Report S-76-2. Waterways Experiment Station.
Bieganousky, W. A., and W. F. Marcuson, III. 1977. Liquefaction Potential of Dams and
Foundations: Report 2--Laboratory Standard Penetration Test on Platte River Sand and
Standard Concrete Sand, Report S-762. Waterways Experiment Station.
Bonilla, M. G., R. K. Mark, and J. J. Lienkaemper. 1984. "Statistical Relations Among
Earthquake Magnitude, Surface Rupture Length, and Surface Fault Displacement." Bulletin of
the Seismological Society of America 74(6):2379-2411.
Building Research Institute of Japan. Report on the Damage by the 1978 Off Miyagi Prefecture
Earthquake, Report 86, pp. 319-341.
California Department of Transportation. 1971. The San Fernando Earthquake--Field Investigation of Bridge Damage: Preliminary Report. Sacramento: CALTRANS.
Castro, G. 1975. "Liquefaction and Cyclic Mobility of Saturated Sands." Journal of the ASCE
Geotechnical Engineering Division 101 (GT6):551-569.
Chang, C. Y., M. S. Power, C. M. Mok, Y. K. Tang, and H. T. Tang. 1990. “Analysis of
Dynamic Lateral Earth Pressures Recorded on Lotung Reactor Containment Model Structure,”
in Proceedings, Fourth U.S. National Conference on Earthquake Engineering, pp. 643-652.
EERI.
Dewa, Herose, Saito, and Ohira. 1980. "Earthquake Observations and Analysis of LNG Tank on
Pile Foundations." In Proceedings of the 7th World Conference on Earthquake Engineering,
Istanbul.
Earthquake Engineering Research Institute. 1986. "Geotechnical Engineering." In Reducing
Earthquake Hazards: Lessons Learned from Earthquakes, Publication 86-02, pp. 23-52. El
Cerrito, California: Earthquake Engineering Research Institute.
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Ganev, T., F. Yamazaki, and T. Katayama. 1995. “Observation and Numerical Analysis of SoilStructure Interaction of a Reinforced Concrete Tower.” Journal of Earthquake Engineering and
Structural Dynamics 24: 491-503.
Gerwick, B. C. Jr. 1982. "Seismic Design of Prestressed Concrete Piles." In Proceedings of the
9th Congress of the Federation Internationale de la Preconstrainte, Vol. 2, pp. 60-69.
Gerwick, B. C., Jr., and H. A. Brauner. 1970. Design of High-Performance Prestressed
Concrete Piles for Dynamic Loading. New York: American Society for Testing and Materials.
Hardman, Scott L., and T. Leslie Youd. 1987. State of the Art for Assessing Earthquake
Hazards in the United States, Miscellaneous Paper S-73-1. Vicksburg, Mississippi: U.S. Army
Waterways Experiment Station.
Hart, E. W. 1988. Fault-Rupture Hazard Zones in California, Special Publication 42. Sacramento: California Division of Mines and Geology.
Idriss, I. M. 1989. "Response of Soft Soil Sites During Earthquakes." In Proceedings of the H.
Bolton Seed Memorial Symposium, Vol. 2, pp. 273-289.
Idriss, I. M. 1985 "Evaluating Seismic Risk in Engineering Practice." In Proceedings of the 11th
International Conference on Soil Mechanics and Foundation Engineering, Vol 1, p. 155.
Ishihara, Kenji. 1985. "Stability of Natural Deposits During Earthquakes." In Proceedings of
the 11th International Conference on Soil Mechanics and Foundation Engineering, San Francisco, Vol. 1, pp. 321-376.
Itoh, T. and T. Nogami. 1990. “Effects of Surrounding Soil on Seismic Response of Building
Basements,” in Proceedings, Fourth U.S. National Conference on Earthquake Engineering, pp.
643-652. EERI.
Kishida, Hanazuto, and Nakai. 1980. "Damage of Reinforced Precast Concrete Piles During the
Miyagi-Oki Earthquake of June 12, 1981." In Proceedings of the 7th World Conference on
Earthquake Engineering, Istanbul, Vol. 9, p. 461.
Kishida. "Damage of Reinforced Concrete Buildings in Niigata City with Special Reference to
Foundation Engineering." Soils and Foundations 6(1):71-86.
Koester, J. P., and A. G. Franklin. 1985. Current Methodologies for Assessing the Potential for
Earthquake Induced Liquefaction in Soils, Report NUREG/CR-4430 RA. Washington, D.C.:
U.S. Nuclear Regulatory Commission.
Lam, I., and G. R. Martin. 1986. Seismic Design of Highway Bridge Foundations -- Design
Procedures and Guidelines, Report FHWA/RD-86/102. Washington, D.C.: FHWA.
Ledbetter, R. H. 1985. Improvement of Liquefiable Foundation Conditions Beneath Existing
Structures, Technical Report REMR-GT-2. Vicksburg, Mississippi: U.S. Army Waterways
Experiment Station.
Makdisi, F. T., and H. B. Seed. 1978. "Simplified Procedure for Estimating Dam and Embankment Earthquake-Induced Deformations." Journal of Geotechnical Engineering ASCE
104(GT7):849-867.
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Margason, E., and M. Holloway. 1977. "Pile Bending During Earthquakes." In Proceedings of
the 6th World Conference on Earthquake Engineering, New Dehli.
Nadim, F. 1982. A Numerical Model for Evaluation of Seismic Behavior of Gravity Retaining
Walls, Doctoral Thesis Research Report R82-33, Department of Civil Engineer, Massachusetts
Institute of Technology, Cambridge.
National Research Council, Committee on Earthquake Engineering. 1985. Liquefaction of Soils
During Earthquakes. Washington, D.C.: National Academy Press.
Nodar, Setsuo, and Satoshi Hayashi. 1980. "Damage to Port Structures in the 1978 MiyagiKenoki Earthquake." In Proceedings of the 7th World Conference on Earthquake Engineering,
Istanbul, Vol. 9, p. 415.
Norris, G. M. 1994. “Seismic Bridge Pile Foundation Behavior,” in Proceedings, International
Conference on Design and Construction of Deep Foundations, FHWA, Vol. 1.
Park, R., and T. J. Falconer. 1983. "Ductility of Prestressed Concrete Piles Subjected to Seismic
Loading." PCI Journal (September/October).
Poulos, S. J., G. Castro, and J. W. France. 1985. "Liquefaction Evaluation Procedures." ASCE
Journal of Geotechnical Engineering 111(6).
Prakash, S., Editor. 1996. Analysis and Design of Retaining Structures Against Earthquakes,
Geotechnical Special Publication 601. Washington, D.C.: American Society of Civil Engineers.
Prakash, S. 1981. Soil Dynamics. New York: McGraw-Hill Book Co.
Richards, R. J., and D. Elms. 1979. “Seismic Behavior of Gravity Retaining Walls.” Journal of
the Geotechnical Engineering Divisions, ASCE 105(GT4): 449-464.
Rollins, Kyle M. 1987. The Influence of Buildings on Potential Liquefaction Damage. Ph.D.
dissertation, University of California, Berkeley.
Seed, H. B. 1987. "Design Problems in Soil Liquefaction." ASCE Journal of Geotechnical
Engineering 113(8).
Seed, H. B., I. Arango, and C. K. Chan. 1975. Evaluation of Soil Liquefaction Potential During
Earthquakes, Report EERC 75-28. Berkeley, California: Earthquake Engineering Research
Center.
Seed, H. B., and W. H. Peacock. 1971. "Test Procedures for Measuring Soil Liquefaction
Characteristics." Journal of the ASCE Soil Mechanics and Foundation Division 97(SM8), Paper
8330.
Seed, H. B., and I. M. Idriss. 1971. "Simplified Procedure for Evaluating Soil Liquefaction
Potential." Journal of the ASCE Soil Mechanics and Foundations Division 97(SM9):1249-1273.
Seed, H. B., Mori, Kenji, and C. K. Chan. 1977. "Influence of Seismic History on the Liquefaction of Sands." Journal of the ASCE Geotechnical Engineering Division (April).
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Seed, H. B. 1976. "Evaluation of Soil Liquefaction Effects of Level Ground During Earthquakes." In Liquefaction Problems in Geotechnical Engineering, Proceedings of the ASCE
National Convention.
Seed, H. B., K. Tokimatsu, L. F. Harder, and R. M. Chung. 1985. The Influence of SPT
Procedures in Soil Liquefaction Resistance Evaluations, Report UBC/EERC-84/15. Berkeley,
California: Earthquake Engineering Research Center.
Seed, H. B., I. M. Idriss, and I. Arango. 1983. "Evaluation of Liquefaction Potential Using Field
Performance Data." ASCE Journal of Geotechnical Engineering 109(3):458-482.
Seed, H. B., and I. M. Idriss. 1982. Ground Motions and Soil Liquefaction During Earthquakes, Vol. 5 of Engineering Monographs on Earthquake Criteria, Structural Design, and
Strong Motion Records. El Cerrito, California: Earthquake Engineering Research Institute.
Seed, R. B., and L. F. Harder. 1989. "SPT-Based Analysis of Cyclic Pore Pressure Generation
and Undrained Residual Strength." In Proceedings of the H. Bolton Seed Memorial Symposium,
Vol. 2, pp. 351-376.
Seed, H. B., and R. V. Whitman. 1970. “Design of Earth Retaining Structures for Dynamic
Loads,” in Proceedings, ASCE Specialty Conference on Lateral Stresses in the Ground and
Design of Earth-Retaining Structures, pp. 103-147. Ithaca, New York: Cornell University.
Shepard, D. A. 1983. "Seismic Design of Concrete Piling." PCI Journal (March/April).
Slemmons, D. B., P. Bodin, and X. Zhang. 1989. "Determination of Earthquake Size from
Surface Faulting Events." In Proceedings of the International Seminar on Seismic Zonation,
Guangahou, China, pp. 157-169.
Soydemir, C., and M. Celebi. 1992. “Seismic Design of Buildings with Multi-Level Basements,”
in Proceedings, Tenth World Conference on Earthquake Engineering, Madrid Spain, Vol. 6, pp.
1731-1734.
Sugimura. 1981. "Earthquake Damage and Design Method of Piles." In Proceedings of the 10th
International Conference on Soil Mechanics and Foundation Engineering, Stockholm.
Sugimura. 1980. "Participation Factor of Horizontal Force Applied to Pile Foundation." In
Proceedings of the 7th World Conference on Earthquake Engineering, Istanbul, Vol. 3, p. 443.
Tamura et al. 1973. "Damage of Bridges During the Tokachi-Oki Earthquake (1968)." In
Proceedings of the Japan Earthquake Engineering Symposium, pp. 147-153.
Tokimatsu, K., and H. B. Seed. 1987. "Evaluation of Settlements in Sands Due to Earthquake
Shaking." Journal of Geotechnical Engineering ASCE 113(8):861-878.
Whitman, R. V. 1991. “Seismic Design of Earth Retaining Structures,” in Proceedings, Second
International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil
Dynamics, St. Louis, Missouri.
Whitman, R. V. 1990. “Seismic Design and Behavior of Gravity Retaining Walls,” in Proceedings, ASCE Specialty Conference on Lateral Stresses in the Ground and Design of EarthRetaining Structures, pp. 817-842. Ithaca, New York: Cornell University.
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Wood, J. H. 1973. Earthquake-Induced Soil Pressures on Structures, Report EERL 73-05.
Pasadena: California Institute of Technology.
Yong, P. M. F. 1985. Dynamic Earth Pressure Against a Rigid Earth Retaining Wall, Central
Laboratory Report 5.8515. Lower Hutt, New Zealand: Ministry of Work and Development.
Youd, T. L. 1993. Liquefaction-Induced Lateral Spread Displacement. NCEL Technical Note
N-1862. Washington, D.C.: U.S. Navy.
Youd, T. L. 1989. "Ground Failure Damage to Buildings During Earthquakes." In Foundation
Engineering--Current Principles and Practices, Vol. 1, pp. 758-770. New York: American
Society of Civil Engineers.
176
Chapter 8 Commentary
STEEL STRUCTURE DESIGN REQUIREMENTS
8.1 REFERENCE DOCUMENTS: The reference documents presented in this section are the
current specifications for the design of steel members, systems, and components in buildings as
approved by the American Institute of Steel Construction (AISC), the American Iron and Steel
Institute (AISI), the American Society of Civil Engineers (ASCE) and the Steel Joist Institute
(SJI).
8.2 SEISMIC REQUIREMENTS FOR STEEL STRUCTURES:
8.3 SEISMIC DESIGN CATEGORIES A, B, AND C: Stlructures assigned to Seismic
Design Categories A, B, and C do not require the same level of ductility capacity to provide the
required performance as those assigned to the higher categories. For this reason, such structures
are permitted to be designed using the requirements of any of the listed references, provided that
the lower R value specified in Table 5.2.2 is used. Should the registered deisgn professional
choose to use the higher R values in the table, it is required that the detailing requiorements for
the higher Seismic Design Categories be used.
8.4 SEISMIC DESIGN CATEGORIES D, E, AND F: Structures assigned to these categories
must be designed in anticipation of significant ductility demands that may be placed on the
structures during their useful life. Therefore, structures in these categories are required to be
designed to meet special detailing requirements as referenced in this section.
8.5 COLD-FORMED STEEL SEISMIC REQUIREMENTS: The allowable stress and
allowable load levels in Ref. 8-4 are incompatible with the force levels in Chapter 5 of the
Provisions. It is therefore necessary to modify the provisions of Ref. 8-4 for use with the
Provisions. Ref. 8-5 and 8-6 are both based on LRFD and thus are consistent with the force
levels in Chapter 5 of the Provisions. As such, only minor modifications are needed to correlate
those load factors for seismic loads to be consistent with these provisions. The modifications of
all of the reference documents affect only designs involving seismic loads.
8.6 LIGHT-FRAMED WALLS: The provisions of this section apply to buildings framed with
cold-formed steel studs and joists. Lateral resistance is typically provided by diagonal braced
(braced frames) or wall sheathing material. This section is only required for use in Seismic Design
Categories D, E, and F. The required strength of connections is intended to assure that inelastic
behavior will occur in the connected members prior to connection failure. Since pull-out of
screws is a sudden or brittle type of failure, designs using pull-out to resist seismic loads are not
permitted. Where diagonal members are used to resist lateral forces, the resulting uplift forces
must be resolved into the foundation or other frame members without relying on the bending
resistance of the track web. This often is accomplished by directly attaching the end stud(s) to the
foundation, frame, or other anchorage device.
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1997 Commentary, Chapter 8
Table 8.6 presents nominal shear values for plywood and oriented strand board attached to steel
stud wall assemblies. Design values are determined by multiplying the nominal values by a phi (N)
factor as presented in Sec. 8.6.5. These nominal values are based upon tests performed at Santa
Clara University (Serrette, 1996). The test program included both cyclic and static tests;
however, the values presented in Table 8.6 are based upon the cyclic tests as they are intended for
use in seismic resistance. In low seismic areas where wind loads dominate, nominal values have
been recommended for wind resistance by AISI based upon monotonic tests (AISI, 1996). The
cyclic tests were performed using the assemblies that were determined to be the most critical from
the static tests. The assemblies cyclically tested consisted of 3.5 x 1.625 inch C studs fabricated
with ASTM A446 Grade A (33 ksi) with a minimum base metal thickness of 0.033 inch. Since
the tests were conducted, ASTM A446 Grade A has been redesignated ASTM A653 SQ Grade
33. The test panels were four feet wide and 8 feed high, the sheathing material was applied
vertically to only a single side of the studs, and there was no sheathing or bracing applied to the
other side.
The cyclic tests were performed using a sequential phase displacement protocol under development at the time of the test by an ad hoc Committee of the Structural Engineers Association of
Southern California. Nominal values were conservatively established by taking the lowest load in
the last set of stable hysteretic loops. It is expected that subsequent testing of steel stud shear
wall assemblies will reduce or modify some of the restrictive limits currently proposed for the use
of the system such as the nominal maximum thickness of the studs of 0.043 inch, the aspect ratio
of 2:1, and the ability to use sheathing on both sides of the wall.
8.7 SEISMIC REQUIREMENTS FOR STEEL DECK DIAPHRAGMS: Since the design
values for steel deck are based on allowable loads, it is necessary to present a method of deriving
design strengths. Two N values are presented — 0.60 for steel deck that is mechanically attached
and 0.50 for welded steel deck. These factors are consistent with current proposals being
circulated for inclusion in updates of Ref. 8-5.
8.5 STEEL CABLES: The provisions of Sec. 8.5 are virtually unchanged from previsous
editions. Although the provisions in Ref. 8-8 are dated, they are the only ones available and there
was no sentiment to eliminate them from the Provisions. The allowable stress levels of steel cable
structures specified in Ref. 8.8 are modified for seismic load effects. The value of 1.5T4 was
chosen as a reasonable value to compare with increases given to other working stress levels.
REFERENCES:
Serette. 1996. Shear Wall Values for Light Weight Steel Framing. American Iron and Steel
Institute.
178
Chapter 9 Commentary
CONCRETE STRUCTURE DESIGN REQUIREMENTS
9.1 REFERENCE DOCUMENT: The main concern of Chapter 9 is the proper detailing of
reinforced concrete construction for earthquake resistance. The bulk of the detailing requirements
in this chapter are contained in Ref. 9-1, Building Code Requirements for Reinforced Concrete,
ACI 318-95. The commentary for ACI 318-95 contains a valuable discussion of the rationale
behind detailing requirements that is not repeated here.
9.1.1 MODIFICATIONS TO REF. 9-1: The modifications noted for ACI 318-95 are: changes
in load factors necessary to coordinate with the equivalent yield basis of this document; additional
definitions necessary for seismic design requirements for structural systems composed of precast
elements; and changes that incorporate certain features of the detailing requirements for
reinforced concrete that have been adopted into the 1997 Uniform Building Code.
Included as Sec. 9.1.1.5 is a statement on reinforced concrete structural systems incorporating
precast concrete elements. One design alternative is emulation of monolithic reinforced concrete
construction. The other alternative is the use of the unique properties of precast elements
interconnected predominately by dry joints. For the first alternative Sec. 9.1.1.5, 9.1.1.6 and
9.1.1.9 define design procedures ensuring that the resulting structural systems have strength and
stiffness characteristics equivalent to those for monolithic reinforced concrete construction. The
existing code requirements for monolithic construction then apply for all but the connections. The
second alternative, the Appendix to Chapter 9, is included for information and for trial design by
users.
Procedures for structural system composed from precast elements interconnected predominately
by dry joints are included as an appendix because the existing state of knowledge makes it
premature to propose code requirements based on that information. The complexity of structural
systems, configurations and details possible with precast concrete elements requires:
1.
Selecting functional and compatible details for connections and members that are reliable
and can be built with acceptable tolerances;
2.
Verifying experimentally the inelastic force-deformation relationships for welded, bolted,
or grouted connections proposed for the seismic resisting elements of the structure; and
3.
Analyzing the structure using those connection relationships and the inelastic reversed
cyclic loading effects imposed by the anticipated earthquake ground motions.
Research conducted to date (Cheok and Lew, 1991; Elliott et al., 1992; Englekirk, 1987; French
et al., 1989; BSSC, 1987; Hawkins and Englekirk, 1987; Jayashanker and French, 1988; Mast,
1992; Nakaki and Englekirk, 1991; Neille, 1977; New Zealand Society, 1991; Pekau and Hum,
179
1997 Commentary, Chapter 9
1991; Powell et al., 1993; Priestley and Tao, 1991; Stanton et al., 1986; Stanton et al., 1991)
documents concepts for design using dry connections and the behavior of structural systems and
subassemblages composed of precast elements both at and beyond peak strength levels for
nonlinear reversed cyclic loadings, and provides the basis for the appendix.
Emulation of Monolithic Construction Using Strong Connections: For emulation of the behavior
of monolithic reinforced concrete construction, Sec. 9.1.1.6 provides two alternatives. Sec.
21.2.2.6 in Sec. 9.1.1.6 covers structural systems with "wet" connections. Sec. 21.2.2.7 in Sec.
9.1.1.6 covers structural systems with "strong" connections.
For frame systems that use strong connections, Sec. 21.2.2.7 and 21.2.7, the different connection
categories envisaged are shown in Figure C9.1.1-1. Considerable freedom is given to locating the
nonlinear action zones (plastic hinges) along the length of the precast member. However, those
hinges must be separated from the connection by a distance of at least three quarters of the
member's depth. Wet-joint connections are permitted at the strong connection but not at the
hinge location.
Provision 21.2.7.2 makes the strength required for a strong connection dependent on the
distances hinges are separated from that connection, the strengths of those hinges and the
nonlinear deformation mechanism envisaged. The conditions described by Sec. 21.2.7.2 for a
beam to continuous column connection are shown in Figure C9.1.1-2, which is an adaptation of
Figure R21.3.4 of Ref. 9-1. Because the strong connection must not yield or slip; its nominal
strengths, Sn CONNECTION, in both flexure and shear must be greater than those corresponding
to development of the probable strengths Mpr1 and Mpr2 at the hinge locations. Figure
C9.1.1-2b, illustrates the situation for flexure. Per Ref. 9-1 moments Mpr1 and Mpr2 are
determined using a strength reduction factor of 1.0 and reinforcing steel stresses of at least 1.25fy.
For columns above the ground floor, moments at a joint may be limited by flexural strengths of
the beams framing into that joint. However, for a strong column-weak beam deformation
mechanism, dynamic inelastic analysis and studies of strong motion measurements have shown
that beam end moments are not equally divided between top and bottom columns even where
those columns have equal stiffness. Elastic analysis predicts moments as shown in Figure
C9.1.1-3a while the actual situation is likely to be as shown in Figure C9.1.1-3b. Accordingly,
provision 21.2.7.3 is included for the midheight column connection.
Use of Prestressing Tendons: Sec. 9.1.1.7 defines conditions under which prestressing tendons
can be used, in conjunction with deformed reinforcing bars, in frames resisting earthquake forces.
As documented by Ishizuka and Hawkins (1987), if those conditions are met no modification is
necessary to the R and Cd factors of Table 2.2.2 when prestressing is used. Satisfactory seismic
performance can be obtained when prestressing amounts greater than those permitted by Sec.
9.1.1.6 are used. For example, the connection of Figure C9A-3 has a satisfactory performance for
a probable strength of 180 kips and a reversed deformation of 10 mm. However, as documented
by Park and Thompson (1977) and Thompson and Park (1980) and required by the combination
of New Zealand Standards 3101:1982 and 4203:1992, ensuring that satisfactory performance
requires modification of the R and Cd factors.
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Concrete Structure Design Requirements
Figure C9.1.1-1 Connection categories
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1997 Commentary, Chapter 9
Figure C9.1.1-2 Design forces for strong connections between beams and continuous
columns.
Figure C9.1.1-3 Moments at beam-to-column connections.
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Concrete Structure Design Requirements
9.1.1.12: The minimum thicknesses for concrete diaphragms reflect current usage in joist and
waffle systems and topping slabs for precast floor and roof systems. Bonding of the top slab
provides restraint against slab buckling which, for an untopped joist or waffle system, is provided
by the webs.
9.1.1.14 Coupling Beams: Short-span coupling beams between shear walls, under reversing
loads simulating earthquakes, have been experimentally investigated by many researchers (Barney
et al., 1978; Bertero and Popov, 1975; Brown and Jirsa, 1971; Hirosawa et al., 1973; Ma et al.,
1976; Paulay and Binney, 1974; Scribner and Wight, 1978; Shiu et al., 1978; Wight and Sozen,
1975).
The tests indicate that short flexural members with small clear-span-to-effective-depth ratios
behave differently than slender flexural members. When short coupling beam specimens were
subjected to inelastic load cycles, large flexural cracks formed at both beam ends. With increasing
load reversals, cracks from each direction of loading interconnected, forming a vertical plane of
weakness at each end of a beam. Thus, instead of a conventional truss mechanism, shear transfer
across the plane of weakness was provided primarily by aggregate interlock and shear friction.
Under subsequent inelastic load cycles, the shear resisting mechanism deteriorated rapidly
resulting in a loss of load capacity by "sliding shear."
It was found (Barney et al. , 1978; Paulay and Binney, 1974; Shiu et al., 1978) that increasing the
number of hoop stirrups was not effective in improving resistance against sliding shear.
Therefore, various configurations of special shear reinforcement to improve seismic performance
of short coupling beams were tested (Barney et al., 1978; Bertero and Popov, 1974; Paulay and
Binney, 1974; Scribner and Wight, 1978). Full-length diagonal reinforcement was found to
produce the greatest energy dissipation and deformation capacities.
In tests at the Portland Cement Association (Shiu et al., 1978), beam specimens with
clear-span-to-effective-depth ratio of 2.8 were able to sustain over 34 reversing load cycles. A
maximum nominal shear stress of 12.5%ðfEQ \O(_,c) was recorded and a maximum imposed
deformation of over nine times yield deflection was measured. In addition, the specimen was able
to dissipate three times more energy than a comparable specimen without diagonal shear
reinforcement. No sign of sliding shear was observed at completion of testing. Similar results
have been reported by Paulay and Binney (1974). In both investigations, diagonal shear
reinforcement was designed by the proposed equation to resist the total shear force by truss
action.
Based on test results, Paulay (1977) recommended that diagonal shear reinforcement be used to
carry 75 percent of the induced shear in flexural members when nominal design shear stress under
load reversals is larger than 3%ðfEQ \O(_,c) psi. When nominal shear stress exceeds 4.5%ðfEQ
\O(_,c) psi, he recommended that diagonal reinforcement should carry 100 percent of the induced
cyclic shear forces.
For clear-span-to-effective-depth ratios greater than 4.0, PCA tests (Barney et al., 1978; Shiu et
al., 1978) indicated that specimens with conventional longitudinal reinforcement resisted over 46
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1997 Commentary, Chapter 9
inelastic reversing load cycles. At the same time, maximum deformation of 13 times yield
deflection was measured. Therefore, experimental data have shown that beams with
clear-span-to-effective-depth ratios greater than 4.0 do not require diagonal reinforcement. In
addition, diagonal reinforcement is not very effective in beams with clear-span-to-effective-depth
ratios greater than 4.0.
It is permitted to use reinforcement arrangements without the diagonal bars if it is assumed that
the coupling beams may fail early in the design earthquake and the consequences of such failures
are taken into account. The consequences include, but are not necessarily limited to, reductions in
stiffness due to lack of coupling between walls as well as debris or deformed members blocking
exits and damage or failure of nonstructural components and cladding and the connections thereto
due to increased drifts. The vertical load carrying capacity of the structure must not be impaired.
Even if the coupling beams are expected to fail early in the earthquake, the full design strength of
the coupling beams still must be provided if they are assumed to be part of the seismic force
resisting system. This is to reduce the likelihood that the transition of shear wall behavior from
coupled to uncoupled action will begin at unduly low seismic loads.
9.2 BOLTS AND HEADED STUD ANCHORS IN CONCRETE: The allowable loads on
anchor bolts have been chosen to suit the capacity reduction factors in this document.
9.2.2 BOLTS AND HEADED STUD ANCHORS: These requirements follow those given in
the Uniform Building Code modified by recent improvements in shear capacity calculation given
in the PCI Design Handbook, Fourth Edition.
While the requirements do not prohibit the use of single anchor connections, it is considered
necessary to use at least two anchors in any load-carrying device whose failure might lead to
collapse.
The requirements generally relate to groups of anchors attaching a loaded steel plate to the
concrete surface. The thickness of this connector plate also is a design consideration and must be
adequate to allow the anchors to perform in group action if the calculated design strengths are to
be realized. A plate thickness of not less than one half the diameter of the anchor shank is
recommended.
These requirements are intended to provide proper strength of connections. To achieve adequate
ductility for seismic or other dynamic loads, use of auxiliary reinforcement for confining concrete
or for direct load transfer should be considered. A number of tests have shown that hairpins or
similar reinforcement confining the concrete engaged by the anchors and running through the
failure surface into the adjacent concrete provides enhancement of a connection's ductility under
dynamic loading. No clear recommendations as to the design of such reinforcement have been
suggested, but its use is highly recommended in all anchor connections and particularly those
subject to seismic or other dynamic loading.
Tests have shown that there are consistent shear ductility variations between bolts anchored to
drilled or punched plates with nuts and connections using welded, headed studs.
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Concrete Structure Design Requirements
Recommendations for design are not presently available, but this should also be considered in
critical connections subject to dynamic or seismic loading.
9.3 CLASSIFICATION OF MOMENT FRAMES:
9.3.1.1 ORDINARY MOMENT FRAMES: Since ordinary frames are permitted only in
Categories A and B, they are not required to meet any particular seismic requirements. Attention
should be paid to the often overlooked requirement for joint reinforcement in Sec. 11.12.1 of Ref.
9-1.
9.3.1.2 INTERMEDIATE MOMENT FRAMES AND 9.3.1.3 SPECIAL MOMENT
FRAMES: The concept of moment frames for various levels of hazard zones and of
performance is changed somewhat from the requirements of Ref. 9-1. Two sets of moment frame
detailing requirements are defined in Ref. 9-1, one for "regions of high seismic risk" and the other
for "regions of moderate seismic risk." For the purposes of this document, the "regions" are made
equivalent to Seismic Design Categories in which "high risk" means Categories D and E and
"moderate risk" means Category C. This document labels these two frames the "special moment
frame" and the "intermediate moment frame," respectively.
The level of inelastic energy absorption of the two frames is not the same. These requirements
introduce the concept that the R factors for these two frames should not be the same. The
preliminary version of these requirements (ATC 3-06) assigned the R for ordinary frames to what
is now called the intermediate frame. In spite of the fact that the R factor for the intermediate
frame is less than the R factor for the special frame, use of the intermediate frame is not permitted
in the higher Seismic Design Categories (D, E, and F). On the other hand, this arrangement of the
Provisions encourages consideration of the more stringent detailing practices for the special frame
in Category C because the reward for use of the higher R factor can be weighed against the higher
cost of the detailing requirements. These requirements also introduce the concept that an
intermediate frame may be a part of a dual system in Category C.
The differences in the performance basis of the requirements for the two types of frames might be
briefly summarized as follows (see the commentary of Ref. 9-1 for a fuller discussion of the
requirement for the special frame):
1.
The shear strength of beams and columns shall not be less than that required when the
member has yielded at each end in flexure. For the special frame, strain hardening and
other factors are considered by raising the effective tensile strength of the bars to 125
percent of specified yield. For the intermediate frame, an escape clause is provided in that
the calculated shear using double the prescribed seismic force may be substituted. Both
types require the same minimum amount and maximum spacing of transverse
reinforcement throughout the member.
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1997 Commentary, Chapter 9
2.
The shear strength of joints is limited and special requirements for anchoring bars in joints
exist for special moment frames but not intermediate frames. Both frames require
transverse reinforcement in joints although less is required for the intermediate frame.
3.
Closely spaced transverse reinforcement is required in regions of potential hinging
(typically the ends of beams and columns) to control lateral buckling of longitudinal bars
after the cover has spalled. The spacing limit is slightly more stringent for columns in the
special frame.
4.
The amount of transverse reinforcement in regions of hinging for special frames is
empirically tied to the concept of providing enough confinement of the concrete core to
preserve a ductile response. These amounts are not required in the intermediate frame
and, in
fact, stirrups in lieu of hoops may be used in beams.
186
Concrete Structure Design Requirements
5.
The special frame must follow the strong column/weak beam rule. Although this is not
required for the intermediate frame, it is highly recommended for multistory construction.
6.
The maximum and minimum amounts of reinforcement are limited to prevent rebar
congestion and assure a nonbrittle flexural response. Although the precise limits are
different for the two types of frames, a great portion of practical, buildable designs will
satisfy either.
7.
Minimum amounts of continuous reinforcement to account for moment reversals are
required by placing lower limits on the flexural strength at any cross section.
Requirements for the two types of frames are similar.
8.
Locations for splices of reinforcement are more tightly controlled for the special frame.
9.
In addition, the special frame must satisfy numerous other requirements beyond the
intermediate frame to assure that member proportions are within the scope of the present
research experience on seismic resistance and that the analysis, the design procedures, the
qualities of the materials, and the inspection procedures are at the highest level of the state
of the art.
9.4 SEISMIC DESIGN CATEGORY A: Construction qualifying under Category A may be
built with no special detailing requirements for earthquake resistance. Special details for ductility
and toughness are not required in Category A.
9.5 SEISMIC DESIGN CATEGORY B: Special details for ductility and toughness are not
required in Category B.
9.6 SEISMIC DESIGN CATEGORY C: A frame used as part of the lateral force resisting
system in Category C as identified in Table 2.2.2 is required to have certain details that are
intended to help sustain integrity of the frame when subjected to deformation reversals into the
nonlinear range of response. Such frames must have attributes of intermediate moment frames.
Structural (shear) walls of structures in Category C are to be built in accordance with the
requirements of Ref. 9-1 except the requirements of Sec. 21.6 of Ref. 9-1 do not apply.
9.7 SEISMIC DESIGN CATEGORIES D, E, AND F: The requirements conform to current
practice in the areas of highest seismic hazard.
REFERENCES
Precast/Prestressed Concrete
ACI Committee 550. 1993. "Design Recommendations for Precast Concrete Structures," ACI
Structural Journal 90 (1):115-121.
Applied Technology Council. 1981. Proceedings of a Workshop on Design of Prefabricated
Concrete Buildings for Earthquake Loads, Report ATC-8.
187
1997 Commentary, Chapter 9
Building Seismic Safety Council. 1987. Guide to Use of the NEHRP Recommended Provisions in
Earthquake Resistant Design of Buildings, 1985 Edition. Washington, D.C.: FEMA.
Cheok, G. S., and H. S. Lew. 1991. "Performance of Precast Concrete Beam-to-Column
Connections Subject to Cyclic Loading," PCI Journal 36 (3):56-67.
Clough, D. P. 1986. "A Seismic Design Methodology for Medium-Rise Precast Concrete
Buildings." In Proceedings of Seminar on Precast Construction in Seismic Zones, JSPS/NSF,
Tokyo, October.
Englekirk, R. E. 1987. "Concepts for the Development of Earthquake Resistant Ductile Frames
of Precast Concrete," PCI Journal 32(1).
Elliott, K. S., G. Davies, and W. Omar. 1992. "Experimental and Theoretical Investigation of
Precast Concrete Hollow-Cored Slabs Uses as Horizontal Floor Diaphragms," The Structural
Engineer 70(10):175-187.
French, C. W., M. Hafner, and V. Jayashanker. 1989. "Connections Between Precast Elements
Failure Within Connection Region," ASCE Journal of Structural Engineering 115(12):3171-3192.
Hawkins, N. M., and R. E. Englekirk. 1987. "U.S.-Japan Seminar on P/C Concrete Construction
in Seismic Zones," PCI Journal 32(2).
Jayashanker, V., and C. E. French. 1988. An Interior Moment Resistant Connection Between
Precast Elements Subjected to Cyclic Lateral Loads, Structural Engineering Report 87-10.
Mineapolis: University of Minnesota.
Mast, R. F. 1992. "A Precast Concrete Frame System for Seismic Zone Four," PCI Journal
37(1):50-64.
Mattock, A. H. 1977. Shear Transfer Under Cyclically Reversing Loading Across an Interface
Between Concretes Cast at Different Times, Report SM 77-1. Seattle: Department of Civil
Engineering, University of Washington.
Mattock, A. H. 1974. The Shear Transfer Behavior of Cracked Monolithic Concrete Subject to
Cyclically Reversing Shear, Report SM 74-4. Seattle: Department of Civil Engineering,
University of Washington.
Menegotto, M. 1994. "Seismic Diaphgram Behavior of Untopped Hollow-Core Floors." In
Proceedings, Federation International of Prestressing (FIP) Congress, Washington, D.C., May.
Mueller, P. 1989. "Hysteric Behavior of Precast Panel Walls." In Proceedings, Seminar on
Precast Concrete Construction in Seismic Zones, JSPS/NSF, Tokyo, 1989, Vol. I, pp. 127-142.
Nakaki, S. D., and R. E. Englekirk. 1991. "PRESS Industry Seismic Workshops: Concept
Development," PCI Journal 36(5):54-61.
188
Concrete Structure Design Requirements
Neille, D. S. 1977. Behavior of Headed Stud Connections for Precast Concrete Connections for
Precast Concrete Panels Under Monotonic and Cycled Shear Loading, thesis submitted in partial
fulfillment of the requirements of Doctor of Philosophy, University of British Columbia.
New Zealand Society of Earthquake Engineering. 1991. Guidelines for the Use of Structural
Precast Concrete in Buildings.
Pekau, O. A., and D. Hum. 1991. "Seismic Response of Friction-Jointed Precast Panel Shear
Walls," PCI Journal 36(2):56-71.
Powell, G., F. Filippou, V. Prakash, and S. Campbell. 1993. "Analytical Platform for Precast
Structural Systems." In Proceedings, ASCE Structures Congress '93. New York: ASCE.
Priestley, M. J. N. 1991. "Overview of PRESS Research Program," PCI Journal 36(4):50-57.
Priestley, M. J. N., and J. R. Tao. 1991. "Seismic Response of Precast-Prestressed Frames with
Partially Debonded Tendons," Second Meeting of US-Japan Joint Technical Coordinating
Committee for PRESS, Tsukuba, Japan, November.
Stanton, J. F., R. G. Anderson, C. W. Dolan, and D. E. McClearly. 1986. Moment-Resisting and
Simple Connections, Research Report 1/4, Prestressed Concrete Institute.
Warnes, C. E. 1992. "Precast Concrete Connection Details for All Seismic Zones," Concrete
International 14(11):36-44.
Wood, S. L. 1990. "Shear Strength of Low-Rise Reinforced Concrete Walls," ACI Structural
Journal 87(1):99-107.
Yee, A. A. 1991. "Design Considerations for Precast Prestressed Concrete Building Structures
in Seismic Areas," PCI Journal 36(3):40-55.
Prestressed and Partially Prestressed Concrete
ACI-ASCE Committee 423. 1985. "Recommendations for Concrete Members Prestressed with
Unbonded Tendons," ACI Manual for Concrete Practice, Part 3, p. 423.3-1 - 423.3-R16. Detroit,
Michigan: American Concrete Institute.
Hirosawa, M., M. Ozaki, and M. Wakabayashi. 1973. "Experimental Study on Large Models of
Reinforced Concrete Columns." In Proceedings of the Fifth World Conference on Earthquake
Engineering, Rome.
Ishizuka, T., and N. M. Hawkins. 1987. Effect of Bond Deterioration on the Seismic Response
of Reinforced and Partially Prestressed Concrete and Ductile Moment Resistant Frames, Report
SM 87-2. Seattle: University of Washington, Department of Civil Engineering.
JSPS/NSF. 1986. Proceedings of the Seminar on Precast Construction in Seismic Zones, Vol. 1.
189
1997 Commentary, Chapter 9
Ma, S. M., V. V. Bertero, and E. P. Popov. 1976. Experimental and Analytical Studies on the
Hysteretic Behavior of Reinforced Concrete Rectangular and T-Beams, Report EERC 72-2.
Berkeley: University of California.
Park, R., and K. J. Thompson. 1977. "Cyclic Load Tests on Prestressed and Partially Prestressed
Beam-Column Joints," PCI Journal 22(5):84-110.
Paulay, T. 1977. "Capacity Design of Reinforced Concrete Ductile Frames." In Proceedings of
the Workshop on ERCBC, University of California.
Paulay, T., and J. R. Binney. 1974. "Diagonally Reinforced Coupling Beams of Shear Walls." In
Shear in Reinforcement Concrete, Vol. 2, Publication SP-42, pp. 579-598. Detroit, Michigan:
American Concrete Institute.
Scribner, C. F., and J. K. Wight. 1978. Delaying Shear Strength Decay in Reinforced Concrete
Flexural Members Under Large Load Reversals, University of Michigan, Department of Civil
Engineering Report UMEE 78R2 to the National Science Foundation.
Shiu, K. N., G. B. Barney, A. E. Fiorato, and W. G. Corley. 1978. "Reversing Load Tests of
Reinforced Concrete Coupling Beams." In Proceedings of the Central American Conference on
Earthquake Engineering, San Salvador.
Thompson, K. J., and R. Park. 1980. "Seismic Response of Partially Prestressed Concrete,"
ASCE Journal of the Structural Division 106(ST8):1755-1775.
190
Concrete Structure Design Requirements
191
1997 Commentary, Chapter 9
Appendix to Chapter 9
REINFORCED CONCRETE STRUCTURAL SYSTEMS
COMPOSED OF INTERCONNECTED PRECAST ELEMENTS
PREFACE: The provisions for reinforced concrete structural systems composed of precast
elements in the body of the 1997 Provisions are for precast systems emulating monolithic
reinforced concrete construction. However, one of the principal characteristics of precast
systems is that they often are assembled using dry joints where connections are made by
bolting, welding, post-tensioning, or other similar means. Research conducted to date
documents concepts for design using dry joints and the behavior of subassemblages
composed from interconnected precast elements both at and beyond peak strength levels
for nonlinear reversed cyclic loadings (Applied Technology Council, 1981; Cheok and Lew,
1992; Clough, 1986; Eliott et al., 1987; Hawkins and Englekirk, 1987; Jayashanker and
French, 1988; Mast, 1992; Nakaki and Englekirk, 1991; Neille, 1977; New Zealand Society,
1991; Pekau and Hum, 1991; Powell et al., 1993; Priestley, 1991; Priestley and Tao, 1992;
Stanton et al., 1986; Stanton et al., 1991). This appendix is included for information and as
a compilation of the current understanding of the performance under seismic loads of
structural systems composed from interconnected precast elements. It is considered
premature to base code provisions on this resource appendix; however, user review, trial
designs, and comment on this appendix are encouraged. Please direct such feedback to the
BSSC.
The only design approach currently validated adequately for codification for construction using
precast elements is that of emulation of monolithic reinforced concrete construction. Yet, in
regions of moderate and low seismicity, it is reasonable to expect that structural systems of
adequate strength, stiffness, and energy dissipation can be constructed from precast concrete
elements using bolting, welding, or similar means that involve dry connections only. The
objective of this appendix is to provide a framework within which such systems can begin to be
codified for design purposes.
Tests by Stanton et al. (1986) have demonstrated that many of the moment resisting dry
connections typically utilized for precast concrete construction for gravity loadings have adequate
behavior for monotonic but not reversed loading. For reversed loadings, such connections lack
ductility whenever the connection is made by welding or bolting. For example, as illustrated in
Figure C9A-1, the corbel connection shown in (a) functions well for gravity loadings (b) but not
for loading reversals (c) through (f). In the latter case, the corbel is an impediment to ductility
because the connection is not detailed carefully enough. For negative moment loading, the beam
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Concrete Structure Design Requirements
end rotates about the edge of the corbel causing a prying action on the negative moment
connection to the column and marked secondary stresses in the reinforcement, inserts, or welds
that make up that connection. The prying causes kinking of the reinforcement initiating spalling
or splitting of the concrete surrounding that reinforcement. That action, combined with splitting
of the beam end or the corbel edge for positive moment loadings, results in the strength rapidly
dropping below acceptable levels with load reversals and increasing rotations. However, that
does not mean that all corbels per se are bad for reversed loadings. Shown in Figure C9A-2 is an
inverted T-beam detail that has been developed to provide positive connection to the corbel while
minimizing difficulties associated with beam shrinkage and beam loading reversals.
Figure C9A-1 Dry connections (PCI Research Project/4-86, Moment Connection BC16A).
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1997 Commentary, Chapter 9
Figure C9A-2 Inverted tee beam to corbel connection (Shockey Bros., Winchester,
Virginia).
It is essential that connection detailing recognize the load-deformation behavior imposed on
connections by requirements of the seismic force resisting structural system selected and possible
uncertainties in the connection's response. Ideally, appropriate detailing concepts should be
developed through analytical modeling and verified by physical testing. Without physical testing,
the uncertainties in the response make it imperative that the response modification coefficients and
deflection amplification factors used for lateral-force-resisting systems constructed with dry
connections be less than those for the comparable monolithic reinforced concrete structural
systems. However, regardless of the R and Cd values used it is essential to identify:
1.
The magnitude of the deformation demands to which each connection will be subjected in
order for the structural system to achieve the overall deformation required of it;
2.
The ability of each connection to provide that deformation and the associated probable
strength without failure; and
3.
The ability of each connection and the associated connection region to provide the
necessary system stiffness and energy dissipation.
Sec. 9A.2 addresses identification of the relation between the deformation demand imposed on
the connections, their deformation capacity, and the deformation demands, (R and Cd) selected
for the structure. The designer is required to study those relationships and identify the potential
for prying actions or undesirable rocking motions. Use of computer programs, such as
Drain-2DX developed by Powell et al. (1993), usually will be necessary to study the relation
between the deformation selected for the structure and the resultant deformation demands placed
on the connections.
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Concrete Structure Design Requirements
Sec. 9A.3 addresses limitations on the R and Cd factors that can be used for the
lateral-force-resisting structural framing system. Values as large as those for monolithic
construction can be used if the connection's response characteristics have been established by
physical testing. If, however, those characteristics have been determined only through analytical
modeling, values are required to be less than those for monolithic construction because of
uncertainties about response of the actual connections.
For Table 9A.3.3, it is intended that the values selected for Rj and Cdj should have the same
relation to one another as the values specified for R and Cd in Table 2.2.2. For example, for a
special moment frame of reinforced concrete, R and Cd are specified in Table 2.2.2 as 8 and 5.5,
respectively. For the same frame, connections of Category C are required by this appendix.
Thus, by Table 9A.3.3, Rj can range for the precast frame from 4 to 7 and Cdj can range from
2.25 to 4.5. The footnote to Table 9A.3.3 requires that Rj and Cdj be varied in step so that the
designer can choose, depending on the detailing practice used, Rj and Cdj values coupled as
shown in Table C9A.3.3.
TABLE C9A.3.3 Restricted R and Cd for Connection Category C
Restricted Response Modification
Coefficient, Rj
Restricted Deflection Amplification
Factor, Cdj
4
2.25
5
3.00
6
3.75
7
4.5
Sec. 9A.4 provides a framework for evaluating connection performance. The connection is
identified as having three factors contributing to its characterization: the connector, its
anchorage, and the surrounding connection region. Three Connection Performance Categories
are identified. For Connection Performance Category A, there are no special requirements but
those connections can be used only for lateral-force-resisting systems of Seismic Design Category
A. Further, any dry connection for which there is not direct transfer of tensile or shear force from
the connector's anchorage to the principal reinforcement of the precast element by welding,
bolting, or adequate lap length must be assigned to Category A. Performance Category B
connections must exhibit stable inelastic capacities with increasing reversed cyclic deformation
demands. However, such connections do not have to have the energy dissipation hysterectic
characteristics normally associated with monolithic concrete connections. For example, the
connection illustrated in Figure C9A-3 has a satisfactory performance for a probable strength of
180 kips and a reversed deformation of 10 mm. A connection cannot be used for
lateral-force-resisting systems of Seismic Design Category C, D, or E unless that connection is of
Connection Performance Category C for which the stressed area at the interface must be greater
than 30 percent of that for the closest adjacent area of uniform stress in the precast element. This
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1997 Commentary, Chapter 9
restriction is aimed at minimizing the concentration of inelastic deformations in the element by
forcing connections to be designed so as to activate a significant volume of the adjacent precast
element and, therefore, more closely replicate the behavior for monolithic reinforced concrete
construction.
The majority of the requirements of this appendix are intended to apply to precast elements and
nonlinear action location connections that are part of the lateral force resisting system. However,
Sec. 9A.5.2 also addresses the special case where there is a connector that is on the
lateral-load-resisting path and is not to be part of the nonlinear action response. Further, it is also
very important to consider the integrity and flexibility of all other connections in the structure to
determine that their behavior is compatible with the anticipated lateral movements of the building.
Figure C9A-3 Dry connections (R.Spencer, Earthquake Resistance Connections for Low Rise Precast Concrete
Buildings, JSPS Seminar on Precast Concrete Construction in Seismic Design).
REFERENCES
Applied Technology Council. 1981. Proceedings of a Workshop on Design of Prefabricated
Concrete Buildings for Earthquake Loads, Report ATC-8.
Cheok, G. S., and H. S. Lew. 1991. "Performance of Precast Concrete Beam-to-Column
Connections Subject to Cyclic Loading," PCI Journal 36 (3):56-67.
Clough, D. P. 1986. "A Seismic Design Methodology for Medium-Rise Precast Concrete
Buildings." In Proceedings of Seminar on Precast Construction in Seismic Zones, JSPS/NSF,
Tokyo, October.
Elliott, K. S., et al. 1987.
Hawkins, N. M., and R. E. Englekirk. 1987. "U.S.-Japan Seminar on P/C Concrete Construction
in Seismic Zones," PCI Journal 32(2).
Jayashanker, V., and C. E. French. 1988. An Interior Moment Resistant Connection Between
196
Concrete Structure Design Requirements
Precast Elements Subjected to Cyclic Lateral Loads, Structural Engineering Report 87-10.
Mineapolis: University of Minnesota.
Mast, R. F. 1992. "A Precast Concrete Frame System for Seismic Zone Four," PCI Journal
37(1):50-64.
Nakaki, S. D., and R. E. Englekirk. 1991. "PRESS Industry Seismic Workshops: Concept
Development," PCI Journal 36(5):54-61.
Neille, D. S. 1977. Behavior of Headed Stud Connections for Precast Concrete Connections for
Precast Concrete Panels Under Monotonic and Cycled Shear Loading, thesis submitted in partial
fulfillment of the requirements of Doctor of Philosophy, University of British Columbia.
New Zealand Society of Earthquake Engineering. 1991. Guidelines for the Use of Structural
Precast Concrete in Buildings.
Pekau, O. A., and D. Hum. 1991. "Seismic Response of Friction-Jointed Precast Panel Shear
Walls," PCI Journal 36(2):56-71.
Powell, G., F. Filippou, V. Prakash, and S. Campbell. 1993. "Analytical Platform for Precast
Structural Systems." In Proceedings, ASCE Structures Congress '93. New York: ASCE.
Priestley, M. J. N. 1991. "Overview of PRESS Research Program," PCI Journal 36(4):50-57.
Priestley, M. J. N., and J. R. Tao. 1992.
Stanton, J. F., R. G. Anderson, C. W. Dolan, and D. E. McClearly. 1986. Moment-Resisting and
Simple Connections, Research Report 1/4, Prestressed Concrete Institute.
Stanton, J. F., T. R. Hicks, and N. M. Hawkins. 1991. PRESSS Project 1.3: Connection
Classification and Evaluation. PCI Journal 36(5):62-71.
197
Chapter 10 Commentary
COMPOSITE STEEL AND CONCRETE STRUCTURE
DESIGN REQUIREMENTS
The 1994 Edition of the NEHRP Recommended Provisions included a new chapter on composite steel
and concrete structures. These provisions have been updated and incorporated in Part II of the 1997
Edition of the AISC Seismic Provisions. This edition of the NEHRP Recommended Provisions
includes by reference Part II of the AISC Seismic Provisions (1997), together with the underlying
AISC-LRFD (1993) and ACI-318 (1995) standards. Part II of the AISC Seismic Provisions provides
definitions for composite systems consistent with the system designations in Table 5.2.2 and specifies
requirements for the seismic design of composite systems and components.
In general, available research shows that properly detailed composite elements and connections
can perform as well, or better, than structural steel and reinforced concrete components. However,
due to the lack of design experience with certain types of composite structures in high seismic risk
areas, usage of composite systems in Seismic Design Categories D and above requires documentation
(substantiating evidence) that the proposed system will perform as intended by Part II of the AISC
Seismic Provisions and implied by the R values in Table 5.2.2. It is intended that the substantiating
evidence consist of a rational analysis that considers force transfer between structural steel, reinforced
concrete and composite elements and identifies locations in the structure required to sustain inelastic
deformations and dissipate seismic energy. Design of composite members and connections to sustain
inelastic deformations shall be based on models and criteria substantiated by test data. For many
composite components, test data and design models are available and referenced in the commentary to
the AISC Seismic Provisions – Part II (1997).
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Chapter 11 Commentary
MASONRY STRUCTURE DESIGN REQUIREMENTS
11.1 GENERAL:
11.1.1 Scope: The provisions of Chapter 11 govern design and construction of all types of masonry.
Quality assurance is covered with a reference to Chapter 3. Reinforced and plain (unreinforced)
masonry elements that are part of the basic structural system and those that are not part of the basic
structural system are included.
11.1.2 Reference Documents: Design and construction standards cited in Chapter 11 are listed in
Sec. 11.1.2. The materials standards are specifically listed to include only those materials permitted by
the provisions. The listing includes the document's designation, the year of the edition and the title of
the document.
11.1.3 Definitions: Terms used in the provisions which have a specific meaning which differs from
the dictionary definition are defined in Sec. 11.1.3. All other terms are defined by the dictionary.
11.1.4 Notations: Notations used in the provisions are defined in Sec. 11.1.4. English units of
measure are stated followed by the metric unit in parenthesis for each term.
11.2 CONSTRUCTION REQUIREMENTS:
11.2.1 General: Ref. 11-2 is a standard specification prepared under consensus procedures. It was
developed by members representing construction, design, materials, and research of masonry
structures. The document is intended to be incorporated into contract documents used to construct
masonry structures.
This standard specification was developed to be used in conjunction with Building Code Requirements
for Masonry structures, Ref. 11-1. Appropriate standards for materials and test methods are
referenced. In addition to a general section, there are sections on masonry, reinforcement and metal
accessories, and grout.
The materials listed in Ref. 11-2 have been restricted in order to obtain more predictable behavior and
better performance required for strength design. Construction provisions found in Chapter 11 override
those found in Ref. 11-2.
11.2.2 Quality Assurance: See Chapter 3 of the Provisions and Commentary. Quality assurance
requirements for masonry structures include testing of masonry components (mortar, grout, and units)
or testing of masonry assemblages. Industry guidelines for materials testing are listed below.
1. Brick Institute of America, 11490 Commerce Park Drive, Reston, Virginia 22091, Technical Notes
on Brick Construction:
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1997 Commentary, Chapter 11
No. 39 Revised, "Testing for Engineered Brick Masonry: Brick, Mortar and Grout," January 1987.
No. 39A, "Testing for Engineered Brick Masonry: Determination of Allowable Design Stresses,"
December 1987.
No. 39B, "Testing for Engineering Brick Masonry: Quality Assurance," March 1988.
2. National Concrete Masonry Association, 2302 Horse Pen Road, Herndon, Virginia 22071-3499:
TEK 22A, Prism Testing for Engineered Concrete Masonry, 1979.
TEK 107, Laboratory and Field Testing of Mortar and Grout, 1979.
TEK 108, Testing Concrete Masonry Assemblages, 1979.
Industry guidelines for field inspection are listed below.
1. Brick Institute of America, 11490 Commerce Park Drive, Reston, Virginia 22091, Technical
Notes on Brick Construction:
No. 17C, "Reinforced Brick Masonry: Inspectors' Guide," May 1986.
2. National Concrete Masonry Association, 2302 Horse Pen Road, Herndon, Virginia 22071-3499:
TEK 65, Field Inspection of Engineered Concrete Masonry, 1975.
TEK 132, Inspector's Guide for Concrete Masonry Construction, 1983.
11.3 GENERAL DESIGN REQUIREMENTS:
11.3.1 Scope: This chapter offers three different methods for designing masonry structures. Any
method, used within the limitations imposed, provides acceptable masonry construction with
acceptable seismic resistance characteristics.
11.3.2 Empirical Masonry Design: Empirical design methods are based on the successful
performance of masonry buildings. Prescriptive requirements and limited exposure to loads are
necessary to ensure compliance.
The design process results in sizes and proportions of masonry elements using minimum thicknesses
and maximum spans. Although rudimentary stress calculations are made, empirical masonry design
does not require a complete structural analysis.
11.3.3 Plain (Unreinforced) Masonry Design: Design methods for plain masonry, often referred to
as unreinforced masonry. The procedures utilize working stress design requirements using principles
of mechanics.
11.3.4 Reinforced Masonry Design: Reinforcing steel complements the high compressive strength
of masonry with high tensile strength. Increased load-carrying capacity and greater ductility result
from the use of reinforcing steel.
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Masonry Structure Design Requirements
11.3.5 - 11.3.9 Seismic Design Categories A through F: Any type of masonry shear wall is
permitted in Seismic Design Categories A and B. Detailed plain masonry shear walls or intermediate
reinforced masonry shear walls are required for Seismic Design Category C. Special reinforced
masonry shear walls are required for Seismic Design Categories D, E, or F. Minimum requirements for
each type of masonry shear wall are given in Sec. 11.11. These requirements are consistent with
intended inelastic deformation capacities that are the bases for the R, S, and Cd factors given in Table
5.2.2. Additional requirements for construction of masonry elements other than shear walls are given
for each Seismic Design Category in Sec. 11.3.5 through 11.3.9
11.3.6 Seismic Design Category B: The use of empirical masonry design, Sec. 11.3.2, for the lateral
load resisting system is not appropriate for Seismic Design Category B. Masonry walls that are not
part of the lateral load resisting system may be designed by the empirical method.
11.3.10: Properties of Materials:
11.3.10.1 Steel Reinforcement Modulus of Elasticity: The given modulus of elasticity of steel
reinforcement is taken from previous codes and is consistent with established design values. Design
may be based on tested values of modulus of elasticity; however, these tests are rarely performed
because it is impractical to test materials to be used in the construction at the time when the project is
being designed.
11.3.10.2 Masonry Modulus of Elasticity: Modulus of elasticity of masonry is used in determining
stiffness of structural components prior to cracking. Therefore, the modulus is taken from the elastic
portion of the stress strain curve. The modulus of elasticity of masonry is not clearly related to any
property of mortar, unit, grout or prism h/t, but is influenced by all of these. TS5 concluded it was best
to relate the value of Em to the specified compressive strength of masonry. This is because fmN is also
influenced by these parameters. The 750 multiplier is used rather than lower multipliers reported
(Wolde-Tinsae, 1993) since the actual compressive strength of masonry must exceed the specified
compressive strength.
11.3.10.4 Masonry Compressive Strength: Research has been performed on structural masonry
components having a compressive strength in the range of 1,500 to 6,000 psi (10 to 41 MPa). Design
criteria are based on these research results. Design values therefore are limited to compressive
strengths in the range of 1,500 to 4,000 psi (10 to 28 MPa) for concrete masonry and 1,500 to 6,000
psi (10 to 41 MPa) for clay masonry.
11.3.10.5 Modulus of Rupture: Modulus of rupture values in Table 11.3.10 are based on allowable
working stress values for flexural tension multiplied by 2.0 to approximate the lower limit of strength
values. See the Commentary to Ref. 11-1 for discussion. Stack bond masonry has historically been
assumed to have no flexural bond strength across the head joints; thus, the grout area alone is used.
11.3.10.6 Reinforcement Strength: Research conducted on reinforced masonry components used
Grade 60 reinforcement. To be consistent with laboratory documented performance, design is based
on a steel yield strength that does not exceed 60,000 psi (413 MPa).
11.3.11 Section Properties: Section properties of masonry members are available in masonry design
publications. Design is based on specified dimension. Actual dimensions may vary within the tolerance
range given in the construction requirement (i.e., Ref. 11-2). The strength reduction factors are based
in part on an anticipated variation in the specified (design) dimensions.
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1997 Commentary, Chapter 11
11.3.12 Headed and Bent-Bar Anchor Bolts: This section covers cast-in-place headed anchor bolts
and bent-bar anchors (J- or L-bolts) in grout. General background information on this topic is given in
CEB, 1995.
The tensile capacity of a headed anchor bolt is governed by yield and fracture of the anchor steel or by
breakout of a roughly conical volume of masonry starting at the anchor head and having a fracture
surface oriented at 45 degrees to the masonry surface. Steel capacity is calculated conventionally using
the effective tensile stress area of the anchor (i.e., including the reduction in area of the anchor shank
due to threads). Masonry breakout capacity is calculated using expressions adapted from concrete
design, which use a simplified design model based on a stress of 4%fm’ uniformly distributed over the
area of that right circular cone, projected onto the surface of the masonry. Reductions in breakout
capacity due to nearby edges or adjacent anchors are computed in terms of reductions in those
projected areas (Brown and Whitlock, 1983).
The tensile capacity of a bent-bar anchor bolt (J- or L-bolt) is governed by yield and fracture of the
anchor steel, by tensile cone breakout of the masonry, or by straightening and pullout of the anchor
from the masonry. Capacities corresponding to the first two failure modes are calculated as for headed
anchor bolts. Pullout capacity is calculated as proposed by Shaikh, 1996. Possible contributions to
tensile pullout capacity due to friction are neglected.
The tensile breakout capacity of a headed anchor is usually much greater than the pullout capacity of a
J- or L-bolt. the designer is encouraged to use headed anchors when anchor tensile capacity is critical.
The shear capacity of a headed or a bent-bar anchor bolt is governed by yield and fracture of the
anchor steel or by masonry shear breakout. Steel capacity is calculated conventionally using the
effective tensile stress area (i.e., threads are conservatively assumed to lie in the critical shear plane).
Shear breakout capacity is calculated as proposed ;by Brown and Whitlock, 1983.
Under static shear loading, bent-bar anchor bolts (J- or L-bolts) do not exhibit straightening and
pullout. Under reversed cyclic shear, however, available research suggests that straightening and
pullout may occur. Headed anchor bolts are recommended for such applications (Malik et al., 1982).
11.5 STRENGTH AND DEFORMATION REQUIREMENTS:
11.5.3 Design Strength: The design strength of a member and its connections is calculated by
engineering principles and materials strength and yield values. This calculated strength is the nominal
strength of the member. The nominal strength is less than the expected or mean strength because
minimum guaranteed values or specified strengths are used for the calculations of nominal strength. A
strength reduction factor, N, is used to reduce the nominal strength to a design strength. The strength
reduction factor, N, is a variable that is dependent on the material and material behavior. Flexural
strength of reinforced members is reduced less by the N factor than is shear strength. Exceeding of the
flexural strength of a reinforced member causes yielding of the reinforcement but not strength
degradation. Exceeding of the shear strength results in a strength degradation.
202
Masonry Structure Design Requirements
Flexure Without Axial Load: The strength reduction factor for reinforced masonry is greater than
for plain masonry because plain masonry after cracking lacks ductile performance.
Axial Load and Axial Load with Flexure: If the axial load results in balanced strain conditions
(flexure produces strain in the reinforcement equal to the yield strain and strain in the masonry equal to
the maximum usable strain, ,mu) and the flexural reinforcement is minimal, an increase in flexural
moment can cause compressive stresses in excess of the compressive strength. The failure will not be
ductile; therefore, the strength reduction factor is more severe. Linear interpolation of the strength
reduction factor is allowed since the required axial strength due to factored load, Pu, decreases from the
axial load resulting in balanced strain conditions to zero, so as to make the transition linear from axial
load with flexure to flexure without axial load.
The strength reduction factor for the vertical members of wall frames is more restrictive than for shear
walls or coupled shear walls. The strength reduction factor for the vertical members of wall frames
does not have a linear variation to its value. When Pu/AnfNm is equal to 0.1, the strength reduction
factor will be equal to 0.65.
The strength reduction factor for plain masonry members is unchanged from that factor that is applied
for flexure only. Axial load increases the flexural capacity of plain masonry but does not significantly
change its lack of ductility.
Shear: Strength reduction factors for calculation of design shear strength are commonly more severe
than those factors used for calculation of design flexural strength. This concept is partially supported
by the wider variance of shear capacities that have been obtained from experimental testing. The
variance of the results of each experiment from the body of data is due not only to the variability of the
masonry materials, the test apparatus and test methods, and the shear strength parameters tested but
also to the greater sensitivity of shear resistance mechanisms to those factors.
Bearing: Exceeding of the bearing capacity causes crushing and spalling of bearing surfaces. The
strength reduction factors given are those established for elements that have strength degradation.
11.5.4 Deformation Requirements: Stiffness of a structural element is as important or more
important than strength. Stiffness is critical for serviceability and control of displacements. Drift of an
element is the movement of one story of the building relative to the adjacent stories or the
displacement of the shear wall relative to its fixed base. Drift of the top level of a shear wall is affected
by foundation flexibility but the structural stresses and strains in the wall would not be increased by
foundation flexibility.
The product of the effective moment of inertia, I, and the effective modulus of elasticity, E, is usually
used as a variable for the calculation of the deformation of reinforced elements. The variability in I is
caused by tensile cracking of the masonry cross section. If tensile cracking is not acceptable, as for
plain masonry, I has a single value and the compressive modulus of elasticity and the moment of inertia
of the gross cross section is used for the calculation of deformation.
If tensile cracking in anticipated, such as for reinforced masonry, the effective I at every cross section
of the wall or beam is dependent on the curvature of the cross section and the shear deformation of
each increment of the member length. Several nonlinear finite element programs have the capability of
determining the stiffness degradation of reinforced masonry elements, but the effective stiffness, I, can
be determined by use of Eq. 11.5.4.3.
203
1997 Commentary, Chapter 11
The cracking moment is calculated using the section modulus of the gross section of wall times the
modulus of rupture of masonry, fr. The moment of inertia of the cracked section is calculated about
the neutral axis of the section, using the masonry properties, and transforming the reinforcement into
equivalent masonry areas by use of the ratio of the compressive modulus of steel and masonry. The
cracked moment of inertia, Icr, and the compressive modulus of masonry, Em, is used to calculate the
effective moment of inertia, Ieff.
Eq. 11.5.4.3 has been used as a means of providing a transition in stiffness between gross moment of
inertia and a totally cracked section. Abboud (1987), Abboud and Hamid (1987), Abboud et al. (1990
and 1993), Hamid et al. (1989), and Horton and Tadros (1990) give additional insight and behavior for
computing deflection for masonry components.
11.6 FLEXURE AND AXIAL LOADS:
11.6.2 Design Requirements of Reinforced Masonry Members: The design principles listed are
those that traditionally have been used for reinforced masonry members. The theory used for design of
normally proportioned flexural members has limited applicability to deep flexural members. Shear
warping of the cross section and a combination of diagonal tension stress and flexural tension stresses
in the body of the deep flexural members require that deep beam theory be used for members that
exceed the specified limits of span to depth ratio.
11.6.2.2: Longitudinal reinforcement in flexural members is limited to a maximum amount to ensure
that masonry compressive strains will not exceed ultimate values. For all masonry components other
than walls bending in the out-of-plane sense, maximum reinforcement is limited in accordance with a
prescribed strain distribution based on a tensile strain equal to five times the yield strain for the
reinforcing bar closest to the edge of the member, and a maximum masonry compressive strain equal to
0.0025 for concrete masonry or 0.0035 for clay-unit masonry. By limiting longitudinal reinforcement in
this manner, inelastic curvature capacity is easily depicted as the slope of this strain distribution.
Because axial force is implicitly considered in the determination of maximum longitudinal
reinforcement, inelastic curvature capacity can be relied on no matter what the level of axial
compressive force. Thus, the capacity reduction factors, N, for axial load and flexure can be the same
as for flexure alone. Also, confinement reinforcement is not required because the maximum masonry
compressive strain will be less than ultimate values.
Calculated tensile force in the reinforcement is based on a stress equal to 1.25 times the yield stress to
account for differences between the actual yield strength and the minimum specified strength, and the
possibility of strain hardening. This increase of stress beyond yield also compensates for effects of
discontinuous tensile strain fields that develop as a result of tensile cracking.
The masonry compressive force is estimated using a rectangular stress block defined with parameters
based on recent research done with the Technical Coordination Committee for Masonry Research
(TCCMaR).
204
Masonry Structure Design Requirements
For walls bending out-of-plane, the limit on maximum reinforcement is relaxed by considering a strain
distribution based on 1.3 times the yield strain for the reinforcing bar closest to the member edge. This
limiting strain distribution is less severe than that adopted for in-plane bending. It is based on research
done by Blondet and Mayes (1991).
Maximum reinforcement per the requirements of Sec. 11.6.2.2.1 for an in-plane wall with uniformly
distributed vertical reinforcement can be derived using simple equilibrium concepts to give:
)
Dmax '
0.64 f m " &
Pg
bd
1.25f y (1 & ") & 0.5fs
(C11.6.2.2-1)
max "
where Dmax is the total amount of vertical steel divided by b and d; b is the width of the section; d is the
distance from the extreme compressive fiber to the location of the tensile vertical bar closest to the
edge of the member; " is equal to the depth of the compression zone divided by the effective depth, d;
Pg is equal to the unfactored gravity compressive force; fy is the specified yield stress of the
reinforcement, and fs max is the maximum compressive stress in the vertical reinforcement.
Similarly, maximum reinforcement per the requirements of Sec. 11.6.2.2.2 for an out-of-plane wall
with a single layer of vertical reinforcement centered on the wall section reduces to:
)
Dmax '
0.64 f m " &
Pg
bd
(C11.6.2.2-2)
2.50f y
where Dg max is the total amount of vertical steel divided by the gross area of the wall section.
Maximum percentages of reinforcement as given by Equations C11.6.2.2-1 and C11.6.2.2-2 are
plotted versus vertical compressive stress in Figures C11.6.2.2-1 and C11.6.2.2-2 for clay-unit and
concrete masonry of typical compressive strengths (3000 psi for clay-unit masonry and 2000 psi for
concrete masonry). For in-plane walls, average vertical compressive stress across the section is taken
to be the same as Pg /bd since the distance d is close to the total section depth. Maximum
reinforcement is limited by out-of-plane criteria for lower vertical axial compressive stresses and by inplane criteria for higher axial stresses.
For calibration purposes, maximum longitudinal reinforcement per Sec. 2108.2.3.3 of the 1997
Uniform Building Code is also plotted in Figures C11.6.2.2-1 and C11.6.2.2-2. The UBC criterion
limits longitudinal reinforcement to no more than one-half of that resulting in a balanced condition
where ultimate masonry compressive stress is equal to 0.003 and reinforcement is at its yield strain. A
rectangular stress block is to be used with a stress equal to 0.85 times f’m and a stress block depth equal
to 0.85 times the compressed zone. No increase in the yield stress is specified by the UBC to account
for increases due to higher expected strengths, strain hardening, or flexural cracking. The UBC
criterion also considers axial force when limiting maximum reinforcement. However, in addition to
205
1997 Commentary, Chapter 11
gravity forces, axial forces due to earthquake effects times a load factor of 1.4 are also considered.
Using the same procedure as used to derive the two former equations, the UBC criterion reduces to:
)
Dg
max
'
1
2
0.723 f m " &
Pu
bd
(C11.6.2.2-3)
2.00 f y
where Pu is the factored axial load (1.0D + 1.0L +1.4E).
The UBC criterion results in a more restrictive limit on maximum reinforcement for axial compressive
stress less than 381 psi for clay-unit masonry and 103 psi for concrete masonry (with the assumed
values of f’m). For axial compressive stresses above these values, the in-plane criterion per Sec.
11.6.2.2 results in a more restrictive limit on maximum reinforcement.
FIGURE C11.6.2.2-1 Maximum reinforcement for clay-unit masonry walls.
206
Masonry Structure Design Requirements
FIGURE C11.6.2.2-2 Maximum reinforcement for concrete masonry walls.
For further discussion, see He and Priestley (1992), Leiva and Klingner (1991), Limin and Priestley
(1988), Merryman et al. (1989), Seible et al. (1992), and Shing et al. (1991).
11.6.3 Design of Plan (Unreinforced) Masonry Members:
11.6.3.5: The axial load strengths given by Eq. 11.6.3.5-1 and 11.6.3.5-2 are based on analysis of the
results of axial load tests performed on clay and concrete masonry elements. For members having an
h/r ratio not exceeding 99, the specimens failed at loads less than the Euler buckling load. Eq.
11.6.3.5-1 was empirically fit to test data for these members. For h/r values in excess of 99, the limited
test data is adequately approximated by the Euler buckling equation.
11.7 SHEAR:
11.7.3 Design of Reinforced Masonry Members: The development of strength design procedures
for masonry requires a reasonably simplified and accurate equation that is capable of predicting the
ultimate shear strength of a masonry wall. Once agreed upon, this equation, together with appropriate
N factors, will form a key part of strength design procedures.
Over the past two decades many hundreds of tests have been performed in the U.S., Japan and New
Zealand to determine the strength and ductility of concrete block and clay brick shear walls subjected
to cyclic lateral load patterns. From these tests come equations to predict the shear strength of walls
usually calibrated to the tests carried out by the particular researcher. Fattal and Todd (1991)
compared the predictions of four different equations with available experimental results. The only flaw
in this work was that they included the UBC design equations with the inference that the UBC
equations were predictive equations for the ultimate shear strength of masonry. This is not the intent of
207
1997 Commentary, Chapter 11
the UBC equations. They were developed and then modified as part of the code development process
to provide a lower bound on the shear capacity of masonry walls. Two other reports/papers were
reviewed as part of preparing this overview document; Blondet et al. (1989) and Anderson and
Priestley (1992) also looked at predictive equations which were more simplified than those included in
the Fattal and Todd review. As a consequence, a total of six different predictive methods have been
reviewed.
In summary, the methods include two or more of the following components:
Vu ' Vm % Vsh % Vsv % Vp
(C11.7.3-1)
where:
Vm = contribution of the masonry
Vsh = contribution of the horizontal steel
Vsv = contribution of the vertical steel
Vp = contribution of the axial load
The report by Fattal and Todd (1991) is quite thorough and the test data used to assess the Shing,
Matsamura, and Architectural Institute of Japan (AIJ) predictive equations were also used to assess the
methods proposed by Blondet et al. (1989) and Anderson and Priestley (1992) and the final TCCMaR
equations that were developed as part of the TCCMaR study. The form of these equations are given in
Table C11.7.3-2. Rather than present the details of each of the test results that were developed, a
statistical summary is provided in Table C11.7.3-1. This provides the overall average, standard
deviation and coefficient of variation for all 62 tests included in the Fattal and Todd report. The values
given in Table C11.7.3-1 are the ratio of the shear strength obtained by the predictive equation divided
by the ultimate strength obtained from the test. A perfect prediction has a ratio of 1 and a conservative
prediction has a ratio less than 1.
TABLE C11.7.3-1
Tests
Shing
Okamoto
Matsamura
Blondet
et al.
Anderson &
Priestley
TCCMaR
All 62 tests
Mean
0.83
0.81
0.91
1.03
1.06
1.02
Standard
Deviation
0.23
0.27
0.20
0.24
0.23
0.24
Coefficient of
Variation
0.05
0.07
0.04
0.06
0.05
0.05
Mean Values
Tests 1-10
(Shing)
0.94
1.25
0.93
0.88
1.02
0.87
Tests 11-27
(Matsumura)
0.89
0.82
0.99
1.10
1.13
1.07
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Masonry Structure Design Requirements
Tests
Shing
Okamoto
Matsamura
Blondet
et al.
Anderson &
Priestley
TCCMaR
Tests 27-38
(Okamoto)
0.65
0.76
0.75
0.80
0.86
0.81
Tests 39-62
(Sveinsson)
0.82
0.66
0.91
1.13
1.11
1.12
Also included in Table C11.7.3-1 are the mean values of the four different sets of tests. Test 1-10 are
from Shing et al. (1991), Tests 11-28 are from Matsamura (1987), Tests 29-37 are from Okamoto et
al. (1987), and Tests 38-62 are from Sveinsson et al. (1985).
TABLE C11-7.3-2
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1997 Commentary, Chapter 11
As part of the TCCMaR studies, it was decided to use a combination of the Blondet et al. and
Anderson and Priestley equations. In comparing the manner in which the two methods account for
contribution of the masonry component, it was decided to use the Blondet form. As part of the
Berkeley tests (Mayes et al., 1976, Chen et al., 1978, Hidalgo et al., (1978, 1979), it was concluded
that the M/Vd ratio should be part of the masonry equation rather than just a straight function of 2.9
%fmN as in the Anderson and Priestley equation. Furthermore, there was very little numerical difference
in the values used to account for the vertical load contribution. As a consequence, it was decided to
use the more simplified form of 0.25Fc used by Anderson and Priestley. The final form of the
TCCMaR equation was given as:
)
v ' (4 & 1.75 M/Vd) f m % 0.5Dh f yh % 0.25Fc
(C11.7.3-2)
The metric equivalent of Eq. C11.7.3-2 is:
)
v ' 0.083(4 & 1.75 M/Vd) f m % 0.5Dh f yh % 0.25Fc
Some members of TCCMaR believed that some contribution of vertical steel should be included and
this issue was investigated. Many of the test specimens only included jamb steel and consequently two
different vertical steel contributions were investigated: 1/4Dvfyv and 1/4Dvifyvi where Dv is the total
vertical steel and Dvi is only the interior vertical steel and neglects the jamb steel. The correlation and
the test results were not as good when a contribution from vertical steel was included and consequently
it was decided not to include it in the recommended TCCMaR shear equation.
Application of the shear strength equation to partially grouted masonry was based in part on Fattal
(1993a and 1993b).
11.8 SPECIAL REQUIREMENTS FOR BEAMS:
11.8.1: Masonry beams may be loaded normal to their plane by wind or earthquake forces. The beam
must have adequate strength to span between support points under the action of the out-of-plane loads.
The arbitrary limits of 50 and 32 were judged to be adequate absolute limits on the unbraced span to
beam width ratios for the conditions listed.
11.8.2: Gravity loading of a masonry beam may be applied eccentrically to its vertical centroidal plane.
The lateral supports of the masonry building should restrain the beam from rotation under the eccentric
action of the gravity load.
If the beam is supported laterally at one edge only (top or bottom), then the lateral support should have
the moment capacity to restrain the rotation caused by loading normal to the face of the beam that is
eccentric to the support point.
11.8.3: A minimum amount of flexural reinforcement in the positive moment zone of the beam is
specified. This minimum is specified as a ratio, D, of the quantity of the reinforcement to the cross-
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Masonry Structure Design Requirements
sectional area of the beam. The minimum ratio specified is intended to require that the post-cracked
moment capacity exceeds the uncracked moment capacity of the section.
These requirements for a minimum quantity of positive moment reinforcement assumes that cracking
has occurred in zones of negative moment and that the change in beam stiffness has increased the
positive moment. However, if the positive moment capacity of the reinforced section exceeds the
uncracked positive moment capacity, transfer of moment to this zone is accommodated.
If a section of the adjacent concrete floor serves as the compression flange of the beam, minimum
reinforcement is based on the masonry section which is in tension due to positive moment.
11.8.4 DEEP FLEXURAL MEMBERS: The theory used for design of beams has a limited
applicability to deep beams. Shear warping of the cross section and a combination of diagonal tension
stress and flexural tension stress in the body of the deep beam requires that deep beam theory be used
for design of members that exceed the specified limits of span to depth ratio. Analysis of wall sections
that are used as beams generally will result in a distribution of tensile stress that requires the lower onehalf of the beam section to have uniformly distributed reinforcement. The uniform distribution of
reinforcement resists tensile stress caused by shear as well as flexural moment.
The flexural reinforcement for deep beams must meet or exceed the minimum flexural reinforcement
ratio of Sec. 11.8.3. Additionally, horizontal and vertical reinforcement must be distributed
throughout the length and depth of deep beams and must provide reinforcement ratios of at least
0.001. Distributed flexural reinforcement may be included in the calculations of the minimum
distributed reinforcement ratios.
Flexural reinforcement that is lumped entirely at the bottom and/or top of a deep flexural member,
however, should be ignored when calculating the distributed horizontal reinforcement ratio. In such a
case, the lumped flexural steel must provide a minimum flexural reinforcement ratio of 120/fy in
accordance with Sec. 11.8.3. For Grade 60 steel, this requirement is equivalent to a minimum flexural
reinforcement ratio of 0.002.
Although this flexural reinforcement ratio results in twice the ratio required by Sec. 11.8.4.3, the
flexural steel is lumped at the top and/or bottom of the beam and is not uniformly distributed. Since the
intent of Sec. 11.8.4.3 is to ensure a minimum quantity of uniformly distributed reinforcement
throughout the depth of the deep beam, the lumped flexural steel is not considered when calculating the
minimum distributed reinforcement ratios.
11.9 SPECIAL REQUIREMENTS FOR COLUMNS:
11.9.1: Maximum and minimum limitations on the area of longitudinal reinforcement for columns are
traditional values that have been in codes for many years. Minimum areas are limited so that creep of
the masonry, which tends to transfer load from masonry to reinforcing steel will not result in increasing
the stress in the steel to yield level. The maximum area limitation represents a practical limit on the
amount of reinforcing steel in terms of economy and steel placement. No testing or research has been
done to justify changes in these traditional values.
11.9.2: The minimum number of bars in columns also is a traditional number. It is obviously
appropriate, however, to suit rectangular or square column shapes and tying requirements.
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1997 Commentary, Chapter 11
11.9.3: The lateral tie restrictions in this section are also traditional. The column tie bending
requirements of Part c are to be as shown.
Reinforcement is restricted to an amount below the area required for flexural bending only in order to
preserve a ductile failure condition (i.e., steel will reach ultimate yield strain before concrete reaches
ultimate yield strain which would be defined as a brittle failure). It is therefore important to keep the
reinforcement ratio low.
11.10 SPECIAL REQUIREMENTS FOR WALLS:
11.10.1: The flexural strength of reinforced walls loaded normal to the surface is required to exceed
the uncracked flexural strength. The basis for this requirement is that a static over load on the wall may
cause very large displacements before strain hardening in the reinforcement increases the cracked
flexural strength to the value of the uncracked flexural strength.
11.11 SPECIAL REQUIREMENTS FOR SHEAR WALLS:
11.11.1 through 11.11.5: Detailing requirements for masonry shear walls have been reorganized for
1997 in Sec. 11.11.1 through 11.11.5 to provide direct correlations with those categories given as line
items in Table 5.2.2: ordinary plain masonry shear walls, detailed plain masonry shear walls, ordinary
reinforced masonry shear walls, intermediate reinforced masonry shear walls, and special reinforced
masonry shear walls. This was done so that variable R, S, and Cd factors could be given for each shear
wall category rather than specifying detailing requirements per the Seismic Design Category as was
done in previous editions of the Provisions. This reorganization is more consistent with the other
material chapters, which are organized by type of lateral-force-resisting elements (e.g., ordinary,
intermediate, or special moment resisting frames).
The word “plain” refers to the condition when a wall is unreinforced or tensile stresses in
reinforcement, if any, are neglected. The word “reinforced” refers to the condition when tensile
stresses in reinforcement are considered in the design process. “Detailed plain” and “intermediate
reinforced” walls much have minimum reinforcement per Seismic Design Category C whereas
“ordinary plain” and “ordinary reinforced” walls do not need to have any minimum reinforcement.
Reinforcement requirements for “special reinforced” walls follow the requ9irements for Seismic
Design Categories D and E. Requirements in each Seismic Design Category that are not germane to
masonry walls have bene retained in Sec. 11.3.5 through 11.3.9. in newly.
11.11.6 Flanged Shear Walls: Tests on flanged shear walls (Priestley and Limin, 1990; Sieble et al.,
1992) have indicated that if the conditions of Sec. 11.11.3.1 are satisfied, the flange will act in
conjunction with the web as a part of the flexural member.
The tributary flange widths defined in Sec. 11.11.3.3 and 11.11.3.4 are considered to be values
appropriate for predicting flexural behavior and strength. The values were taken from experimental
results. This has significance when calculating probable shear force on the wall, which is related to the
probable maximum flexural strength. For the calculation of maximum allowable reinforcement ratios,
the reinforcement in the flange of the width specified in Sec. 11.11.3.4 must be considered as part of
the maximum reinforcement ratio.
11.11.7 Coupled Shear Walls: Coupled shear walls are defined as shear walls in a common wall
plane that are interconnected or coupled by spandrel beams. These beams are typically at each floor
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Masonry Structure Design Requirements
level. The coupling beams can be a section of a reinforced concrete floor that has continuity with the
shear walls. Caution should be exercised to distinguish between coupled shear walls and walls with
openings. In a coupled wall system, the yield limit state is allowed only in the coupling beam and at the
base of the shear wall. If the flexure or shear yield state occurs in the wall between coupling beams, the
system is a wall with openings. This system has very limited ductility and should be redesigned to
prevent yielding in the reinforced wall at points other than the base of the shear wall.
Conformance with the requirement that the coupling beams reach their moment limit state at or before
the shear wall reaches its moment limit state need not be checked if the ratio of the depth of the shear
wall to the depth of the coupling beams exceeds 3 or more and the length of the coupling beams is less
than one-half of the story height. Linear elastic analyses of the coupled wall system are inadequate to
determine the yield status of the shear wall and the coupling beams. The stiffness of the shear wall will
degrade rapidly in the first story. The shear walls in the upper stories may be uncracked.
11.11.7.2 Shear Strength of Coupling Beams: The nominal shear strength of coupling beams must
be equal to the shear caused by development of a full yield hinge at each end of the coupling beams.
This nominal shear strength is estimated by dividing the sum of the calculated yield moment capacity of
each end of the coupling beams, 91 and 92, by the clear span length, L.
A coupling beam may consist of a masonry beam and a part of the reinforced concrete floor system.
Reinforcement in the floor system parallel to the coupling beam should be considered as a part of the
coupling beam reinforcement. The limit of the minimum width of floor that should be used is six times
the floor slab thickness. This quantity of reinforcement may exceed the limits of Sec. 11.6.2.2 but
should be used for the computation of the normal shear strength.
11.13 GLASS-UNIT MASONRY AND MASONRY VENEER: Chapters 11 and 12 of ACI 53095/ASCE 5-95/TMS 402-95 have been newly introduced in the 1997 Provisions to address design of
glass-unit masonry and masonry veneer. Direct reference is made to these chapters for design
requirements. Investigations of seismic performance have shown that architectural components
meeting these requirements perform well (Jalil, Kelm and Klingner, 1992, and Klingner, 1994).
REFERENCES
Abboud, B. E., X. Lu, and F. C. Schmitt. 1993. "An Evaluation of Three Current Deflection
Methods for Predicting the Lateral Deflection of Masonry walls." In Proceedings of the Sixth North
American Masonry Conference, Philadelphia, Pennsylvania, pp.73-85.
Abboud, B. E., and A. A. Hamid. 1987. "A Comparative Study of the Flexural Behavior of
Reinforced Block Masonry and Reinforced concrete Using Small Scale Model walls." In Proceedings
of the Fourth North American Masonry Conference, Los Angeles, California.
Abboud, B. E., A. A. Hamid, and H. G. Harris. 1990. "Small Scale Modeling of Concrete Block
Masonry structures," ACI Structural Journal 87(2):145-155.
Abboud, B. E. 1987. The Use of Small Scale Direct Models for Concrete Block Masonry
Assemblages and Slander Reinforced walls Under Out-of-Plane Loads, Doctoral Thesis, Drexel
University, Philadelphia, Pennsylvania.
213
1997 Commentary, Chapter 11
Blondet, J. M., R. L. Mayes, T. E. Kelley, R. R. Villablanca, and R. E. Klingner. 1989. Performance
of Engineered Masonry in the Chilean Earthquake of March 3, 1985: Implications for U.S. Design
Practice. Austin: Phil Ferguson Structural Engineering Laboratory, University of Texas.
Brown, Russell H., and A. Rhett Whitlock. 1983. “Strength of Anchor Bolts in Grouted Masonry,”
ASCE Journal of Structural Engineering 109(6) June.
CEB. 1995. “Design of Fastenings in Concrete (Draft CEB Guide, Parts 1 to 3)” and “Fastenings for
Seismic Retrofitting (State-of-the-Art Report on Design and Application),” Task Group 3.5
(Embedments), Euro-International Concrete Committee (CEB), CEB Bulletin D’Information No. 226,
Comite Euro-International du Beton, August.
Chen, S. W., P. A. Hidalgo, R. L. Mayes, R. W. Clough, and H. D. McNiven. 1978. Cyclic Loading
Tests of Masonry Single Piers, Vol. 2, EERC Report UCB/EERC-78/27. Berkeley: University of
California Earthquake Engineering Research Center.
Fattal, S. G. 1993a. strength of Partially Grouted Masonry shear walls, Report NIST 5147.
Gaithersburg, Maryland: National Institute of Standards and Technology.
Fattal, S. G. 1993b. The Effect of Critical Parameters on the Behavior of Partially Grouted
Masonry shear walls Under Lateral Loads, NIST Report NISTIR 5116. Gaithersburg, Maryland:
National Institute of Standards and Technology.
Fattal, S. G., and D. R. Todd. 1991. Ultimate strength of Masonry walls: Prediction vs. Test Results,
NIST Report NISTIR 4633. Gaithersburg, Maryland: National Institute of Standards and
Technology.
Hamid, A. A., M. K. Hatem, H. G. Harris, and B. E. Abboud. 1990. Hysteretic Response and
Ductility of Reinforced concrete Masonry walls Under Out-of-Plane Loading." In Proceedings of the
Fifth North American Conference, Urbana, Illinois, pp. 397-410.
Hamid, A. A., B. E. Abboud, M. W. Farah, M. K. Hatem, and H. G. Harris. 1989. "Response of
Reinforced Block Masonry walls to Out-of-Plane Static Loads," in U.S.-Japan Coordinated Program
For Masonry Building Research Report 3.2(a), Drexel University.
He, L., and N. Priestley. 1992. Seismic Behavior of Flanged Masonry shear walls - Final Report,
TCCMaR Report 4.1-2.
Hidalgo, P. A., R. L. Mayes, H. D. McNiven, and R. W. Clough. 1978. Cyclic Loading Tests of
Masonry Single Piers, Vol. 1 and 3, EERC Report UCB/EERC-78/27 and 79/12. Berkeley,
University of California Earthquake Engineering Research Center.
Horton, R. T., and M. K. Tadros. 1990. "Deflection of Reinforced Masonry Members," ACI
Structural Journal 87(4):73-85.
Jail, I., W. Kelm, and R. E. Klingner. 1992. Performance of Masonry and Masonry Veneer Buildings
in the Loma Prieta Earthquake. PMSEL Report 92-1. Austin: University of Texas, Department of
Civil Engineering.
Klingner, R. E. 1994. Performance of Masonry Structures in the Northridge, California, Earthquake
of January 17, 1991. Boulder, Colorado: The Masonry Society.
214
Masonry Structure Design Requirements
Leiva, G., and R. Klingner. 1991. In-Plane Seismic Resistance of Two-story Concrete Masonry shear
walls with Openings, TCCMaR Report 3.1(c)-2.
Limin, H., and N. Priestley. 1988. Seismic Behavior of Flanged Masonry shear walls, TCCMaR
Report 4.1-1.
Malik, J. B., J. A. Mendonca, and R. E. Klingner. 1982. “Effect of Reinforcing Details on the Shear
Resistance of Short Anchor Bolts Under Reversed Cyclic Loading,” Journal of the American Concrete
Institute, Proceedings, Vol. 79, No. 1, January-February, pp 3-11.
Matsumura, A. 1987. "Shear strength of Reinforced Hollow Unit Masonry walls." In Proceedings of
the 4th North American Masonry Conference, Los Angeles, California.
Mayes, R. L., Y. Omote, and R. W. Clough. 1976. Cyclic Shear Tests of Masonry Piers, Vol. 1,
Report 76-8. Berkeley: University of California Earthquake Engineering Research Center.
Merryman, K., G. Leiva, N. Antrobus, and R. Klingner. 1989. In-Plane Seismic Resistance of Twostory Concrete Masonry Coupled Shear Walls, TCCMaR Report 3.1(c)-1.
Okamoto, S., Y. Yamazaki, T. Kaminosono, M. Teshigawara, and H. Hirashi. 1987. "Seismic
Capacity of Reinforced Masonry walls and Beams in Wind and Seismic Effects." In Proceedings of
the 18th joint Meeting, U.S. - Japan Panel on Wind and Seismic Effects, ed. by N. J. Raufaste, Report
NBSIR 87-3540. Gaithersburg, Maryland: National Institute of Standards and Technology.
Priestley, N., and H. Limin. 1990. "Seismic Response of T-Section Masonry shear walls." TMS
Journal 9(1):10-19.
Seible, F., G. A. Hegemier, M. J. N. Priestley, G. R. Kingsley, A. Kurkchubasche, and A. Igarashi.
1992. The U.S. - TCCMaR Five-story Full Scale Masonry Research Building Test - Preliminary
Report, TCCMaR Report 9.2-4.
Shaikh, A. Fattah. 1996. “Design of Hooked Anchor bolts in Concrete and Masonry: Proposed Code
Provisions and Commentary,” prepared for the National Codes and Standards Council, 1996.
Shing, P. B., J. Noland, H. Spaeh, E. Klamerus, and M. Schuller. 1991. Response of Single-story
reinforced masonry shear walls to In-Plane Lateral Loads, TCCMaR Report 3.1(a)-2.
Shing, P. B., M. Schuller, and V. S. Hoskere. 1990. "In-Plane Resistance of Masonry shear walls,"
ASCE Journal of Structural Engineering 116(3).
Sveinsson, B. I., H. D. McNiven, and H. Sucuoglu. 1985. Cyclic Shear Tests of Masonry Piers, Vol
4, Report UCB/EERC 85-15. Berkeley: University of California Earthquake Engineering Research
Center.
Wolde-Tinsae, A. M., R. H. Atkinson, and A. A. Hamid. 1993. "State-of-the-Art Modulus of
Elasticity of Masonry." In Proceedings of the Sixth North American Masonry Conference. Boulder,
Colorado: The Masonry Society.
The following Technical Coordinating Committee for Masonry Research task reports not specifically
cited providing the substantiating data for the strength design criteria presented in this chapter are
available through the Earthquake Engineering Research Center Library in Richmond, California (phone
415-231-9403):
215
1997 Commentary, Chapter 11
Task No. Author(s) and Title
1.1-1:
Atkinson and Kingsley, Comparison of the Behavior of Clay & Concrete Masonry in
Compression, September 1985. 151 pgs.
1.2(a)-1:
Hamid, A. A., G. F. Assis, and H. G. Harris, Material Models for Grouted Block
Masonry, August 1988. 67 pgs.
1.2(a)-2:
Assis, G. F., A. A. Hamid, and H. G. Harris, Material Models for Grouted Block
Masonry, August 1989. 134 pgs.
1.2(b)-1:
Young, J. M., and R. H. Brown, Compressive Stress Distribution of Grouted Hollow Clay
Masonry Under Strain Gradient, May 1988, 170 pgs.
1.3-1:
Atkinson, R. H., An Assessment of Current Material Test Standards for Masonry Limit
States Design Methods, June 1991. 38 pgs.
2.1-1:
Hart, G., and M. Basharkhah, Slender Wall Structural Engineering Analysis Computer
Program (Shwall, Version 1.01), September 1987. 68 pgs.
2.1-2:
Hart, G., and M. Basharkhah, Shear Wall Structural Engineering Analysis Computer
Program (Shwall, Version 1.01). September 1987. 75 pgs.
2.1-3:
Nakaki, D., and G. Hart, Uplifting Response of Structures Subjected to Earthquake
Motions, August 1987. 200 pgs.
2.1-4:
Hart, G., N. Sajjad, and M. Basharkhah, Inelastic Column Analysis Computer Program
(INCAP, Version 1.01), March 1988.
2.1-5:
Hong, W. K., G. C. Hart, and R. E. Englekirk, Force-Deflection Evaluation and Models
for University of Colorado Flexural walls, December 1989.
2.1-6:
Hart, G. C., J. W. Jaw, and Y. K. Low, SCM Model for University of Colorado Flexural
Walls, December 1989. 31 pgs.
2.1-7:
Hart, G. C., N. Sajjad, and M. Basharkhah, Inelastic Masonry Flexural Shear Wall
Analysis Computer Program, February 1990. 41 pgs.
2.1-8:
Hart, G. C., R. Englekirk, M. Srinivasan, S. C. Huang, and D. J. Drag, Seismic
Performance Study, DPC Gymnasium, Elastic Time History Analysis Using SAP90,
February 1992. 41 pgs.
2.1-9:
Hart, G. C., R. Englekirk, M. Srinivasan, S. C. Huang, and D. J. Drag, Seismic
Performance Study, TMS Shopping Center, Elastic Time History Analysis Using SAP90,
February 1992. 42 pgs.
2.1-10:
Hart, G. C., R. Englekirk, J. W. Jaw, M. Srinivasan, S. C. Huang, and D. J. Drag, Seismic
Performance Study, RCJ Hotel, February 1992. 51 pgs.
2.1-11:
Hart, G. C., R. Englekirk, M. Srinivasan, S. C. Huang, and D. J. Drag, Performance
Study, 2-Story Masonry Wall-Frame Building, February 1992. 112 pgs.
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Masonry Structure Design Requirements
2.1-12:
Hart, G. C., R. Englekirk, J. W. Jaw, M. Srinivasan, S. C. Huang, and J. D. Drag, Seismic
Performance Study, Designed by Tentative Limit Sates Design Standard, February 1992.
75 pgs.
2.2-1:
Ewing, R. D., A. El-Mustapha, and J. Kariotis, FEM/I - A Finite Element Computer
Program for the Nonlinear Static Analysis of Reinforced Masonry Building Components,
December 1987 (Revised June 1990). 124 pgs.
2.2-2:
Ewing, R. D., Parametric Studies on Reinforced Masonry Shear Walls Using FEM/I, A
Nonlinear Finite Element Analysis Program, March 1992.
2.2-3:
Ewing, R. D., Finite Element Analysis of Reinforced Masonry Building Components
Designed by a Tentative Masonry Limit States Design Standard, March 1992. 48 pgs.
2.3-1:
Ewing, R., J. Kariotis, and A. El-Mustapha, LPM/I, A Computer Program for the
Nonlinear, Dynamic Analysis of Lumped Parameter Models, August, 1987. 200 pgs.
2.3-2:
Kariotis, J., A. El-Mustapha, and R. Ewing, Influence of Foundation Model on the
Uplifting of Structures, July 1988. 50 pgs.
2.3-3:
Kariotis, J., A. Rahman, and A. El-Mustapha, Investigation of Current Seismic Design
Provisions for Reinforced Masonry Shear Walls, January 1990. 48 pgs.
2.3-4:
Kariotis, J., A. Rahman, O. Waqfi, and R. Ewing, Version 1.03 LPM/I - A Computer
Program for the Nonlinear, Dynamic Analysis of Lumped Parameter Models, February
1992. 227 pgs.
2.3-5:
Kariotis, J., O. Waqfi, and R. Ewing, R., A Computer Program Using Beam Elements for
the Nonlinear, Dynamic Analysis of Lumped Parameter Models, February 1992. 96 pgs.
2.3-6:
Kariotis, J., and O. Waqfi, Comparison of the Dynamic Response of a Damped MDOF
Nonlinear Beam Model with an Equivalent SDOF Hysteretic Model, April 1992. 88 pgs.
2.3-7:
Kariotis, J., and O. Waqfi, Recommended Procedure for Calculation of the Balanced
Reinforcement Ratio, February 1992. 73 pgs.
2.4(b)-1:
Button, M. R., and R. L. Mayes, Out-of-Plane Seismic Response of Reinforced Masonry
Walls: Correlation of Full-Scale Test and Analytical Model Results, March 1991. 65 pgs.
3.1(a)-1:
Scrivener, J., Summary of Findings of Cyclic Tests on Masonry Piers, June 1986. 7 pgs.
3.1(a)-2:
Shing, P. B., J. Noland, H. Spaeh, E. Klamerus, and M. Schuller, Response of Single-Story
Reinforced Masonry Shear Walls to In-Plane Lateral Loads, January 1991. 136 pgs.
3.1(b)-1:
Seible, F., and H. LaRovere, Summary of Pseudo Dynamic Testing, February 1987. 46
pgs.
3.1(b)-2:
Igarashi, A., F. Seible, and G. Hegemier, Development of the Generated Sequential
displacement Procedure and the Simulated Seismic Testing of the TCCMaR Three-Story
In-Plane Walls, June 1993.
3.1(c)-1:
Merryman, K., G. Leiva, B. Antrobus, and R. Klingner, In-Plane Seismic Resistance of
Two-Story Concrete Masonry Coupled shear walls, September 1989. 176 pgs.
217
1997 Commentary, Chapter 11
3.1(c)-2:
Leiva, G., and R. Klingner, In-plane Seismic Resistance of Two-story Concrete Masonry
Shear Walls with Openings, August 1991. 326 pgs.
3.2(a)-1:
Hamid, A., B. Abboud, M. Farah, K. Hatem, and H. Harris, Response of Reinforced Block
Masonry Walls to Out-of-Plane Static Loads; September 1989. 120 pgs.
3.2(b)-1:
Agbabian, M., S. Adham, S. Masri, V. Avanessian, and V. Traina, Out-of-Plane Dynamic
Testing of Concrete Masonry Walls, Vol. 1 and 2, July 1989. 220 pgs.
3.2(b)-2:
Blondet, M., and R. L. Mayes, The Transverse Response of Clay Masonry Walls
Subjected to Strong Motion Earthquakes, Vol. 1: General Information, April 1991. 172
pgs.
3.2(b)-2:
Blondet, M., and R. L. Mayes, The Transverse Response of Clay Masonry Walls
Subjected to Strong Motion Earthquakes, Vol. 2: Walls No. 4 and 6 (Group 1), April
1991. 267 pgs.
3.2(b)-2:
Blondet, M., and R. L. Mayes, The Transverse Response of Clay Masonry Walls
Subjected to Strong Motion Earthquakes, Vol. 3: Walls No. 8, 9, 10 and 11 (Group 2),
April 1991. 310 pgs.
3.2(b)-2:
Blondet, M., and R. L. Mayes, The Transverse Response of Clay Masonry Walls
Subjected to Strong Motion Earthquakes, Vol. 4: Walls No. 3, 5, and 7 (Group 3), April
1991. 256 pgs.
4.1-1:
He, L., and M. J. N. Priestley, Seismic Behavior of Flanged Masonry Shear Walls, May
1988. 119 pgs.
4.1-2:
He, L., and M. J. N. Priestley, Seismic Behavior of Flanged Masonry Shear Walls - Final
Report, November 1992. 279 pgs.
4.2-1:
Hegemier, G., and H. Murakami, On the Behavior of Floor-to-Wall Intersections in
Concrete Masonry Construction: Part I: Experimental.
4.2-2:
Hegemier, G., and H. Murakami, On the Behavior of Floor-to-Wall Intersections in
Concrete Masonry Construction: Part II: Theoretical.
5.1-1:
Porter, M., and A. Sabri, Plank Diaphragm Characteristics, July 1990. 226 pgs.
5.2-1:
Porter, M., F. Yeomans, and A. Johns, Assembly of Existing Diaphragm Data, July 1990.
142 pgs.
6.2-1:
Scrivener, J., Bond of Reinforcement in Grouted Hollow-Unit Masonry: A State-of-theArt, June 1986. 53 pgs.
6.2-2:
Soric, Z., and L. Tulin, Bond Splices in Reinforced Masonry, August 1987. 296 pgs.
7.1-1:
Paulson, T., and D. Abrams, Measured Inelastic Response of Reinforced Masonry
Building structures to Earthquake Motions, October 1990. 294 pgs.
8.1-1:
Hart, G., A Limit State Design Method for Reinforced Masonry, June 1988.
218
8.1-2:
Hart, G., Expected Value Design in the Context of a Limit Sate Design Methodology,
February 1990.
8.2-1:
Hart, G., and G. T. Zorapapel, Reliability of Concrete Masonry Wall Structures,
December 1991. 229 pgs.
8.2-2:
Hart, G., and N. Sajjad, Confinement in Concrete Masonry, December 1990.
8.2-3:
Hart, G., and J. Jang, Seismic Performance of Masonry Wall Frames, December 1991.
9.1-1:
Kariotis, J. C., and A. W. Johnson, Design of Reinforced Masonry Research Building,
September 1987. 42 pgs.
9.1-2:
Kariotis, J. C., and O. M. Waqfi, Trial Designs Made in Accordance with Tentative Limit
States Design Standards for Reinforced Masonry Buildings, February 1992. 184 pgs.
9.2-1:
Seible, F., Report on Large Structures Testing Facilities in Japan, September 1985. 120
pgs.
9.2-2:
Seible, F., Design and Construction of the Charles Lee Powell Structural Systems
Laboratory, November 1986. 65 pgs.
9.2-3:
Seible, F., The Japanese Five-story Full Scale Reinforced Masonry Building Test,
January 1988. 100 pgs.
9.2-4:
Seible, F., G. A. Hegemier, M. J. N. Priestley, G. R. Kingsley, A. Kurkchubasche, and A.
Igarashi, The U.S. - TCCMaR Five-story Full Scale Masonry Research Building Test Preliminary Report, October 1992. 58 pgs.
11.1-1:
TCCMaR, Summary Report: U.S. Coordinated Program for Masonry Building
Research, September 1985 to August 1986. 190 pgs.
11.1-2:
TCCMaR, Status Report: U.S. Coordinated Program for Masonry Building Research,
November 1988. 170 pgs.
Appendix to Chapter 11 Commentary
ALTERNATIVE MASONRY STRUCTURE DESIGN REQUIREMENTS
11A.1.2 REFERENCE DOCUMENTS: This section references the Building Code Requirements
for Masonry Structures (Ref. 11A-1), which covers all types of masonry (clay, concrete, glass, stone,
etc.). Construction and quality assurance requirements are included by reference to Specifications for
Masonry Structures (ACI 530.1/ASCE 6/TMS 602). These design and construction documents
reference nationally recognized testing standards and material standards developed by the American
Society for Testing and Materials (ASTM) and others.
Concern has been expressed about the area of vertical reinforcement permitted in Sec. 3.1.2 of Ref.
11A-1. The percentage of the area of the grout space (minimum grout area) and the cover and
clearance requirements in Chapter 11 of Ref. 11A-1 provide reasonable assurance that the strength of
the reinforcement can be developed.
11A.1.2.1 Modifications to Appendix A of Reference 11A-1: Appendix A requirements of ACI
530/ASCE 5/TMS 402, "Special Provisions for Seismic Design," are based on seismic zones defined
by ASCE 7, Minimum Design Loads for Buildings and Other Structures. To be consistent with the
NEHRP Recommended Provisions, Table 11A.1.1 correlates seismic zones to Seismic Design
Categories.
11A.2 STRENGTH OF MEMBERS AND CONNECTIONS: The strength of members and
connections is based on working stress procedures multiplied by a factor to approximate typical
capacity. Capacity is approximated to equal the allowable stress determined by Ref. 11A-1 multiplied
by a 1.33 factor for load combinations that include wind or earthquake (Ref. 11A-1, Sec. 5.3.2) and
further multiplied by a 2.5 factor.
The resulting approximate capacity is 3.3 times the allowable stress. The design strength is equal to the
approximated capacity times the strength reduction factor, N, to achieve a reliable design level value.
11A.2.1: Splice length of reinforcement is based on the allowable stress in the reinforcement in
accordance with Ref. 11A-1, Sec. 11.5.7. This allowable stress is not modified by the 2.5 factor from
Sec. 11A-2 or by a strength reduction factor, N. Splice lengths required by these provisions are
therefore identical to the splice length required by Ref. 11A-1.
11A.3 RESPONSE MODIFICATION COEFFICIENTS: Masonry designed in accordance with
Chapter 10 of Ref. 11A-1 is required to have reinforcement to resist tension as well as minimum levels
of reinforcement and detailing based on seismic zone (i.e., NEHRP Recommended Provisions Seismic
Design Category). These requirements are intended to provide a level of inelastic cyclic straining
capacity consistent with the response modification coefficients of Table 2.2.2 for reinforced masonry.
Unreinforced masonry shear walls designed in accordance with Chapter 9 of Ref. 11A-1 that do not
tolerate inelastic straining without loss of strength use lower response modification coefficients to
ensure that unreinforced masonry shear walls remain within the elastic range when subjected to design
level seismic forces.
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Alternative Masonry Structure Design Requirements
11A.4 SEISMIC DESIGN CATEGORY A: Ref. 11A-1 permits three design methods for
masonry:
1. Design allowing tensile stresses in masonry (Chapter 9, Reinforced Masonry),
2. Design neglecting tensile strength of masonry (Chapter 10, Unreinforced Masonry), and
3. Empirical design of masonry (Chapter 11, Empirical).
Any of the three methods are considered appropriate for designs in Category A.
11A.5 SEISMIC DESIGN CATEGORY B: Masonry may be designed by Methods 1, 2, or 3
described above; however, in Category B, design of the basic structural system must be based on a
structural analysis in accordance with Methods 1 or 2 described above.
11A.6 SEISMIC DESIGN CATEGORY C: In addition to the requirements of Category B,
minimum levels of reinforcement and detailing are required in accordance with Appendix A of Ref.
11A-1. Further, noncomposite wythes (i.e., cavity walls) and screen walls must meet the detailing
requirements of Sec. 11A.6.1.1 and Sec. 11A.6.1.2, respectively.
11A.7 SEISMIC DESIGN CATEGORY D: In addition to the requirements of Category C, the
area and spacing of shear reinforcement for shear walls must meet the requirements of Sec. 11A.7.2.
Special inspection is required in accordance with Sec. 1.6.2.5.
11A.8 Seismic Design Category E: The additional requirements of Category E are intended to
ensure that the structure remains functional after the earthquake.
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Chapter 12 Commentary
WOOD STRUCTURE DESIGN REQUIREMENTS
12.1 REFERENCE DOCUMENTS: Wood construction practices have not been codified in a form
that is standard throughout the country. A major change for the 1997 Provisions is the incorporation
by reference of the Load and Resistance Factor Standard for Engineered Wood Construction
(LRFD), ASCE 16-95 (Ref. 12-1). Engineered wood strength design as prescribed in these Provisions
generally follows the LRFD specification (Ref. 12-1). Conventional light frame construction practice
as prescribed in these Provisions generally follows the requirements of the One- and Two-Family
Dwelling Code, Ref. 12-5, jointly sponsored by the three model code organizations. The One- and
Two-Family Dwelling Code is a revised and updated version of the Federal Housing Administration's
(FHA) Minimum Property Standards.
References 12-3 and 12-11 indicate that the term "structural-use panel" has replaced the term
“plywood” and this change in terminology was reflected in the 1991 and 1994 Provisions and is
continued in this 1997 edition. The term “structural-use panel” includes wood-based products
manufactured to meet a performance standard (Ref. 12-11). One requirement of this performance
standard is bracing or lateral force resistance capability. These products include oriented strand board
(OSB), plywood, and composite panels.
Many wood frame structures are a combination of engineered wood and “conventional” light frame
construction. Wood also is used in combination with other materials (American Institute of Timber
Construction, 1985; Breyer, 1993; Faherty and Williamson, 1989; Hoyle and Woeste, 1989; Somayaji,
1992; Stalnaker and Harris, 1989). The requirements of the model building codes were used as a
resource in developing the requirements introduced in the 1991 Provisions and further modified in this
edition.
The general requirements of Chapter 12 cover construction practices necessary to provide a
performance level of seismic resistance consistent with the purposes stated in Chapter 1. These
requirements also may be related to gravity load capacity and wind force resistance which is a natural
outgrowth of any design procedure.
For the 1997 Provisions, the reference documents for this chapter have been reordered and regrouped
according to their primary focus into three subsections: Sec. 12.1.2.1, Engineered Wood
Construction; Sec. 12.1.2.2, Conventional Construction; and Sec. 12.1.2.3, Materials Standards. This
was purely an editorial change to make the chapter consistent with the other materials chapters.
12.1.3 Definitions: Definitions are provided in Chapter 2 of the Provisions. The intent is to make the
definitions used in this chapter compatible with those used in other chapters and Ref. 12-1.
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The term “tie-down (hold-down)” is specifically defined to emphasize that the intent of the tie-down is
to resist load and deflection in the form of uplift of the chord of the shear wall. These devices are
required to show zero slip relative to the chord before load is resisted. Unlike devices that are attached
to the tie-down post with bolts fitted through over-drilled holes, the intent is that there be tight-fitting
holes so that load is immediately resisted. Examples of such devices are those attached to the tie-down
post with nails and then bolted through the sill plate to the foundation or stud of a wall below. If bolts
or similar types of fasteners are used to connect the device to the chord, cyclic tests using sequential
phased displacement tests procedure, or the equivalent, are required to show that no loss in wall
capacity, ductility, or stiffness is experienced. The bolts shall be “finger tight” and friction between the
tie-down device and the chord shall not be counted on to resist load. The tests shall also simulate the
deformation patterns expected in a wall assembly during a seismic event.
12.1.4 Notations: These variable definitions are included to assist the reader in understanding the
equations and tables used in the chapter. To the extent possible, these definitions are compatible with
the usage of the symbols in other chapters of the Provisions and Ref. 12-1. The definition of “factored
resistance” has been added as the values of 8ND to account for the time effect factor and resistance
factor. This is the basis of all values presented in this chapter.
12.2 DESIGN METHODS: Prior to the publication of Ref. 12-1, typical design of wood frame
structures followed the American Forest and Paper Association (AF&PA) National Design
Specification for Wood Construction (NDS) (AF&PA, 1991). The NDS is based on “allowable”
stresses and implied factors of safety. However, the design procedure provided by the Provisions was
developed on the premise of the resistance capacity of members and connections at the yield level
(ASCE, 1988; Canadian Wood Council, 1990 and 1991; Keenan, 1986). In order to accommodate
this difference in philosophy, the 1994 and prior editions of the Provisions made adjustments to the
tabulated “allowable” stresses in the reference documents.
With the completion of the Load and Resistance Factor Standard for Engineered Wood Construction
(ASCE, 1995), the modifications and use of an “allowable” stress based standard is no longer
necessary. Therefore, the 1997 Provisions includes the LRFD standard by reference (Ref. 12-1) and
uses it as the primary design procedure for engineered wood construction. The one difference between
the LRFD reference document and the Provisions is the use of the shear wall and diaphragm tables in
the Provisions. The resistances shown in Tables 12.4.3-2a and b were reduced 10 percent to account
for capacity reductions observed in cyclic testing of shear walls. (Dolan, 1996; Rose, 1996). This
reduction should be reviewed during the 2000 revision of the Provisions when additional test data are
available. However, the capacities provided for diaphragms were not reduced because the severe,
repeated racking damage that occurred in shear walls has not been noted in diaphragms in recent
earthquakes.
Conventional light-frame construction, a prescriptive method of constructing wood structures, is
allowed for some performance categories. These structures must be constructed according to the
requirements set forth in Sec. 12.5 and Ref. 12-5. If the construction deviates from these prescriptive
requirements, then the engineered design requirements of Sec. 12.3 and 12.4 and Ref. 12-1 shall be
followed. If a structure that is classified as conventional construction contains some structural
elements that do not meet the requirements of conventional construction, the elements in question can
be engineered in accordance with Section 12.2.2.1 without changing the rest of the structure to
engineered construction. The extent of design to be provided must be determined by the responsible
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registered design professional; however, the minimum acceptable extent is often taken to be force
transfer into the element, design of the element, and force transfer out of the element. This does not
apply to a structure that is principally an engineered structure with minor elements that could be
considered conventional. When more than one braced wall line or diaphragm in any area of a
conventional residence requires design, the nature of the construction may have changed, and
engineered design might be appropriate for the entire lateral-force-resisting system. The absence of a
ceiling diaphragm may also create a configuration that is non-conventional. The requirement for
engineering portions of a conventional construction structure to maintain lateral-force resistance and
stiffness is added to provide displacement compatibility. This is similar to the requirement in
Sec. 12.3.3.
Alternate Strength of Members and Connections: It remains the intent of the Provisions that load
and resistance factor design be used. This is the first time that a strength based standard has been
available for reference. When allowable stress design is to be used, however, the factored resistance of
members and connections subjected to seismic forces acting alone or in combination with other
prescribed loads shall be determined using a capacity reduction factor, N, times 2.16 times the
allowable stresses permitted in the National Design Specification for Wood Construction (NDS) and
supplements (AF&PA, 1991). The allowable stresses used shall not include a duration of load factor,
CD. The value of the capacity reduction factor, N, shall be as follows:
Wood members
In flexure
In compression
In tension
In shear and torsion
Connectors
Anchor bolts, bolts, lag bolts, nails, screws, etc.
Bolts in single shear in members that are part of a
seismic-force-resisting system
N = 1.00
N = 0.90
N = 1.00
N = 1.00
N = 0.85
N = 0.40
These “soft” conversions from allowable stress design values to load and resistance factor design
values appeared in Sec. 9.2 in the 1994 Provisions. For the 1997 Provisions, the factored resistance of
shear walls and diaphragms shall be in accordance with Tables 12.4.3-1a and b and Tables 12.4.3-2a
and b.
An alternative method of calculating soft conversions is provided in ASTM D5457-93. The reader is
cautioned, however, that the loads and load combinations to be used for conversion are not specified
so it is incumbent upon the user to determine appropriate conversion values.
12.3 ENGINEERED WOOD CONSTRUCTION: Engineered construction for wood structures
as defined by these Provisions encompasses all structures that cannot be classified as conventional
construction. Therefore, any structure exceeding the height limitations or having braced walls spaced
at greater intervals than prescribed in Table 12.5.1-1 or not conforming to the requirements in Sec.
12.5 must be engineered using standard design methods and principles of mechanics. Framing
members in engineered wood construction are sized based on calculated capacities to resist the loads
and forces imposed. Construction techniques that utilize wood for lateral force resistance in the form
of diaphragms or shear walls are discussed further in Sec. 12.4. Limitations have been set on the use of
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wood diaphragms that are used in combination with concrete and masonry walls or where torsion is
induced by the arrangement of the vertical resisting elements. A load path must be provided to
transmit the lateral forces from the diaphragm through the vertical resisting elements to the foundation.
It is important for the registered design professional to follow the forces down, as for gravity loads,
designing each connection and member along the load path.
Although wood moment resisting frames are not specifically covered in the Provisions, they are not
excluded by them. There are several technical references for their design, and they have been used in
Canada, Europe, and New Zealand. Wood moment resisting frames are designed to resist both vertical
loads and lateral forces. Detailing at columns to beam/girder connections is critical in developing frame
action and must incorporate effects of member shrinkage. Detailed information can be obtained from
the national wood research laboratories.
There are many references that describe the engineering practices and procedures used to design wood
structures that will perform adequately when subjected to lateral forces. The list at the end of this
Commentary chapter gives some, but by no means all, of these.
Changes in the 1997 Provisions include editorial changes to improve clarity or enforceability or to
make the provisions more compatible with the LRFD specification. Significant additions to Sec. 12.3
include displacement compatibility requirements and provisions governing horizontal distribution of
shear.
12.3.2 Framing Requirements: All framing that is designed as part of an engineered wood structure
must be designed with connectors that are able to transfer the required forces between various
components. These connectors can be either proprietary hardware or some of the more conventional
connections used in wood construction. However, the capacity of these connectors should be designed
according to accepted engineering practice to ensure that they will have the capacity to resist the
forces. The requirement of columns and posts being framed to full end bearing requires that the force
transfer from the column to the base be accomplished through end grain bearing of the wood, not
through placing the bolts or other connectors in shear. This requirement is included to ensure adequate
capacity for transfer of the vertical forces due to both gravity and overturning moment. Alternatively,
the connection can be designed to transfer the full loading through placing the bolts or other
connectors in shear neglecting all possible bearing.
12.3.3 Deformation Compatibility Requirements: The intent of this section is to require the
registered design professional to visualize the deformed shape of the structure to ensure that the
connections provide the necessary ductility to allow the probable deflection demand placed on the
structure. Unlike steel or other metal structures, wood is not a ductile material and virtually all of the
ductility achieved in the structure is from the metal connections. The planned failure mechanism of
wood structures must be through the connections, including the nailing of structural panels, otherwise
the failure will be brittle in nature. The philosophy of strong elastic columns and yielding beams cannot
be projected from steel to wood structures. To enable a wood structure to deform and dissipate
energy during a seismic event, the connections must be the weak link in the structure and be ductile.
Recent earthquakes, such as that in Northridge, California, have shown failures due to the fact that
consideration of deformation compatibility was neglected.
As an example of a compatibility issue, consider the deformation compatibility between a tie-down
connector to the tie-down post and the edge nailing of shear wall sheathing to the tie-down post and
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Wood Structure Design Requirements
adjacent bottom plate. Recent testing and observations from the Northridge earthquake have
suggested that the tie-down post experiences notable displacement before significant load can be
carried through the tie-down connector. This is due, among other things, to the oversizing of the bolt
holes in the tie-down post and the deformation and rotation of the tie-down bracket. Anchor bolts
connecting the bottom plate to the foundation below tend to attempt to carry the shear wall uplift as
the tie-down post moves. The sheathing, however, is nailed to both the bottom plate, which is held in
place, and the tie-down post, which is being pulled up. The result is a large deformation demand being
placed on the nails connecting the sheathing to the framing. This often results in the nails pulling out of
the sheathing at the tie-down post corner and sometimes results in an unzipping effect where a
significant portion of the remaining sheathing nailing fails as high loads cause one nailed connection to
fail and move on to overstress the next nail. The most effective solution currently known is to limit the
slip and deformation at the tie-down post by using a very stiff nailed or screwed tie-down.
Because this is an area where understanding of compatibility issues is just starting to develop, the Sec.
12.3.3 provision uses the wording “shall be considered in design” in lieu of the originally proposed
“provision shall be made to ensure...” The intent is to provide guidance while not requiring the
impossible. Equations for estimating diaphragm and shear wall deflections are discussed in Sec. 12.4.1
of this commentary.
If necessary, the stiffness of the wood diaphragms and shear walls can be increased with the use of
adhesives (if adhesives are to be used, see Commentary Sec. 12.4). However, it should be noted that
there are no rational methods for determining deflections in diaphragms that are constructed with nonwood sheathing materials. If the nail stiffness values or shear stiffness of non-wood sheathing materials
is determined in a scientific manner, such as through experimental cyclic testing (e.g., see Sec. 12.4 of
the Commentary), the calculations for determining the stiffness of shear panels will be considered
validated.
12.3.4 Design Limitations: Again, the consideration of deformation compatibility is a primary
consideration in engineered wood construction. This is especially true if the effective stiffnesses at load
levels experienced are not compatible between lateral load resisting elements considered. The interstory drift limits also must be considered when designing the structure.
12.3.4.1 Wood Members Resisting Horizontal Seismic Forces Contributed by Masonry and
Concrete: Due to the significant difference in in-plane stiffness between wood and masonry or
concrete systems, the use of wood members to resist the seismic forces produced by masonry and
concrete is not allowed. This is due to the probable torsional response such a structure will exhibit.
There are two exceptions where wood can be considered to be part of the lateral-load-resisting system.
The first is when the wood is in the form of a horizontal truss or diaphragm and the lateral loads do not
produce rotation of the horizontal member. The second exception is in structures of two stories or less
in height. In this case, the capacity of the wood shear walls will be sufficient to resist the lower
magnitude loads imposed. Five restrictions are imposed on these structures to ensure hat the structural
performance will not include rotational response and the drift will not cause failure of the masonry or
concrete portions of the structure.
12.3.4.2 Horizontal Distribution of Shear: This section of the Provisions is intended to define
when a diaphragm can be considered to be flexible or rigid. The purpose is to determine whether the
diaphragm should have the loads proportioned according to tributary area or stiffness. For flexible
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1997 Commentary, Chapter 12
diaphragms, the loads should be distributed according to tributary area whereas for rigid diaphragms,
the loads should be distributed according to stiffness. The remainder of the intent of this section is
covered in the general discussion for Sec. 12.3.4 above.
The distribution of seismic forces to the vertical elements (shear walls) of the lateral force resisting
system is dependent, first, on the relative stiffness of the vertical elements versus the horizontal
elements and, second, on the relative stiffness of the vertical elements when they have varying
deflection characteristics. The first issue is discussed in detail in the Provisions, which define when a
diaphragm can be considered flexible or rigid and set limits on diaphragms that act in rotation or that
cantilever. The second is largely an issue of engineering mechanics, but is discussed in Sec. 12.4 of this
commentary because significant variations in engineering practice currently exist.
In situations where a series of vertical elements of the lateral force resisting system are aligned in a row,
seismic forces will distribute to the different elements according to their relative stiffness.
Typical current design practice is to distribute seismic forces to a line of structural-use panel sheathed
walls in proportion to the lengths of the wall segments such that each segment carries the same unit
load. Structural-use panel sheathed wall segments without openings can generally be calculated to
have a stiffness in proportion to the wall length when: the tie-down slip is ignored, the structural-use
panel sheathing is selected from Tables 12.4.3-2a and b, and the aspect ratio limits of these provisions
are satisfied. For stiffness to be proportional to the wall length, the average load per nail for a given
nail size must be approximately equal. Conversely, a wall could be stiffened by adding nails and
reducing the calculated average load per nail. When including tie-down (hold-down) slip from anchors
with negligible slip (1/16 in, 2 mm or less), the assumption of wall stiffness proportional to length is still
fairly reasonable. For larger tie-down slip values, wall stiffness will move towards being proportional
to the square of the wall length; more importantly, however, the anchorage will start exhibiting
displacement compatibility problems as discussed in Sec. 12.3.3. For shear walls with aspect ratios
higher than 2/1, the stiffness is no longer in proportion to the length and equations are not available to
reasonably calculate the stiffness. For a line of walls with variations in tie-down slip, chord framing,
unit load per nail, or other aspects of construction, distribution of load to wall segments will need to be
based on a deflection analysis. The shear wall and diaphragm deflection equations that are currently
available are not always accurate. As testing results become available, the deflection calculation
formulas will need to be updated and design assumptions for distribution of forces reviewed.
Torsional Diaphragm Force Distribution: Sec. 12.3.4.2 defines a diaphragm as being flexible when
the maximum lateral deformation of the diaphragm is more than two times the average story drift.
Conversely, a diaphragm will be considered rigid when the diaphragm deflection is equal to or less than
two times the story drift. This is based on a model building code definition that applies to all materials.
For flexible diaphragms, seismic forces should be distributed to the vertical resisting elements
according to tributary area or simple beam analysis. Although rotation of the diaphragm may occur
because lines of vertical elements have different stiffnesses, the diaphragm is not considered stiff
enough to redistribute seismic forces through rotation. The diaphragm can be visualized as a singlespan beam supported on rigid supports.
For diaphragms defined as rigid, rotational or torsional behavior is expected and results in
redistribution of shear to the vertical-force-resisting elements. Requirements for horizontal shear
distribution are in Sec. 5.3.5. Torsional response of a structure due to irregular stiffness at any level
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Wood Structure Design Requirements
within the structure can be a potential cause of failure. As a result, dimensional and diaphragm ratio
limitations are provided for different categories of rotation. Also, additional requirements apply when
the structure is deemed to have a torsional irregularity in accordance with Table 5.2.3.2, Item 1.
In order to understand limits placed on diaphragms acting in rotation, it is helpful to consider two
different categories of diaphragms. Category I includes rigid diaphragms that rely on force transfer
through rotation to maintain stability. An example would be an open front structure with shear walls
on the other three sides. For this more structurally critical category, applicable limitations are:
C
Sec. 12.3.4.1 -- Diaphragm not to be used to resist forces contributed by masonry or concrete in
structures over one story.
C
Sec. 12.3.4.2, second paragraph -- The length of the diaphragm normal to the opening not to
exceed 25 feet ( to perpendicular shear walls), and diaphragm l/w ratios limited as noted.
C
Sec. 12.3.4.2, fourth paragraph -- Additional limitations when rotation is significant enough to be
considered a torsional irregularity.
Category II includes rigid diaphragms that have two or more supporting shear walls in each of two
perpendicular directions but, because the center of mass and center of rigidity do not coincide,
redistribute forces to shear walls through rotation of the diaphragm. These can be further divided into
Category IIA where the center of rigidity and mass are separated by a small portion of the structure’s
least dimension and the magnitude of the rotation is on the order of the accidental rotation discussed in
Sec. 5.3.5.2. For this level of rotation, Sec. 12.3.4.1 Exception 1 might be considered applicable and,
as a result, no particular limitations would be placed on diaphragm rotation for Category IIA.
Category IIB, rigid diaphragms with eccentricities larger than those discussed in Sec. 5.3.5.2, are
subject to the following limitations:
C
Sec. 12.3.4.1 -- Diaphragm not to be used to resist forces contributed by masonry or concrete in
structures over one story.
C
Sec. 12.3.4.2, fourth paragraph -- Additional limitations when rotation is significant enough to be
considered a torsional irregularity.
Sec. 12.4 and Tables 12.4.3-1a and b provide limits for diaphragm ratios. Because flexible diaphragms
have very little capacity for distributing torsional forces, further limitation of aspect ratios is used to
limit diaphragm deformation such that rigid behavior will occur. The resulting deformation demand on
the structure also is limited. Where diaphragm ratios are further limited, exceptions permit higher
ratios where calculations demonstrate that higher diaphragm deflections can be tolerated. In this case,
it is important to determine the effect of diaphragm rigidity on the horizontal distribution and also the
ability of other structural elements to withstand resulting deformations.
Proposals to prohibit wood diaphragms acting in rotation were advanced following the 1994
Northridge earthquake. To date, however, the understanding is that the notable collapses in the
Northridge Earthquake occurred in part because of lack of deformation compatibility between the
various vertical resisting elements rather than because of the inability of the diaphragm to act in
rotation.
Diaphragm Cantilever: Limitations concerning diaphragms that cantilever horizontally past the
outermost shear wall (or other vertical element) are related to but distinct from those imposed because
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of diaphragm rotation. Such diaphragms can be flexible or rigid and for rigid diaphragms can be
Category I, IIA or IIB. Both the limitations based on diaphragm rotation (if applicable) and the
following limit on diaphragm cantilever must be considered:
C
Sec. 12.3.4.2, third paragraph -- Diaphragm cantilever not to exceed the lesser of 25 feet or two
thirds of the diaphragm width.
Relative Stiffness of Vertical Elements: In situations where a series of vertical elements of the
lateral force resisting system are aligned in a row, the forces will distribute to the different elements
according to their relative stiffnesses. This behavior needs to be taken into account whether it involves
a series of structural-use panel shear walls of different lengths, a mixture of structural-use panel shear
walls with diagonal lumber or non-wood sheathed shear walls, or a mixture of wood shear walls with
walls of some other material such as concrete or masonry. See the Commentary Sec. 12.3.3 for a
discussion of deflection compatibility of structural elements.
12.3.4.3 Framing and Anchorage Limitations: The anchorage connections used in engineered
wood construction must be capable of resisting the forces that will occur between adjacent members
(beams and columns) and elements (diaphragms and shear walls). These connections can utilize
proprietary hardware or be designed in accordance with principles of mechanics. Connections are
often the cause of structural failures in wood structures, and the registered design professional is
cautioned to use conservative values for allowable capacities since most published values are based on
monotonic, not cyclic, load applications (National Oceanic and Atmospheric Administration, 1971).
Testing has shown that some one-sided bolted connections subject to cyclic loading, such as tie-down
devices, do not perform well. This was substantiated by the poor performance of various wood frame
elements in structures in the January 1994 Northridge earthquake.
Anchorage of Concrete or Masonry Walls Concrete or masonry wall anchorages using toe nails or
nails subject to withdrawal are prohibited by these Provisions. It has been shown that these types of
connections are inadequate and do not perform well (U.S. Department of Agriculture, National
Oceanic and Atmospheric Administration, 1971). Ledgers subjected to cross-grain bending or tension
perpendicular to grain also have performed poorly in past earthquakes, and their use is now prohibited
by these Provisions.
12.3.4.4 Shear Wall Anchorage: Tie-down devices that permit significant vertical movement
between the tie-down and the tie-down post can cause failure in the nails connecting the shear wall
sheathing to the sill plate. High tension and tie-down rotation due to eccentricity can cause the bolts
connecting the tie-down bracket to the tie-down post to pull through and split the tie-down post.
Devices that permit such movement include heavily loaded one-sided bolted connections with small
dimensions between elements resisting rotation due to eccentricity. Any device that uses over-drilled
holes such as most bolted connections will also allow significant slip to occur between the device and
the tie-down post before load is restrained. Both the NDS and the steel manual specify that bolt holes
will be over-drilled as much as 1/16 in (2 mm). This slip is what causes much of the damage to the
nails connecting the sheathing to the sill plate. Friction between the tie-down post and the device
cannot be counted on to resist load because relaxation in the wood will cause a loss of clamping and,
therefore, a loss in friction over time. This is why all tests should be conducted with the bolts “finger
tight” as opposed to tightening with a wrench.
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Cyclic tests of tie-down connections shall follow a pattern similar to the sequential phased displacement
(SPD) tests used by Dolan (1996) and Rose (1996). These tests used full wall assemblies and therefore
induced deflection patterns similar to those expected during an earthquake. If full wall assembly tests
are not used to test the tie-down devices, it must be shown that the expected rotation as well as tension
and compression are used. This is to ensure that walls using the devices will be able to deform in the
intended manner. This allows the registered design professional to consider compatibility of
deformations when designing the structure.
Splitting of the bottom plate of the shear walls has been observed in tests as well as in structures
subjected to earthquakes. Splitting of plates remote from the end of the shear wall can be caused by
the rotation of individual sheathing panels inducing upward forces in the nails at one end of the panel
and downward forces at the other. With the upward forces on the nails and a significant distance
perpendicular to the wall to the downward force produced by the anchor bolt, high cross-grain bending
stresses occur. Splitting can be reduced or eliminated by use of large plate washers sufficiently stiff to
reduce the eccentricity and by using thicker sill plates. Thicker sill plates (3 in. nominal, 65 mm) are
required for all shear walls for which Tables 12.4.3-2a and b require 3 in. nominal (65 mm) framing to
prevent splitting due to close nail spacing. This is to help prevent failure of the sill plate due to high
lateral loading and cross-grain bending.
The tendency for the nut on a tie-down bracket anchor bolt to loosen significantly during cycled
loading has been observed in some testing. One tested method of limiting the loosening is to apply
adhesive between the nut and tie-down bolt.
A logical load path for the structure must be provided so that the forces induced in the upper portions
of the structure are transmitted adequately through the lower portions of the structure to the
foundation.
12.4 DIAPHRAGMS AND SHEAR WALLS: Many wood-framed structures resist seismic forces
by acting as a "box system." The forces are transmitted through diaphragms, such as roofs and floors,
to reactions provided by shear walls. The forces are, in turn, transmitted to the lower stories and to the
final point of resistance, the foundations. A shear wall is a vertical diaphragm generally considered to
act as a cantilever from the foundation.
A diaphragm is a nearly horizontal structural unit that acts as a deep beam or girder when flexible in
comparison to its supports and as a plate when rigid in comparison to its supports. The analogy to a
girder is somewhat more appropriate since girders and diaphragms are made up as assemblies
(American Plywood Association, 1991; Applied Technology Council, 1981). Sheathing acts as the
"web" to resist the shear in diaphragms and is stiffened by the framing members, which also provide
support for gravity loads. Flexure is resisted by the edge elements acting like "flanges" to resist induced
tension or compression forces. The “flanges” may be top plates, ledgers, bond beams, or any other
continuous element at the perimeter of the diaphragm.
The "flange" (chord) can serve several functions at the same time, providing resistance to loads and
forces from different sources. When it functions as the tension or compression flange of the "girder," it
is important that the connection to the "web" be designed to accomplish the shear transfer. Since most
diaphragm "flanges" consist of many pieces, it is important that the splices be designed to transmit the
tension or compression occurring at the location of the splice and to recognize that the direction of
application of seismic forces can reverse. It should also be recognized that the shear walls parallel to
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the flanges may be acting with the flanges to distribute the diaphragm shears. When seismic forces are
delivered at right angles to the direction considered previously, the "flange" becomes a part of the
reaction system. It may function to transfer the diaphragm shear to the shear wall(s), either directly or
as a drag strut between segments of shear walls that are not continuous along the length of the
diaphragm.
For shear walls, which may be considered to be deep vertical cantilever beams, the "flanges" are
subjected to tension and compression while the "webs" resist the shear. It is important that the "flange"
members, splices at intermediate floors, and the connection to the foundation be detailed and sized for
the induced forces. The shear wall aspect ratios, h/w, have been limited to 2/1 in light of the poor
performance of walls with larger aspect ratios in recent tests and in the January 1994 Northridge
earthquake, and the results of recent research (Applied Technology Council, 1995; White and Dolan,
1996).
The "webs" of diaphragms and shear walls often have openings. The transfer of forces around
openings can be treated similarly to openings in the webs of steel girders. Members at the edges of
openings have forces due to flexure and the higher web shear induced in them and the resultant forces
must be transferred into the body of the diaphragm beyond the opening.
In the past, wood sheathed diaphragms have been considered to be flexible by many registered design
professionals and model code enforcement agencies. The newer versions of the model codes now
recognize that the determination of rigidity or flexibility for determination of how forces will be
distributed is dependent on the relative deformations of the horizontal and vertical resisting elements.
Wood sheathed diaphragms in structures with wood frame shear walls with various types of sheathing
may be relatively rigid compared with the vertical resisting system and, therefore, capable of
transmitting torsional lateral forces. A relative deformation of the diaphragm of two or more when
compared with the vertical resisting system deformation under the same force is used to define a
diaphragm as being flexible.
Discussions of these and other topics related to diaphragm and shear wall design, such as cyclic testing,
and pitched or notched diaphragms, may be found in the references.
12.4.1 Diaphragm and Shear Wall Aspect Ratios: The aspect ratio limits of Sec. 12.4.3.1 through
12.4.3.4 are unchanged from previous editions of the Provisions; however, definitions of the aspect
ratios have been added. The l/w for a diaphragm and h/w for a shear wall discussed in the first
paragraph are intended to be the typical definitions for aspect ratio. The diaphragm span, l, is measured
perpendicular to the direction of applied force, either for the full dimension of the diaphragm or
between supports as appropriate. The width, w, is parallel to the applied force (see Figure C12.4.1-1).
The h of the shear wall is the clear story height (see Figure C12.4.1-2). The alternate definition of
aspect ratio is only to be used where specific design and detailing is provided for force transfer around
the openings. It is required that the individual wall piers meet the aspect ratio requirement (see Figure
C12.4.1-3) and that the overall perforated wall also meet the aspect ratio requirement. Use of the
alternate definition involves the design and detailing of chord and collector elements around the
opening, and often results in the addition of blocking, strapping and special nailing. As noted, the
design for force transfer around the opening must use a rational analysis, and in accordance with ASCE
16 which discusses design principles for shear walls, diaphragms and boundary elements.
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Wood Structure Design Requirements
FIGURE C12.4.1-1. Diaphragm dimension definitions.
Deflections: The mid-span deflection of a simple-span blocked structural-use panel diaphragm
uniformly nailed throughout may be calculated by use of the following formula:
) '
() X)
5v l 3
v l
%
% 0.188 l en % j c
8wEA
4Gt
2w
where:
)
= the calculated deflection, in millimeters, or inches.
v
= maximum shear due to factored design loads in the direction under consideration, in
kilonewtons per meter, or pounds per lineal foot.
l
= diaphragm length, in meters, or feet.
w
= diaphragm width, in meters, or feet.
E
= elastic modulus of chords, in megapascals, or pounds per square inch.
A
= area of chord cross-section, in square millimeters, or square inches.
Gt
= panel rigidity through the thickness, in Newtons per millimeter, or pounds per inch.
en
= nail deformation, in millimeters, or inches
E ()cX)
= sum of individual chord-splice slip values on both sides of the diaphragm, each
multiplied by its distance to the nearest support, in millimeters, or inches.
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1997 Commentary, Chapter 12
FIGURE C12.4.1-2 Typical shear wall height-to-width ratio.
If not uniformly nailed, the constant 0.188 in the third term must be modified accordingly (See ATC-7,
Applied technology Council, 1981).
This formula was developed based on engineering principles and monotonic testing. Therefore, it
provides an estimate of diaphragm deflection due to loads applied in the factored resistance shear
range. The effects of cyclic loading and resulting energy dissipation may alter the values for nail
deformation in the third term as well as chord splice effects of the fourth term, if mechanically-spliced
wood chords are used. The formula is not applicable to partially-blocked diaphragms.
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Wood Structure Design Requirements
FIGURE C12.4.1-3 Alternate shear wall height-to-width ratio with design for force transfer
around openings.
The deflection of a blocked structural-use panel shear wall may be calculated by use of the following
formula.
) '
8v h3
v h
h
%
% 0.75h en %
d
wEA
Gt
w a
where:
) = the calculated deflection, in millimeters, or inches.
v
= maximum shear due to factored design loads at the top of the wall, in kilonewtons per meter,
or pounds per lineal foot.
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1997 Commentary, Chapter 12
h = shear wall height, in meters, or feet.
w = shear wall width, in meters, or feet.
E = elastic modulus of boundary element (vertical member at shear wall boundary),in megapascals,
or pounds per square inch.
A = area of boundary element cross-section (vertical member at shear wall boundary), in square
millimeters, or square inches.
Gt = panel rigidity through the thickness, in Newtons per millimeter, or pounds per inch.
en = nail deformation, in millimeters, or inches.
da = deflection due to anchorage details ( rotation and slip at hold downs),in millimeters, or inches.
Guidance for use of the above two equations can be found in References 12-2, 12-3, and 12-4, and
ATC-7 (Applied Technology Council, 1981).
The capacity of shear walls shall be determined either from tabulated values that are based on
experimental results or from standard principles of mechanics. The tables of allowable values for shear
walls sheathed with other than wood or wood-based structural-use panels were eliminated in the 1991
Provisions as a result of re-learning the lessons from past earthquakes and testing on the performance
of structures sheathed with these materials during the Northridge earthquake. The values for capacity
for shear walls sheathed with wood-based structural-use panels have been reduced from monotonic test
values by 10 percent to account for the reduction in capacity observed during cyclic tests. Capacities
for diaphragms were not reduced from the monotonic test values because the severe damage that
occurred in shear walls has not been noted in diaphragms in recent earthquakes.
One stipulation is that there are no accepted rational methods for calculating deflections for
diaphragms and shear walls that are sheathed with materials other than wood-based structural-use
panel products fastened with nails. Therefore, if a rational method is to be used, the capacity of the
fastener in the sheathing material must be validated by acceptable test procedures employing cyclic
forces or displacements. Validation must include correlation between the overall stiffness and capacity
predicted by principles of mechanics and that observed from test results. A diaphragm or shear wall
sheathed with dissimilar materials on the two faces should be designed as a single-sided wall using the
capacity of the stronger of the materials and ignoring the weaker of the materials.
The Provisions are based on assemblies having energy dissipation capacities which were recognized in
setting the R factors. For diaphragms and shear walls utilizing wood framing, the energy dissipation is
almost entirely due to nail bending. Fasteners other than nails and staples have not been extensively
tested under cyclic load application. When screws or adhesives have been tested in assemblies
subjected to cyclic loading, they have had a brittle mode of failure. For this reason, adhesives are
prohibited for wood framed shear wall assemblies and only the tabulated values for nailed or stapled
sheathing are recommended. Analysis and design of shear wall sheathing applied with adhesives is
beyond the scope of the Provisions. If one wished to use shear wall sheathing attached with adhesives,
as an alternate method of construction in accordance with Sec. 1.2.5, caution should be used (Dolan
and White, 1992; Foschi and Filiatrault, 1990). The increased stiffness will result in larger forces being
attracted to the structure. The anchorage connections and adjoining assemblies must, therefore, be
designed for these increased forces. Due to the brittle failure mode, these walls should be designed to
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Wood Structure Design Requirements
remain elastic, similar to unreinforced masonry. The use of adhesives for attaching sheathing for
diaphragms increases their stiffness, and could easily change the diaphragm response from flexible to
rigid.
12.4.2 Shear Resistance Based on Principals of Mechanics: Discussion of cyclic test protocol is
included in ATC (1995), Dolan (1996), and Rose (1996).
12.4.3 Sheathing Requirements: Sheathing nails should be driven flush with the surface of the
panel, and not further. This could result in the nail head creating a small depression in, but not
fracturing, the first veneer. This requirement is imposed because of the significant reduction in
capacity and ductility observed in shear walls constructed with over-driven nails. It is advised that the
edge distance for sheathing nails be increased as much as possible along the bottom of the panel to
reduce the potential for the nails to pull through the sheathing.
12.4.3.3 and 12.4.3.4 Single and Double Diagonally Sheathed Lumber Diaphragms and Shear
Walls: Diagonally sheathed lumber diaphragms and shear walls are presented in the Provisions
because they are still used for new construction in some regions. The 1994 Provisions contain
allowable stress design values. The design values in the 1997 Provisions are expressed in terms of the
factored shear resistance (8ND) in order to provide consistency with the tables for structural-use
panels. The factored shear resistance is based on a soft conversion from the model code allowable
stress loads and capacities to Provisions strength loads for regions with high effective peak
accelerations. This will allow users in the western states, were this construction is currently being used,
to continue with little or no change in requirements; at the same time, reasonable values are provided
for regions with lower effective peak accelerations.
12.5 CONVENTIONAL LIGHT-FRAME CONSTRUCTION: These provisions intend that a
structure using conventional construction methods and complying with the requirements of this section
be deemed capable of resisting the seismic forces imposed by the Provisions. Repetitive framing
members such as joists, rafters, and studs together with sheathing and finishes comprise conventional
light-frame construction. The subject of conventional construction is addressed in each of the model
codes. It is acknowledged and accepted that, for the most part, the conventional construction
provisions in the model codes concerning framing members and sheathing that carry gravity loads are
adequate. This is due to the fact that the tables in the model codes giving allowable spans have been
developed using basic principles of mechanics. For seismic lateral force resistance, however, experience
has shown that additional requirements are needed.
To provide lateral force resistance in vertical elements of structures, wall bracing requirements have
been incorporated in conventional construction provisions of the model codes. With a few exceptions,
these generally have been adequate for single family residences for which conventional construction
requirements were originally developed. While the model building codes have been quite specific as to
the type of bracing materials to be used and the amount of bracing required in any wall, no limits on the
number or maximum separation between braced walls have been established. This section of the
Provisions introduces the concept of mandating the maximum spacing of braced wall lines. By
mandating the maximum spacing of braced wall lines and thereby limiting the lateral forces acting on
these vertical elements, these revisions provide for a lateral-force-resisting system that will be less
prone to overstressing and that can be applied and enforced more uniformly than previous model
building code requirements. While specific elements of light-frame construction may be calculated to be
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1997 Commentary, Chapter 12
overstressed, there is typically a great deal of redundancy and uncounted resistance in such structures
and they have generally performed well in past earthquakes. The experience in the Northridge
earthquake was, however, less reassuring, especially for those residences relying on gypsum board or
stucco for lateral force resistance. The light weight of conventional construction, together with the
large energy dissipation capacity of the multiple fasteners used and inherent redundancy of the system
are major factors in the observed good performance where wood or wood-based panels were used.
The scope of this section specifically excludes prescriptive design of structures with concrete or
masonry walls above the basement story, with the exception of veneer, in order to maintain the light
weight of construction that the bracing requirements are based on. Wood braced wall panels and
diaphragms as prescribed in this section are not intended to support lateral forces due to masonry or
concrete construction. Prescriptive (empirical) design of masonry walls is allowed for in Chapter 11;
however, design of structures combining masonry wall construction and wood roof and floor
diaphragm construction must have an engineered design. In regions of high seismic activity, past
earthquakes have demonstrated significant problems with structures combining masonry and wood
construction. While engineered design requirements do address these problems, the prescriptive
requirements in the model codes do not adequately address these problems. Masonry and concrete
basement walls are permitted to be constructed in accordance with the requirements of Ref. 12-5.
12.5.1.1 Irregular Structures: This section was added to the 1997 Provisions to clarify the
definition of irregular (unusually shaped) structures that would require the structure to be designed for
the forces prescribed in Chapter 5 in accordance with the requirements of Sec. 12.3 and 12.4. The
descriptions and diagrams provide the registered design professional with several typical irregularities
that produce torsional response, or result in forces considered high enough to require an engineered
design and applies only to Seismic Design Category C and D structures.
Structures with geometric discontinuities in the lateral force resisting system have been observed to
sustain more earthquake and wind damage than structures without discontinuities. They have also
been observed to concentrate damage at the discontinuity location. For Seismic Design Categories C
and D, this section translates applicable irregularities from Tables 5.2.3.2 and 5.2.3.3 into limitations
on conventional light-frame construction. When a structure falls within the description of irregular, it is
required that either the entire structure or the non-conventional portions be engineered in accordance
with the engineered design portions of these Provisions. The irregularities are based on similar model
code requirements. While conceptually these are equally applicable to all Seismic Design Categories,
they are more readily accepted in areas of high seismic risk, where damage due to irregularities has
repeatedly been observed.
The engineered design of non-conventional portions in lieu of the entire structure is a common practice
in some regions. The registered design professional is left to judge the extent of the portion to be
designed. This often involves design of the nonconforming element, force transfer into the element,
and a load path from the element to the foundation. A nonconforming portion will sometimes have
enough of an impact on the behavior of a structure to warrant that the entire lateral-force-resisting
system receive an engineered design.
12.5.1.1.1: This limitation is based on Item 4 of Table 5.2.3.3 and applies when braced wall panels are
offset out-of-plane from floor to floor. In-plane offsets are discussed in another item. Ideally braced
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Wood Structure Design Requirements
wall panels would always stack above of each other from floor to floor with the length stepping down
at upper floors as less length of bracing is required.
Because cantilevers and set backs are very often incorporated into residential construction, the
exception offers rules by which limited cantilevers and setbacks can be considered conventional. Floor
joists are limited to 2 by 10 (actual 1½ by 9¼ in., 38 by 235 mm) or larger and doubled at braced wall
panel ends in order to accommodate the vertical overturning reactions at the end of braced wall panels.
In addition the ends of cantilevers are attached to a common rim joist to allow for redistribution of
load. For rim joists that cannot run the entire length of the cantilever, the metal tie is intended to
transfer vertical shear as well as provide a nominal tension tie. Limitations are placed on gravity loads
to be carried by cantilever or setback floor joists so that the joist strength will not be exceeded. The
roof loads discussed are based on the use of solid sawn members where allowable spans limit the
possible loads. Where engineered framing members such as trusses are used, gravity load capacity of
the cantilevered or setback floor joists should be carefully evaluated.
12.5.1.1.2: This limitation is based in Item 1 of Table 5.2.3.2, and applies to open-front structures or
portions of structures. The conventional construction bracing concept is based on using braced wall
lines to divide a structure up into a series of boxes of limited dimension, with the seismic force to each
box being limited by the size. The intent is that each box be supported by braced wall lines on all four
sides, limiting the amount of torsion that can occur. The exception, which permits portions of roofs or
floors to extend past the braced wall line, is intended to permit construction such as porch roofs and
bay windows. Walls with no lateral resistance are allowed in areas where braced walls are prohibited.
12.5.1.1.3: This limitation is based on Item 4 of Table 5.2.3.3 and applies when braced wall panels are
offset in-plane. Ends of braced wall panels supported on window or door headers can be calculated to
transfer large vertical reactions to headers that may not be of adequate size to resist these reactions.
The exception permits a 1 foot extension of the braced wall panel over a 4 by 12 (actual 3½ by 11¼
in., 89 by 286 mm) header on the basis that the vertical reaction is within a 45 degree line of the header
support and therefore will not result in critical shear or flexure. All other header conditions require an
engineered design. Walls with no lateral resistance are allowed in areas where braced walls are
prohibited.
12.5.1.1.4: This limitation results from observation of damage that is somewhat unique to split-level
wood frame construction. If floors on either side of an offset move in opposite directions due to
earthquake or wind loading, the short bearing wall in the middle becomes unstable and vertical support
for the upper joists can be lost, resulting in a collapse. If the vertical offset is limited to a dimension
equal to or less than the joist depth, then a simple strap tie directly connecting joists on different levels
can be provided, and the irregularity eliminated. CABO One- and Two-Family Dwelling Code Sec.
502.4.1 provides requirements for tying of floor joists.
12.5.1.1.5: This limitation is based on Item 5 of Table 5.2.3.3 and applies to nonperpendicular braced
wall lines. When braced wall lines are not perpendicular to each other, further evaluation is needed to
determine force distributions and required bracing.
12.5.1.1.6: This limitation is based on Item 3 of Table 5.2.3.2 and attempts to place a practical limit on
openings in floors and roofs. Because stair openings are essential to residential construction and have
long been used without any report of life-safety hazards resulting, these are felt to be acceptable
conventional construction. See Sec. 12.5.3.7 for detailing requirements for permitted openings.
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1997 Commentary, Chapter 12
12.5.1.1.7: This limits a condition that can cause a torsional irregularity per Item 1 of Table 5.2.3.2.
Where heights of braced wall panels vary significantly, distribution of lateral forces will also vary. If a
structure on a hill is supported on 2 foot high braced cripple wall panels on one side and 8 foot high
panels on the other, torsion and redistribution of forces will occur. An engineered design for this
situation is required in order to evaluate force distribution and provide adequate wall bracing and
anchor bolting. This limitation applies specifically to walls from the foundation to the floor. While
gable-end walls have similar variations in wall heights, this has not been observed to be a significant
concern in conventional construction. See Sec. 12.5.3.6 for detailing requirements for permitted
foundation stepping.
12.5.2.1 Spacing Between Braced Wall Lines: Table 12.5.1-1 prescribes the spacing of braced wall
lines and number of stories permitted for conventional construction structures. Figures C12.5.2.1-1
and C12.5.2.1-2 illustrate the basic components of the lateral bracing system. Information in Tables
12.5.1-1 and 12.5.2-1 was first included in the 1991 Edition.
12.5.2.2 Braced Wall Line Sheathing Requirements: Table 12.5.2-1 prescribes the minimum
length of bracing along each 25 ft (7.6 m) length of braced wall line. (See Commentary Sec. 12.4
regarding adhesive attachment.) Total height of structures has been reduced to limit overturning of the
braced walls so that significant uplift is not generally encountered. The height limit will accommodate
8 to 10 ft (2.4 to 3 m) story heights.
12.5.3 DETAILING REQUIREMENTS: The intent of this section is to rely on the traditional
light-frame conventional construction materials and fastenings as prescribed in the references for this
chapter. Braced wall panels are not required to be aligned vertically or horizontally (within the limits
prescribed in Sec. 12.5.1.1) but stacking is desirable where possible. With the freedom provided for
non-alignment it becomes important that a load path be provided to transfer lateral forces from upper
levels through intermediate vertical and horizontal resisting elements to the foundation. Connections
between horizontal and vertical resisting elements are prescribed. In structures two or three stories in
height, it is desirable to have interior braced wall panels supported on a continuous foundation. See
Figures C12.5.3-1 through C12.5.3-11 for examples of connections.
The 1997 Provisions incorporates some of the wall anchorage, top plate, and braced wall panel
connection requirements from the model building codes. These are included for completeness of the
document and to clarify the requirement for the registered design professional. Additional
requirements for foundations supporting braced wall panels has also been added to provide guidance
and clarity for the registered design professional.
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Wood Structure Design Requirements
FIGURE C12.5.2.1-1 Acceptable one-story bracing example.
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1997 Commentary, Chapter 12
FIGURE C12.5.2.1-2 Acceptable two-story bracing example.
242
Wood Structure Design Requirements
FIGURE C12.5.3-1 Wall anchor detail.
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1997 Commentary, Chapter 12
FIGURE C12.5.3-2 Double top plate splice.
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Wood Structure Design Requirements
FIGURE C12.5.3-3 Single top plate splice.
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1997 Commentary, Chapter 12
FIGURE C12.5.3-4 Full bearing on bottom plate.
246
Wood Structure Design Requirements
FIGURE C12.5.3-5 Exterior braced wall.
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1997 Commentary, Chapter 12
FIGURE C12.5.3-6 Interior braced wall at perpendicular joist.
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Wood Structure Design Requirements
FIGURE C12.5.3-7 Interior braced wall at parallel joist.
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1997 Commentary, Chapter 12
FIGURE C12.5.3-8 Offset at interior braced wall.
250
Wood Structure Design Requirements
FIGURE C12.5.3-9 Diaphragm connection to braced wall below.
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1997 Commentary, Chapter 12
FIGURE C12.5.3-10 Post base detail.
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Wood Structure Design Requirements
FIGURE C12.5.3-11 Wood beam connection to post.
12.5.3.1 Braced Wall Panel Connections: The exception provided in this section of the Provisions is
included due to the difficulty in providing a mechanism to transfer the diaphragm loads from a truss
roof system to the braced wall panels of the top story. This problem has been considered by the
Clackamas County, Oregon Building Codes Division, and an alternate to the CABO Building Code
Sec. 402.10 was written in 1993, and revised September 5, 1995. The details shown in Figure
C12.5.3.1-1 through C12.5.3.1-4 are provided as suggested methods for providing positive transfer of
the lateral forces from the diaphragm through the web sections of the trusses to the top of the braced
wall panels below.
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1997 Commentary, Chapter 12
FIGURE C12.5.3.1-1. Suggested methods for transfering roof diaphragms loads to braced wall
panels.
FIGURE C12.5.3.1-2. Alternate gable end brace.
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Wood Structure Design Requirements
FIGURE C12.5.3.1-3 Wall parallel to truss bracing detail.
FIGURE C12.5.3.1-4 Wall parallel to truss alternate bracing detail.
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1997 Commentary, Chapter 12
12.6 SEISMIC DESIGN CATEGORY A: Wood frame structures assigned to Seismic Design
Category A, other than one- and two-family dwellings, must conform with Sec. 12.5 or if engineered
need only comply with the reference documents and Sec. 5.2.6.1.2. Exceptions addressing one- and
two-family detached dwellings appear in Sec. 1.2.1.
12.7 SEISMIC DESIGN CATEGORIES B, C, AND D: In the 1997 Provisions, Seismic Design
Categories B, C, and D, have been combined. At the same time, subsections on material limitations
and anchorage requirements have been moved to Sec. 12.3 and 12.4. This was based on the
philosophy that detailing requirements should vary based on R values, not Seismic Design Categories.
Other changes made in the 1997 Provisions were editorial (i.e., for clarification or consistency).
Structures assigned to Seismic Design Categories B, C, and D are required to meet the minimum construction requirements of Sec. 12.5 (Sherwood and Stroh, 1989) or must be engineered using standard
design methods and principles of mechanics. Conventional light-frame construction requirements were
modified in the 1991 Provisions to limit the spacing between braced wall lines based on calculated
capacities to resist the loads and forces imposed.
Engineered structures assigned to Seismic Design Categories B, C, and D are required to conform to
the provisions of Sec. 12.3, Engineered Wood Construction, and Sec. 12.4, Diaphragms and Shear
Walls. Included in these sections are general design limitations, limits on wood resisting forces
contributed by concrete or masonry, shear wall and diaphragm aspect ratio limitations, and
requirements for distribution of shear to vertical resisting elements. See Commentary Sec. 12.3 and
12.4.
In the 1997 Provisions, Sec. 12.4.1 has been modified to improve the clarity and enforceability of the
Provisions. The requirements for Seismic Design Categories C and D were moved into the same
section as Seismic Design Category B with the triggers for restrictions such as materials limitations
associated with Seismic Design Categories C and D being moved to Sections 12.3 and 12.4.
12.8 SEISMIC DESIGN CATEGORIES E and F: Seismic Design Category F structures require an
engineered design. Conventional construction is not considered rigorous enough for structures
expected to be functional following a major seismic event. For Seismic Design Category E and F
structures, close attention to load path and detailing is required.
Structures assigned to Seismic Design Category E and F require blocked diaphragms. Structural-use
panels must be applied directly to the framing members; the use of gypsum wallboard between the
structural-use panels and the framing members is prohibited because of the poor performance of nails in
gypsum. Restrictions on allowable shear values for structural-use shear panels when used in
conjunction with concrete and masonry walls are intended to provide for deformation compatibility of
the different materials.
Changes made in the 1997 Provisions to this section were to provide consistent terminology or were
additions taken from the LRFD standard.
REFERENCES
American Institute of Timber Construction. 1994. Timber Construction Manual. New York, New
York: John Wiley and Sons, Inc.
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Wood Structure Design Requirements
American Forest & Paper Association. 1991. ANSI/NFoPA NDS-1991. National Design
Specification for Wood Construction. Washington, D.C.: AF&PA.
American Forest & Paper Association. 1996. Manual of Wood Construction: Load and Resistance
Factor Design (LRFD). Washington, D.C.: AF&PA.
American Plywood Association. 1991. Design/Construction Guide--Diaphragms. Tacoma,
Washington: APA.
American Plywood Association. 1994. Northridge California Earthquake, T94-5. Tacoma,
Washington.: APA.
American Plywood Association. 1996. Proposed Cyclic Testing Standard for Shear Walls. Tacoma,
Washington: APA.
ASCE Committee on Wood Load and Resistance Factor Design for Engineered Wood Construction.
1988. A Pre Standard Report American Society of Civil Engineers. New York, New York: ASCE.
ASCE. 1995. Standard For Load and Resistance Factor For Engineered Wood Construction.
AF&PA/ASCE 16-95. American Society of Civil Engineers, New York, New York.
Applied Technology Council. 1981. Guidelines for the Design of Horizontal Wood Diaphragms,
ATC-7. Redwood City, California: ATC.
Applied Technology Council. 1995. Cyclic Testing of Narrow Plywood Shear Walls, ATC-R-1.
Redwood City, California: ATC.
Breyer, Donald E. 1993. Design of Wood Structures, Third Edition. New York, New York:
McGraw-Hill Book Company.
Canadian Wood Council. 1990. Wood Design Manual. Ottawa: Canadian Wood Council.
Canadian Wood Council. 1991. Wood Reference Handbook. Ottawa: Canadian Wood Council.
Council of American Building Officials (CABO). 1995. One- and Two-Family Dwelling Code.
Country Club Hills, Illinois: BOCA.
Department of the Army, Navy and Air Force. 1992. Seismic Design for Buildings, TM5-809-10
(Tri-Services Manual). Washington, D.C.: U.S. Government Printing Office.
Dolan, J.D., 1994. “Proposed Test Method for Dynamic Properties of Connections Assembled with
Mechanical Fasteners.” ASTM Journal of Testing and Evaluation 22(6):542-547.
Dolan, J. D., and M. W. White. 1992. "Design Considerations for Using Adhesives in Shear Walls."
ASCE Journal of Structural Engineering 118(12):3473-3480.
Dolan, J. D. and Johnson, A. C. 1996. “Monotonic Tests of Long Shear Walls with Openings”
Virginia Polytechnic Institute and State University, Timber Engineering Report No. TE-1996-001.
Blacksburg, Virginia: VPISU.
Dolan, J. D. and Johnson, A. C. 1996. “Cyclic Tests of Long Shear Walls with Openings” Virginia
Polytechnic Institute and State University, Timber Engineering Report No. TE-1996-002. Blacksburg,
Virginia: VPISU.
257
1997 Commentary, Chapter 12
Earthquake Engineering Research Institute. 1996. Northridge earthquake reconnaissance report,
Chapter 6, Supplement C to Volume 11, pp 125 et seq., Earthquake Spectra.
Faherty, Keith F., and T. G. Williamson. 1989. Wood Engineering and Construction Handbook.
New York, New York: McGraw-Hill.
U.S. Department of Housing and Urban Development, 1984. HUD Minimum Property Standards,
Vol. I, II, and III. Washington, D.C.: United States Government Printing Office.
Forest Products Laboratory. 1986. Wood: Engineering Design Concepts. University Park:
Materials Education Council, The Pennsylvania State University.
Foschi, R. O., and A. Filiatrault. 1990. Performance Evaluation of 3M Scotch Grip Wood Adhesive
5230 for the Static and Dynamic Design of Timber Shear Walls and Diaphragms, Vancouver:
Department of Civil Engineering Report, University of British Columbia.
Goetz, Karl-Heinz, Dieter Hoor, Karl Moehler, and Julius Natterer. 1989. Timber Design and
Construction Source Book: A Comprehensive Guide to Methods and Practice. New York, New
York: McGraw-Hill.
Hoyle and Woeste. 1989. Wood Technology and Design of Structures. Iowa State University Press.
International Conference of Building Officials (ICBO). 1994. Uniform Building Code. Whittier,
California: ICBO.
Keenan, F. J. 1986. Limit States Design of Wood Structures. Morrison Hershfield Limited.
Rose, J. D., 1996. Research Report 158. “Preliminary Testing of Wood Structural Panel Shear Walls
Under Cyclic (Reversed) Loading.” Tacoma, Washington: APA.
Sherwood and Stroh. 1989. "Wood-Frame House Construction" in Agricultural Handbook 73.
Washington, D.C.: U.S. Government Printing Office.
Somayaji, Shan. 1992. Structural Wood Design. St. Paul, Minnesota: West Publishing Co.
Stalnaker, Judith J., and E. C. Harris. 1996. Structural Design in Wood, Second Edition. New York,
New York: McGraw-Hill.
U.S. Department of Agriculture, National Oceanic and Atmospheric Administration. 1971. San
Fernando, California, Earthquake of February 9, 1971. Washington, D.C.: NOAA.
White, M. W. and J. D. Dolan, 1996. “Seismic Response of Timber Shear Walls. Part I: Aspect
Ratios.” Paper submitted for publication in ASCE Journal of Structural Engineering.
Additional references are listed in John Peterson. 1983. "Bibliography on Lumber and Wood Panel
Diaphragms" , Journal of Structural Engineering, Vol. 109 No. 12. American Society of Civil
Engineers, New York, New York.
258
Wood Structure Design Requirements
259
Chapter 13 Commentary
SEISMICALLY ISOLATED STRUCTURES
Seismic isolation, commonly referred to as base isolation, is a design concept based on the
premise that a structure can be substantially decoupled from potentially damaging earthquake
motions. By decoupling the structure from the ground motion, the level of response in the
structure can be significantly reduced from the level that would otherwise occur in a conventional
fixed-base building. Conversely, seismic isolation permits designing with a reduced level of
earthquake load to achieve the same degree of seismic protection and reliability as a conventional
fixed-base building.
The potential advantages of seismic isolation and the recent advancements in isolation-system
products already have led to the design and construction of over 100 seismically isolated buildings
and bridges in the United States. A significant amount of research, development, and application
activity has occurred over the past 20 years. The following references provide a summary of
some of the work that has been performed: Applied Technology Council (1986, 1993), ASCE
Structures Congress (1989, 1991, 1993 and 1995), EERI Spectra (1990), Skinner, et al. (1993),
U.S. Conference on Earthquake Engineering (1990 and 1994), and World Conference on
Earthquake Engineering (1988, 1992 and 1996).
In the mid-1980s, the initial applications identified a need to supplement existing codes with
design requirements developed specifically for seismically isolated buildings. Code development
work occurred throughout the late 1980s. The status of U.S. seismic isolation design requirements as of October 1996 is as follows:
1. In late 1989, the Structural Engineers Association of California (SEAOC) State Seismology
Committee adopted an "Appendix to Chapter 2" of the SEAOC Blue Book entitled, "General
Requirements for the Design and Construction of Seismic-Isolated Structures." These
requirements were submitted to the International Conference of Building Officials (ICBO) and
were adopted by ICBO as an appendix of the 1991 Uniform Building Code (UBC). The
isolation appendix of the UBC has been updated on an annual basis since that time and the
most current version of these regulations may be found in the 1997 UBC.
2. In the late 1980s, the building Safety Board (BSB) of California, Office of the State Architect, adopted An Acceptable Method for Design and Review of Hospital Buildings Utilizing
Base Isolation based on recommendations of SEAOC. These methods were used for
regulation of California hospitals until the BSB replaced them with the 1991 UBC appendix
(with slight modification). The current version of these regulations may be found in 1995
California Building Code.
3. In 1991 the Federal Emergency Management Agency (FEMA) initiated a 6-year program to
develop a set of nationally applicable guidelines for seismic rehabilitation of existing buildings.
These guidelines (known as the NEHRP Guidelines for the Seismic Rehabilitation of
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1997 Commentary, Chapter 13
Buildings) are now available as FEMA 273. The design and analysis methods of the NEHRP
Guidelines parallel closely methods required by the NEHRP Recommended Provisions for
new buildings, except that more liberal design is permitted for the superstructure of a
rehabilitated building.
During development of the 1994 Provisions, it was decided to use the latest version (1993
approved changes) of the SEAOC/UBC provisions as a basis for the development of the requirements included in the Provisions. The only significant changes involved an appropriate conversion to strength design and making the requirements applicable on a national basis. For the 1997
Provisions, it was decided to incorporate the latest version of the SEAOC/UBC provisions (1997
UBC). Since the 1997 UBC is now based on strength design, the 1997 UBC and the 1997
Provisions are almost identical, except for seismic criteria. The seismic criteria of the Provisions
are based on the new national earthquake maps (developed by the Seismic Design Procedures
Group) which can be substantially different from the seismic criteria of the 1997 UBC.
A general concern has long existed regarding the applicability of different types of isolation
systems. Rather than addressing a specific method of base isolation, the Provisions provides
general design requirements applicable to a wide range of possible seismic isolation systems.
Although remaining general, the design requirements rely on mandatory testing of isolationsystem hardware to confirm the engineering parameters used in the design and to verify the
overall adequacy of the isolation system. Some systems may not be capable of demonstrating
acceptability by test and, consequently, would not be permitted. In general, acceptable systems
will: (1) remain stable for required design displacements, (2) provide increasing resistance with
increasing displacement, (3) not degrade under repeated cyclic load, and (4) have quantifiable
engineering parameters (e.g., force-deflection characteristics and damping).
FIGURE C13 Idealized force-deflection relationships for isolation
systems (stiffness effects of sacrificial wind-restraint systems not
shown for clarity).
Conceptually, there are four basic
types of isolation system forcedeflection relationships. These
idealized relationships are shown in
Figure C13 with each idealized
curve having the same design
displacement, DD, for the design
earthquake. A linear isolation
system is
represented by Curve A and has the
same isolated period for all
earthquake load levels. In addition,
the force generated in the superstructure is directly proportional to
the displacement across the
isolation system.
A hardening isolation system is
represented by Curve B. This system is soft initially (long effective period) and then stiffens
(effective period shortens) as the earthquake load level increases. When the earthquake load level
induces displacements in excess of the design displacement in a hardening system, the
260
Seismically Isolated Structures
superstructure is subjected to higher forces and the isolation system to lower displacements than a
comparable linear system.
A softening isolation system is represented by Curve C. This system is stiff initially (short
effective period) and softens (effective period lengthens) as the earthquake load level increases.
When the earthquake load level induces displacements in excess of the design displacement in a
softening system, the superstructure is subjected to lower forces and the isolation system to higher
displacements than a comparable linear system.
A sliding isolation system is represented by Curve D. This system is governed by the friction
force of the isolation system. Like the softening system, the effective period lengthens as the
earthquake load level increases and loads on the superstructure remain constant.
The total system displacement for extreme displacement of the sliding isolation system, after
repeated earthquake cycles, is highly dependent on the vibratory characteristics of the ground
motion and may exceed the design displacement, DD . Consequently, minimum design requirements do not adequately define peak seismic displacement for seismic isolation systems governed
solely by friction forces.
13.1 GENERAL: The design requirements permit the use of one of three different analysis
procedures for determining the design-basis seismic loads. The first procedure uses a simplelateral-force formula (similar to the lateral-force coefficient now used in conventional building
design) to prescribe peak lateral displacement and design force as a function of spectral acceleration and isolated-building period and damping. The second and third methods, which are required
for geometrically complex or especially flexible buildings, rely on dynamic analysis procedures
(either response spectrum or time history) to determine peak response of the isolated building.
The three procedures are based on the same level of seismic input and require a similar level of
performance from the building. There are benefits in performing a more complex analysis in that
slightly lower design forces and displacements are permitted as the level of analysis becomes more
sophisticated. The design requirements for the structural system are based on the design
earthquake, a severe level of earthquake ground motion defined as two-thirds of the maximum
considered earthquake. The isolation system, including all connections, supporting structural
elements and the "gap," is required to be designed (and tested) for 100 percent of maximum
considered earthquake demand. Structural elements above the isolation system are not required
to be designed for the full effects of the design earthquake , but may be designed for slightly
reduced loads (i.e., loads reduced by a factor of up to 2.0) if the structural system has sufficient
ductility, etc., to respond inelastically without sustaining significant damage. A similar fixed-base
structure would be designed for loads reduced by a factor of 8 rather than 2.
Ideally, lateral displacement of an isolated structure will result, predominantly due to the
deformations of the isolation system, rather than in distortion of the structure above. Accordingly, the lateral-load-resisting system of the structure above the isolation system should be
designed to have sufficient stiffness and strength to avoid large, inelastic displacements. For this
reason, the Provisions contains criteria that limit the inelastic response of the structure above the
isolation system. Although damage control for the design-basis earthquake is not an explicit
objective of the Provisions, an isolated structure designed to limit inelastic response of the
structural system also will reduce the level of damage that would otherwise occur during an
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1997 Commentary, Chapter 13
earthquake. In general, isolated structures designed in conformance with the Provisions should
be able to:
1. Resist minor and moderate levels of earthquake ground motion without damage to structural
elements, nonstructural components, or building contents and
2. Resist major levels of earthquake ground motion without failure of the isolation system,
without significant damage to structural elements, without extensive damage to nonstructural
components, and without major disruption to facility function.
The above performance objectives for isolated structures considerably exceed the performance
anticipated for fixed-base structures during moderate and major earthquakes. Table C13.1
provides a tabular comparison of the performance expected for isolated and fixed-base structures
designed in accordance with the Provisions. Loss of function is not included in Table C13.1. For
certain (fixed-base) facilities, loss of function would not be expected to occur until there is
significant structural damage causing closure or restricted access to the building. In other cases,
the facility could have only limited or no structural damage but would not be functional as a result
of damage to vital nonstructural components and contents. Isolation would be expected to
mitigate structural and nonstructural damage and protect the facility against loss of function.
The requirements of Chapter 13 provide isolator design displacements, structure-design-shear
forces, and other specific requirements for seismically isolated structures. All other design
requirements including loads (other than seismic), load combinations, allowable forces and
stresses, and horizontal-shear distribution are covered by the applicable sections of the Provisions
for conventional fixed-base structures.
TABLE C13.1 Protection Provided by NEHRP Recommended Provisions for Minor,
Moderate and Major Levels of Earthquake Ground Motion
Earthquake Ground Motion Level
Risk Category
Minor
Moderate
Major
Life safetya
F/I
F/I
F/I
Structural damageb
F/I
F/I
I
Nonstructural damagec (contents damage)
F/I
I
I
a
b
c
Loss of life or serious injury is not expected for fixed-base (F) or isolated (I) buildings.
Significant structural damage is not expected for fixed-base (F) or isolated (I) buildings.
Significant nonstructural (contents) damage is not expected for fixed-base (F) or isolated (I) buildings.
13.2 CRITERIA SELECTION: This section delineates the requirements for the use of the
equivalent-lateral-force and dynamic methods of analysis and the conditions for developing a sitespecific response spectrum. The limitations on the simplified lateral-force design procedure are
quite severe at this time. Limitations cover the site location with respect to active faults; soil
conditions of the site, the height, regularity and stiffness characteristics of the building; and the
characteristics of the isolation system. In fact, the current limitations will necessitate a dynamic
analysis for most isolated structures. Additionally, time-history analysis is required to determine
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Seismically Isolated Structures
the design displacement of the isolation system (and the structure above) for the following
isolated structures:
1. Isolated structures with a "nonlinear" isolation system including, but not limited to, isolation
systems utilizing friction or sliding surfaces, isolation systems with effective damping values
greater than about 30 percent of critical, isolation systems not capable of producing a
significant restoring force, and isolation systems that restrain or limit extreme earthquake
displacement;
2. Isolated structures with a "nonlinear" structure (above the isolation system) including, but not
limited to, structures designed for forces that are less than those specified by the Provisions
for "essentially-elastic" design; and
3. Isolated structures located on Class E or F sites (i.e., very soft soil).
The restrictions placed on the use of equivalent-lateral-force design procedures effectively require
dynamic analysis for virtually all isolated structures. However, lower-bound limits on isolation
system design displacements and structural-design forces are specified by the Provisions in Sec.
13.4 as a percentage of the values prescribed by the equivalent-lateral-force design formulas, even
when dynamic analysis is used as the basis for design. These lower-bound limits on key design
parameters ensure consistency in the design of isolated structures and serve as a "safety net"
against gross under-design. Table C13.2 provides a summary of the lower-bound limits on
dynamic analysis specified by the Provisions.
TABLE C13.2 Lower-Bound Limits on Dynamic Analysis Specified as a Percentage of
Static-Analysis Design Requirements
Dynamic Analysis
Static Analysis
Response
Spectrum
Time History
DD = (g/4B2)(SD1TD/BD)
–
–
DT $ 1.1D
$ 0.9DT
$ 0.9DT
DM = (g/4B2)(SM1TM/BM)
–
–
Total Maximum Displacement - DTM
DTM $ 1.1DM
$ 0.8DTM
$ 0.8DTM
Design Shear - Vb
(at or below the Isolation System)
Vb = kDmaxDD
$ 0.9Vb
$ 0.9Vb
Design Shear - Vs
("Regular" Superstructure)
Vs = kDmaxDD/RI
$ 0.8Vs
$ 0.6Vs
Design Shear - Vs
("Irregular" Superstructure)
Vs = kDmaxDDRI
$ 1.0Vs
$ 0.8Vs
0.015hsx
0.015hsx
0.020hsx
Design Parameter
Design Displacement - DD
Total Design Displacement - DT
Maximum Displacement - DM
Drift (calculated using RI for Cd)
Site-specific design spectra must be developed for both the design earthquake and the maximum
considered earthquake if the site is within 10 km of an active fault or if the Site Class is E or F
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1997 Commentary, Chapter 13
Lower limits are placed on these site-specific spectra and they must not be less than 80 percent of
those given in Sec. 13.4.4.
13.3 EQUIVALENT-LATERAL-FORCE DESIGN PROCEDURE: The lateral displacement given by Equation 13.3.3.1 approximates peak design earthquake displacement of a singledegree-of-freedom, linear-elastic system of period, TD, and equivalent viscous damping, βD, and
the lateral displacement given by Equation 13.3.3.3 approximates peak maximum considered
earthquake displacement of a single-degree-of-freedom, linear-elastic system of period, TM, and
equivalent viscous damping, βDM.
13.3.3 Minimum-Lateral Displacements: Equation 13.3.3.1 is an estimate of peak displacement in the isolation system for the design earthquake. In this equation, the spectral acceleration
term, SD1, is the same as that required for design of a conventional fixed-base structure of period,
TD. A damping term, BD, is used to decrease (or increase) the computed displacement when the
equivalent damping coefficient of the isolation system is greater (or smaller) than 5 percent of
critical damping. Values of coefficient, BD (or BM for the maximum considered earthquake), are
given in Table 13.3.3.1. for different values of isolation system damping, βD (or βM).
A comparison of values obtained from Equation 13.3.3.1 and those obtained from nonlinear timehistory analyses are given in references by Kircher et al. (1988), Lashkari and Kircher (1993) and
Constantinou et al. (1993).
Consideration should be given to possible differences in the properties of the isolation system
used for design and the properties of isolation system actually installed in the building. Similarly,
consideration should be given to possible changes in isolation system properties due to different
design conditions or load combinations. If the true deformational characteristics of the isolation
system are not stable or vary with the nature of the load (i.e., rate, amplitude or time dependent),
the design displacements should be based on deformational characteristics of the isolation system
that give the largest possible deflection (kDmin) and the design forces should be based on deformational characteristics of the isolation system that give the largest possible force (kDmax). If the
true deformational characteristics of the isolation system are not stable or vary with the nature of
the load (i.e., rate, amplitude or time dependent), the damping level used to determine design
displacements and forces should be based on deformational characteristics of the isolation system
that represent the minimum amount of energy dissipated during cyclic response at the design level.
The configuration of the isolation system for a seismically isolated building or structure should be
selected in such a way as to minimize any eccentricity between the center of mass of the superstructure and the center of rigidity of the isolation system. In this way, the effect of torsion on the
displacement of isolation elements will be reduced. As for conventional structures, allowance for
accidental eccentricity in both horizontal directions must be considered. Figure C13.3.3 defines
the terminology used in the Provisions. Equation 13.3.3.5-1 (or Equation 13.3.3.5-2 for the
maximum considered earthquake) provides a simplified formulae for estimating the response due
to torsion in lieu of a more refined analysis. The additional component of displacement due to
torsion increases the design displacement at the corner of the structure by about 15 percent (for a
perfectly square building in plan) to about 30 percent (for a very long, rectangular building) if the
eccentricity is 5 percent of the maximum plan dimension. Such additional displacement, due to
torsion, is appropriate for buildings with an isolation system whose stiffness is uniformly
264
Seismically Isolated Structures
distributed in plan. Isolation systems that have stiffness concentrated toward the perimeter of the
building or certain sliding systems that minimize the effects of mass eccentricity will have reduced
displacements due to torsion. The Provisions permits values of DT as small as 1.1DD, with proper
justification.
13.3.4 Minimum-Lateral Forces: Figure
C13.3.4 defines the terminology below and above
the isolation system. Equation 13.3.4.1 gives
peak seismic shear on all structural components at
or below the seismic interface without reduction
for ductile response. Equation 13.3.4.2 specifies
the peak seismic shear for design of structural
systems above the seismic interface. For structures that have appreciable inelastic-deformation
capability, this equation includes an effective reduction factor of up to 2 for response beyond the
strength-design level.
FIGURE C13.3.3 Displacement terminology.
The basis for the reduction factor is that the design of the structural system is based on strengthdesign procedures. A factor of at least 2 is assumed to exist between the design-force level and
the true-yield level of the structural system. An
investigation of 10 specific buildings indicated
that this factor varied between 2 and 5 (Applied
Technology Council, 1982). Thus, a reduction
factor of 2 is appropriate to ensure that the structural system remains essentially elastic for the
design earthquake .
In Sec. 13.3.4.3, the limitations given on VS ensure that there is at least a factor of 1.5 between
the nominal yield level of the superstructure and
(1) the yield level of the isolation system, (2) the
ultimate capacity of a sacrificial-wind-restraint
system which is intended to fail and release the
superstructure during significant lateral load, or
(3) the break-away friction level of a sliding sysFIGURE C13.3.4 Isolation system terminol- tem.
ogy.
These limitations are essential to ensure that the
superstructure will not yield prematurely before
the isolation system has been activated and significantly displaced.
The design shear force, VS, specified by the requirements of this section ensures that the structural
system of an isolated building will be subjected to significantly less inelastic demands than a
conventionally designed structure. Further reduction in VS, such that the inelastic demand on a
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1997 Commentary, Chapter 13
seismically isolated structure would be the same as the inelastic demand on a conventionally
designed structure, was not considered during development of these requirements but may be
considered in the future.
If the level of performance of the isolated structure is desired to be greater than that implicit in
these requirements, then the denominator of Equation 13.3.4.2 may be reduced. Decreasing the
denominator of Eq. 13.3.4.2 will lessen or eliminate inelastic response of the superstructure for
the design-basis event.
13.3.5 Vertical Distribution of Force: Equation 13.3.5 describes the vertical distribution of
lateral force based on an assumed triangular distribution of seismic acceleration over the height of
the structure above the isolation interface. References by Button (1993) and Constantinou et al.
(1993) provide a good summary of recent work which demonstrates that this vertical distribution
of force will always provide a conservative estimate of the distributions obtained from moredetailed-nonlinear analysis studies.
13.3.6 Drift Limits: The maximum interstory drift permitted for design of isolated structures
varies depending on the method of analysis used, as summarized in Table C13.3.6. For comparison, the drift limits prescribed by the Provisions for fixed-base structures also are summarized in
Table C13.3.6.
TABLE C13.3.6 Comparison of Drift Limits for Fixed-Base and Isolated Structures
Structure
Seismic Use Group
Fixed-Base
Isolated
Buildings (other than
masonry) four stories
or less in height with
component drift design
I
0.025hsx/(Cd/R)
0.015hsx
II
0.020hsx/(Cd/R)
0.015hsx
III
0.015hsx/(Cd/R)
0.015hsx
I
0.020hsx/(Cd/R)
0.015hsx
II
0.015hsx/(Cd/R)
0.015hsx
III
0.010hsx/(Cd/R)
0.015hsx
Other (non-masonry)
buildings
Drift limits in Table C13.3.6 are divided by Cd/R for fixed-base structures since displacements
calculated for lateral loads reduced by R. are factored by Cd before checking drift. The Cd term is
used throughout the Provisions for fixed-base structures to approximate the ratio of actual
earthquake response to response calculated for "reduced" forces. Generally, Cd is 1/2 to 4/5 the value
of R. For isolated structures, the RI factor is used both to reduce lateral loads and to increase
displacements (calculated for reduced lateral loads) before checking drift. Equivalency would be
obtained if the drift limits for both fixed-base and isolated structures were based on their
respective R factors. It may be note that the drift limits for isolated structures are generally more
conservative than those of conventional fixed-base structures, even when fixed-base structures are
designed as Seismic Use Group III buildings.
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Seismically Isolated Structures
13.4 DYNAMIC LATERAL RESPONSE PROCEDURE: This section specifies the
requirements and limits of a dynamic analysis. The design displacement and force limits on a
response-spectrum and time-history analysis are given in Table C13.2.
A more-detailed or refined study can be performed in accordance with the analysis procedures
described in this section. The intent of this section is to provide analysis procedures which are
compatible with the minimum requirements of Sec. 13.3. Reasons for performing a more-refined
study include:
1. The importance of the building.
2. The need to analyze possible structure/isolation-system interaction when the fixed-base period
of the building is greater than one third of the isolated period.
3. The need to explicitly model the deformational characteristics of the lateral-force-resisting
system when the structure above the isolation system is irregular.
4. The desirability of using site-specific ground-motion data, especially for soft soil types (Site
Class E or F) or for sites located within 10 kilometers of an active fault.
5. The desirability of explicitly modeling the deformational characteristics of the base-isolation
system. This is especially important for systems that have damping characteristics that are
amplitude, rather than velocity, dependent, since it is difficult to determine an appropriate
value of equivalent viscous damping for these systems.
Additionally, time-history analysis is required to determine the design displacement of the
isolation system (and the structure above) for the following isolated structures:
1. Isolated structures with a "nonlinear" isolation system including, but not limited to, isolation
systems utilizing friction or sliding surfaces, isolation systems with effective damping values
greater than about 30 percent of critical, isolation systems not capable of producing a
significant restoring force, and isolation systems that restrain or limit extreme earthquake
displacement.
2. Isolated structures with a "nonlinear" structure (above the isolation system) including, but not
limited to, structures designed for forces that are less than those specified by the
SEAOC/UBC provisions for "essentially-elastic" design.
3. Isolated structures located on Class E or F sites (i.e., very soft soil).
When time-history analysis is used as the basis for design, the design displacement of the isolation
system and design forces in elements of the structure above are to be based on the maximum of
the results of not less than three separate analyses, each using a different pair of horizontal time
histories. Each pair of horizontal time histories is to:
1. Be of a duration consistent with the design earthquake or the maximum considered earthquake ,
2. Incorporate near-field phenomena, as appropriate, and
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1997 Commentary, Chapter 13
3. Have response spectra whose square-root-sum-of-the-squares combination of the two
horizontal components equals or exceeds 1.3 times the "target" spectrum at each spectral
ordinate.
The average value of seven time histories is a standard required by the nuclear industry and is
considered appropriate for nonlinear time-history analysis of seismically isolated structures.
13.5 NONSTRUCTURAL COMPONENTS: To accommodate the differential movement
between the isolated building and the ground, provision for flexible utility connections should be
made. In addition, rigid structures crossing the interface, (i.e., stairs, elevator shafts and walls,
should have details to accommodate differential motion at the isolator level without sustaining
damage sufficient to threaten life safety.
13.6 DETAILED SYSTEM REQUIREMENTS: Environmental conditions that may adversely
effect isolation system performance should be thoroughly investigated. Significant research has
been conducted on the effects of temperature, aging, etc., on isolation systems since the 1970s in
Europe, New Zealand, and the United States.
13.6.2.2 Wind Forces: Lateral displacement over the depth of the isolator zone resulting from
wind loads should be limited to a value similar to that required for other story heights.
13.6.2.3 Fire Resistance: In the event of a fire, the isolation system should be capable of
supporting the weight of the building, as required for other vertical-load-supporting elements of
the structure, but may have diminished functionality for lateral (earthquake) load.
13.6.2.4 Lateral-restoring Force: The isolation system should be configured with a lateralrestoring force sufficient to avoid significant residual displacement as a result of an earthquake,
such that the isolated structure will not have a stability problem and be in a condition to survive
aftershocks and future earthquakes.
13.6.2.5 Displacement Restraint: The use of a displacement restraint is not encouraged by the
Provisions. Should a displacement restraint system be implemented, explicit analysis of the
isolated structure for maximum considered earthquake is required to account for the effects of
engaging the displacement restraint.
13.6.2.6 Vertical-load Stability: The vertical loads to be used in checking the stability of any
given isolator should be calculated using bounding values of dead load and live load and the peak
earthquake demand of the maximum considered earthquake. Since earthquake loads are
reversible in nature, peak earthquake load should be combined with bounding values of dead and
live load in a manner which produces both the maximum downward force and the maximum
upward force on any isolator. Stability of each isolator should be verified for these two extreme
values of vertical load at peak maximum considered earthquake displacement of the isolation
system.
13.6.2.7 Overturning: The intent of this requirement is to prevent global, structural overturning
and overstress of elements due to local uplift. Uplift in a braced frame or shear wall is acceptable,
provided the isolation system does not disengage from its horizontal-resisting connection detail.
The connection details used in some isolation systems are such that tension is not permitted on the
system. If the tension capacity of an isolation system is to be utilized on resisting uplift forces,
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Seismically Isolated Structures
then component tests should be performed to demonstrate the adequacy of the system on
resisting-tension forces at the design displacement.
13.6.2.8 Inspection and Replacement: Although most isolation systems will not need to be
replaced after an earthquake, it is good practice to provide for inspection and replacement. After
an earthquake, the building should be inspected and any damaged elements should be replaced or
repaired. It is advised that periodic inspections be made of the isolation system.
13.6.2.9 Quality Control: A test and inspection program is necessary for both fabrication and
installation of the isolation system. Because base isolation is a developing technology, it may be
difficult to reference standards for testing and inspection. Reference can be made to standards for
some materials such as elastomeric bearings (ASTM D4014). Similar standards are required for
other isolation systems. Special inspection procedures and load testing to verify manufacturing
quality should be developed for each project. The requirements will vary with the type of
isolation system used.
13.6.3.2 Building Separations: A minimum separation between the isolated structure and a
rigid obstruction is required to allow free movement in all lateral directions of the superstructure
during an earthquake. Provision should be made for lateral motion greater than the design
displacement, since the exact upper limit of displacement cannot be precisely determined.
13.8 DESIGN AND CONSTRUCTION REVIEW: Design review of the design and analysis
of the isolation system and design review of the isolator testing program is mandated by the
Provisions for two key reasons:
1. The consequences of isolator failure could be catastrophic.
2. Isolator design and fabrication technology is evolving rapidly and may be based on technologies unfamiliar to many design professionals.
These Provisions requires review to be performed by a team of registered design professionals
that are independent of the design team and other project contractors. The review team should
include individuals with special expertise in one or more aspects of the design, analysis and
implementation of seismic isolation systems.
The review team should be formed prior to the development of design criteria (including sitespecific ground shaking criteria) and isolation system design options. Further, the review team
should have full access to all pertinent information and the cooperation of the design team and
regulatory agencies involved with the project.
13.9 REQUIRED TESTS OF THE ISOLATION SYSTEM: The design displacements and
forces developed from the Provisions are predicated on the basis that the deformational characteristics of the base isolation system have been previously defined by a comprehensive set of tests. If
a comprehensive amount of test data are not available on a system, then major design alterations
in the building may be necessary after the tests are complete. This would result from variations in
the isolation-system properties assumed for design and those obtained by test. Therefore, it is
advisable that prototype systems be tested during the early phases of design, if sufficient test data
is not available on an isolation system.
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1997 Commentary, Chapter 13
Typical force-deflection or hysteresis loops are shown in Figure C13.9; also included are the
definitions of values used in Sec. 13.9.3.
The required sequence of tests will experimentally verify:
1. The assumed stiffness and capacity of the wind-restraining mechanism;
2. The variation in the isolator's deformational characteristics with amplitude and with vertical
load, if it is a vertical load-carrying member;
3. The variation in the isolator's deformational characteristics for a realistic number of cycles of
loading at the design displacement; and
4. The ability of the system to carry its maximum and minimum vertical loads at the maximum
displacement.
FIGURE 13.9 The effect of stiffness on an isolation bearing.
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Seismically Isolated Structures
Force-deflection tests are not required if similarly-sized components have been previously tested
using the specified sequence of tests.
Variations in effective stiffness greater than ±15 percent over 3 cycles of loading at a given
amplitude, or ±20 percent over the larger number of cycles at the design displacement, would be
cause for rejection. The variations in the vertical loads required for tests of isolators which carry
vertical, as well as lateral, load are necessary to determine possible variations in the system
properties with variations in overturning force. The appropriate dead loads and overturning forces
for the tests are defined as the average loads on a given type and size of isolator for determining
design properties and are the absolute maximum and minimum loads for the stability tests.
13.9.5.1 Effective Stiffness: The effective stiffness is determined from the hysteresis loops
shown in Figure C13.9). Stiffness may vary considerably as the test amplitude increases but
should be reasonably stable (±15 percent) for more than 3 cycles at a given amplitude.
The intent of these requirements is to ensure that the deformational properties used in design
result in the maximum design forces and displacements. For determining design displacement, this
means using the lowest damping and effective-stiffness values. For determining design forces, this
means using the lowest damping value and the greatest stiffness value.
13.9.5.2 Effective Damping: The determination of equivalent viscous damping is reasonably
reliable for systems whose damping characteristics are velocity dependent. For systems that have
amplitude-dependent, energy-dissipating mechanisms, significant problems arise in determining an
equivalent viscous-damping value. Since it is difficult to relate velocity and amplitude-dependent
phenomena, it is recommended that when the equivalent-viscous damping assumed for the design
of amplitude-dependent, energy-dissipating mechanisms (e.g., pure-sliding systems) is greater than
30 percent, then the design-basis force and displacement should be determined by the timehistory-analysis method, as specified in Sec. C13.2.
REFERENCES
Applied Technology Council. 1982. An Investigation of the Correlation Between Earthquake
Ground Motion and Building Performance, ATC Report 10. Redwood City, California: ATC
Applied Technology Council. 1986. Proceedings of a Seminar and Workshop on Base Isolation
and Passive Energy Dissipation, ATC Report 17. Redwood City, California: ATC.
Applied Technology Council. 1993. Proceedings of Seminar on Seismic Isolation, Passive
Energy Dissipation, and Active Control, ATC 17-1. Redwood City, California: ATC.
American Society of Civil Engineers. 1989, 1991, 1993, and 1995. Seismic Engineering:
Research and Practice. New York City: ASCE.
Constantinou, M. C., C. W. Winters, and D. Theodossiou. 1993. "Evaluation of SEAOC and
UBC analysis procedures, Part 2: Flexible superstructure," in Proceedings of a Seminar on
Seismic Isolation, Passive Energy Dissipation and Active Control, ATC Report 17-1. Redwood
City, California: ATC.
Earthquake Engineering Research Institute. 1990. "Seismic isolation: from idea to reality,"
Earthquake Spectra Journal 6:2.
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1997 Commentary, Chapter 13
Kircher, C. A., B. Lashkari, R. L. Mayes, and T. E. Kelly. 1988. "Evaluation of nonlinear
response in seismically isolated buildings," in Proceedings of a Symposium on Seismic, Shock and
Vibration Isolation, ASMA PVP Conference.
Lashkari, B., and C. A. Kircher. 1993. "Evaluation of SEAOC & UBC analysis procedures, part
1: stiff superstructure," in Proceedings of a Seminar on Seismic Isolation, Passive Energy
Dissipation and Active Control, ATC Report 17-1. Redwood City, California: ATC.
Lashkari, B., and C. A. Kircher. 1990. Proceedings of Fourth U.S. National Conference on
Earthquake Engineering. Berkeley, California: Earthquake Engineering Research Institute.
Skinner, R. I., W. H. Robinson, and G. H. McVerry. 1993. An Introduction to Seismic Isolation.
Sussex, England: Wiley and Sons.
272
Chapter 14 Commentary
NONBUILDING STRUCTURE DESIGN REQUIREMENTS
14.1 GENERAL:
14.1.1 Scope: Requirements concerning nonbuilding structures were originally added to the
1994 Provisions by the 1991-94 Provisions Update Committee (PUC) at the request of the BSSC
Board of Direction to provide building officials with needed guidance. In recognition of the
complexity, nuances and importance of nonbuilding structures, the BSSC Board established 199497 PUC Technical Subcommittee 13 (TS13), Nonbuilding Structures, in 1995. The duties of
TS13 were to review the 1994 Provisions and Commentary and recommend changes for the 1997
Edition. The subcommittee was composed of individuals possessing considerable expertise
concerning various specialized nonbuilding structures and representing a wide variety of
industries concerned with nonbuilding structures.
Building codes traditionally have been perceived as minimum standards of care for the design of
nonbuilding structures and building code compliance of these structures is required by building
officials in many jurisdictions. However, requirements in the industry standards are often at odds
with building code requirements. In some cases, the industry standards need to be altered while in
other cases the building codes need to be modified. Registered design professionals are not
always aware of the numerous accepted standards within an industry or if the accepted standards
are adequate. It is hoped that the 1997 Provisions requirements for nonbuilding structures
appropriately bridge the gap between building code and existing industry standards.
One of TS13's goals was to review and list appropriate industry standards to serve as a resource.
These standards had to be included in the appendix. The subcommittee also has attempted to
provide an appropriate link so that the accepted industry standards can be used with the seismic
ground motions established in the Provisions. It should be noted that some nonbuilding structures are very similar to a building and can be designed employing sections of the Provisions
directly whereas other nonbuilding structures require special analysis unique to the particular type
of nonbuilding structure.
The ultimate goal of TS13 was to provide guidance to develop requirements consistent with the
intent of the Provisions while allowing the use of accepted industry standards. Some of the
referenced standards are consensus documents while others are not.
One good example of the dilemma posed by the conflicts between the Provisions and accepted
design practice for nonbuilding structures are steel multilegged water towers. Historically, such
towers have performed well when properly designed per American Water Works Association
(AWWA) standards, but these standards differ from the Provisions because tension-only rods are
required and the connection forces are not amplified. However, industry practice requires upset
rods that are preloaded at the time of installation, and the towers tend to perform well in
earthquake areas.
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1997 Commentary, Chapter 14
In an effort to provide the appropriate interface between the Provision's requirements for building
structures, nonstructural components, and nonbuilding structures; TS13 recommended that
nonbuilding structure requirements be placed in a separate chapter. The PUC agreed with this
change. The 1997 Provisions Chapter 14 now provides registered design professionals responsible for designing nonbuilding structures with a single point of reference.
Note that building structures, vehicular and railroad bridges, nuclear power plants, and dams are
excluded from the scope of the nonbuilding structure requirements. The excluded structures are
covered either by other sections of the Provisions or by other well established design criteria
(vehicular and railroad bridges, nuclear power plants, and dams).
14.1.2: This section has been developed to provide an appropriate link between the requirements
for nonbuilding structures and those for inclusion in the rest of the Provisions, especially the
requirements for architectural, mechanical, and electrical components.
14.1.5: The rational methods for period calculation contained in the Provisions were developed
for building structures. If the nonbuilding structure has dynamic characteristics similar to a
building, the difference in period is insignificant. If the nonbuilding structure is not similar to a
building structure, other techniques for period calculation will be required. Some of the references in the Appendix of this chapter for specific types of nonbuilding structures may contain
more accurate methods for period determination.
14.2 STRUCTURAL DESIGN REQUIREMENTS:
14.2.1 Design Basis: The subcommittee wanted to employ the new seismic ground motion maps
and the new methodology for establishing seismic design and detailing contained in the 1997
Provisions.
14.2.1.1 Seismic Factors: Table 14.2.1.1 has been formulated to be consistent with the
Provisions. The values listed here are generally lower than the values for buildings. Lower
values are assigned in recognition of the structural performance of nonbuilding structures as
opposed to building structures. Nonbuilding structures tend to be lightly damped, less redundant,
and more given to performance failure when the structure exhibits nonlinear performance.
14.2.1.2 Importance Factors and Seismic Importance Group Classifications: The Importance Factors and Seismic Use Group classifications assigned nonbuilding structures vary from
those assigned building structures. Buildings are designed to protect occupants inside the
structure whereas nonbuilding structures are not normally “occupied” in the same sense as
buildings, but need to be designed in a special manner because they pose a different sort of risk in
regard to public safety (i.e., they may contain very hazardous compounds or be essential
components in critical lifeline systems). For example, tanks and vessels may contain materials that
are essential for lifeline functions following a seismic event (i.e., fire fighting, potable water),
potentially harmful or hazardous to the environment or general health of the public, biologically
lethal or toxic, or explosive or flammable (threat of consequential or secondary damage).
If not covered by the authority having jurisdiction, Table 14.2.1.2.1 may be used to select the
importance factor (I). The value selected should be the highest value that applies to the structure
or siting situation. The importance factor is not intended for use in making economic evaluations
regarding the level of damage, probabilities of occurrence, or cost to repair the structure. These
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Nonbuilding Structures
economic decisions should be made by the owner and other interested parties (insurers, financiers,
etc). Following are examples demonstrating how this table may be applied:
Example 1:
A water storage tank used to provide pressurized potable water for a process within a chemical
plant where the tank is located away from personnel working within the facility.
TABLE 14.2.1.2 Importance Factor (I) and Seismic Use Group Classification
for Nonbuilding Structures
Importance Factor
I = 1.0
I = 1.25
I = 1.5
Seismic Use Group
I
II
III
Hazard
H-I
H - II
H - III
Function
F-I
F - II
F - III
Address each of the issues implied in the matrix:
Seismic Use Group — Neither the structure nor the contents are critical, therefor use Seismic Use
Group I.
Hazard — The contents are not hazardous, therefore use H - I.
Function — The water storage tank is not ancillary to and is not a Seismic Use Group III
structure, therefor use F - I.
This tank has an importance factor of 1.0.
Example 2:
A steel storage rack is located in a retail store in which the customers have direct access to the
aisles. Merchandise is stored on the upper racks. The rack is supported from a slab on grade.
TABLE 14.2.1.2 Importance Factor (I) and Seismic Use Group Classification
for Nonbuilding Structures
Importance Factor
I = 1.0
I = 1.25
I = 1.5
Seismic Use Group
I
II
III
Hazard
H-I
H - II
H - III
Function
F-I
F - II
F - III
\Address each of the issues in the matrix.:
Seismic Use Group — Neither the structure nor the contents are critical, therefor use Seismic Use
Group I.
Hazard — The contents are not hazardous, therefor use H - I.
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1997 Commentary, Chapter 14
Function — The storage rack is not ancillary to and is not a Seismic Use Group III structure,
therefor use F - I.
Within the steel storage rack section in the Provisions there exists a link back to Sec. 6.9 and to
Sec. 6.1.5 requiring an Ip or I of 1.5.
Use an importance factor of 1.5 for this structure.
Example 3:
A water tank is located within an office building complex to supply the fire sprinkler system.
TABLE 14.2.1.2 Importance Factor (I) and Seismic Use Group Classification
for Nonbuilding Structures
Importance Factor
I = 1.0
I = 1.25
I = 1.5
Seismic Use Group
I
II
III
Hazard
H-I
H - II
H - III
Function
F-I
F - II
F - III
Address each of the issues in the matrix.:
Seismic Use Group — The office building is Seismic Use Group I.
Hazard — The contents are not hazardous, therefore use H - I.
Function — The water tank is required to provide water for fire fighting, therefor use F - III.
Use an importance factor of 1.5 for this structure.
Example 4:
A gasoline storage tank is to be constructed within a refinery tank farm. Impoundment diking is
provided to control liquid spills.
Table 14.2.1.2
Importance Factor (I) and Seismic Use Group Classification
for Nonbuilding Structures
Importance Factor
I = 1.0
I = 1.25
I = 1.5
Seismic Use Group
I
II
III
Hazard
H-I
H - II
H - III
Function
F-I
F - II
F - III
Address each of the issues in the matrix.:
Seismic Use Group — The structure is classified as Seismic Use Group I.
Hazard — The contents constitute a high explosion and fire hazard, therefore use H - III.
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Nonbuilding Structures
Function — The tank is not required to provide water critical facilities, therefore use F - I.
Use an importance factor of 1.5 for this structure.
14.2.2: Rigid Nonbuilding Structures: The equation included in the 1994 Provisions did not
agree with the formulas contained in the 1994 Uniform Building Code (UBC). The Seismic
Design Procedure Group recommended using the SDS factor and eliminating the Ca factor. The
appropriate changes are incorporated in the 1997 Provisions.
14.3 NONBUILDING STRUCTURES SIMILAR TO BUILDINGS: This general class of
nonbuilding structures exhibits behavior similar to that of building structure; however, function
and performance are different. The Provisions were used as the primary basis for design with
industry-driven exceptions, modifications, and additions.
14.3.3 Steel Storage Racks: The Rack Manufacturers Institute (RMI) has developed and
maintained a specification that is utilized by much of the storage rack industry. The RMI is in the
process of obtaining ANSI consensus.
An attempt has been made to incorporate this specification in the 1997 Provisions in a manner
ensuring that the applicable requirements of the Provisions also are met. All storage racks can be
designed in accordance with the Rack Manufacturers Institute specification provided that design
force requirements are not less than those required by the force requirements for architectural
systems and components.
In addition, storage racks located at grade may be designed to the same requirements as building
structures provided that all the force and detailing requirements of Chapters 5 and 9 are met.
Based on storage rack performance experienced during the 1994 Northridge earthquake, it is
judged to be necessary to account for 67 percent of the rated rack load in determining the seismic
weight.
14.3.4 Electrical Power Generating Facilities: Electrical power plants closely resemble
building structures, and their performance in seismic events has been good. For reasons of
mechanical performance, lateral drift of the structure must be limited. The lateral bracing system
of choice has been the concentrically braced frame. The height limits on braced frames in
particular can be an encumbrance to the design of large power generation facilities. For this
reason, the exception to height limits in Sec. 14.2.1 was required.
14.3.6 Piers and Wharves: Although previous editions of the Provisions did not include a
specific section on piers and wharves, the inclusion of these structures was deemed necessary to
properly account for the effect of hydrodynamic and liquefaction effects unique to these types of
structures.
14.4 NONBUILDING STRUCTURES NOT SIMILAR TO BUILDINGS: This general class
of nonbuilding structures exhibits behavior markedly different from that of building structures.
Most of these types of structures have industry standards that address their unique structural
performance and behavior. The new elements of the 1997 Provisions regarding ground motion
required that a prudent link to the industry standards be developed.
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1997 Commentary, Chapter 14
14.4.1 General:
14.4.2 Earth Retaining Structures:
14.4.3 Tanks and Vessels: The majority of this section was moved to the Appendix
14.4.5 Telecommunication Towers:
14.4.5.1 General: The majority of this section was moved to the Appendix
14.4.6 Stacks and Chimneys: The design of stacks and chimneys to resist natural hazards is
generally governed by wind design considerations. The exceptions to this general rule involve
locations with high seismicity, stacks and chimneys with large elevated masses, and stacks and
chimneys with unusual geometries. It is prudent to evaluate the effect of seismic loads in all but
those areas with the lowest seismicity. Although not specifically required, it is recommended that
the special seismic details required elsewhere in the Provisions be evaluated for applicability to
stacks and chimneys.
Guyed steel stacks and chimneys are generally light weight. As such the design loads due to
natural hazards are generally governed by wind. On occasion, large flares or other elevated
masses located near the top may require an in-depth seismic analysis. Although Chapter 6,
"Multilevel Guyed Stacks" in Tubular Steel Structures by M. S. Troitsky does not specifically
address seismic loading, it remains an applicable methodology for resolution of seismic forces that
can be determined in these Provisions.
14.4.10 Inverted Pendulums: These structures are those that support an elevated lumped mass
but water tanks are excluded.
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Appendix to Chapter 14
PREFACE: The following sections were originally intended to be part of the Nonbuilding
Structures Chapter of this Commentary. The Provisions Update Committee felt that given
the complexity of the issues, the varied nature of the resource documents, and the lack of
supporting consensus resource documents, time did not allow a sufficient review of the
proposed sections required for inclusion into the main body of the chapter.
The Nonbuilding Structures Technical Subcommittee, however, expressed that what is
presented herein represents the current industry accepted design practice within the
engineering community that specializes in these types of nonbuilding structures.
The Commentary sections are included here so that the design community specializing in
these nonbuilding structures can have the opportunity to gain a familiarity with the
concepts, update their standards, and send comments on this appendix to the BSSC.
It is hoped that the various consensus design standards will be updated to include the
design and construction methodology presented in this Appendix. It is also hoped that
industry standards that are currently not consensus documents will endeavor to move their
standards through the consensus process facilitating building code inclusion.
A14.1.8 References and Standards: TS13 believed it essential to provide a controlled link
between the Provisions and industry design standards. While the subcommittee wanted to employ
the new seismic ground motion maps and the new methodology for establishing seismic design
and detailing contained in the 1997 Provisions, it did not want to abandon the design methodologies established by the various industries involved with nonbuilding structures.
As previously stated, some of the Chapter 14 references are not consensus documents; however,
the cited references do represent the current state of structural design practice in the various
industries. The references are divided into three categories: standards, industry standards, and
industry references. There is ample precedence to incorporate ANSI or ASCE consensus
documents (as contained in the standards section) into an international code, and local code
officials often include industry standards in their local jurisdictions. With the lack of any form of
consensus standards, some industries use other industry references as a method of establishing a
structural design methodology.
A14.1.9 Industry Design Standards and Recommended Practice: This section was added to
aid the registered design professionals in identifying the industry standards for a particular type of
nonbuilding structure. There are no industry standards for some nonbuilding structures and, in
those cases, the commonly accepted design approaches contained in reference and text books are
cited.
The list of petrochemical facilities includes: structures that have a dynamic response similar to
building structures that support equipment such as air coolers, horizontal vessels, heat exchangers, heaters, vertical vessels, and reactors; equipment behaving similar to structures with
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1997 Commentary
integral supports such as stand-alone vertical vessels/heaters, tanks, spheres, boilers, chimneys,
and stacks; concrete or steel pipe ways; and cooling towers.
A14.4.3 Tanks and Vessels:
A14.4.3.1 General: The standards for tanks and vessels have specific requirements to safeguard
against catastrophic failure of the primary structure based on observed behavior in seismic events
since the 1930s. The procedures used to design flat bottom storage tanks and liquid containers is
based on the work of Housner (Ref A14-43) and Wozniak and Mitchell (Ref A14-40).
The requirements in Sec 14.4.3 are intended to link the latest procedures for determining design
level seismic loads with the allowable stress design procedures based on the UBC methods. These
requirements, which in many cases identify specific substitutions to be made in the design
equations of the national standards, will assist users of the Provisions in making consistent
interpretations.
A14.4.3.1.1 Strength and Ductility: As is the case for building structures, ductility and
redundancy in the lateral support systems for tanks and vessels are desirable and necessary for
good seismic performance. Tanks and vessels are not highly redundant structural systems and,
therefore, ductile materials and well designed connection details are needed to increase the
capacity of the vessel to absorb more energy without failure. The critical performance of many
tanks and vessels is governed by shell stability requirements rather than by yielding of the
structural elements.
A14.4.3.1.2 Flexibility of Piping Attachments: The performance of piping connections under
seismic deformations is one of the primary weaknesses observed in recent seismic events. Tank
leakage and damage occurs when the piping connections cannot accommodate the movements the
tank experiences during the a seismic event. Contrary to the design methods used by many piping
designers, which impart mechanical loading to the tank shell, piping systems in seismic areas
should be designed in such a manner as to impose negligible mechanical loads on the tank
connection for the values shown in Table 14.4.3.1.2.
In addition, interconnected equipment, walkways, and bridging between multiple tanks must be
designed to resist the loads and displacements imposed by seismic forces. Unless multiple tanks
are founded on a single rigid foundation, walkways, piping, bridges and other connecting
structures must be designed to allow for the calculated differential movements between connected
structures due to seismic loading assuming the tanks and vessels are out of phase.
A14.4.3.1.3 Anchorage: Many steel tanks can be designed without anchors by using the
annular plate procedures given in the national standards. Those tanks that must be anchored
because of overturning potential could be susceptible to shell tearing if not properly designed.
Ideally, the proper anchorage design will provide both a shell attachment and embedment detail
that will yield the bolt without tearing the shell or pulling the bolt out the foundation. Properly
designed anchored tanks retain greater reserve strength to resist seismic overload than unanchored tanks.
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Appendix to Chapter 14
A14.4.3.2
Ground Supported Storage Tanks for Liquids:
A14.4.3.2.1 General: The response of ground storage tanks to earthquakes is well documented
by Housner, Mitchell and Wozniak, Valets, and others. Unlike building structures, the structural
response is strongly influenced by the fluid-structure interaction. Fluid-structure interaction
forces are categorized as sloshing (convective mass) and rigid (impulsive mass) forces. The
proportion of these forces depends on the geometry (height to diameter ratio) of the tank. Ref
A14-1 and A14-4 provide the necessary data to determine the relative masses and moments for
each of these contributions.
A14.4.3.2.1.1 Freeboard: Performance of ground storage tanks in past earthquakes has
indicated that sloshing of the contents can cause leakage and damage to the roof and internal
components. While the effect of sloshing often involves only the cost and inconvenience of
making repairs, not catastrophic failure, even this limited damage can be prevented or significantly
mitigated when the following aspects are considered:
1.
Effective masses and hydro-dynamic forces in the container
2.
Impulsive and pressure loads.
3.
a.
Sloshing zone (i.e. the upper shell and edge of roof system).
b.
Internal supports (roof support columns, tray-supports, etc.).
c.
Equipment (distribution rings, access tubes, pump wells, risers, etc.).
Freeboard (depends on the sloshing wave height).
Tanks and vessels storing biologically or environmentally benign materials do not typically require
freeboard to protect the public health and safety. However, providing freeboard in areas of
frequent seismic occurrence for vessels normally operated at or near top capacity may lessen
damage (and the cost of subsequent repairs) to the roof and upper container.
The estimate given in the Provision Sec 14.4.3.2.1.1 is based on a median response spectrum
rather than on the one standard deviation response spectra found in Ref. A14-42. It is also based
on the seismic design event as defined by the Provisions. Estimates for the sloshing height
contained in national standards are based on the one standard deviation spectra applied at a
working stress level. Users of the Provisions may estimate slosh heights different from those
recommended in the national standards.
A14.4.3.2.1.4 Sliding Resistance: Steel ground-supported tanks full of product have not been
found to slide off foundations. Resistance to sliding is obtained from the frictional resistance
between the steel bottom and the sand cushion on which bottoms are placed. Because tank
bottoms usually are crowned upward toward the tank center and are constructed of overlapping
fillet welded individual steel plates (resulting in a rough bottom), it can be considered reasonably
conservative to take the ultimate coefficient of friction as 0.70 (U.S. Nuclear Regulatory
Commission, 1989, pg A-50) and, therefore, a value of tan 30o (0.577) is used. The vertical
weight of the tank and contents reduced by the component of vertical acceleration provides the
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1997 Commentary
net vertical load. An orthogonal combination of vertical and horizontal seismic forces following
the procedure in Sec.2.2.5.3.2 may be used.
A14.4.3.2.2 Water and Water Treatment Structures:
A14.4.3.2.2.1 Welded Steel: The AWWA design requirements for of ground-supported steel
water storage structures is based on an allowable stress method that utilizes an effective mass
procedure considering two response modes of the tank and its contents:
1.
The high-frequency amplified response to seismic motion of the tank shell, roof, and
impulsive mass (portion of liquid content of the tank that moves in unison with the shell) and
2.
The low frequency amplified response of the convective mass (portion of the liquid contents
in the fundamental sloshing mode).
The two-part AWWA equation incorporates the above modes, appropriate damping, site
amplification, allowable stress response modification and zone coefficients. In practice, the
typical ground storage tank and impulsive contents will have a natural period, Te, of 0.1 to 0.3
sec. The sloshing period typically will be greater than 1 sec (usually 3 to 5 sec depending on tank
geometry). Thus, the substitution in the Provisions uses a short- and long-period response as it
applies to the appropriate constituent term in the AWWA equations.
A14.4.3.2.3 Petrochemical and Industrial Liquids:
A14.4.3.2.3.1 Welded Steel: See Sec. 14.4.3.2.2.1 for description of the Sas and Sal substitution.
The American Petroleum Institute (API) also uses an allowable stress design procedure and the
API equation has incorporated an Rw factor equal to 4.1667 for all API designed ground storage
structures. Therefore, one will not find an R for API 620 or 650 flat bottom storage structures in
Table 14.2.1.1.
A14.4.4 Electrical Transmission, Substation, and Distribution Structures: The design of
electrical transmission, substation wire support, and distribution structures is typically controlled
by high wind, ice-wind combinations, and unbalance longitudinal wire loads (Agrawal and
Kramer, 1976; ASCE, 1991; IEEE, 1997). Distribution structures typically support equipment
with low mass and seismic loads do not control their design. Earthquake performance of these
structures has demonstrated that seismic loads can be resisted based on traditional electrical
transmission, substation, and distribution wire support structure loading (Steinhardt, 1981).
These structures may be used in special situations were seismic loads should be considered in their
design. The special situations for transmission and substation wire support structures may include
site specific low wind velocity and ice load, and no designed unbalance longitudinal wire load.
For distribution structures, the number of supported transformers may result in significant seismic
load.
Earthquake-related damage to electrical transmission, substation wire support, and distribution
structures typically is caused by large displacements of the foundations due to landslides, ground
failure, and liquefaction (FEMA, 1990). These situations have resulted in structural failure or
damaged structural members without complete loss of structure function.
The fundamental frequency of these structure types typically ranges from 0.5 to 6 Hz. Single pole
type structures have fundamental mode frequencies in the 0.5 to 1.5 Hz range. H-frame struc-
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Appendix to Chapter 14
tures have fundamental mode frequencies in the 1 to 3 Hz ranges, with the lower frequencies in
the direction normal to the plane of the structure and the higher frequencies in plane. Four legged
lattice structures have fundamental mode frequencies in the range of 2 to 6 Hz. Lattice tangent
structures typically have lower frequencies with the higher frequencies being representative of
angle and dead end structures. These frequency ranges can be used to determine if earthquake
loading should be a design consideration. If it is determined that earthquake loads are significant
then a more detailed evaluation of the structure vibration frequencies and mode shapes should be
performed. This can be accomplished using available commercial finite element computer
programs. The default viscous damping value to be used in such an analysis should be 2 percent.
A higher damping value can be used if determined using sound engineering data.
A minimum importance factor (I) of 1.0 should be used to provide the necessary seismic
resistance. An I of 1.0 is required to minimize the loss of function after an earthquake event even
though these systems are normally redundant.
The R values shown in Table 14.2.1.1 reflect the inelastic reserve strength of the structural
systems during an earthquake event. The values presented for these types of structures were
determined based on a review of published values established for building structures and
nonbuilding structures. An analysis of lattice (truss) type transmission towers dictated R values in
the range of 3 to 8 (Lyver, Mueller and Kempner, 1996). The value of 3 for truss systems shown
in Table 14.2.1.1 represents the lower bound value of R. In general, the remaining R values
shown reflect the earthquake performance of these structural systems and engineering judgment.
Other values may be appropriate if determined using sound engineering data.
The Cd and S values shown in Table 14.2.1.1 for these types of structures are presented for
information only and to be consistent with parameters presented for other facilities covered by the
Provisions. The Cd value is a factor used to estimate the peak inelastic deflection (dinel) during a
seismic event when the elastic displacements (del) from a static analysis using seismic loads are
known (dinel = delCd). The S values represent a component force factor to be used to provide
increased reliability in strength for a critical component (component force times S). The
magnitude of this factor is currently specified (when used) by the industry design standards and
recommended practices specified in Sec. 14.1.8.
Traditionally, wire supported mass and dynamic effects have not been included in the evaluation
of structural response ( Long, 1973). Some studies have suggested that for long spans the seismic
contribution of the wires should not be neglected (Li et al., 1991; Li et al. 1994). Reasons for
neglecting the supported wires are the order of magnitude difference between the wire system
natural frequency and that of the supporting structures and the method of connection between
these two systems. The spatial distribution of the structural system (varying wire spans, tower
location and geometry, and seismic ground motion) also helps mitigate the effects of dynamic
coupling. The satisfactory performance of these structures during earthquakes does not justify the
additional loading as a result of the wire dynamics. Engineering judgment should be used to
determined the inclusion or the significance of the wire mass.
A14.4.5 Telecommunication Towers: The design of telecommunication towers is typically
controlled by extreme wind, ice and wind combinations, and restrictive deflection (serviceability)
limits (EIA TIA 222; Canadian Standards Association, 1994). Earthquake performance of these
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1997 Commentary
structures has demonstrated that seismic loads can be resisted based on traditional
telecommunication loading (Lum et al., 1983). As a minimum, this requirement should be to
determine the significance of seismic loads in the design of the tower. Seismic lateral loads in
combination with long-term ice loads should be considered. Recommendations for combined load
effects can be found in ANSI/ASCE 7.
A general industry survey indicated that the seismic performance of these structures to earthquake
loading has been acceptable. Reported earthquake damage has been limited to failure of building
mounted towers and shifting of mounted antennas resulting in misalignment of the signal path
(FEMA, 1990; Lum et al, 1983; NCEER, 1995; Steinhardt, 1981).
The fundamental frequency of these structural types typically ranges from 0.5 to 10 Hz. If it is
determined that earthquake loads are significant then a more detailed evaluation of the structure’s
vibration frequencies and mode shapes should be performed. This can be accomplished using
available commercial finite element computer programs. The default viscous damping value to be
used with such an analysis should be 2 percent. A higher damping value can be used if determined
using sound engineering data.
Recent studies (Galvez and McClure, 1995) have suggested that a linear lateral force distribution
(k = 1) is not an accurate representation for self-supporting telecommunication towers. The
lateral force distribution being studied accounts for the mass participation of the lowest three
flexural modes of vibration of the tower. Until further studies have been completed and a final
recommendation is available it is recommended that a linear distribution be used with the
Provisions when a refined lateral force distribution is required.
The R values shown in Table 14.2.1.1 reflect the inelastic reserve strength of the structural
systems during an earthquake event. The values presented for these types of structures were
determined based on a review of published values established for building structures and
nonbuilding structures. Other values may be appropriate if determined using sound engineering
data.
The Cd and S values shown in Table 14.2.1.1 for these types of structures are presented for
information only and to be consistent with parameters presented for other facilities covered by the
Provisions. The Cd value is a factor used to estimate the peak inelastic deflection (dinel) during a
seismic event when the elastic displacements (del) from a static analysis using seismic loads are
known (dinel = delCd). The S values represent a component force factor to be used to provide
increased reliability in strength for a critical component (component force times S). The
magnitude of this factor is currently specified (when used) by the industry design standards and
recommended practices specified in Sec. 14.1.8.
Guyed towers taller than 66 m should be evaluated using modal analysis procedures. Modeling of
a guyed tower must allow for geometric nonlinearities and potential interactions between the mast
and the guy wires (Amiri and McClure, 1996). The significant earthquake effect will be due to
the dynamic interaction between the mast and the guy wires. The analysis of guyed towers can be
accomplished using available commercial finite element computer programs.
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Appendix to Chapter 14
Reference A14-12C has an informative appendix that provides guidance on when earthquake
design of guyed and self-supporting telecommunication towers may be appropriate. The
following information is obtained from this document.
1.
Steel lattice and guyed towers are less sensitive to earthquake loads than most other
structure types.
2.
Self-supporting lattice towers up to 100 m high and having insignificant mass concentrations
less than 25 percent of their total mass need not be designed for earthquakes.
3.
Self-supporting lattice towers of insignificant mass and over 100 m high or lesser height with
significant mass concentrations may experience base shears and base overturning moments
approaching those caused by ultimate wind loads.
4.
Self-supporting lattice towers and guyed steel masts that are in earthquake design zones
should be designed considering the vertical component of ground motion. For very tall
guyed towers, some vertical ground motion differentials between the mast base and guy
anchorage points may be an important design consideration depending on local seismicity.
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Commentary Appendix A
DEVELOPMENT OF MAXIMUM CONSIDERED EARTHQUAKE
GROUND MOTION MAPS 1 THROUGH 24
BACKGROUND
The Building Seismic Safety Council (BSSC) has been working for several years to replace the
1994 Provisions Maps 1 through 4, which were developed about 20 years ago. The 1994 Provisions Maps 1 through 4 provided the Aa (effective peak acceleration coefficient) and A v (effective
peak velocity-related acceleration coefficient) values to use for design. The BSSC has always
recognized that the maps and coefficients would change with time as the profession gained more
knowledge about earthquakes and their resulting ground motions and as society gained greater
insight into the process of establishing acceptable risk.
In the past 20 years, significant additional earthquake data have been obtained that make the Aa
and Av maps out of date. The first significant changes were made in 1982 and these changes
were later included in the “Appendix to Chapter 1" in the 1988 Provisions. In the 1991 Provisions, that appendix was revised to introduce new spectral maps and procedures for review and
comment. For the 1994 Provisions, that appendix was again revised to describe recent and future
mapping, design values panel efforts, and improved spectral maps.
For these 1997 Provisions, a joint effort involving the BSSC, the Federal Emergency Management Agency (FEMA), and the U.S. Geological Survey (USGS) was conducted to develop both
new maps for use in design and new design procedures reflecting the significant advances made in
the past 20 years. The BSSC’s role in this joint effort was to develop new ground motion maps
for use in design and design procedures based on new USGS seismic hazard maps.
The BSSC appointed a 15-member Seismic Design Procedure Group (SDPG) to develop the
seismic ground motion maps and design procedures. The SDPG membership was composed of
representatives of different segments of the design community as well as two earth science members designated by the USGS, and the membership was representative of the different geographical regions of the country. Also the BSSC, with input from FEMA and USGS, appointed a fivemember Management Committee (MC) to guide the efforts of the SDPG. The MC was geographically balanced insofar as practicable and was composed of two seismic hazard definition
experts and three engineering design experts, including the chairman of the 1997 Provisions Update Committee (PUC). The SDPG and the MC worked closely with the USGS to define the
BSSC mapping needs and to understand how the USGS seismic hazard maps should be used to
develop the BSSC seismic ground motion maps and design procedures.
For a brief overview of how the USGS developed its hazard maps, see Appendix B to this Commentary volume. A detailed description of the development of the maps is contained in the USGS
Open-File Report 96-532, National Seismic-Hazard Maps: Documentation, June 1996, by Frank-
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1997 Commentary, Appendix A
el, et al. (1996). The USGS hazard maps also can be viewed and printed from a USGS Internet
site at http://gldage.cr.usgs.gov/eq/html/finmain.shtml.
The goals of the SDPG were as follows:
1. To replace the existing effective peak acceleration and velocity-related acceleration design
maps with new ground motion spectral response maps based on new USGS seismic hazard
maps.
2. To develop the new ground motion spectral response maps within the existing framework of
the Provisions with emphasis on uniform margin against the collapse of structures.
3. To develop design procedures for use with the new ground motion spectral response maps.
PURPOSE OF THE PROVISIONS
The purpose of the Provisions is to present criteria for the design and construction of new structures subject to earthquake ground motions in order to minimize the risk to life for all structures,
to increase the expected performance of higher occupancy structures as compared to ordinary
structures, and to improve the capability of essential structures to function after an earthquake.
To this end, the Provisions provides the minimum criteria considered prudent for structures subjected to earthquakes at any location in the United States and its territories. The Provisions generally considers property damage as it relates to occupant safety for ordinary structures. For high
occupancy and essential structures, damage limitation criteria are more strict in order to better
provide for the safety of occupants and the continued functioning of the structure. Some structural and nonstructural damage can be expected as a result of the “design ground motions” because the Provisions allows inelastic energy dissipation by utilizing the deformability of the structural system. For ground motions in excess of the design levels, the intent is that there be a low
likelihood of collapse. These goals of the Provisions are the guiding principles for developing the
design maps.
POLICY DECISIONS FOR SEISMIC GROUND MOTION MAPS
The 1997 Provisions maps reflect the following policy decisions that depart from past practice
and the 1994 Provisions:
1. The maps define the maximum considered earthquake ground motion for use in design procedures,
2. The use of the maps for design provide an approximately uniform margin against collapse for
ground motions in excess of the design levels in all areas.
3. The maps are based on both probabilistic and deterministic seismic hazard maps, and
4. The maps are response spectra ordinate maps and reflect the differences in the short-period
range of the response spectra for the areas of the United States and its territories with different ground motion attenuation characteristics and different recurrence times.
These policy decisions reflect new information from both the seismic hazard and seismic engineering communities that is discussed below.
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Development of Maximum Considered Earthquake Ground Motion Maps 1 -24
In the 1994 Provisions, the design ground motions were based on an estimated 90 percent probability of not being exceeded in 50 years (about a 500 year mean recurrence interval) (ATC 3-06
1978). The 1994 Provisions also recognized that larger ground motions are possible and that the
larger motions, although their probability of occurrence during a structure’s life is very small,
nevertheless can occur at any time. The 1994 Provisions also defined a maximum capable earthquake as “the maximum level of earthquake ground shaking that may ever be expected at the
building site within the known geologic framework.” It was additionally specified that in certain
map areas ($ Aa = 0.3), the maximum capable earthquake was associated with a motion that has a
90 percent probability of not being exceeded in 100 years (about a 1000 year mean recurrence
interval). In addition to the maximum capable earthquake definition, sample ground motion maps
were prepared with 90 percent probabilities of not being exceeded in 250 years (about a 2500
year mean recurrence interval).
Given the wide range in return periods for maximum magnitude earthquakes throughout the
United States and its territories (100 years in parts of California to 100,000 years or more in
several other locations), current efforts have focused on defining the maximum considered earthquake ground motions for use in design (not the same as the maximum capable earthquake defined in the 1994 Provisions). The maximum considered earthquake ground motions are determined in a somewhat different manner depending on the seismicity of an individual region; however, they are uniformly defined as the maximum level of earthquake ground shaking that is considered as reasonable to design structures to resist. Focusing on ground motion versus earthquake size facilitates the development of a design approach that provides an approximately uniform margin against collapse throughout the United States.
As noted above, the 1994 Provisions generally uses the notation of 90 percent probability of not
being exceeded in a certain exposure time period (50, 100, or 250 years), which can then be used
to calculate a given mean recurrence interval (500, 1000, or 2500 years). For the purpose of
these 1997 Provisions, the single exposure time period of 50 years has been commonly used as a
reference period over which to consider loads on structures (after 50 years of use, structures may
require evaluation to determine future use and rehabilitation needs). With this in mind, different
levels of probability or return period are expressed as percent probability of exceedance in 50
years. Specifically, 10 percent probability of exceedance in 50 years is a mean recurrence interval
of about 500 years, 5 percent probability of exceedance in 50 years is a mean recurrence interval
of about 1000 years, and 2 percent probability of exceedance in 50 years is a mean recurrence
interval of about 2500 years. The above notation is used throughout the Provisions.
Review of modern probabilistic seismic hazard results, including the maps prepared by the USGS
to support the effort resulting in the 1997 Provisions, indicates that the rate of change of ground
motion versus probability is not constant throughout the United States. For example, the ground
motion difference between the 10 percent probability of exceedance and 2 percent probability of
exceedance in 50 years in coastal California is typically smaller than the difference between the
two probabilities in less active seismic areas such as the eastern or central United States. Because
of these differences, questions were raised concerning whether definition of the ground motion
based on a constant probability for the entire United States would result in similar levels of seismic safety for all structures. Figure A1 plots the 0.2 second spectral acceleration normalized at 2
percent probability of exceedance in 50 years versus the annual frequency of exceedance. Figure
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1997 Commentary, Appendix A
A1 shows that in coastal California, the ratio between the 0.2 second spectral acceleration for the
2 and the 10 percent probabilities of exceedance in 50 years is about 1.5 whereas, in other parts of
the United States, the ratio varies from 2.0 to 5.0.
In answering the questions, it was recognized that seismic safety is the result of a number of steps
in addition to defining the design earthquake ground motions, including the critical items generally
defined as proper site selection, structural design criteria, analysis and procedures, detailed design
requirements, and construction.
FIGURE A1 Relative hazard at selected sites for 0.2 sec spectral response acceleration. The hazard curves are
normalized at 2 percent probability of exceedance in 50 years.
The conservatism in the actual design of the structure is often referred to as the “seismic margin.”
It is the seismic margin that provides confidence that significant loss of life will not be caused by
actual ground motions equal to the design levels. Alternatively, the seismic margin provides a
level of protection against larger, less probable earthquakes although at a lower level of confidence.
The collective opinion of the SDPG was that the seismic margin contained in the Provisions provides, as a minimum, a margin of about 1.5 times the design earthquake ground motions. In other
words, if a structure experiences a level of ground motion 1.5 times the design level, the structure
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Development of Maximum Considered Earthquake Ground Motion Maps 1 -24
should have a low likelihood of collapse. The SDPG recognizes that quantification of this margin
is dependent on the type of structure, detailing requirements, etc., but the 1.5 factor is a conservative judgment appropriate for structures designed in accordance with the Provisions. This seismic
margin estimate is supported by Kennedy et al. (1994), Cornell (1994), and Ellingwood (1994)
who evaluated structural design margins and reached similar conclusions.
The USGS seismic hazard maps indicate that in most locations in the United States the 2 percent
probability of exceedance in 50 years ground motion values are more than 1.5 times the 10 percent probability of exceedance in 50 years ground motion values. This means that if the 10 percent
probability of exceedance in 50 years map was used as the design map and the 2 percent probability of exceedance in 50 years ground motions were to occur, there would be low confidence
(particularly in the central and eastern United States) that structures would not collapse due to
these larger ground motions. Such a conclusion for most of the United States was not acceptable
to the SDPG. The only location where the above results seemed to be acceptable was coastal
California (2 percent probability of exceedance in 50 years map is about 1.5 times the 10 percent
probability of exceedance in 50 years map) where structures have experienced levels of ground
shaking equal to and above the design value.
The USGS probabilistic seismic hazard maps for coastal California also indicate the 10 percent
probability of exceedance in 50 years seismic hazard map is significantly different from (in most
cases larger) the design ground motion values contained in the 1994 Provisions. Given the
generally successful experience with structures that comply with the recent editions of the Uniform Building Code whose design map contains many similarities to the 1994 Provisions design
map, the SDPG was reluctant to suggest large changes without first understanding the basis for
the changes. This stimulated a detailed review of the probabilistic maps for coastal California.
This review identified a unique issue for coastal California in that the recurrence interval of the
estimated maximum magnitude earthquake is less than the recurrence interval represented on the
probabilistic map, in this case the 10 percent probability of exceedance in 50 years map (i.e.,
recurrence interval for maximum magnitude earthquake is 100 to 200 years versus 500 years.)
Given the above, one choice was to accept the change and use the 10 percent probability of
exceedance in 50 years probabilistic map to define the design ground motion for coastal California
and, using this, determine the appropriate probability for design ground motion for the rest of the
United States that would result in the same level of seismic safety. This would have resulted in
the design earthquake being defined at 2 percent probability of exceedance in 50 years and the
need for development of a 0.5 to 1.0 percent probability of exceedance in 50 years map to show
the potential for larger ground motions outside of coastal California. Two major problems were
identified. The first is that requiring such a radical change in design ground motion in coastal
California seems to contradict the general conclusion that the seismic design codes and process
are providing an adequate level of life safety. The second is that completing probabilistic estimates of ground motion for lower probabilities (approaching those used for critical facilities such
as nuclear power plants) is associated with large uncertainties and can be quite controversial.
An alternative choice was to build on the observation that the maximum earthquake for many
seismic faults in coastal California is fairly well known and associated with probabilities larger
than a 10 percent probability of exceedance in 50 years (500 year mean recurrence interval).
Given this, a decision was made to develop a procedure that would use the best estimate of
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1997 Commentary, Appendix A
ground motion from maximum magnitude earthquakes on seismic faults with high probabilities of
occurrence (short return periods). For the purposes of the Provisions, these earthquakes are
defined as “deterministic earthquakes.” Following this approach and recognizing the inherent
seismic margin contained in the Provisions, it was determined that the level of seismic safety
achieved in coastal California would be approximately equivalent to that associated with a 2 to 5
percent probability of exceedance in 50 years for areas outside of coastal California. In other
words, the use of the deterministic earthquakes to establish the maximum considered earthquake
ground motions for use in design in coastal California results in a level of protection close to that
implied in the 1994 Provisions and consistent with maximum magnitude earthquakes expected for
those seismic sources. Additionally, this approach results in less drastic changes to ground motion values for coastal California than the alternative approach of using probabilistic based maps.
One could ask why any changes are necessary for coastal California given the positive experience
from recent earthquakes. While it is true that the current seismic design practices have produced
positive results, the current design ground motions in the 1994 Provisions are less than those
expected from maximum magnitude earthquakes on known seismic sources. The 1994 Provisions
reportedly considered maximum magnitude earthquakes but did not directly link them to the
design ground motions (Applied Technology Council, 1978). If there is high confidence in the
definition of the fault and magnitude of the earthquake and the maximum earthquake occurs frequently, then the design should be linked to at least the best estimate ground motion for such an
earthquake. Indeed, it is the actual earthquake experience in coastal California that is providing
increased confidence in the seismic margins contained in the Provisions.
The above approach also is responsive to comments that the use of 10 percent probability of
exceedance in 50 years is not sufficiently conservative in the central and eastern United States
where the earthquakes are expected to occur infrequently. Based on the above discussion and the
inherent seismic margin contained in the Provisions, the SDPG selected 2 percent probability of
exceedance in 50 years as the maximum considered earthquake ground motion for use in design
where the use of the deterministic earthquake approach discussed above is not used.
The maximum considered earthquake ground motion maps are based on two response spectral
values (a short-period and a long-period value) instead of the Aa and Av coefficients. The decision
to use response spectral values is based on earthquake data obtained during the past 20 years
showing that site-specific spectral values are more appropriate for design input than the Aa and Av
coefficients used with standardized spectral shapes. The spectral shapes vary in different areas of
the country and for different site conditions. This is particularly the case for the short-period
portion of the response spectra. Based on the differences in the ground motion attenuation characteristics between the central and eastern and western United States, the USGS used different
ground motion attenuation functions for these areas in developing the seismic hazard maps. The
ground motion attenuation functions in the eastern United States result in higher short-period
spectral accelerations at lower periods for a given earthquake magnitude than the western United
States attenuation functions, particularly compared to the high seismicity region of coastal California. The short-period response spectral values were reviewed in order to determine the most
appropriate value to use for the maximum considered earthquake ground motion maps. Based on
this review, the short-period spectral response value of 0.2 second was selected to represent the
short-period range of the response spectra for the eastern United States. In the western United
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Development of Maximum Considered Earthquake Ground Motion Maps 1 -24
States the most appropriate short-period response spectral value was determined to be 0.3 second, but a comparison of the 0.2 and 0.3 second values indicated that the differences in the response spectral values were insignificant. Based on this and for convenience of preparing the
maximum considered earthquake ground motion maps, the short-period response spectral value of
0.2 second was selected to represent the short-period range of the response spectra for all of the
United States. The long-period response spectral value selected for use is 1.0 second for all of the
United States. Based on the ground motion attenuation functions and the USGS seismic hazard
maps, a 1/T (T = natural period) relationship was selected to define the response spectra from the
short period value to the long-period value. Using the spectral values from the ground motion
maps will allow the different spectral shapes to be incorporated into design.
DEVELOPMENT OF THE MAXIMUM CONSIDERED EARTHQUAKE GROUND
MOTION MAPS FOR USE IN DESIGN
The concept for developing maximum considered earthquake ground motions for use in design
involved two distinct steps:
1. The various USGS probabilistic seismic hazard maps were combined with deterministic hazard maps by a set of rules (logic) to create the maximum considered earthquake ground motion maps that can be used to define response spectra for use in design and
2. Design procedures were developed that transform the response spectra into design values
(e.g., design base shear).
The response spectra defined from the first step represent general “site-dependent” spectra similar
to those that would be obtained by a geotechnical study and used for dynamic analysis except
their shapes are less refined (i.e., shape defined for only a limited number of response periods).
The response spectra do not represent the same hazard level across the country but do represent
actual ground motion consistent with providing approximately uniform protection against the
collapse of structures. The response spectra represent the maximum considered earthquake
ground motions for use in design for Site Class B (rock with a shear wave velocity of 760 meters/second).
The maximum considered earthquake ground motion maps for use in design are based on a defined set of rules for combining the USGS seismic hazard maps to reflect the differences in the
ability to define the fault sources and seismicity characteristics across the regions of the country as
discussed in the policy decisions. Accommodating regional differences allows the maximum
considered earthquake maps to represent ground motions for use in design that provide reasonably consistent margins of preventing the collapse of structures. Based on this, three regions
have been defined:
1. Regions of negligible seismicity with very low probability of collapse of the structure,
2. Regions of low and moderate to high seismicity, and
3. Regions of high seismicity near known fault sources with short return periods.
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1997 Commentary, Appendix A
Regions of Negligible Seismicity With Very Low Probability of Collapse of the Structure
The regions of negligible seismicity with very low probability of collapse have been defined by:
1. Determining areas where the seismic hazard is controlled by earthquakes with Mb (body wave
magnitude) magnitudes less than or equal to 5.5 and
2. Examining the recorded ground motions associated with Modified Mercalli Intensity V.
The basis for the first premise is that in this region, there are a number of examples of earthquakes
with Mb – 5.5 which caused only localized damage to structures not designed for earthquakes.
The basis for the second premise is that Modified Mecalli Intensity V ground motions typically do
not cause structural damage. By definition, Modified Mercalli Intensity V ground shaking is felt
by most people, displaces or upsets small objects, etc., but typically causes no, or only minor,
structural damage in buildings of any type. Modified Mercalli Intensity VI ground shaking is felt
by everyone, small objects fall off shelves, etc., and minor or moderate structural damage occurs
to weak plaster and masonry construction. Life-threatening damage or collapse of structures
would not be expected for either Modified Mercalli Intensities V or VI ground shaking. Based
on an evaluation of 1994 Northridge earthquake data, regions of different Modified Mercalli
Intensity (Dewey, 1995) were correlated with maps of smooth response spectra developed from
instrumental recordings (Sommerville, 1995). The Northridge earthquake provided a sufficient
number of instrumental recordings and associated spectra to permit correlating Modified Mercalli
Intensity with response spectra. The results of the correlation determined the average response
spectrum for each Modified Mercalli Intensity region. For Modified Mercalli Intensity V, the
average response spectrum of that region had a spectral response acceleration of slightly greater
than 0.25g at 0.3 seconds and a spectral response acceleration of slightly greater than 0.10g at 1.0
seconds. On the basis of these values and the minor nature of damage associated with Modified
Mercalli Intensity V, 0.25g (short-period acceleration) and 0.10g (acceleration at a period of 1
second, taken proportional to 1/T) is deemed to be a conservative estimate of the spectrum below
which life-threatening damage would not be expected to occur even to the most vulnerable of
types of structures. Therefore, this region is defined as areas having maximum considered earthquake ground motions with a 2 percent probability of exceedance in 50 years equal to or less than
0.25g (short period) and 0.10g (long period). The seismic hazard in these areas is generally the
result of Mb – 5.5 earthquakes. In these areas, a minimum lateral force design of 1 percent of the
dead load of the structure shall be used in addition to the detailing requirements for the Seismic
Design Category A structures.
In these areas it is not considered necessary to specify seismic-resistant design on the basis of a
maximum considered earthquake ground motion. The ground motion computed for such areas is
determined more by the rarity of the event with respect to the chosen level of probability than by
the level of motion that would occur if a small but close earthquake actually did occur. However,
it is desirable to provide some protection, both against earthquakes as well as many other types of
unanticipated loadings. The requirements for Seismic Design Category A provide a nominal
amount of structural integrity that will improve the performance of buildings in the event of a
possible, but rare earthquake. The result of design to Seismic Design Category A is that fewer
buildings would collapse in the vicinity of such an earthquake.
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Development of Maximum Considered Earthquake Ground Motion Maps 1 -24
The integrity is provided by a combination of requirements. First, a complete load path for lateral
forces must be identified. Then it must be designed for a lateral force equal to a 1% acceleration
on the mass. Lastly, the minimum connection forces specified for Seismic Design Category A
must be satisfied.
The 1 percent value has been used in other countries as a minimum value for structural integrity.
For many structures, design for the wind loadings specified in the local building codes will normally control the lateral force design when compared to the minimum structural integrity force on
the structure. However, many low-rise heavy structures or structures with significant dead loads
resulting from heavy equipment may be controlled by the nominal 1 percent acceleration. Also,
minimum connection forces may exceed structural forces due to wind in additional structures.
The regions of negligible seismicity will vary depending on the Site Class on which structures are
located. The Provisions seismic ground motion maps (Maps 1 through 19 ) are for Site Class B
conditions and the region of negligible seismicity for Site Class B is defined where the maximum
considered earthquake ground motion short-period values are #0.25g and the long-period values
are # 0.10g. The regions of negligible seismicity for the other Site Classes are defined by using
the appropriate site coefficients to determine the maximum considered earthquake ground motion
for the Site Class and then determining if the short-period values are # 0.25g and the long-period
values are # 0.10g. If so, then the site of the structure is located in the region of negligible seismicity for that Site Class.
Regions of Low and Moderate to High Seismicity
In regions of low and moderate to high seismicity, the earthquake sources generally are not well
defined and the maximum magnitude estimates have relatively long return periods. Based on this,
probabilistic hazard maps are considered to be the best means to represent the uncertainties and to
define the response spectra for these regions. The maximum considered earthquake ground motion for these regions is defined as the ground motion with a 2 percent probability of exceedance
in 50 years. The basis for this decision is explained in the policy discussion.
Consideration was given to establishing a separate region of low seismicity and defining a minimum level of ground motion (i.e., deterministic minimum ground motions). This was considered
because in the transition between the regions of negligible seismicity to the regions of low seismicity, the ground motions are relatively small and may not be very meaningful for use in seismic
design. The minimum level was also considered because the uncertainty in the ground motion
levels in the regions of low seismicity is larger than in the regions of moderate to high seismicity.
This larger uncertainty may warrant consideration of using higher ground motions (or some minimum level of ground motion) than provided by the maximum considered earthquake ground
motions shown on the maps.
The studies discussed above for the regions of negligible seismicity by Dewey (1995) and
Sommerville (1995), plus other unpublished studies (to date), were evaluated as a means of determining minimum levels of ground motion for used in design. These studies correlated the Modified Mercalli Intensity data with the recorded ground motions and associated damage. The studies
included damage information for a variety of structures which had no specific seismic design and
determined the levels of ground motion associated with each Modified Mercalli Intensity. These
studies indicate that ground motion levels of about 0.50g short-period spectral response and
295
1997 Commentary, Appendix A
0.20g long-period spectral response are representative of Modified Mercalli Intensity VII damage.
Modified Mercalli Intensity VII ground shaking results in negligible damage in buildings of good
design and construction, slight to moderate damage in well-built ordinary buildings, considerable
damage in poorly-built or badly designed buildings, adobe houses, old walls (especially where laid
up without mortar), etc. In other words, Modified Mercalli VII ground shaking is about the level
of ground motion where significant structural damage may occur and result in life safety concerns
for occupants. This tends to suggest that designing structures for ground motion levels below
0.50g short-period spectral response and 0.20g long-period spectral response may not be meaningful.
One interpretation of this information suggests that the ground motion levels for defining the
regions of negligible seismicity could be increased. This interpretation would result in much
larger regions that require no specific seismic design compared to the 1994 Provisions.
Another interpretation of the information suggests establishing a minimum level of ground Motion
(at about the Modified Mercalli VII shaking) for regions of low seismicity, in order to transition
from the regions of negligible seismicity to the region of moderate to high seismicity. Implementation of a minimum level of ground motion, such as 0.50g for the short-period spectral response
and 0.20g for the long-period spectral response, would result in increases (large percentages) in
ground motions used for design compared to the 1994 Provisions.
Based on the significant changes in past practices resulting from implementing either of the above
interpretations, the SDPG decided that additional studies are needed to support these changes.
Results of such studies should be considered for future editions of the Provisions.
Regions of High Seismicity Near Known Fault Sources With Short Return Periods
In regions of high seismicity near known fault sources with short return periods, deterministic
hazard maps are used to define the response spectra maps as discussed above. The maximum
considered earthquake ground motions for use in design are determined from the USGS deterministic hazard maps developed using the ground motion attenuation functions based on the median
estimate increased by 50 percent. Increasing the median ground motion estimates by 50 percent is
deemed to provide an appropriate margin and is similar to some deterministic estimates for a
large magnitude characteristic earthquake using ground motion attenuation functions with one
standard deviation. Estimated standard deviations for some active fault sources have been determined to be higher than 50 percent, but this increase in the median ground motions was considered reasonable for defining the maximum considered earthquake ground motions for use in design.
Maximum Considered Earthquake Ground Motion Maps for Use in Design
Considering the rules for the three regions discussed above, the maximum considered earthquake
ground motion maps for use in design were developed by combining the regions in the following
manner:
1. Where the maximum considered earthquake map ground motion values (based on the 2 percent probability of exceedance in 50 years) for Site Class B adjusted for the specific site
296
Development of Maximum Considered Earthquake Ground Motion Maps 1 -24
conditions are # 0.25g for the short-period spectral response and # 0.10g for the long period
spectral response, then the site will be in the region of negligible seismicity and a minimum
lateral force design of 1 percent of the dead load of the structure shall be used in addition to
the detailing requirements for the Seismic Design Category A structures.
2. Where the maximum considered earthquake ground motion values (based on the 2 percent
probability of exceedance in 50 years) for Site Class B adjusted for the specific site conditions
are greater than 0.25g for the short-period spectral response and 0.10g for the long-period
spectral response, the maximum considered earthquake ground motion values (based on the 2
percent probability of exceedance in 50 years adjusted for the specific site conditions) will be
used until the values equal the present (1994 NEHRP) ceiling design values increased by 50
percent (short period = 1.50g, long period = 0.60g). The present ceiling design values are
increased by 50 percent to represent the maximum considered earthquake ground motion
values. This will define the sites in regions of low and moderate to high seismicity.
3. To transition from regions of low and moderate to high seismicity to regions of high seismicity
with short return periods, the maximum considered earthquake ground motion values based
on 2 percent probability of exceedance in 50 years will be used until the values equal the present (1994 Provisions) ceiling design values increased by 50 percent (short period = 1.50g,
long period = 0.60g). The present ceiling design values are increased by 50 percent to represent maximum considered earthquake ground motion values. When the 1.5 times the ceiling
values are reached, then they will be used until the deterministic maximum considered earthquake map values of 1.5g (long period) and 0.60g (short period) are obtained. From there,
the deterministic maximum considered earthquake ground motion map values will be used.
In some cases there are regions of high seismicity near known faults with return periods such that
the probabilistic map values (2 percent probability of exceedance in 50 years) will exceed the
present ceiling values of the 1994 Provisions increased by 50 percent and will be less than the
deterministic map values. In these regions, the probabilistic map values will be used for the maximum considered earthquake ground motions.
The basis for using present ceiling design values as the transition between the two regions is because earthquake experience has shown that regularly configured, properly designed structures
performed satisfactorily in past earthquakes. The most significant structural damage experienced
in the Northridge and Kobe earthquakes was related to configuration, structural systems, inadequate connection detailing, incompatibility of deformations, and design or construction deficiencies -- not due to deficiency in strength (Structural Engineers Association of California, 1995).
The earthquake designs of the structures in the United States (coastal California) which have
performed satisfactorily in past earthquakes were based on the criteria in the Uniform Building
Code. Considering the site conditions of the structures and the criteria in the Uniform Building
Code, the ceiling design values for these structures were determined to be appropriate for use
with the Provisions maximum considered earthquake ground motion maps for Site Class B.
Based on this, the equivalent maximum considered earthquake ground motion values for the
ceiling were determined to be 1.50g for the short period and 0.60g for the long period.
As indicated above there also are some regions of high seismicity near known fault sources with
return periods such that the probabilistic map values (2 percent probability of exceedance in 50
297
1997 Commentary, Appendix A
years) will exceed the present ceiling values of the 1994 Provisions increased by 50 percent and
also be less than the deterministic map values. In these regions, the probabilistic map values will
be used for the maximum considered earthquake ground motions.
The near source area in the high seismicity regions is defined as the area where the maximum
considered earthquake ground motion values are $ 0.75g on the 1.0 second map. In the near
source area, Provisions Sec. 5.2.3 through 5.2.6 impose additional requirements for certain
structures unless the structures are fairly regular, do not exceed 5 stories in height, and do not
have a period of vibration over 0.5 seconds. For the fairly regular structures not exceeding 5
stories in height and not having a period of vibration over 0.5 seconds, the maximum considered
earthquake ground motion values will not exceed the present ceiling design values increased by 50
percent. The basis for this is because of the earthquake experience discussed above.
These development rules for the maximum considered earthquake ground motion maps for use in
design are illustrated in Figures A2 and A3. The application of these rules result in the 1997
Provision maximum considered earthquake ground motion maps (Maps 1 through 24).
FIGURE A2 Development of the maximum considered earthquake ground motion
map for spectral acceleration of T = 1.0, Site Class B.
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Development of Maximum Considered Earthquake Ground Motion Maps 1 -24
STEP 1 -- DEFINE POTENTIAL SEISMIC SOURCES
A.
Compile Earth Science Information -- Compile historic seismicity and fault characteristics including earthquake magnitudes
and recurrence intervals.
B.
Prepare Seismic Source Map -- Specify historic seismicity and faults used as sources.
STEP 2 -- PREPARE PROBABILISTIC AND DETERMINISTIC SPECTRAL RESPONSE MAPS
A. Develop Regional Attenuation Relations
(1)
Eastern U.S. ( Toro, et al., 1993, and Frankel, 1996)
(2)
Western U.S. (Boore et al., 1993 &1994, Campbell and Bozorgnia, 1994, and Sadigh, 1993 for PGA. Boore et al., 1993
&1994, and Sadigh, 1993 for spectral values)
(3)
Deep Events (™35km) (Geomatrix et al., 1993)
(4) Cascadia Subduction Zone (Geomatrix et al., 1993, and Sadigh, 1993)
B.
Prepare Probabilistic Spectral Response Maps (USGS Probabilistic Maps) -- Maps showing SS and S1 where SS and S1 are
the short and 1 second period ground motion response spectral values for a 2 percent chance of exceedence in 50 years inferred
for sites with average shear wave velocity of 760 m/s from the information developed in Steps 1A and 1B and the ground
motion attenuation relationships in Step 2A.
C.
Prepare Deterministic Spectral Response Maps (USGS Deterministic Maps) -- Maps showing SS and S1 for faults and
maximum earthquakes developed in Steps 1A and 1B and the median ground motion attenuation relations in Step 2A increased
by 50% to represent the uncertainty.
STEP 3 -- PREPARE EARTHQUAKE GROUND MOTION SPECTRAL RESPONSE MAPS FOR PROVISIONS
(MAXIMUM CONSIDERED EARTHQUAKE GROUND MOTION MAP)
Region 1 -- Regions of Negligible Seismicity with Very Low Probability of Collapse of the Structure (No Spectral Values)
Region definition: Regions for which SS < 0.25g and S1 < 0.10g from Step 2B.
Design values: No spectral ground motion values required. Use a minimum lateral force level of 1 percent of the dead load for
Seismic Design Category A.
Region 2 -- Regions of Low and Moderate to High Seismicity (Probabilistic Map Values)
Region definition: Regions for which 0.25g < SS < 1.5g and 0.25g < S1 < 0.60g from Step 2B.
Maximum considered earthquake map values: Use SS and SI map values from Step 2B.
Transition Between Regions 2 and 3 - Use MCE values of SS = 1.5g and S1 = 0.60g
Region 3 -- Regions of High Seismicity Near Known Faults (Deterministic Values)
Region definition: Regions for which 1.5g < SS and 0.60g < S1 from Step 2C.
Maximum considered earthquake map values: Use SS and SI map values from Step 2C.
FIGURE A3 Methodology for development of the maximum considered earthquake ground motion maps (Site
Class B).
299
1997 Commentary, Appendix A
Use of the Maximum Considered Earthquake Ground Motion Maps in the Design Procedure: The 1994 Provisions define the seismic base shear as a function of the outdated effective
peak velocity-related acceleration Av, and effective peak acceleration, Aa. For the 1997 Provisions, the base shear of the structure is defined as a function of the maximum considered earthquake ground motion maps where SS = maximum considered earthquake spectral acceleration in
the short-period range for Site Class B; S1 = maximum considered earthquake spectral acceleration at the 1.0 second period for Site Class B; SMS = FaSS, maximum considered earthquake spectral acceleration in the short-period range adjusted for Site Class effects where Fa is the site coefficient defined in Provisions Sec. 4.1.2.4; SM1 = FvS1, maximum considered earthquake spectral
acceleration at 1.0 second period adjusted for Site Class effects where Fv is the site coefficient
defined in Provisions Sec. 4.1.2.4; SDS = (2/3) SMS, spectral acceleration in the short-period range
for the design ground motions; and SD1 = (2/3) SM1, spectral acceleration at 1.0 second period for
the design ground motions.
As noted above, the design ground motions SDS and SD1 are defined as 2/3 times the maximum
considered earthquake ground motions. The 2/3 factor is based on the estimated seismic margins
in the design process of the Provisions as previously discussed (i.e., the design level of ground
motion is 1/1.5 or 2/3 times the maximum considered earthquake ground motion).
Based on the above defined ground motions, the base shear is:
V ' Cs W
where C s '
S DS
and SDS = the design spectral response acceleration in the short period range as
R/I
determined from Sec. 4.1.2.5, R = the response modification factor from Table 5.2.2, and I = the
occupancy importance factor determined in accordance with Sec. 1.4.
The value of Cs need not exceed C s '
SD1
T (R/I)
but shall not be taken less than
Cs ' 0.1 SD1 or, for buildings and structures in Seismic Design Categories E and F,
Cs '
0.5 S1
R/I
where I and R are as defined above and SD1 = the design spectral response acceler-
ation at a period of 1.0 second as determined from Sec. 4.1.2.5, T = the fundamental period of
the structure (sec) determined in Sec. 5.3.3, and S1 = the mapped maximum considered earthquake spectral response acceleration determined in accordance with Sec. 4.1.
Where a design response spectrum is required by these Provisions and site-specific procedures are
not used, the design response spectrum curve shall be developed as indicated in Figure A4 and as
follows:
300
Development of Maximum Considered Earthquake Ground Motion Maps 1 -24
1. For periods less than or equal to T0, the design spectral response acceleration, Sa, shall be
taken as given by Eq. 4.1.2.6-1:
S a ' 0.6
S DS
T0
(4.1.2.6-
T % 0.4 SDS
1)
2. For periods greater than or equal to T0 and less than or equal to TS, the design spectral response acceleration, Sa, shall be taken as equal to SDS.
3. For periods greater than TS, the design spectral response acceleration, Sa, shall be taken as
given by Eq. 4.1.2.6-3:
Sa '
SD1
(4.1.2.6-3)
T
where:
SDS = the design spectral response acceleration at short periods;
SD1 = the design spectral response acceleration at 1 second period;
T
= the fundamental period of the structure (sec);
T0 = 0.2SD1/SDS; and
TS = SD1/SDS.
Figure A4 Design response spectrum.
301
1997 Commentary, Appendix A
Site-specific procedures for determining ground motions and response spectra are discussed in
Sec. 4.1.3 of the Provisions.
REFERENCES
Applied Technology Council. 1978. Tentative Provisions for the Development of Seismic Regulations for Building, Report ATC-3-06. Redwood City, California: Applied Technology Council.
Structural Engineers Association of California. 1995. Vision 2000, Performance Based Seismic
Engineering of Buildings. Prepared by the Structural Engineers Association of California, Vision
2000 Committee, for the California Office of Emergency Services.
Kennedy, R. P. and S. A. Short. 1994. Basis for Seismic Provisions of DOE-STD-1020,
UCRL-CR-111478. Prepared for Lawrence Livermore National Laboratory. Livermore: University of California.
Ellingwood, B. R. 1994. “Probability-based Codified Design for Earthquakes.” Engineering
Structures 17(7).
Cornell, C. A. 1994. Risk-Based Structural Design, Proceedings of Symposium on Risk Analysis, Ann Arbor: University of Michigan.
Dewey, J. W., B. G. Reagor, L. Dengler, and K. Moley. 1995. Intensity Distribution and
Isoseismal Maps for the Northridge, California, Earthquake of January 17, 1994, U. S. Geological Survey Open-File Report 95-92.
Sommerville, P. 1995. “Smooth Site Response Spectra Contours Developed for ATC-33,"
Appendix B of Guidelines for Seismic Rehabilitation of Existing Buildings, Northridge Earthquake Case-Study Report, ATC-33-1. Redwood City, California: Applied Technology Council.
Frankel, et al. 1996. National Seismic-Hazard Maps: Documentation, June 1996, U.S. Geological Survey Open-File Report 96-532.
302
Commentary Appendix B
DEVELOPMENT OF THE USGS SEISMIC MAPS
INTRODUCTION
The 1997 NEHRP Recommended Provisions uses new design procedures based on the use of
spectral response acceleration rather than the traditional peak ground acceleration and/or peak
ground velocity. The use of spectral ordinates and their relationship to building codes has been
described by Leyendecker et al (1995). The spectral response accelerations used in the new design approach are obtained from combining probabilistic maps (Frankel, et al, 1996) prepared by
the U.S. Geological Survey (USGS) with deterministic maps using procedures developed by the
Building Seismic Safety Council’s Seismic Design Procedures Group (SDPG). The SDPG recommendations are based on using the 1996 USGS probabilistic hazard maps with additional modifications based on review by the SDPG and the application of engineering judgement. This
appendix summarizes the development of the USGS maps and describes how the 1997 Provisions
design maps were prepared from them using SDPG recommendations. The SDPG effort has
sometimes been referred to as Project ‘97.
DEVELOPMENT OF PROBABILISTIC MAPS FOR THE UNITED STATES
New seismic hazard maps for the conterminous United States were completed by the USGS in
June, 1996 and placed on the Internet World Wide Web (http://geohazards.cr.usgs. gov/eq/). The
color maps can be viewed on the Web and/or downloaded to the user's computer for printing.
Paper copies of the maps are also available (Frankel et al, 1997a, 1997b).
New seismic hazard maps for Alaska were completed by the USGS in January 1998 and placed on
the USGS web site (http://geohazards.cr.usgs. gov/eq/). Both documentation and printing of the
maps are in progress (U. S. Geological Survey, 1998a, 1998b).
New probabilistic maps are in preparation for Hawaii using the methodology similar to that used
for the rest of the United States. and described below. These maps will be to be completed in
early 1998. Probabilistic maps for Puerto Rico, Culebra, Vieques, St. Thomas, St. John, St.
Croix, Guam, and Tutuila needed for the 1997 Provisions are not expected during the current
cycle of USGS map revisions (development of design maps for these areas is described below).
This appendix provides a brief description of the USGS seismic hazard maps, the geologic/seismologic inputs to these maps, and the ground-motion relations used for the maps. It is based on
the USGS map documentation for the central and eastern United States (CEUS) and the western
United States prepared by Frankel et al (1996). The complete reference document, also available
on the USGS Web site, should be reviewed for detailed technical information.
The hazard maps depict probabilistic ground acceleration and spectral response acceleration with
10 percent, 5 percent, and 2 percent probabilities of exceedance (PE) in 50 years. These maps
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Development of the USGS Seismic Maps
correspond to return times of approximately 500, 1000, and 2500 years, respectively.* All spectral response values shown in the maps correspond to 5 percent of critical damping. The maps are
based on the assumption that earthquake occurrence is Poissonian, so that the probability of occurrence is time-independent. The methodologies used for the maps were presented, discussed,
and substantially modified during 6 regional workshops for the conterminous United States convened by the USGS from June 1994-June 1995. A seventh workshop for Alaska was held in
September 1996.
The methodology for the maps (Frankel et al., 1996) includes three primary features:
1. The use of smoothed historical seismicity is one component of the hazard calculation. This is
used in lieu of source zones used in previous USGS maps. The analytical procedure is described in Frankel (1995).
2. Another important feature is the use of alternative models of seismic hazard in a logic tree
formalism. For the central and eastern United States (CEUS), different models based on
different reference magnitudes are combined to form the hazard maps. In addition, large
background zones based on broad geologic criteria are used as alternative source models for
the CEUS and the western United States (WUS). These background zones are meant to
quantify hazard in areas with little historic seismicity, but with the potential to produce major
earthquakes. The background zones were developed from extensive discussions at the regional workshops.
3. For the WUS, a big advance in the new maps is the use of geologic slip rates to determine
fault recurrence times. Slip rates from about 500 faults or fault segments were used in preparing the probabilistic maps.
The hazard maps do not consider the uncertainty in seismicity or fault parameters. Preferred
values of maximum magnitudes and slip rates were used instead. The next stage of this effort is
the quantification of uncertainties in hazard curves for selected sites. These data will be included
on the Internet as they become available.
The USGS hazard maps are not meant to be used for Mexico, areas north of 49 degrees north
latitude, and offshore the Atlantic and Gulf of Mexico coasts of the United States.
CEUS and WUS Attenuation Boundary
Attenuation of ground motion differs between the CEUS and the WUS. The boundary between
regions was located along the eastern edge of the Basin and Range province (Figure B1). The
*
Previous USGS maps (e.g. Algermissen, et al, 1990 and Leyendecker, et al, 1995) and earlier editions of the
Provisions expressed probability as a 10 percent probability of exceedance in a specified exposure time. Beginning
with the 1996 maps, probability is being expressed as a specified probability of exceedance in a 50 year time period.
Thus, 5 percent in 50 years and 2 percent in 50 years used now correspond closely to 10 percent in 100 years and 10
percent in 250 years, respectively, that was used previously. This same information may be conveyed as annual
frequency. In this approach 10 percent probability of exceedance (PE) in 50 years corresponds to an annual frequency
of exceedance of 0.0021; 5 percent PE in 100 years corresponds to 0.00103; and 2 percent PE in 50 years corresponds
to 0.000404.
304
1997 Commentary, Appendix B
previous USGS maps (e.g., Algermissen et al., 1990) used an attenuation boundary further to the
east along the Rocky Mountain front.
FIGURE B1 Attenuation boundary for eastern and western attenuation function.
Separate hazard calculations were done for the two regions using different attenuation relations.
Earthquakes west of the boundary used the WUS attenuation relations and earthquakes east of
the boundary used CEUS attenuation relations. WUS attenuation relations were used for WUS
earthquakes, even for sites located east of the attenuation boundary. Similarly CEUS attenuations
were used for CEUS earthquakes, even for sites located west of the attenuation boundary. It
would have been computationally difficult to consider how much of the path was contained in the
attenuation province. Also, since the attenuation relation is dependent on the stress drop, basing
the relation that was used on the location of the earthquake rather than the receiver is reasonable.
Hazard Curves
The probabilistic maps were constructed from mean hazard curves, that is the mean probabilities
of exceedance as a function of ground motion or spectral response. Hazard curves were obtained
for each site on a calculation grid.
A grid (or site) spacing of 0.1 degrees in latitude and longitude was used for the WUS and 0.2
degrees for the CEUS. This resulted in hazard calculations at about 65,000 sites for the WUS
runs and 35,000 sites for the CEUS runs. The CEUS hazard curves were interpolated to yield a
set of hazard curves on a 0.1 degree grid. A grid of hazard curves with 0.1 degree spacing was
thereby obtained for the entire conterminous United States. A special grid spacing of 0.05 de-
305
Development of the USGS Seismic Maps
grees was also done for California, Nevada, and western Utah because of the density of faults
warranted increased density of data. These data were used for maps of this region.
Figure B2 is a sample of mean hazard curves used in making the 1996 maps. The curves include
cities from various regions in the United States. It should be noted that in some areas the curves
are very sensitive to the latitude and longitude selected. A probabilistic map is a contour plot of
the ground motion or spectral values obtained by taking a “slice” through all 150,000 hazard
curves at a particular probability value. The gridded data obtained from the hazard curves that
was used to make each probabilistic map is located at the USGS Web site. Figure B2 also shows
the general difference in slope of the hazard curves of the CEUS versus the WUS. This difference
has been noted in other studies.
FIGURE B2 Hazard curves for selected cities.
CENTRAL AND EASTERN UNITED STATES
The basic procedure for constructing the CEUS portion of the hazard maps is diagramed in Figure
B3. Four models of hazard are shown on the left side of the figure. Model 1 is based on mb 3.0
and larger earthquakes since 1924. Model 2 is derived from mb 4.0 and larger earthquakes since
1860. Model 3 is produced from mb 5.0 and larger events since 1700. In constructing the hazard
maps, model 1 was assigned a weight twice that of models 2 and 3.
306
1997 Commentary, Appendix B
The procedure described by Frankel (1995) is used to construct the hazard maps directly from the
historic seismicity (models 1 - 3). The number of events greater than the minimum magnitude are
counted on a grid with
spacing of 0.1 degrees in
latitude and longitude. The
logarithm of this number
represents the maximum
likelihood a-value for each
grid cell. Note that the
maximum likelihood
method counts a mb 5 event
the same as a mb 3 event in
the determination of a-value. Then the gridded avalues are smoothed using
a Gaussian function. A
Gaussian with a correlation
distance of 50 km was used
for model 1 and 75 km for
models 2 and 3. The 50
km distance was chosen
FIGURE B3 Seismic hazard models for the central and eastern United
because it is similar in
States. Smoothed seismicity models are shown on the left and fault models
width to many of the trends are shown on the right.
in historic seismicity in the
CEUS. In addition, it is
comparable to the error in location of mb 3 events in the period of 1924-1975, before the advent
of local seismic networks. A larger correlation distance was used for models 2 and 3 since they
include earthquakes further back in time with poorer estimates of locations.
Model 4 consists of large background source zones. This alternative is meant to quantify hazard
in areas with little historical seismicity but with the potential to generate damaging earthquakes.
These background zones are detailed in a later section of this text. The sum of the weights of
models 1-4 is one. For a weighting scheme that is uniform in space, this ensures that the total
seismicity rate in the combined model equals the historic seismicity rate. A spatially-varying
weighting scheme which slightly exceeds the historic seismicity rate was used in the final map for
reasons which are described later.
A regional b-value of 0.95 was used for models 1-4 in all of the CEUS except Charlevoix, Quebec. This b-value was determined from a catalog for events east of 105 degrees W. For the
Charlevoix region a b-value of 0.76 was used based on the work of John Adams, Stephen Halchuck and Dieter Weichert of the Geologic Survey of Canada (see Adams et al., 1996).
Figure B4 shows a map of the CEUS Mmax values used for models 1-4 (bold M refers to moment
magnitude). These Mmax zones correspond to the background zones used in model 4. Most of the
CEUS is divided into a cratonic region and a region of extended crust. An Mmax of 6.5 was used
for the cratonic area. A Mmax of 7.5 was used for the Wabash Valley zone in keeping with magni-
307
Development of the USGS Seismic Maps
tudes derived from paleoliquefaction evidence (Obermeier et al., 1992). An Mmax of 7.5 was used
in the zone of extended crust outboard of the craton. An Mmax of 6.5 was used for the Rocky
Mountain zone and the Colorado Plateau, consistent with the magnitude of the largest historic
events in these regions. An Mmax of 7.2 was used for the gridded seismicity within the Charleston
areal source zone. A minimum mb of 5.0 was used in all the hazard calculations for the CEUS.
FIGURE B4 Central and eastern U.S. maximum magnitude zones.
Model 5 (Figure B3, right) consists of the contribution from large earthquakes (M>7.0) in four
specific areas of the CEUS: New Madrid, Charleston, South Carolina, the Meers fault in southwest Oklahoma, and the Cheraw Fault in eastern Colorado. This model has a weight of 1. The
treatment of these special areas is described in B.3.1. There are three other areas in the CEUS
that are called special zones: eastern Tennessee, Wabash Valley, and Charlevoix. These are described in B.3.1.
Special Zones
New Madrid: To calculate the hazard from large events in the New Madrid area, three parallel
faults in an S-shaped pattern encompassing the area of highest historic seismicity were considered.
These were not meant to be actual faults; they are simply a way of expressing the uncertainty in
the source locations of large earthquakes such as the 1811-12 sequence. A characteristic rupture
model with a characteristic moment magnitude M of 8.0, similar to the estimated magnitudes of
the largest events in 1811-12 (Johnston, 1996a,b) was assumed. A recurrence time of 1000 years
308
1997 Commentary, Appendix B
for such an event was used as an average value, considering the uncertainty in the magnitudes of
pre-historic events.
An areal source zone was used for New Madrid for models 1-3, rather than spatially-smoothed
historic seismicity. This zone accounts for the hazard from New Madrid events with moment
magnitudes less than 7.5.
Charleston, South Carolina: An areal source zone was used to quantify the hazard from large
earthquakes. The extent of the areal source zone was constrained by the areal distribution of
paleoliquefaction locations, although the source zone does not encompass all the paleoliquefaction sites. A characteristic rupture model of moment magnitude 7.3 earthquakes, based on the
estimated magnitude of the 1886 event (Johnston, 1996b) was assumed. For the M7.3 events a
recurrence time of 650 years was used, based on dates of paleoliquefaction events (Amick and
Gelinas, 1991; Obermeier et al., 1990, Johnston and Schweig, written comm., 1996).
Meers Fault: The Meers fault in southwestern Oklahoma was explicitly included. The segment
of the fault which has produced a Holocene scarp as described in Crone and Luza (1990) was
used. A characteristic moment magnitude of 7.0 and a recurrence time of 4000 years was used
based on their work.
Cheraw Fault: This eastern Colorado fault with Holocene faulting based on a study by Crone et
al. (1996) was included. The recurrence rate of this fault was obtained from a slip rate of 0.5
mm/yr. A maximum magnitude of 7.1 was found from the fault length using the relations of Wells
and Coppersmith (1994).
Eastern Tennessee Seismic Zone: The eastern Tennessee seismic zone is a linear trend of seismicity that is most obvious for smaller events with magnitudes around 2 (see Powell et al., 1994).
The magnitude 3 and larger earthquakes tend to cluster in one part of this linear trend, so that
hazard maps are based just on smoothed mb3.
Wabash Valley: Recent work has identified several paleoearthquakes in the areas of southern
Indiana and Illinois based on widespread paleoliquefaction features (Obermeier et al., 1992). An
areal zone was used with a higher Mmax of 7.5 to account for such large events. The sum of the
gridded a-values in this zone calculated from model 1 produce a recurrence time of 2600 years for
events with mb 6.5. The recurrence rate of M6.5 and greater events is estimated to be about
4,000 years from the paleoliquefaction dates (P. Munson and S. Obermeier, pers. comm., 1995),
so it is not necessary to add additional large events to augment models 1-3. The Wabash Valley
Mmax zone in the maps is based on the Wabash Valley fault zone.
Charlevoix, Quebec: As mentioned above, a 40 km by 70 km region surrounding this seismicity
cluster was assigned a b-value of 0.76, based on the work of Adams, Halchuck and Weichert.
This b-value was used in models 1-3.
Background Source Zones (Model 4)
The background source zones (see Figure B5) are intended to quantify seismic hazard in areas
that have not had significant historic seismicity, but could very well produce sizeable earthquakes
in the future. They consist of a cratonic zone, an extended margin zone, a Rocky Mountain zone,
and a Colorado Plateau zone. The Rocky Mountain zone was not discussed at any workshop, but
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Development of the USGS Seismic Maps
is clearly defined by the Rocky Mountain front on the east and the areas of extensional tectonics
to the west, north and south. As stated above, the dividing line between the cratonic and extended margin zone was drawn by Rus Wheeler based on the westward and northern edge of
rifting during the opening of the Iapetan ocean. One justification for having craton and extended
crust zones is the work done by Johnston (1994). They compiled a global survey of earthquakes
in cratonic and extended crust and found a higher seismicity rate (normalized by area) for the
extended areas.
FIGURE B5 Central and eastern U.S. background zones.
For each background zone, a-values were determined by counting the number of mb3 and larger
events within the zone since 1924 and adjusting the rate to equal that since 1976. A b-value of
0.95 was used for all the background zones, based on the b-value found for the entire CEUS.
Adaptive Weighting for CEUS
The inclusion of background zones lowers the probabilistic ground motions in areas of relatively
high historic seismicity while raising the hazard to only low levels in areas with no historic seismicity. The June 1996 versions of the maps include the background zones using a weighting
scheme that can vary locally depending on the level of historic seismicity in that cell of the a-value
grid. Spatially-varying weighting was suggested by Allin Cornell in the external review of the
interim maps. The "adaptive weighting" procedure avoids lowering the hazard in higher seismicity areas to raise the hazard in low seismicity areas. This was implemented by looping through the
a-value grid and checking to see if the a-value for each cell from the historic seismicity was
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1997 Commentary, Appendix B
greater than the a-value from the background zone. For the CEUS the a-value from the historic
seismicity was derived by weighting the rates from models 1, 2, and 3 by 0.5, 0.25, 0.25 respectively. If this weighted sum was greater than the rate from the appropriate background zone, then
the rate for that cell was determined by weighting the rates from models 1-3 by 0.5, .25, .25 (i.e.,
historic seismicity only, no background zone). If the weighted sum from the historic seismicity
was less than the rate of the background zone, then a weighting of 0.4, 0.2, 0.2, 0.2 for models 14, respectively (including the background zone as model 4). This procedure does not make the
rate for any cell lower than it would be from the historic seismicity (models 1-3). It also incorporates the background zones in areas of low historic seismicity. The total seismicity rate in the
resulting a-value grid is only 10 percent larger than the observed rate of mb3's since 1976. This is
not a major difference. Of course, this procedure produces substantially higher ground motions
(in terms of percentage increase) in the seismically quiet areas as compared to no background
zone. These values are still quite low in an absolute sense.
CEUS Catalogs and B-Value Calculation
The primary catalog used for the CEUS for longitudes east of 105 degrees is Seeber and
Armbruster (1991), which is a refinement of the EPRI (1986) catalog. This was supplemented
with the PDE catalog from 1985-1995. In addition, PDE, DNAG, Stover and Coffman (1993),
Stover, Reagor, and Algermissen (1984) catalogs were searched to find events not included in
Seeber and Armbruster (1991). Mueller et al. (1996) describes the treatment of catalogs, adjustment of rates to correct for incompleteness, the removal of aftershocks, and the assignment of
magnitudes.
Attenuation Relations for CEUS
The reference site condition used for the maps is specified to be the boundary between NEHRP
classes B and C (Martin and Dobry, 1994), meaning it has an average shear-wave velocity of 760
m/sec in the top 30m. This corresponds to a typical "firm-rock" site for the western United States
(see WUS attenuation section below), although many rock sites in the CEUS probably have much
higher velocities. The motivation for using this reference site is that it corresponds to the average
of sites classified as "rock" sites in WUS attenuation relations. In addition, it was considered less
problematic to use this site condition for the CEUS than to use a soil condition. Most previouslypublished attenuation relations for the CEUS are based on a hard-rock site condition. It is less of
a problem to convert these to a firm-rock condition than to convert them to a soil condition, since
there would be less concern over possible non-linearity for the firm-rock site compared to the soil
site.
Two equally-weighted, attenuation relations were used for the CEUS. Both sets of relations
were derived by stochastic simulations and random vibration theory. First the Toro et al. (1993)
attenuation for hard-rock was used. The attenuation relations were multiplied by frequency-dependent factors developed by USGS to convert them from hard-rock to firm-rock sites. The
factors used 1.52 for PGA, 1.76 for 0.2 sec spectral response, 1.72 for 0.3 sec spectral response
and 1.34 for 1.0 sec spectral response. These factors were applied independently of magnitude
and distance.
The second set of relations was derived by USGS (Frankel et al., 1996) for firm-rock sites. These
relations were based on a Brune source model with a stress drop of 150 bars. The simulations
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Development of the USGS Seismic Maps
contained frequency-dependent amplification factors derived from a hypothesized shear-wave
velocity profile of a CEUS firm-rock site. A series of tables of ground motions and response
spectral values as a function of moment magnitude and distance was produced instead of an equation.
For CEUS hazard calculations for models 1-4, a source depth of 5.0 km was assumed when using
the USGS ground motion tables. Since a minimum hypocentral distance of 10 km is used in the
USGS tables, the probabilistic ground motions are insensitive to the choice of source depth. In
the hazard program, when hypocentral distances are less than 10 km the distance is set to 10 km
when using the tables. For the Toro et al. (1993) relations, the fictitious depths that they specify
for each period are used, so that the choice of source depth used in the USGS tables was not
applied.
For both sets of ground motion relations, values of 0.75, 0.75, 0.75 and 0.80 were used for the
natural logarithms of the standard deviation of PGA, 0.2 sec, 0.3 sec and 1.0 sec spectral responses, respectively. These values are similar to the aleatory standard deviations reported to the
Senior Seismic Hazard Analysis Committee (1996).
A cap in the median ground motions was placed on the ground motions within the hazard code.
USGS was concerned that the median ground motions of both the Toro et al. and the new USGS
tables became very large (>2.5 g PGA) for distances of about 10 km for the M 8.0 events for New
Madrid. Accordingly the median PGA's was capped at 1.5 g. The median 0.3 and 0.2 sec values
were capped at 3.75 g which was derived by multiplying the PGA cap by 2.5 (the WUS
conversion factor). This only affected the PGA values for the 2 percent PE in 50 year maps for
the area directly above the three fictitious faults for the New Madrid region. It does not change
any of the values at Memphis. The capping did not significantly alter the 0.3 and 0.2 sec values
in this area. The PGA and spectral response values did not change in the Charleston region from
this capping. Note that the capping was for the median values only. As the variability (sigma) of
the ground motions was maintained in the hazard code, values larger than the median were
allowed. USGS felt that the capping recognizes that values derived from point source simulations
are not as reliable for M8.0 earthquakes at close-in distances (< 20 km).
Additional Notes for CEUS
One of the major outcomes of the new maps for the CEUS is that the ground motions are about a
factor of 2-3 times lower, on average, than the PGA values in Algermissen et al. (1990) and the
spectral values in Algermissen et al. (1991) and Leyendecker et al. (1995). The primary cause of
this difference is the magnitudes assigned to pre-instrumental earthquakes in the catalog.
Magnitudes of historic events used by Algermissen et al were based on Imax (maximum observed
intensity), using magnitude-Imax relations derived from WUS earthquakes. This overestimates the
magnitudes of these events and, in turn, overestimates the rates of M4.9 and larger events. The
magnitudes of historic events used in the new maps were primarily derived by Seeber and
Armbruster (1991) from either felt area or Imax using relations derived from CEUS earthquakes
(Sibol et al., 1987). Thus, rates of M4.9 and larger events are much lower in the new catalog,
compared to those used for the previous USGS maps.
It is useful to compare the new maps to the source zones used in the EPRI (1986) study. For the
areas to the north and west of New Madrid, most of the six EPRI teams had three source zones in
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1997 Commentary, Appendix B
common: 1) the Nemaha Ridge in Kansas and Nebraska, 2) the Colorado-Great Lakes lineament
extending from Colorado to the western end of Lake Superior, and 3) a small fault zone in northern Illinois, west of Chicago. Each of these source zones are apparent as higher hazard areas in
the our maps. The Nemaha Ridge is outlined in the maps because of magnitude 4 and 5 events
occurring in the vicinity. Portions of the Colorado-Great Lakes lineament show higher hazard in
the map, particularly the portion in South Dakota and western Minnesota. The portion of the
lineament in eastern Minnesota has been historically inactive, so is not apparent on the maps. The
area in western Minnesota shows some hazard because of the occurrence of a few magnitude 4
events since 1860. A recent paper by Chandler (1995), argues that the locations and focal mechanisms of these earthquakes are not compatible with them being on the lineament, which is expressed as the Morris Fault in this region. The area in northern Illinois has relatively high hazard
in the maps because of M4-5 events that have occurred there.
Frankel (1995) also found good agreement in the mean PE's and hazard curves derived from
models 1-3 and 4 and those produced by the EPRI (1986) study, when the same PGA attenuation
relations were used.
WESTERN UNITED STATES
The maps for the WUS include a cooperative effort with the California Division of Mines and
Geology. This was made possible, in part, because CDMG was doing a probabilistic map at the
same time the USGS maps were prepared. There was considerable cooperation in this effort. For
example, the fault data base used in the USGS maps was obtained from CDMG. Similarly USGS
software was made available to CDMG. The result is that maps produced by both agencies are
the same.
The procedure for mapping
hazard in the WUS is
shown in Figure B6. On
the left side, hazards are
considered from earthquakes with magnitudes
less than or equal to
moment magnitude 7.0.
For most of the WUS, two
alternative models are used:
1) smoothed historical
seismicity (weight of 0.67)
and 2) large background
zones (weight 0.33) based
on broad geologic criteria
and workshop input.
Model 1 used a 0.1 degree
source grid to count
number of events. The
determination of a-value
was changed somewhat
FIGURE B6 Seismic hazard models for California and the western United
States. Smoothed seismicity models are shown on the left and fault models
are shown on the right.
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Development of the USGS Seismic Maps
from the CEUS, to incorporate different completeness times for different magnitude ranges. The
a-value for each grid cell was calculated from the maximum likelihood method of Weichert
(1980), based on events with magnitudes of 4.0 and larger. The ranges used were M4.0 to 5.0
since 1963, M5.0 to 6.0 since 1930, and M6.0 and larger since 1850. For the first two categories,
completeness time was derived from plots of cumulative number of events versus time. M3
events were not used in the WUS hazard calculations since they are only complete since about
1976 for most areas and may not even be complete after 1976 for some areas. For California
M4.0 to M5.0 since 1933, M5.0 to 6.0 since 1900, and M6.0 and larger since 1850 were used.
The catalog for California is complete to earlier dates compared to the catalogs for the rest of the
WUS (see below).
Another difference with the CEUS is that multiple models with different minimum magnitudes for
the a-value estimates (such as models 1-3 for the CEUS) were not used. The use of such multiple
models in the CEUS was partially motivated by the observation that some mb4 and mb5 events in
the CEUS occurred in areas with few mb3 events since 1924 (e.g., Nemaha Ridge events and
western Minnesota events). It was considered desirable to be able to give such mb4 and mb5
events extra weight in the hazard calculation over what they would have in one run with a
minimum magnitude of 3. In contrast it appears that virtually all M5 and M6 events in the WUS
have occurred in areas with numerous M4 events since 1965. There was also reluctance to use a
WUS model with a-values based on a minimum magnitude of 6.0, since this would tend to double
count events that have occurred on mapped faults included in Figure B6 right.
For model 1, the gridded a-values were smoothed with a Gaussian with a correlation distance of
50 km, as in model 1 for the CEUS. The hazard calculation from the gridded a-values differed
from that in the CEUS, because we considered fault finiteness in the WUS calculations. For each
source grid cell, a fictitious fault for magnitudes of 6.0 and larger was used. The fault was
centered on the center of the grid cell. The strike of the fault was random and was varied for each
magnitude increment. The length of the fault was determined from the relations of Wells and
Coppersmith (1994). The fictitious faults were taken to be vertical.
A maximum moment magnitude of 7.0 was used for models 1 and 2, except for four shear zones
in northeastern California and western Nevada described below. Of course, larger moment
magnitudes are included in the specific faults. A minimum moment magnitude of 5.0 were used
for models 1 and 2. For each WUS site, the hazard calculation was done for source-site distances
of 200 km and less, except for the Cascadia subduction zone, where the maximum distance was
1000 km.
Separate hazard calculations for deep events (> 35 km) were done. These events were culled
from the catalogs. Their a-values were calculated separately from the shallow events. Different
attenuation relations were used.
Regional b-values were calculated based on the method of Weichert (1980), using events with
magnitudes of 4 and larger and using varying completeness times for different magnitudes.
Accordingly, a regional b-value of 0.8 was used in models 1 and 2 for the WUS runs based on
shallow events. For the deep events (>35 km), an average b-value of 0.65 was found. This low
b-value was used in the hazard calculations for the deep events.
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1997 Commentary, Appendix B
We used a b-value of 0.9 for most of California, except for the easternmost portion of California
in our basin and range background zone (see below). This b-value was derived by CDMG.
Faults
The hazard from about 500 Quaternary
faults or fault segments was used for the
maps. Faults were considered where
geologic slip rates have been determined or
estimates of recurrence times have been
made from trenching studies. A table of the
fault parameters used in the hazard
calculations has been compiled and is shown
on the USGS Internet Web site. Figure B7
shows the faults used in the maps. The
numerous individuals who worked on
compilations of fault data are too numerous
to cite here. They are cited, along with their
contribution, in the map documentation
(Frankel, et al, 1996).
Recurrence Models for Faults
The hazard from specific faults is added to the
hazard from the seismicity as shown in Figure
B6. Faults are divided into types A and B,
roughly following the nomenclature of WGCEP
(1995). A fault is classified as A-type if there
have been sufficient studies of it to produce
models of fault segmentation. In California the Atype faults are: San Andreas, San Jacinto,
Elsinore, Hayward, Rodgers Creek, and
Imperial (M. Petersen, C. Cramer, and W.
Bryant, written comm., 1996). The only
FIGURE B7 Western U.S. faults included in the maps.
fault outside of California classified as an Atype is the Wasatch Fault. Single-segment ruptures were assumed on the Wasatch Fault.
For California, the rupture scenarios specified by Petersen, Cramer and Bryant of CDMG, with
input from Lienkaemper of USGS for northern California were used. Single-segment,
characteristic rupture for the San Jacinto and Elsinore faults were assumed. For the San Andreas
fault, multiple-segment ruptures were included in the hazard calculation, including repeats of the
1906 and 1857 rupture zones, and a scenario with the southern San Andreas fault rupturing from
San Bernardino through the Coachella segment. Both single-segment and double-segment
ruptures of the Hayward Fault were included.
For California faults, characteristic magnitudes derived by CDMG from the fault area using the
relations in Wells and Coppersmith (1994) were used. For the remainder of the WUS, the
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Development of the USGS Seismic Maps
characteristic magnitude was determined from the fault length using the relations of Wells and
Coppersmith (1994) appropriate for that fault type.
For the B-type faults, it was felt there were insufficient studies to warrant specific segmentation
boundaries. For these faults, the scheme of Petersen et al. (1996)was followed, using both
characteristic and Gutenberg-Richter (GR; exponential) models of earthquake occurrence. These
recurrence models were weighted equally. The G-R model basically accounts for the possibility
that a fault is segmented and may rupture only part of its length. It was assumed that the G-R
distribution applies from a minimum moment magnitude of 6.5 up to a moment magnitude
corresponding to rupture of the entire fault length.
The procedure for calculating hazard using the G-R model involves looping through magnitude
increments. For each magnitude a rupture length is calculated using Wells and Coppersmith
(1994). Then a rupture zone of this length is floated along the fault trace. For each site, the
appropriate distance to the floating ruptures is found and the frequency of exceedance (FE) is
calculated. The FE's are then added for all the floating rupture zones.
As used by USGS, the characteristic earthquake model (Schwartz and Coppersmith, 1984) is
actually the maximum magnitude model of Wesnousky (1986) Here it is assumed that the fault
only generates earthquakes that rupture the entire fault. Smaller events along the fault would be
incorporated by models 1 and 2 with the distributed seismicity or by the G-R model described
above.
It should be noted that using the G-R model generally produces higher probabilistic ground
motions than the characteristic earthquake model, because of the more frequent occurrence of
earthquakes with magnitudes of about 6.5.
Fault widths (except for California)were determined by assuming a seismogenic depth of 15 km
and then using the dip, so that the width equaled 15 km divided by the sine of the dip. For most
normal faults a dip of 60 degrees is assumed. Dip directions were taken from the literature. For
the Wasatch, Lost River, Beaverhead, Lemhi, and Hebgen Lake faults, the dip angles were taken
from the literature (see fault parameter table on Web site). Strike-slip faults were assigned a dip
of 90 degrees. For California faults, widths were often defined using the depth of seismicity (J.
Lienkaemper, written comm., 1996; M. Petersen, C. Cramer, and W. Bryant, written comm.,
1996). Fault length was calculated from the total length of the digitized fault trace.
Special Cases
There are a number of special cases which need to be described.
Blind thrusts in the Los Angeles area: Following Petersen et al (1996) and as discussed at the
Pasadena workshop, 0.5 weight was assigned to blind thrusts in the L.A. region, because of the
uncertainty in their slip rates and in whether they were indeed seismically active. These faults are
the Elysian Park thrust and the Compton thrust. The Santa Barbara Channel thrust (Shaw and
Suppe, 1994) also has partial weight, based on the weighting scheme developed by CDMG.
Offshore faults in Oregon: A weight of 0.05 was assigned to three offshore faults in Oregon
identified by Goldfinger et al. (in press) and tabulated by Geomatrix (1995): the Wecoma, Daisy
Bank and Alvin Canyon faults. It was felt the uncertainty in the seismic activity of these faults
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1997 Commentary, Appendix B
warranted a low weight, and the 0.05 probability of activity decided in Geomatrix (1995) was
used. A 0.5 weight was assigned to the Cape Blanco blind thrust.
Lost River, Lemhi and Beaverhead faults in Idaho: It was assumed that the magnitude of the
Borah Peak event (M7.0) represented a maximum magnitude for these faults. As with (3), the
characteristic model floated a M7.0 along each fault. The G-R model considered magnitudes
between 6.5 and 7.0. Note that using a larger maximum magnitude would lower the probabilistic
ground motions, because it would increase the recurrence time.
Hurricane and Sevier-Torroweap Faults in Utah and Arizona: The long lengths of these faults
(about 250 km) implied a maximum magnitude too large compared to historical events in the
region. Therefore a maximum magnitude of M7.5 was chosen. The characteristic and G-R models were implemented as in case (3). Other faults (outside of California) where the Mmax was
determined to be greater than 7.5 based on the fault length were assigned a maximum magnitude
of 7.5.
Wasatch Fault in Utah: Recurrence times derived from dates of paleoearthquakes by Black et al.
(1995) and the compilation of McCalpin and Nishenko (1996) were used
Hebgen Lake Fault in Montana: A characteristic moment magnitude of 7.3 based on the 1959
event (Doser, 1985) was used.
Short faults: All short faults with characteristic magnitudes of less than 6.5 were treated with the
characteristic recurrence model only (weight=1). No G-R relation was used. If a fault had a
characteristic magnitude less than 6.0, it was not used.
Seattle Fault: The characteristic recurrence time was fixed at 5000 years, which is the minimum
recurrence time apparent from paleoseismology (R. Bucknam, pers. comm., 1996). Using the
characteristic magnitude of 7.1 derived from the length and a 0.5 mm/yr slip rate yielded a characteristic recurrence time of about 3000 years.
Eglington fault near Las Vegas: The recurrence time for this fault was fixed at 14,000 years,
similar to the recurrence noted in Wyman et al. (1993).
Shear Zones in Eastern California and Western Nevada: Areal shear zones were added along the
western border of Nevada extending from the northern end of the Death Valley fault through the
Tahoe-Reno area through northeast California ending at the latitude of Klamath Falls, Oregon. A
shear rate of 4 mm/yr to zone 1, and 2 mm/yr to zones 2 and 3 was assigned. The shear rate in
zone 1 is comparable to the shear rate observed on the Death Valley fault, but which is not observed in mapped faults north of the Death Valley fault (C. dePolo and J. Anderson, pers. comm.,
1996). For the Foothills Fault system (zone 4) a shear rate of 0.05 mm/yr was used. a-values
were determined for these zones in the manner described in Ward(1994). For zones 1-3, a magnitude range of 6.5-7.3 was used. For zone 4, a magnitude range of 6.0-7 was used. The maximum
magnitude for the calculation of hazard from the smoothed historic seismicity was lowered in
these zones so that it did not overlap with these magnitude ranges. Fictitious faults with a fixed
strike were used in the hazard calculation for these zones. Again, use of these areal zones in
California was agreed upon after consultation with CDMG personnel.
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Development of the USGS Seismic Maps
Cascadia Subduction Zone
Two alternative scenarios for great earthquakes on the Cascadia subduction zone were considered. For both scenarios it was assumed that the recurrence time of rupture at any point along the
subduction zone was 500 years. This time is in or near most of the average intervals estimated
from coastal and offshore evidence (see Atwater and Hemphill-Haley, 1996; Geomatrix, 1995; B.
Atwater, written comm., 1996). Individual intervals, however, range from a few hundred years to
about 1000 years (Atwater et al., 1995).
The first scenario is for moment magnitude 8.3 earthquakes to fill the subduction zone every 500
years. Based on a rupture length of 250 km (see Geomatrix, 1995) for an M8.3 event and the
1100 km length of the entire subduction zone, this requires a repeat time of about 110 years for
an M8.3 event. However, no such event has been observed in the historic record of about 150
years. This M8.3 scenario is similar to what was used in the 1994 edition of the USGS maps (see
Leyendecker et al., 1995) and it is comparable to the highest weighted scenario in Geomatrix
(1995). A M8.3 rupture zone was floated along the strike of the subduction zone to calculate the
hazard. A weight of 0.67 was assigned for this scenario in the maps.
The second scenario used is for a moment magnitude 9.0 earthquake to rupture the entire Cascadia subduction zone every 500 years on average. No compelling reason was seen to rule out
such a scenario. This scenario would explain the lack of M8s in the historic record. It is also
consistent with a recent interpretation of Japanese tsunami records by Satake et al. (1996). By
ruling out alternative source regions, Satake et al. (1996) reported that a tsunami in 1700 could
have been produced by a M9.0 earthquake along the Cascadia subduction zone. A weight of 0.33
was assigned to the M9.0 scenario in the maps.
The subduction zone was specified as a dipping plane striking north-south from about Cape
Mendocino to 50 degrees north. It was assumed that the plane reached 20 km depth at a longitude of 123.8 degrees west, just east of the coastline. This corresponds roughly to the 20 km
depth contour drawn by Hyndman and Wang (1995) and is consistent with the depth and location
of the Petrolia earthquake in northern California. A dip of 10 degrees was assigned to the plane
and a width of 90 km. The seismogenic portion of the plane was assumed to extend to a depth of
20 km.
Background Source Zones
The background source zones for the WUS (model 2) were based on broad geologic criteria and
were developed by discussion at the Salt Lake City (SLC) workshop (except for the Cascades
source zone). These zones are shown in Figure B8. Note that there are no background source
zones west of the Cascades and west of the Basin and Range province. For those areas, model 1
was used with a weight of 1.
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1997 Commentary, Appendix B
FIGURE B8 Western U.S. background zones.
At the SLC workshop there was substantial sentiment for a Yellowstone Parabola source zone
(see, e.g., Anders et al., 1989) that would join up seismically-active areas in western Wyoming
with the source areas of the Bora Peak and Hebgen Lake earthquakes. It was felt that the relatively seismically-quiescent areas consisting of the Snake River Plain and Colorado Plateau should
be separate source zones because of the geologic characteristics. An area of southwest Arizona
was suggested as a separate source zone by Bruce Scheol, based partly on differences in the age
and length of geologic structures compared with the Basin and Range Province (see Edge et al.,
1992). A Cascades source zone was added since it was felt that was a geologically-distinct area.
The remaining background source zone includes the Basin and Range Province, the Rio Grande
Rift, areas of Arizona and New Mexico, portions of west Texas, and areas of eastern Washington
and northern Idaho and Montana. The northern border of this zone follows the international
border. As stated above, this seems to be a valid approach since the hazard maps are being based
on the seismicity rate in the area of interest.
This large background zone is intended to address the possibility of having large earthquakes (M6
and larger) in areas with relatively low rates of seismicity in the brief historic record. It is important to have a large zone that contains areas of high seismicity in order to quantify the hazard in
relatively quiescent areas such as eastern Oregon and Washington, central Arizona, parts of New
Mexico, and west Texas. One can see the effect of this large background zone by noting the
contours on the hazard maps in these areas. The prominence of the background zones in the
maps is determined by the weighting of models 1 and 2.
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Development of the USGS Seismic Maps
Adaptive Weighting for the WUS
The adaptive weighting procedure was used to include the background zones in the WUS without
lowering the hazard values in the high seismicity areas. As with the CEUS, the a-value was
checked for each source cell to see whether the rate from the historic seismicity exceeded that
from the appropriate background zone. If it did, the a-value was used from the historic seismicity.
If the historic seismicity a-value was below the background value, then a rate derived from using
0.67 times the historic rate plus 0.33 times the background rate was used. This does not lower
the a-value in any cell lower than the value from the historic seismicity. The total seismicity rate
in this portion of the WUS in the new a-value grid is 16 percent above the historic rate (derived
from M4 and greater events since 1963).
WUS Catalogs
For the WUS, except for California, the Stover and Coffman (1993), Stover, Reagor, and
Algermissen (1984), PDE, and DNAG catalogs (with the addition of Alan Sanford's catalog for
New Mexico) were used. For California, a catalog compiled by Mark Petersen of California
Division of Mines and Geology (CDMG) was used. Mueller et al. (1996) describes the processing of the catalogs, the removal of aftershocks, and the assignment of magnitudes. Utah coalmining events were removed from the catalog (see Mueller et al., 1996). Explosions at NTS and
their aftershocks were also removed from the catalog.
Attenuation Relations for WUS
Crustal Events: For spectral response acceleration, three equally-weighted attenuation relations
were used: (1) Boore, Joyner, and Fumal (BJF; 1993, 1994a) with later modifications to differentiate thrust and strike-slip faulting (Boore et al., 1994b) and (2) Sadigh et al. (1993). For (1)
ground motions were calculated for a site with average shear-wave velocity of 760 m/sec in the
top 30m, using the relations between shear-wave velocity and site amplification in Boore et al.
(1994a). For (2) their "rock" values were used. Joyner (1995) reported velocity profiles compiled by W. Silva and by D. Boore showing that WUS rock sites basically spanned the NEHRP
B/C boundary. When calculating ground motions for each fault, the relations appropriate for that
fault type (e.g, thrust) were used. All of the relations found higher ground motions for thrust
faults compared with strike slip faults.
All calculations included the variability of ground motions. For 1) the sigma values reported in
BJF (1994b) were used. For 2) the magnitude-dependent sigmas found in those studies were
used.
The distance measure from fault to site varies with the attenuation relation and this was accounted
for in the hazard codes (see B.5 for additional detail on distance measures).
Deep events (> 35 km): Most of these events occurred beneath the Puget Sound region. although
some were in northwestern California. For these deep events, only one attenuation relation was
used -- i.e., by Geomatrix (1993; with recent modification for depth dependence provided by R.
Youngs, written comm., 1996) which is based on empirical data of deep events recorded on rock
sites. The relations of Crouse (1991) were used because they were for soil sites. It was found
that the ground motions from Geomatrix (1993) are somewhat smaller than those from Crouse
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1997 Commentary, Appendix B
(1991), by an amount consistent with soil amplification. These events were placed at a depth of
40 km for calculation of ground motions.
Cascadia subduction zone: For M8.3 events on the subduction zone, two attenuation relations
(with equal weights) were used following the lead of Geomatrix (1993): 1) Sadigh et al. (1993)
for crustal thrust earthquakes and 2) Geomatrix (1993) for interface earthquakes. For the M9.0
scenario, Sadigh et al. (1993) formulas could not be used since they are invalid over M8.5.
Therefore, only Geomatrix (1993) was used. Again the values from Geomatrix (1993) were
somewhat smaller than the soil values in Crouse (1991).
ALASKA
The basic procedure, shown in Figure B9, for constructing the Alaska hazard maps is similar to
that previously described for the Western United States. The maps have been completed and both
the maps and documentation (USGS, 1998a,
1998b) have been placed
on the USGS internet site
(http://geohazards.cr.usgs.
gov/eq/); printing of the
maps is in progress.
Faults
The hazard from nine faults
was used for the maps (Figure B10). Faults were included in the map when an
estimated slip rate was
available. The seismic
hazard associated with
faults not explicitly included
in the map is captured to a
large degree by the
smoothed seismicity
model. Specific details on FIGURE B9 Seismic hazard models for Alaska. Smoothed seismicity
models are shown on the left and fault models are shown on the right.
the fault parameters are
given in USGS., 1997a.
All of the faults except one were strike-slip faults.
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Development of the USGS Seismic Maps
FIGURE B10 Faualts included in the maps. Faults are shown with different line types for clarify. Dipping
faults are shown as closed polygons.
Recurrence Models for Faults
As was done for the western U.S., faults were divided into types A and B. The fault treatment
was the same as the western U.S. Type A faults were the Queen Charlotte, Fairweather offshore,
Fairweather onshore, and Transition fault. Type B faults included western Denali, eastern Denali,
Totshunda, and Castle Mountain.
For the type B faults, both characteristic and Gutenberg-Richter (G-R) models of earthquake
occurrence were used. These recurrence models were weighted equally. The G-R model accounts for the possibility that a fault is segmented and may rupture only part of its length. It was
assumed that the G-R distribution applies from a minimum moment magnitude of 6.5 up to a
moment magnitude corresponding to rupture of the entire fault length.
Special Case
The Transition fault was treated as a Type A fault even though its segmentation is unknown.
Although the rationale for this treatment is documented in USGS, 1998a, it should be pointed out
that the parameters, such as segmentation and slip rate, associated with this fault are highly uncertain.
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1997 Commentary, Appendix B
Megathrust
The Alaska-Aleutian megathrust was considered in four parts, shown in Figure B11. Specific
rationale for the use of these boundaries is complex and is described in USGS, 1998a.
FIGURE B11 Subduction zones included in the maps.
Alaska Catalogs
A new earthquake catalog was built by combining Preliminary Determination of Epicenter, Decade of North American Geology, and International Seismological Centre catalogs with USGS
interpretations of catalog reliability. Mueller et al. (1997) describes the processing of the catalogs, the removal of aftershocks, and the assignment of magnitudes.
Attenuation Relations for Alaska
Crustal Events: For spectral response acceleration, two equally-weighted attenuation relations
were used: (1) Boore, Joyner, and Fumal (BJF; 1997) and (2) Sadigh et al. (1997). For (1)
ground motions were calculated for a site with average shear-wave velocity of 760 m/sec in the
top 30m. For (2) their "rock" values were used. These are recent publication of the attenuations
cited for the western U.S. The attenuations are the same. When calculating ground motions for
each fault, the relations appropriate for that fault type (e.g, strike slip) were used. All calculations
included the variability of ground motions.
Deep events (50 - 80 km): For these deep events, only one attenuation relation was used, the
intraslab form of Youngs et al (1997) with a depth fixed at 60 km.
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Development of the USGS Seismic Maps
Deeper events (80 - 120 km): For these deeper events, only one attenuation relation was used,
the intraslab form of Youngs et al (1997) with a depth fixed at 90 km.
Megathrust and Transition Fault: Only one attenuation relation was used, the interslab form of
Youngs et al (1997). It should be noted that the use of this attenuation for the Transition fault
resulted in lower ground motions than would have been obtained using the crustal attenuation
equations.
PROBABILISTIC MAPS
Two of the probabilistic maps were key to the decisions made by the SDPG for developing the
maximum considered earthquake ground motion maps. These are the 0.2 sec and 1.0 sec spectral
response maps for a 2 percent probability of exceedance in 50 years. These are shown in Figures
12 and 13 respectively. The way in which these maps were used is described in the following
sections.
FIGURE B12 Probabilistic map of 0.2 sec spectral response acceleration with a 2% probability of exceedance in
50 years. The reference site material has a shear wave velocity of 750m/sec.
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1997 Commentary, Appendix B
FIGURE B13 Probabilistic map of 1.0 sec spectral response acceleration with a 2% probability of exceedance in
50 years. The reference site material has a shear wave velocity of 750m/sec.
DEVELOPMENT OF NEHRP MAXIMUM CONSIDERED EARTHQUAKE SPECTRAL
ACCELERATION MAPS
The maximum considered earthquake spectral acceleration maps were derived from the 2 percent
in 50 year probabilistic maps shown simplified as Figures 12 and 13 (also see Frankel, et al, 1997),
discussed above, with the application of the SDPG rules also described previously. Additional
detail in applying the rules is described in this section. The 0.2 sec map is used for illustration
purposes. The same procedures and similar comments apply for the 1.0 sec map.
One of the essential features of the SDPG rules was that the recommendations, when applied by
others, would result in the same maps. This procedures allows the use of engineering judgement
to be used in developing the maps, as long as those judgements are explicitly stated. This approach will simplify modification of the recommendations as knowledge improves.
It should be noted that although the maps are termed maximum considered earthquake Ground
Motion maps. These maps are not for a single earthquake. The maps include probabilistic effects
which consider all possible earthquakes up to the plateau level. Above the plateau level, the
contours are included for the deterministic earthquake on each fault (unless the deterministic value
is higher than the probabilistic values).
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Development of the USGS Seismic Maps
Deterministic Contours
The deterministic contours, when included, are computed using the same attenuation functions
used in the probabilistic analysis. However, the deterministic values are not used unless they are
less than the probabilistic values. After study of those areas where the plateau was reached, the
only areas where the deterministic values were less than the probabilistic values were located in
California and along the subduction zone region of Washington and Oregon. Further study indicated that those areas with values in excess of the plateau were located in California. The appropriate attenuation for this area were the Boore-Joyner-Fumal attenuation (1993,1994) and the
Sadigh et al (1993) attenuation.
The form of these attenuations and the distance measures used have an effect on the shape of
these deterministic contours. Accordingly, they are discussed below. The Boore-Joyner-Fumal
equation is:
logY ' bss Gss % bRS GRS % b2 (M & 6) 2 % b4 r % b5 log (r) % b v (logVs % logVa)
where:
Y
= ground motion parameter
M
= earthquake magnitude
bSS, bRS
= coefficients for strike-slip and reverse-slip faults, determined by regression and
different for each ground motion parameter
GSS
= 1.0 for strike-slip fault, otherwise zero
GRS
= 1.0 for reverse-slip fault, otherwise zero
b2, b3, b4, b5
= coefficients determined by regression, different for each spectral acceleration
bV
= coefficient determined by regression, different for each spectral acceleration
VA
= coefficient determined by regression, different for each spectral acceleration
VS
= shear wave velocity for different site category
r
= (d2 +h2)½
d
= closest horizontal distance from the site of interest to the surface projection of
the rupture surface, see Figure B14
h
= fictitious depth determined by regression, different for each ground motion
parameter
Coefficients determined by regression are tabulated in the reports describing the attenuation equation.
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1997 Commentary, Appendix B
FIGURE B14 Measures of distance for strike-slip and dipping faults. A cross section
of strike-slip fault is shown in figure (a) and the shape of a typical deterministic
contour is shown in figure (b). A dipping fault is shown in figure (c) and the shape of
a typical deterministic contour is shown in figure (d).
The Sadigh et al. equation is:
lnY(T) ' F6C1 % C2 M % C3 (8 &'5 &M)2.5 % C4ln[D % exp(C5 % C6 M)] % Cyln(D % 2)>
where:
Y
= spectral response acceleration at period T
M
= earthquake magnitude
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Development of the USGS Seismic Maps
C1, C2, C3....C7
= coefficients determined by regression, different for each ground motion
parameter
D
= closest distance to the fault rupture surface, see Figure B14
F
= Factor for fault type. 1.0 for strike-slip faults, 1.2 for reverse/thrust
faulting, 1.09 for oblique faults
The distance measures are shown in Figure B14 and are discussed in more detail below.
The computation of spectral response (or any ground motion parameter) is a relatively simple
matter for a specific site (or specific distance from a fault) but can become complex when preparing contours since it is difficult to calculate the specific distance at which a particular ground
motion occurs This is due to the complexity of the two attenuation functions and the need to
combine their results. Since the attenuation functions were weighted equally, each contributes
equally to the ground motion at a site. Deterministic contours were determined by preparing
attenuation tables, that is the spectral response was computed at various distances from the fault
or the fault ends for each earthquake magnitude. Contours for specific values were then drawn
by selecting the table for the appropriate magnitude and determining, using interpolation, the
distance from the fault for a given spectral acceleration. This procedure required, as a minimum,
one attenuation table for each fault. Depending on the fault geometry, more than one table was
needed. In order to illustrate this the strike-slip fault is discussed first, followed by a discussion of
dipping faults.
Strike-Slip Faults: The strike-slip fault, shown in Figure B14a, b is the simplest introduction to
application of the SDPG rules. The distance measures are shown for each attenuation function in
Figure B14a. The Boore-Joyner-Fumal equation uses the distance, d4. The term r in equation
includes d4 and the fictitious depth h. Since h is not zero, r > d4, even if the term y in Figure B14a
is zero. The Sadigh et al. equation measures the distance, D, as the closest distance to the rupture surface. In this case to the top of the rupture. If the depth y is zero, then d4 = D4.
It makes little difference in the computations if the fault rupture plane begins at the surface or at
some distance below the surface. For the strike-slip fault the contour for a particular spectral
acceleration is a constant distant from the fault and the contour is as shown in Figure B14b. One
attenuation table (including the effects of both attenuation equations) can be used for either side
of the fault and at the fault ends.
Dipping Faults: The dipping fault, shown in Figures B14c and d, is the most complex case for
preparing deterministic contours. The distance measures are shown for each attenuation function
in Figure B14c. As before, it is a simple matter to compute the spectral values at a specific site,
but not as simple to compute the distance at which a specific spectral acceleration occurs. This is
particularly true at the end of the fault.
On the left side of the fault shown in Figure B14c, an attenuation table is prepared, much as in the
case of the strike-slip fault. This table may also be used to determine the contour around a portion of the fault end as shown in Figure B14d. In this case it is simply one-quarter of a circle.
A separate attenuation table must be prepared for the right side of the fault as shown in Figure
B14d. Since d or D is measured differently, depending on location x, calculations must keep track
328
1997 Commentary, Appendix B
of whether or not the location is inside or outside of the surface fault projection. Note that the
term d is zero when the location x falls within the surface projection, but the fictitious depth h is
not. Outside the fault projection, the distance d is measured from the edge of the projection. The
distance D is calculated differently, as illustrated in Figure B14c, depending on location but it is
always the closest distance to the fault rupture surface.
At the ends of the fault, an attenuation grid was prepared to determine the contour shape shown
dotted in Figure B14d. The contour in this area was digitized using the gridded values and combined with the remainder of the contour determined from the left and right attenuation tables.
This need for digitizing a portion of the contour greatly increased the time required to prepare
each of the contours for dipping faults. In short, each dipping fault required two attenuation
tables and an attenation grid to prepare each deterministic contour. Thus preparation of each
contour is far more time-consuming than preparing a contour for a strike-slip fault. Each contour
is unsymmetrical around the fault, the amount of asymmetry depends on the angle of dip.
It can be argued that the knowledge of fault locations and geometry does not warrant this level of
effort. However, it was considered necessary in order to follow the concept of repeatability in
preparing the maps.
Combining Deterministic Contours: Where two or more faults are nearby, as in Figure B15a, the
deterministic contours were merged (depending on amplitudes) as shown in Figure B15b. The
merging resulted in the sharp “corners”
shown in the figure. Although it can be
argued that these intersections should be
smoothed, it was believed that maintaining
the shape reflected the decision to use deterministic contours.
Combining Deterministic and Probabilistic Contours
The SDPG decision to use a combination
of deterministic and probabilistic contours, although simple in principle, led to
number of problems in preparing the
contour maps.
Figure B16a, b for a single strike-slip fault
illustrates the concept originally envisioned for combining the deterministic
and probabilistic contours. After
combining the two sets of contours shown
FIGURE B15 Procedure for combining deterministic
contours from nearby faults.
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Development of the USGS Seismic Maps
in Figure B16a, the maximum considered earthquake contours would be as shown in Figure
B16b.
In application the situation is more complex, there is frequently more than one fault, with different
magnitudes, different return times, different fault geometry, and different locations with respect to
each other. Examples are shown in Figures 17 and 18 which will be discussed later. The effect
of the variables is illustrated in Figure B16 c and d. The deterministic curve is shown for a single
fault with a return time much larger than that of the map. The deterministic spectral acceleration
is much larger than the spectral acceleration resulting from historical seismicity. The probabilistic
curve is not necessarily symmetrical to the fault. The resulting maximum considered earthquake
curve shown in Figure B16d is a complex mix of the probabilistic and deterministic curves. There
is not always a plateau and the curve is not necessarily symmetrical to the fault, even for a strikeslip fault. Simply stated, the probabilistic curve consider other sources such as historical seismicity and other faults as well as time. The deterministic curve does not consider other sources for
this simple example and does not consider time.
FIGURE B16 Procedure for obtaining maximum considered earthquake ground motion.
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1997 Commentary, Appendix B
The only areas of the United States that have deterministic contours are in California, along the
Pacific coast through Oregon and Washington, and in Alaska. At first review it can be seen that
there are several other areas that have contours in excess of the plateau but do not have plateaus.
In these areas (e.g., New Madrid), the deterministic values exceed the probabilistic ones and thus
were not used.
There were several instances where application of the SDPG rules produced results that appear
counterintuitive and in other instance produced results that were edited. Two examples from
southern California are discussed below. Each example is illustrated with a three-part figure. Part
(a) shows both probabilistic contours (dashed) and deterministic contours (solid) for each fault
which is also shown. Part (b) shows the maximum considered earthquake results produced by
following the SDPG rules. Part (c) shows how part (b) was edited for the final map.
Example 1: The first example in Figure B17 illustrates the occurrence of gaps in the deterministic
contours around a fault and the halt of a deterministic contour before the end of a fault. When the
probabilistic contours and deterministic contours shown in Figure B17a are combined, a gap in
the deterministic contours occurs in the vicinity of 34.6O and 118.8O. Similarly the deterministic
contours stop prior to the end of the fault around 34.65O and 119.4O. Both of these are shown in
Figure B17b.
After study, it is clear that the SDPG rules results in a repeatable, but unusual, set of contours.
The result does not go along with the concept of accounting for near fault effects with the deterministic contours. Because of this undesirable effect, the contours were hand edited to restore the
gaps and produce the result in Figure B17c.
All occurrences similar to this were edited to modify the contours so that the deterministic contours did not have abrupt breaks or stops before the ends of the fault.
Example 2: The second example in Figure B18 illustrates the occurrence of many faults at different orientations to each other and with different return times. Merging of the complex set of
contours is shown in Figure B18b. The contours are greatly simplified. Some small plateaus are
shown along the 150 percent contour, as is a gap along one of the faults around 34.0o and
116.35o. The gap was edited as in example 1. The small plateaus were edited out using the
judgement that their presence was inconsequential (less than a few percent effect on the maps)
and unnecessarily complicated an already complicated map.
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Development of the USGS Seismic Maps
FIGURE B17a Combining contours – Example 1. Both probabilistic and deterministic contours are shwon.
Probabilistic contours are shown dotted.
FIGURE B17b Combining contours – Example 1. Both probabilistic contours are merged using strict
interpretation of committee rules.
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1997 Commentary, Appendix B
FIGURE B17c Combining contours – Example 1. Probabilistic contours are merged with deterministic
contours using strict interpretation of committee rules with subsequent editing.
FIGURE B18a Combining contours – Example 1. Both probabilistic and deterministic contours are shown.
Probabilistic contours are shown dotted.
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Development of the USGS Seismic Maps
FIGURE B18b Combining contours – Example 1. Probabilistic contours are merged using strict
interpretation of committee rules.
FIGURE B18c Combining contours – Example 1. Probabilistic contours are merged with deterministic
contours using strict interpretation of committee rules with subsequent editing.
334
1997 Commentary, Appendix B
Another problem created was that some of the faults have portions of the fault, with a specific
acceleration value, in areas where the contours are less than the fault value. An example occurs
with the fault labeled 248 in the vicinity of 34.4o and 117.2o A footnote was added to the maximum considered earthquake maps to the effect that the fault value was only to be used in areas
where it exceeded the surrounding contours. Although other approaches are possible, such as
showing the unused portion of the fault dashed, the full length of the faults are shown solid in the
maps.
As shown in Figure B18b, a sawtooth contour around 34.15o and 116.3o results from application
of committee rules. Although this appears to be a candidate for smoothing, it was not done as
shown in Figure B18c. Once again there are several possible ways to smooth but it was not done
in the interest of repeatability.
FIGURE B19 Hazard curves for selected cities. The curves are normalized to 2% in 50 years.
Probability Level
The maximum considered earthquake spectral acceleration maps use the 2 percent in 50 maps as a
base; however, the values obtained from the maps are multiplied by 2/3 for use in the design
equation. This implicitly results in a different probability being used in different areas of the
United States. The hazard curves shown in Figure B2 are normalized to the 2 percent in 50 year
value in Figure B19. This figure shows that the slope of the hazard curve varies in different areas
of the United States. In general, the curves are steeper for CEUS cities than for WUS cities with
335
Development of the USGS Seismic Maps
the WUS curves beginning to flatten out earlier than the CEUS cities. Typical curves for a CEUS
and WUS city are shown in Figure B20. This figure shows than when the 2/3 factor is applied,
probabilistic values a for WUS location are close to a 10 percent in 50 year value and probabilities
for CEUS locations reflect a lower probability.
FIGURE B20 Effect on the probability level of multiplying the spectral acceleration by 2/3.
Interpolation
Linear interpolation between contours is permitted using the maximum considered earthquake
maps. To facilitate interpolation, spot values have been provided inside closed contours of increasing or decreasing values of the design parameter. Additional spot values have been provided
where linear interpolation would be difficult. Values have also been provided along faults in the
deterministic areas to aid in interpolation.
Hawaii
The Hawaii State Earthquake Advisory Board (HSEAB), in its ballot on the 1997 Provisions,
proposed different maps from those included in the original BSSC ballot. The HSEAB’s comments were based in part on recent work done to propose changes in seismic zonation for the
1994 and 1997 Uniform Building Code. The HSEAB also was concerned that in early 1998 the
USGS would be completing maps that would be more up to date then those included in the original BSSC ballot. Essentially, the HSEAB’s recommendation was that the maps it submitted or
336
1997 Commentary, Appendix B
the new USGS maps should be used for Hawaii. The USGS maps were completed in March 1998
and were reviewed by the HSEAB, including proposals for incorporation of deterministic contours where the ground motions exceed the plateau levels described previously. The maps were
revised in response to review comments and the modified design maps are included as part of the
Provisions.
Briefly, the probabilistic maps were prepared using a USGS methodology similar to that used for
the western United States. Two attenuation fuctions were used: Sadigh as described earlier and
Munson and Thurber, which incorportes Hawaii data. The Hawaii contour maps (Provisions
Maps 19 and 20) are probabilistic except for two areas on the island of Hawaii. The two areas
(outlined by the heavy border on Maps 19 and 20) are located on the western and southeastern
portion of the island. The two areas are defined by horizontal rupture planes at a 9 km depth.
Within these zones, the spectral accelerations are constant. The western zone uses a magniture
7.0 event while the southwestern zones uses a magnitude 8.2 event. The deterministic values
inside the zone and for the contours were calculated as described in earlier sections.
Documentation for the maps is being prepared. The probabilistic maps and documentation are
available on the USGS internet site (htt\p://geohazards.cr.usgs.gov/eq/)
Additional Maximum Considered Earthquake Ground Motion Maps
Although new probabilistic maps were not available for Puerto Rico, Culebra, Vieques, St.
Thomas, St. John, St. Croix, Guam, and Tutuila maximum considered earthquake maps were
required for use by the Provisions. Maximum considered earthquake spectral response maps for
these areas were prepared as follows.
Maps for Puerto Rico, Culebra, Vieques, St. Thomas, St. John, St. Croix, Guam, and Tutuila,
were prepared using the 1994 NEHRP maps. These were for approximately 10 percent probability of exceedance in 50 years. The ratio of PGA for 2 percent in 50 years to 10 percent in 50
years for the new USGS maps is about two. Accordingly maps for these areas were converted to
2 percent in 50 year maps by multiplying by two. These maps were then converted to spectral
maps by using the factors described below.
A study of the ratios of the 0.2 sec and 1.0 sec spectral responses to PGA was done. Although
approximate, the ratios were about 2.25 to 2.5 for the 0.2 sec spectral acceleration and about 1.0
for the 1.0 sec response. Thus PGA for the above regions was converted to spectral acceleration
by multiplying PGA by 2.5 for the 0.2 sec response and by 1.0 for the 1.0 sec response. It should
be noted that the multiplier for the 1.0 sec response varied over a wider range than the 0.2 sec
response multiplier. It should be used cautiously.
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Johnston, A. 1996b. “Seismic Moment Assessment of Stable Continental Earthquakes, Part 3:
1811-1812 New Madrid, 1886 Charleston, and 1755 Lisbon,” submitted to Geophys. J. Int.
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Johnston, A.C., K.J. Coppersmith, L.R. Kanter, and C.A. Cornell. 1994. The earthquakes of
stable continental regions: assessment of large earthquake potential, EPRI TR-102261, J.F.
Schneider, ed., Electric Power Research Institute, 309 pp.
Johnston, A.C. and E.S. Schweig . 1996. The enigma of the New Madrid earthquakes of 18111812, Annual Review of Earth and Planetary Sciences, v. 24, pp. 339-384.
Joyner, W.B. 1995. Advocacy Paper #4, Soil is the most appropriate reference site condition, in
ATC 35-2, Preprints: National Earthquake Ground Motion Mapping Workshop, Applied Technology Council.
Leyendecker, E.V., D.M. Perkins, S.T. Algermissen, P.C. Thenhaus, and S.L. Hanson. 1995.
USGS Spectral Response Maps and Their Relationship with Seismic Design Forces in Building
Codes, U.S. Geological Survey, Open-File Report 95-596.
Lienkaemper, J.J. 1996. "Fault parameter compilation for northern California", in preparation.
Martin, G.R. and R. Dobry. 1994. Earthquake site response and seismic code provisions,
NCEER Bulletin, v. 8, pp. 1-6.
Mueller, C., M. Hopper, and A. Frankel. 1996. Preparation of earthquake catalogs for the
1996 national seismic hazard maps: documentation, U.S. Geological Survey Open-File Report, in
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Mueller, C., M. A. Frankel, J. Lahr, and M. Wyss.. 1997. Preparation of earthquake catalogs
for the National Seismic Hazard Maps: Alaska, U.S. Geological Survey Open-File Report, in
review.
Obermeier, S.F., R.B. Jacobson, J.P. Smoote, R.E. Weems, G.S. Gohn. J.E. Monroe, and
D.S. Powers. 1990. “Earthquake-induced Liquefaction Features in the Coastal Setting of South
Carolina and in the Fluvial Setting of the New Madrid Seismic Zone,” U.S. Geological Survey,
Prof. Paper 1504, 44 pp.
Obermeier, S.F., P.J. Munson, C.A. Munson, J.R. Martin, A.D. Frankel, T.L. Youd, and E.C.
Pond. 1992. “Liquefaction Evidence for Strong Holocene Earthquake. s in the Wabash Valley of
Indiana-Illinois”, Seismological Research Letters, v. 63, pp. 321-336.
Petersen, M.D. and S.G. Wesnousky. 1994. Fault slip rates and earthquake histories for active
faults in southern California, Bull. Seism. Soc. Am., v. 84, pp. 1608-1649.
Petersen, M.D., C.H. Cramer, W.A. Bryant, M.S. Reichle, and T.R. Toppozada. 1996. Preliminary hazard assessment for Los Angeles, Ventura, and Orange counties, California, affected
by the 17 January 1994 Northridge earthquake, Bull. Seism. Soc. Am., v. 86, pp. S247-S261.
Powell, C., G. Bollinger, M. Chapman, M. Sibol, A. Johnston, and R. Wheeler. 994. A
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THE COUNCIL:
ITS PURPOSE AND
ACTIVITIES
Of the National Institute of Building Sciences
The Building Seismic Safety Council (BSSC) was established in 1979 under the auspices of the National
Institute of Building Sciences as an entirely new type of instrument for dealing with the complex regulatory,
technical, social, and economic issues involved in developing and promulgating building earthquake risk
mitigation regulatory provisions that are national in scope. By bringing together in the BSSC all of the needed expertise and all relevant public and private interests, it was believed that issues related to the seismic
safety of the built environment could be resolved and jurisdictional problems overcome through authoritative
guidance and assistance backed by a broad consensus.
The BSSC is an independent, voluntary membership body representing a wide variety of building community
interests. Its fundamental purpose is to enhance public safety by providing a national forum that fosters improved seismic safety provisions for use by the building community in the planning, design, construction,
regulation, and utilization of buildings. To fulfill its purpose, the BSSC:
#
Promotes the development of seismic safety provisions suitable for use throughout the United States;
#
Recommends, encourages, and promotes the adoption of appropriate seismic safety provisions in voluntary standards and model codes;
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Assesses progress in the implementation of such provisions by federal, state, and local regulatory and
construction agencies;
#
Identifies opportunities for improving seismic safety regulations and practices and encourages public
and private organizations to effect such improvements;
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Promotes the development of training and educational courses and materials for use by design professionals, builders, building regulatory officials, elected officials, industry representatives, other members
of the building community, and the public;
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Advises government bodies on their programs of research, development, and implementation; and
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Periodically reviews and evaluates research findings, practices, and experience and makes recommendations for incorporation into seismic design practices.
The BSSC's area of interest encompasses all building types, structures, and related facilities and includes explicit consideration and assessment of the social, technical, administrative, political, legal, and economic
implications of its deliberations and recommendations. The BSSC believes that the achievement of its purpose is a concern shared by all in the public and private sectors; therefore, its activities are structured to provide all interested entities (i.e., government bodies at all levels, voluntary organizations, business, industry,
the design profession, the construction industry, the research community, and the general public) with the opportunity to participate. The BSSC also believes that the regional and local differences in the nature and
magnitude of potentially hazardous earthquake events require a flexible approach to seismic safety that
allows for consideration of the relative risk, resources, and capabilities of each community.
The BSSC is committed to continued technical improvement of seismic design provisions, assessment of advances in engineering knowledge and design experience, and evaluation of earthquake impacts. It recognizes
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that appropriate earthquake hazard risk reduction measures and initiatives should be adopted by existing
organizations and institutions and incorporated, whenever possible, into their legislation, regulations, practices, rules, codes, relief procedures, and loan requirements so that these measures and initiatives become an
integral part of established activities, not additional burdens. Thus, the BSSC itself assumes no standards-making or -promulgating role; rather, it advocates that code- and standards-formulation organizations
consider the BSSC’s recommendations for inclusion in their documents and standards.
IMPROVING THE SEISMIC SAFETY OF NEW BUILDINGS
The BSSC program directed toward improving the seismic safety of new buildings has been conducted with
funding from the Federal Emergency Management Agency (FEMA). It is structured to create and maintain
authoritative, technically sound, up-to-date resource documents that can be used by the voluntary standards
and model code organizations, the building community, the research community, and the public as the foundation for improved seismic safety design provisions.
The BSSC program began with initiatives taken by the National Science Foundation (NSF). Under an agreement with the National Institute of Standards and Technology (NIST; formerly the National Bureau of
Standards), Tentative Provisions for the Development of Seismic Regulations for Buildings (referred to here
as the Tentative Provisions) was prepared by the Applied Technology Council (ATC). The ATC document
was described as the product of a "cooperative effort with the design professions, building code interests, and
the research community" intended to "...present, in one comprehensive document, the current state of knowledge in the fields of engineering seismology and engineering practice as it pertains to seismic design and construction of buildings." The document, however, included many innovations, and the ATC explained that a
careful assessment was needed.
Following the issuance of the Tentative Provisions in 1978, NIST released a technical note calling for ". . .
systematic analysis of the logic and internal consistency of [the Tentative Provisions]" and developed a plan
for assessing and implementing seismic design provisions for buildings. This plan called for a thorough review of the Tentative Provisions by all interested organizations; the conduct of trial designs to establish the
technical validity of the new provisions and to assess their economic impact; the establishment of a mechanism to encourage consideration and adoption of the new provisions by organizations promulgating national
standards and model codes; and educational, technical, and administrative assistance to facilitate implementation and enforcement.
During this same period, other significant events occurred. In October 1977, Congress passed the
Earthquake Hazards Reduction Act of 1977 (P.L. 95-124) and, in June 1978, the National Earthquake
Hazards Reduction Program (NEHRP) was created. Further, FEMA was established as an independent
agency to coordinate all emergency management functions at the federal level. Thus, the future disposition of
the Tentative Provisions and the 1978 NIST plan shifted to FEMA. The emergence of FEMA as the agency
responsible for implementation of P.L. 95-124 (as amended) and the NEHRP also required the creation of a
mechanism for obtaining broad public and private consensus on both recommended improved building design
and construction regulatory provisions and the means to be used in their promulgation. Following a series of
meetings between representatives of the original participants in the NSF-sponsored project on seismic design
provisions, FEMA, the American Society of Civil Engineers and the National Institute of Building Sciences
(NIBS), the concept of the Building Seismic Safety Council was born. As the concept began to take form,
progressively wider public and private participation was sought, culminating in a broadly representative
organizing meeting in the spring of 1979, at which time a charter and organizational rules and procedures
were thoroughly debated and agreed upon.
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The BSSC provided the mechanism or forum needed to encourage consideration and adoption of the new
provisions by the relevant organizations. A joint BSSC-NIST committee was formed to conduct the needed
review of the Tentative Provisions, which resulted in 198 recommendations for changes. Another joint
BSSC-NIST committee developed both the criteria by which the needed trial designs could be evaluated and
the specific trial design program plan. Subsequently, a BSSC-NIST Trial Design Overview Committee was
created to revise the trial design plan to accommodate a multiphased effort and to refine the Tentative Provisions, to the extent practicable, to reflect the recommendations generated during the earlier review.
Trial Designs
Initially, the BSSC trial design effort was to be conducted in two phases and was to include trial designs for
100 new buildings in 11 major cities, but financial limitations required that the program be scaled down. Ultimately, 17 design firms were retained to prepare trial designs for 46 new buildings in 4 cities with medium
to high seismic risk (10 in Los Angeles, 4 in Seattle, 6 in Memphis, 6 in Phoenix) and in 5 cities with medium
to low seismic risk (3 in Charleston, South Carolina, 4 in Chicago, 3 in Ft. Worth, 7 in New York, and 3 in
St. Louis). Alternative designs for six of these buildings also were included.
The firms participating in the trial design program were: ABAM Engineers, Inc.; Alfred Benesch and Company; Allen and Hoshall; Bruce C. Olsen; Datum/Moore Partnership; Ellers, Oakley, Chester, and Rike, Inc.;
Enwright Associates, Inc.; Johnson and Nielsen Associates; Klein and Hoffman, Inc.; Magadini-Alagia
Associates; Read Jones Christoffersen, Inc.; Robertson, Fowler, and Associates; S. B. Barnes and Associates;
Skilling Ward Rogers Barkshire, Inc.; Theiss Engineers, Inc.; Weidlinger Associates; and Wheeler and Gray.
For each of the 52 designs, a set of general specifications was developed, but the responsible design engineering firms were given latitude to ensure that building design parameters were compatible with local
construction practice. The designers were not permitted, however, to change the basic structural type even if
an alternative structural type would have cost less than the specified type under the early version of the Provisions, and this constraint may have prevented some designers from selecting the most economical system.
Each building was designed twice – once according to the amended Tentative Provisions and again according
to the prevailing local code for the particular location of the design. In this context, basic structural designs
(complete enough to assess the cost of the structural portion of the building), partial structural designs (special studies to test specific parameters, provisions, or objectives), partial nonstructural designs (complete
enough to assess the cost of the nonstructural portion of the building), and design/construction cost estimates
were developed.
This phase of the BSSC program concluded with publication of a draft version of the recommended provisions, the NEHRP Recommended Provisions for the Development of Seismic Regulations for New Buildings, an overview of the Provisions refinement and trial design efforts, and the design firms' reports.
The 1985 Edition of the NEHRP Recommended Provisions
The draft version represented an interim set of provisions pending their balloting by the BSSC member
organizations. The first ballot, conducted in accordance with the BSSC Charter, was organized on a
chapter-by-chapter basis. As required by BSSC procedures, the ballot provided for four responses: "yes,"
"yes with reservations," "no," and "abstain." All "yes with reservations" and "no" votes were to be accompanied by an explanation of the reasons for the vote and the "no" votes were to be accompanied by specific
suggestions for change if those changes would change the negative vote to an affirmative.
All comments and explanations received with "yes with reservations" and "no" votes were compiled, and proposals for dealing with them were developed for consideration by the Technical Overview Committee and,
subsequently, the BSSC Board of Direction. The draft provisions then were revised to reflect the changes
deemed appropriate by the BSSC Board and the revision was submitted to the BSSC membership for balloting again.
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As a result of this second ballot, virtually the entire provisions document received consensus approval, and a
special BSSC Council meeting was held in November 1985 to resolve as many of the remaining issues as
possible. The 1985 Edition of the NEHRP Recommended Provisions then was transmitted to FEMA for
publication in December 1985.
During the next three years, a number of documents were published to support and complement the 1985
Provisions. They included a guide to application of the Provisions in earthquake-resistant building design, a
nontechnical explanation of the Provisions for the lay reader, and a handbook for interested members of the
building community and others explaining the societal implications of utilizing improved seismic safety
provisions and a companion volume of selected readings.
The 1988 Edition
The need for continuing revision of the Provisions had been anticipated since the onset of the BSSC program
and the effort to update the 1985 Edition for reissuance in 1988 began in January 1986. During the update
effort, nine BSSC Technical Committees (TCs) studied issues concerning seismic risk maps, structural design, foundations, concrete, masonry, steel, wood, architectural and mechanical and electrical systems, and
regulatory use. The Technical Committees worked under the general direction of a Technical Management
Committee (TMC), which was composed of a representative of each TC as well as additional members identified by the BSSC Board to provide balance.
The TCs and TMC worked throughout 1987 to develop specific proposals for changes needed in the 1985
Provisions. In December 1987, the Board reviewed these proposals and decided upon a set of 53 for submittal to the BSSC membership for ballot. Approximately half of the proposals reflected new issues while the
other half reflected efforts to deal with unresolved 1985 edition issues.
The balloting was conducted on a proposal-by-proposal basis in February-April 1988. Fifty of the proposals
on the ballot passed and three failed. All comments and "yes with reservation" and "no" votes received as a
result of the ballot were compiled for review by the TMC. Many of the comments could be addressed by
making minor editorial adjustments and these were approved by the BSSC Board. Other comments were
found to be unpersuasive or in need of further study during the next update cycle (to prepare the 1991 Provisions). A number of comments persuaded the TMC and Board that a substantial alteration of some balloted
proposals was necessary, and it was decided to submit these matters (11 in all) to the BSSC membership for
reballot during June-July 1988. Nine of the eleven reballot proposals passed and two failed.
On the basis of the ballot and reballot results, the 1988 Provisions documents were prepared and transmitted
to FEMA for publication in August 1988. A report describing the changes made in the 1985 edition and
issues in need of attention in the next update cycle also was prepared, and efforts to update the complementary reports published to support the 1985 edition were initiated. Ultimately, the following publications were
updated to reflect the 1988 Edition and reissued by FEMA: the Guide to Application of the Provisions, the
handbook discussing societal implications (which was extensively revised and retitled Seismic Considerations for Communities at Risk), and several Seismic Considerations handbooks (which are described
below).
The 1991 Edition
During the effort to produce the 1991 Provisions, a Provisions Update Committee (PUC) and 11 Technical
Subcommittees addressed seismic hazard maps, structural design criteria and analysis, foundations, cast-inplace and precast concrete structures, masonry structures, steel structures, wood structures, mechanical-electrical systems and building equipment and architectural elements, quality assurance, interface with codes and
standards, and composite structures. Their work resulted in 58 substantive and 45 editorial proposals for
change to the 1988 Provisions.
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The PUC approved more than 90 percent of the proposals and, in January 1991, the BSSC Board accepted
the PUC-approved proposals for balloting by the BSSC member organizations in April-May 1991.
Following the balloting, the PUC considered the comments received with "yes with reservations" and "no"
votes and prepared 21 reballot proposals for consideration by the BSSC member organizations. The reballoting was completed in August 1991 with the approval by the BSSC member organizations of 19 of the
reballot proposals.
On the basis of the ballot and reballot results, the 1991 Provisions documents were prepared and transmitted
to FEMA for publication in September 1991. Reports describing the changes made in the 1988 Edition and
issues in need of attention in the next update cycle also were developed.
In August 1992, in response to a request from FEMA, the BSSC initiated an effort to continue its structured
information dissemination and instruction/training effort aimed at stimulating widespread use of the
Provisions. The primary objectives of the effort were to bring several of the publications complementing the
Provisions into conformance with the 1991 Edition in a manner reflecting other related developments (e.g.,
the fact that all three model codes now include requirements based on the Provisions) and to bring instructional course materials currently being used in the BSSC seminar series (described below) into conformance
with the 1991 Provisions.
The 1994 Edition
The effort to structure the 1994 PUC and its technical subcommittees was initiated in late 1991. By early
1992, 12 Technical Subcommittees (TSs) were established to address seismic hazard mapping, loads and
analysis criteria, foundations and geotechnical considerations, cast-in-place and precast concrete structures,
masonry structures, steel structures, wood structures, mechanical-electrical systems and building equipment
and architectural elements, quality assurance, interface with codes and standards, and composite steel and
concrete structures, and base isolation/energy dissipation.
The TSs worked throughout 1992 and 1993 and, at a December 1994 meeting, the PUC voted to forward 52
proposals to the BSSC Board with its recommendation that they be submitted to the BSSC member organizations for balloting. Three proposals not approved by the PUC also were forwarded to the Board because 20
percent of the PUC members present at the meeting voted to do so. Subsequently, an additional proposal to
address needed terminology changes also was developed and forwarded to the Board.
The Board subsequently accepted the PUC-approved proposals; it also accepted one of the proposals submitted under the "20 percent" rule but revised the proposal to be balloted as four separate items. The BSSC
member organization balloting of the resulting 57 proposals occurred in March-May 1994, with 42 of the 54
voting member organizations submitting their ballots. Fifty-three of the proposals passed, and the ballot results and comments were reviewed by the PUC in July 1994. Twenty substantive changes that would require
reballoting were identified. Of the four proposals that failed the ballot, three were withdrawn by the TS
chairmen and one was substantially modified and also was accepted for reballoting. The BSSC Board of
Direction accepted the PUC recommendations except in one case where it deemed comments to be persuasive
and made an additional substantive change to be reballoted by the BSSC member organizations.
The second ballot package composed of 22 changes was considered by the BSSC member organizations in
September-October 1994. The PUC then assessed the second ballot results and made its recommendations to
the BSSC Board in November. One needed revision identified later was considered by the PUC Executive
Committee in December. The final copy of the 1994 Edition of the Provisions including a summary of the
differences between the 1991 and 1994 Editions was delivered to FEMA in March 1995.
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1997 Update Effort
In September 1994, NIBS entered into a contract with FEMA for initiation of the 39-month BSSC 1997
Provisions update effort. Late in 1994, the BSSC member organization representatives and alternate
representatives and the BSSC Board of Direction were asked to identify individuals to serve on the 1997
PUC and its TSs. The 1997 PUC was constituted early in 1995, and 12 PUC Technical Subcommittees were
established to address design criteria and analysis, foundations and geotechnical considerations, cast-inplace/precast concrete structures, masonry structures, steel structures, wood structures, mechanical-electrical
systems and building equipment and architectural elements, quality assurance, interface with codes and standards, composite steel and concrete structures, energy dissipation and base isolation, and nonbuilding structures.
As part of this effort, the BSSC developed for the 1997 Provisions a revised seismic design procedure.
Unlike the design procedure based on U.S. Geological Survey (USGS) peak acceleration and peak velocityrelated acceleration ground motion maps developed in the 1970s and used in earlier editions of the Provisions, the new design procedure involves new design maps based on recently revised USGS spectral response
maps and a process specified within the body of the Provisions. This task was conducted with the cooperation of the USGS (under a Memorandum of Understanding signed by the BSSC and USGS) by the Seismic Design Procedure Group (SDPG) working with the guidance of a five-member Management Committee.
More than 200 individuals participated in the 1997 update effort, and more than 165 substantive proposals
for change were developed. A series of editorial/organizational changes also were made. All draft TS,
SDPG, and PUC proposals for change were finalized in late February 1997, and in early March, the PUC
Chair presented to the BSSC Board of Direction the PUC’s recommendations concerning proposals for
change to be submitted to the BSSC member organizations for balloting. The Board accepted these recommendations, and the first round of balloting was conducted in April-June 1997.
Of the 158 items on the first ballot, only 8 did not pass; however, many comments were submitted with “no”
and “yes with reservations” votes. These comments were compiled for distribution to the PUC, which met in
mid-July to review the comments, receive TS responses to the comments and recommendations for change,
and formulate its recommendations concerning what items should be submitted to the BSSC member organizations for a second ballot. The PUC deliberations resulted in the decision to recommend to the BSSC Board
that 28 items be included in the second ballot. The PUC Chair subsequently presented the PUC’s recommendations to the Board, which accepted those recommendations.
The second round of balloting was completed in October. All but one proposal passed; however, a number of
comments on virtually all the proposals were submitted with the ballots and were immediately compiled for
consideration by the PUC. The PUC Executive Committee met in December to formulate its recommendations to the Board, and the Board subsequently accepted those recommendations.
The PUC concluded its update work by identifying issues in need of consideration during the next update
cycle and technical issues in need of study. The final version of the 1997 Provisions, including an appendix
describing the differences between the 1994 and 1997 edition, was transmitted to FEMA in February 1998.
The contract for the 1997 update effort has been extended by FEMA to June 30, 1998, to permit development
of a CD-ROM for presentation of the design map data.
Code Resource Development Effort
In mid-1996, FEMA asked the BSSC to initiate an effort to generate a code resource document based on the
1997 Provisions for use by the International Code Council (ICC) in adopting seismic provisions for the first
edition of the International Building Code (IBC) to be published in 2000.
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The orientation meeting of the Code Resource Development Committee (CRDC) appointed to conduct this
effort was held in Denver on October 17. At this meeting, the group was briefed on the status of the Provisions update effort and formulated a tentative plan and schedule for its efforts.
The group next met in January 1997 to review a preliminary code language/format version of the 1997 Provisions and to develop additional needed input. As a result of this meeting, several task groups were established to focus on specific topics and to provide revisions to the preliminary draft. A new draft incorporating
these comments then was developed for further refinement by the CRDC. A copy also was delivered to the
members of the IBC Structural Subcommittee so that they could begin determining how the seismic provisions would fit into their code requirements.
The CRDC met again in February to review the second draft of the code language/format version of the 1997
Provisions. This meeting was held just preceding a PUC meeting and changes made by the PUC subsequently were incorporated into the CRDC draft. The CRDC Chair presented this composite draft to the IBC
Structural Subcommittee in March 1997.
In July, the CRDC met to develop comments on the IBC working draft to be submitted to the ICC in preparation for an August public comment forum. These comments generally reflected actions taken by the PUC in
response to comments submitted with the first ballot on the changes proposed for the 1997 Provisions as
well as CRDC recommendations concerning changes made by the IBC Structural Subcommittee in the original CRDC submittal. CRDC representatives then attended the August forum to support the CRDC recommendations.
The CRDC then met in December to prepare “code change proposals” on the first published version of the
IBC. The proposed “code changes” developed by the committee were submitted to the IBC on January 5,
1998. The group is next meeting in March to review the changes to the draft IBC proposed by others to
assist CRDC representatives who will attend a public hearing on the draft IBC in April 1998. Subsequent
efforts are expected to focus on supporting the CRDC-developed provisions throughout the code adoption
process.
In addition, a task group of the CRDC was established in late-1997 to provide the ICC committee developing
the the International Residential Dwelling Code with input concerning seismic requirements reflecting the
1997 Provisions. The activities of this task group are expected to parallel those of the CRDC with the IBC.
The 2000 Edition
In September 1997, NIBS entered into a contract with FEMA for initiation of the 48-month BSSC effort to
update the 1997 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other
Structures for re-issuance in 2000 and prepare code changes based on the 2000 Provisions for submittal to
the IBC. The BSSC member organization representatives and alternate representatives and the BSSC Board
of Direction were asked to identify candidates to participate; the individuals serving on the 1997 update
committees were contacted to determine if they are interested in participating in the new effort; and a press
release on the 2000 update effort was issued.
In lieu of the Seismic Design Procedure Group (SDPG) used in the 1997 update, the BSSC is re-establishing
Technical Subcommittee 1, Seismic Design Mapping, which was used in earlier updates of the Provisions.
This subcommittee will be composed of an equal number of representatives from the earth science community, including representatives from the USGS, and the engineering community. A sufficient number of
members of the SDPG will be included to ensure a smooth transition.
An additional 11 subcommittees will address seismic design and analysis, foundations and geotechnical
considerations, cast-in-place and precast concrete structures, masonry structures, steel structures, wood
structures, mechanical-electrical systems and building equipment and architectural elements, quality assurance, composite steel and concrete structures, base isolation and energy dissipation, and nonbuilding struc-
349
tures and one ad hoc task group to develop appropriate anchorage requirements for concrete/masonry/wood
elements. Unlike earlier updates, it is not anticipated that a technical subcommittee will be appointed to serve
as the interface with codes and standards; rather, the PUC will appoint a task group to serve as the liaison
with the the model code and standards organizations and three model code representatives will serve on the
PUC.
The BSSC, through the PUC and its TSs, will identify major technical issues to be addressed during the 2000
update of the Provisions, assess the basis for change to the 1997 Edition, resolve technical issues, and develop proposals for change. The results of recent relevant research and lessons learned from earthquakes
occurring prior to and during the duration of the project will be given consideration at all stages of this process. Particular attention will be focused on those technical problems identified but unresolved during the
preparation of the 1997 Edition. Attention also will be given to the improvement of criteria to eventually
allow for design based on desired building performance levels, an approach taken in the NEHRP Guidelines
for the Seismic Rehabilitation of Buildings.
The PUC also will coordinate its efforts with those individuals working with the ICC to develop the IBC.
Changes recommended by those individuals will be submitted to the PUC for consideration and changes
developed by the PUC will be formatted for consideration in the IBC development process.
As part of the update process, the BSSC also will develop a simplified design procedure in order to facilitate
use of the Provisions in areas of low and moderate seismic hazard. This process will be performed by a
separate task group reporting directly to TS2, Seismic Design and Analysis.
As in previous update efforts, two rounds of balloting by the BSSC member organizations are planned, and
delivery of the final consensus-approved 2000 Provisions is expected to occur in December 2000. An appendix identifying the major differences between the 1997 and the 2000 editions of the Provisions will be included in the Provisions volume and a letter report describing unresolved issues and major technical topics in
need of further study also will be prepared.
Following completion of the 2000 Provisions, the BSSC will establish a procedure whereby code language
versions of changes to the Provisions will be prepared for submittal as proposed code changes for the 2003
Edition of the IBC. These code changes will be developed for PUC consideration and approval by a Code
Liaison Group with the assistance of a consultant experienced in the code change process. In addition, the
BSSC will designate three members of the PUC who, along with the consultant, will formally submit the code
changes prior to the IBC deadline.
The 2000 PUC met for the first time in February 1998 to develop a schedule for its work, review the charges
to the technical subcommittees, and consider how best to deal with unresolved issues and coordinate with the
BSSC work devoted to the IBC effort.
Information Dissemination/Technology Transfer
The BSSC continues in its efforts to stimulate widespread use of the Provisions. In addition to the issuance
of a variety of publications that complement the Provisions, over the past seven years the BSSC has developed materials for use in and promoted the conduct of a series of seminars on application of the Provisions
among relevant professional associations. To date, more than 90 of these seminars have been conducted with
a wide variety of cosponsors and more than 70,000 reports have been distributed.
Other information dissemination efforts have involved the participation of BSSC representatives in a wide
variety of meetings and conferences, BSSC participation in development of curriculum for a FEMA Emergency Management Institute course on the Provisions for structural engineers and other design professionals,
issuance of press releases, development of in-depth articles for the publications of relevant groups, work with
Building Officials and Code Administrators International (BOCA) that resulted in use of the Provisions in
the BOCA National Building Code and the Southern Building Code Congress International’s Standard
350
Building Code, and cooperation with the American Society of Civil Engineers (ASCE) that resulted in use of
the Provisions in the 1993 and 1995 Editions of Standard ASCE 7. In addition, many requests for specific
types of information and other forms of technical support are received and responded to monthly.
During 1996, as part of the efforts of a joint committee of the BSSC, Central U.S. Earthquake Consortium,
Southern Building Code Congress International and Insurance Institute for Property Loss Reduction to develop mechanisms for the seismic training of building code officials, the BSSC contributed its expertise in
the development of a manual for use in such training efforts.
Information dissemination efforts during 1997 have been somewhat curtailed so that resources can be devoted to introduction of the 1997 Provisions and related efforts. In this regard, NIBS has requested and
received an extension of its existing information dissemination contract with FEMA through September 1998
to permit, among other things, work on revised versions of the Nontechnical Explanation of the NEHRP
Recommended Provisions, Guide to Application of the Provisions, and Provisions educational/training
materials to reflect the 1997 Edition. In addition, it is anticipated that development of an Internet/Web site
devoted to the 1997 Provisions and the 2000 update effort will be initiated.
IMPROVING THE SEISMIC SAFETY OF EXISTING BUILDINGS
Guidelines/Commentary Development Project
In August 1991, NIBS entered into a cooperative agreement with FEMA for a comprehensive 6-year program
leading to the development of a set of nationally applicable guidelines for the seismic rehabilitation of existing buildings. Under this agreement, the BSSC serves as program manager with the American Society of
Civil Engineers (ASCE) and the Applied Technology Council (ATC) working as subcontractors. Initially,
FEMA provided funding for a program definition activity designed to generate the detailed work plan for the
overall program. The work plan was completed in April 1992 and in September FEMA contracted with
NIBS for the remainder of the effort.
The major objectives of the project were to develop a set of technically sound, nationally applicable guidelines (with commentary) for the seismic rehabilitation of buildings to serve as a primary resource on the
seismic rehabilitation of buildings for the use of design professionals, model code and standards organizations, state and local building regulatory personnel, and educators; to develop building community consensus
regarding the guidelines; and to develop the basis of a plan for stimulating widespread acceptance and application of the guidelines.
The project work was structured to ensure that the technical guidelines writing effort benefits from: consideration of the results of completed and ongoing technical efforts and research activities as well as societal
issues, public policy concerns, and the recommendations presented in an earlier FEMA-funded report on
issues identification and resolution; cost data on application of rehabilitation procedures; the reactions of
potential users; and consensus review by a broad spectrum of building community interests.
While overall management remained the responsibility of the BSSC, responsibility for conduct of the specific
project tasks were shared by the BSSC with ASCE (which organized user workshops and conducted literature
review and other research activities) and ATC (which was responsible for drafting the Guidelines, its Commentary, and a volume of example applications as well as conducting a study to assess the validity of several
concepts being proposed for use in the Guidelines). Specific BSSC tasks were conducted under the guidance
of a BSSC Project Committee. To ensure project continuity and direction, a Project Oversight Committee
(POC) was responsible to the BSSC Board for accomplishment of the project objectives and the conduct of
project tasks. Further, a Seismic Rehabilitation Advisory Panel was established to review project products
and to advise the POC and, if appropriate, the BSSC Board, on the approach being taken, problems arising or
351
anticipated, and progress being made. In addition, three workshops were held over the course of the project
to provide the Guidelines/Commentary writers with input from potential users of the documents.
The BSSC Board of Direction accepted the 100-percent-complete draft of the Guidelines and Commentary
for consensus balloting in mid-August 1996. The first round of balloting occurred in October-December with
a ballot symposium for the voting representatives held in November 1996.
The Guidelines and Commentary were approved by the BSSC membership; however, a significant number of
comments were received. The ATC Senior Technical Committee reviewed these comments in detail and
commissioned members of the technical teams that developed the Guidelines to develop detailed responses
and to formulate any needed proposals for change reflecting the comments. This effort resulted in 48 proposals for change to be submitted to the BSSC member organizations for a second round of balloting.
Following acceptance of the second ballot materials by the BSSC Board, the voting occurred in June-July
1997. Again the results were compiled for review by ATC. Meeting in September 1997, the Project Oversight Committee received recommendations from ATC regarding comment resolution; it was concluded that
none of the changes proposed in response to ballot comments were sufficiently substantive to warrant
reballoting. Subsequently, the POC conclusion was presented to the BSSC Board, which agreed and
approved finalization of the Guidelines and Commentary for submittal to FEMA for publication. The final
versions of the documents then were prepared and transmitted to FEMA in September 1998 and published
copies became available in March 1998.
During the course of the project, BSSC Project Committee recommendations resulted in the following
additions to the NIBS/BSSC contract with FEMA for the project: the BSSC ballot symposium for voting
representatives mentioned above; the case studies program described below; and an effort to develop the
curriculum for and conduct a series of two-day educational seminars to introduce and provide training in use
of the Guidelines to practicing structural and architectural engineers, seismic engineering educators and
students, building officials and technical staff, interested contractors, hazard mitigation officers, and others.
Case Studies Project
As noted above, the case studies project is an extension of the multiyear project leading to publication of the
NEHRP Guidelines for the Seismic Rehabilitation of Buildings and its Commentary. The project is
expected to contribute to the credibility of the Guidelines by providing potential users with representative
real-world application data and to provide FEMA with the information needed to determine whether and
when to update the Guidelines.
Although the Guidelines documents reflect expert experience, current research, and innovative theories, the
case studies project is expected to answer a number of critical questions: Can the Guidelines and its
Commentary be understood and applied by practicing design professionals of varying levels of experience?
Do the Guidelines result in rational designs generated in a reasonable and logical way? What are the costs
involved in seismically rehabilitating various types of buildings to the optional levels of performance both
above and below the Guidelines’ “basic safety objective”? Are the requirements to achieve the “basic safety
objective” equivalent to, less stringent than, or more stringent than current practice for new construction?
Specifically, the objectives of the project are to: (a) test the usability of the NEHRP Guidelines for the
Seismic Rehabilitation of Buildings in authentic applications in order to determine the extent to which
practicing design engineers and architects find the Guidelines documents, including the structural analysis
procedures and acceptance criteria, to be presented in understandable language and in a clear, logical fashion
that permits valid engineering determinations to be made, and evaluate the ease of transition from current
engineering practices to the new concepts presented in the Guidelines; (b) assess the technical adequacy of
the Guidelines design and analysis procedures to determine if application of the procedures results (in the
judgment of the designer) in rational designs of building components for corrective rehabilitation measures
352
and whether the designs that result adequately meet the selected performance levels when compared to current
practice and in light of the knowledge and experience of the designer; (c) assess whether the Guidelines
acceptance criteria are properly calibrated to result in component designs that provide permissible values of
such key factors as drift, component strength demand, and inelastic deformation at selected performance
levels; (d) develop data on the costs of rehabilitation design and construction to meet the Guidelines’ “basic
safety objective” as well as the higher performance levels included and assess whether the anticipated higher
costs of advanced engineering analysis result in worthwhile savings compared to the cost of constructing
more conservative design solutions arrived at by a less systematic engineering effort; and (e) compare the
acceptance criteria of the Guidelines with the prevailing seismic design requirements for new buildings in the
building location to determine whether requirements for achieving the Guidelines’ “basic safety objective”
are equivalent to or more or less stringent than those expected of new buildings.
It is planned that seismic rehabilitation designs will be developed for approximately 40 buildings selected
insofar as practicable from an inventory of buildings already determined to be seismically deficient under the
implementation program of Executive Order 12941 and considered “typical of existing structures located
throughout the nation.” Where federal buildings from this inventory do not represent the full spectrum of
buildings which need to be studied, case study candidates have been sought from among privately owned
buildings or those owned by other levels of government. Qualified structural engineering or
architectural/engineering (A/E) firms will be engaged to produce detailed designs for seismic rehabilitation of
the lateral-load-resisting systems, foundations, and critical nonstructural elements of the selected buildings,
and to make specified comparisons with current practices and costs. Each design contractor’s products and
experiences using the Guidelines will be assessed in order to generate credible data that will establish the
technical validity of the Guidelines, define their economic impact, and identify any needed changes in the
Guidelines or highlight areas in need of research and investigation before a Guidelines update is planned.
Many parameters and possible combinations thereof will be considered in addition to basic building types and
seismic deficiencies.
The case studies will include consideration of numerous design approaches, options, and determinations to
give a balanced representation, within the resources available, of the following factors: different performance
levels and ranges, both systematic (linear/nonlinear, static/dynamic) and simplified analysis methods as
presented in the Guidelines, alternate designs and cost comparisons for the same building provided by more
than one design firm, different structural systems, varying seismicity (high, medium, and low), short and stiff
versus tall and flexible building types, rehabilitation Guidelines compared to current new construction
practices, geographic dispersion of cases among seismic risk areas, presence of auxiliary energy dispersion
systems or base isolation, and historical preservation status of building.
The project is being guided by the Case Studies Project Committee (CSPC). At its organization meeting in
May 1997, the CSPC reviewed the background and structure of the project, developed an initial work
plan/project schedule, and defined the roles of the various participants. The CSPC also established three
subcommittees to address the development of criteria for building selection, design professional selection,
and contractor requests for proposals. In addition to the architects/engineers who will be engaged to perform
the case studies designs, the project will utilize a paid Project Technical Advisor and a Design Assessment
Panel of professionals knowledgeable about the content and use of the Guidelines.
In July, the CSPC met again to review letters of interest and resumes for the advertised position of the Project
Technical Advisor; initial selection recommendations were developed for action by the BSSC Board and
subsequently resulted in a contract with Andrew T. Merovich of A. T. Merovich and Associates, San
Francisco, California. The subcommittee responsible for development of building selection criteria also
presented a matrix for the selection and matching of available buildings.
353
The case studies project was posted in the Commerce Business Daily and in the Official Proposals section of
Engineering News Record. These postings resulted in receipt of 149 expressions of interest; of these, 133
appeared to be qualified to move into the next stage of the selection process.
The CSPC met again in early December to finalize the list of buildings recommended for study, approve a
draft of the “Request for Qualifications” (RFQ) and contractor selection criteria, and identify individuals to
serve on the Design Assessment Panel.
In December 1997 and January 1998, the qualified design firms were asked to supply additional information
on their detailed qualifications and to identify which of the available case study buildings they wished to
design. Over 120 responses were received.
A CSPC task group met on February 9-11, 1998, to match the candidate firms with the available buildings
based on the experience and interest of the firms and the level of complexity assigned to each building. The
full CSPC met on February 12 to confirm which firms are to be asked for price solicitations, and second and
third choice firms also were identified. Successful firms were notified on February 9. It is anticipated that
bid packages (including statements of work and building data) will be completed by mid-March and furnished
to the first-choice firms shortly thereafter.
The latest project schedule shows the case study designs being accomplished from May through September
1998 with the final project report to be submitted to FEMA by the end of March 1999.
Earlier Projects Focusing on Evaluation and Rehabilitation Techniques
An earlier FEMA-funded project was designed to provide consensus-backed approval of publications on
seismic hazard evaluation and strengthening techniques for existing buildings. This effort involved
identifying and resolving major technical issues in two preliminary documents developed for FEMA by others
– a handbook for seismic evaluation of existing buildings prepared by the Applied Technology Council
(ATC) and a handbook of techniques for rehabilitating existing buildings to resist seismic forces prepared by
URS/John A. Blume and Associates (URS/Blume); revising the documents for balloting by the BSSC membership; balloting the documents in accordance with the BSSC Charter; assessing the ballot results; developing proposals to resolve the issues raised; identifying any unresolvable issues; and preparing copies of the
documents that reflect the results of the balloting and a summary of changes made and unresolved issues.
Basically, this consensus project was directed by the BSSC Board and a 22-member Retrofit of Existing
Buildings (REB) Committee composed of individuals representing the needed disciplines and geographical
areas and possessing special expertise in the seismic rehabilitation of existing buildings. The consensus
approved documents (the NEHRP Handbook for the Seismic Evaluation of Existing Buildings and the
NEHRP Handbook of Techniques for the Seismic Rehabilitation of Existing Buildings) were transmitted to
FEMA in mid-1992.
The BSSC also was involved in an even earlier project with the ATC and the Earthquake Engineering
Research Institute to develop an action plan for reducing earthquake hazards to existing buildings. The
action plan that resulted from this effort prompted FEMA to fund a number of projects, including those
described above.
Assessment of the San Francisco Opera House
In October 1994, the NIBS-BSSC initiated an effort to provide FEMA with objective expert advice
concerning the San Francisco War Memorial Opera House. The Opera House, constructed circa 1920 with a
steel frame clad and infilled with masonry, was damaged in the Loma Prieta earthquake and the city of San
Francisco subsequently petitioned FEMA for supplemental funding of approximately $33 million to cover the
costs of a complete seismic upgrade of the building under the Stafford Act, which provides funding for work
when local building code upgrade requirements are met. In this case, the San Francisco Building Code was
the local code in effect. The effort was structured to involve three phases, if warranted, and was to be
354
conducted by a three-member Independent Review Panel of experts knowledgeable and experienced in
building codes and building code administration.
During Phase I, the Review Panel conducted an unbiased, expert review of the applicable code sections
pertinent to the repair of earthquake damage in order to provide FEMA with a definitive interpretation of
such terms as “how much” change/repair of “what nature” would be sufficient to require complete seismic
upgrading of a building of the same general type and construction as the Opera House. It reviewed all
relevant, immediately available information about the Opera House case provided by FEMA and the city and
the relevant portions of the San Francisco Building Code and other similar building codes pertinent to the
repair of earthquake-caused damage to buildings and prepared and delivered to FEMA in February 1995 a
preliminary report of its findings.
At this point, the Panel was informed by FEMA that the city of San Francisco had rescinded its request
indicating that the “proposed determination on eligibility for funding through review and recommendation by
an independent and impartial review body from NIBS” would not be necessary. Later, however, FEMA
asked that NIBS-BSSC complete Phase I so that it would be better prepared should other similar situations
arise. Thus, the Panel continued and delivered a final report to FEMA in July 1995.
IMPROVING THE SEISMIC SAFETY OF NEW AND EXISTING LIFELINES
Given the fact that buildings continue to be useful in a seismic emergency only if the services on which they
depend continue to function, the BSSC developed an action plan for the abatement of seismic hazards to lifelines to provide FEMA and other government agencies and private sector organizations with a basis for their
long-range planning. The action plan was developed through a consensus process utilizing the special talents
of individuals and organizations involved in the planning, design, construction, operation, and regulation of
lifeline facilities and systems.
Five lifeline categories were considered: water and sewer facilities, transportation facilities, communication
facilities, electric power facilities, and gas and liquid fuel lines. A workshop involving more than 65
participants and the preparation of over 40 issue papers was held. Each lifeline category was addressed by a
separate panel and overview groups focused on political, economic, social, legal, regulatory, and seismic risk
issues. An Action Plan Committee composed of the chairman of each workshop panel and overview group
was appointed to draft the final action plan for review and comment by all workshop participants. The
project reports, including the action plan and a definitive six-volume set of workshop proceedings, were
transmitted to FEMA in May 1987.
In recognition of both the complexity and importance of lifelines and their susceptibility to disruption as a
result of earthquakes and other natural hazards (hurricanes, tornadoes, flooding), FEMA subsequently concluded that the lifeline problem could best be approached through a nationally coordinated and structured program aimed at abating the risk to lifelines from earthquakes as well as other natural hazards. Thus, in 1988,
FEMA asked the BSSC's parent institution, the National Institute of Buildings Sciences, to provide expert
recommendations concerning appropriate and effective strategies and approaches to use in implementing such
a program.
The effort, conducted for NIBS by an ad hoc Panel on Lifelines with the assistance of the BSSC, resulted in a
report recommending that the federal government, working through FEMA, structure a nationally coordinated, comprehensive program for mitigating the risk to lifelines from seismic and other natural hazards that
focuses on awareness and education, vulnerability assessment, design criteria and standards, regulatory
policy, and continuing guidance. Identified were a number of specific actions to be taken during the next
three to six years to initiate the program.
355
MULTIHAZARD ACTIVITIES
Multihazard Assessment Forum
In 1993, FEMA contracted with NIBS for the BSSC to organize and hold a forum intended to explore how
best to formulate an integrated approach to mitigating the effects of various natural hazards under the National Earthquake Hazards Reduction Program. More than 50 experts in various disciplines concerning natural
hazards risk abatement participated in the June 1994 forum and articulated the benefits of pursuing an integrated approach to natural hazards risk abatement. A BSSC steering committee then developed a report, An
Integrated Approach to Natural Hazards Risk Mitigation, based on the forum presentations and discussion
that urged FEMA to initiate an effort to create a National Multihazard Mitigation Council structured and
charged to integrate and coordinate public and private efforts to mitigate the risk from natural hazards. This
report was delivered to FEMA in early 1995.
Multihazard Council Program Definition and Initiation
In September 1995, the BSSC negotiated with FEMA a modification of an existing contract to provide for
conduct of the first phase of a longer term effort devoted to stimulating the application of technology and
experience data in mitigating the risks to buildings posed by multiple natural hazards and development of
natural hazard risk mitigation measures and provisions that are national in scope for use by those involved in
the planning, design, construction, regulation, and utilization of the built environment. During this first
phase, the BSSC is conducting a program definition and initiation effort expected to culminate in the
establishment of a National Multihazard Mitigation Council (NMMC) to integrate and coordinate public and
private efforts to mitigate the risks associated with natural hazards as recommended in the report cited above.
To conduct the project, the BSSC established a 12-member "blue ribbon" Multihazard Project Steering
Committee (MPSC) composed of well-respected leaders in the natural hazards risk mitigation community.
The MPSC, which met in July and December 1996 and February 1997, to developed an organizational
structure for the proposed council, a draft charter, a draft mission statement, and a preliminary outline for a
work plan. Due consideration has been given to the fact that the proposed council will need to maximize the
use of resources through mitigation of risks utilizing common measures; promote cost-effective loss
reduction, effective technology transfer, conflict identification, and coordination of performance objectives;
improve efficiency in the development of codes and standards; provide an open forum for articulation of
different needs and perspectives; facilitate policy adoption and implementation; fill educational and public
awareness needs; and provide a single credible source for recommendations and directions. In addition, the
MPSC is responsible for formulating and directing implementation of a strategy for effectively stimulating
the level of interest and degree of cooperation among the various constituencies needed to establish the
proposed council.
One of the major project milestones was the organization and conduct of a September 8-10 forum to review
the proposed charter, mission statement, and five-year plan. Almost 80 individuals attended. Following
background presentations and status reports on current mitigation-related activities, the forum was devoted
primarily to presentation and discussion of the preliminary goals and objectives of the proposed council; the
proposed NMMC Charter, home/organization, and membership; proposed activities to be included in the
five-year plan for the NMMC; and the Steering Committee’s candidates for the initial NMMC board. In
essence, the forum participants gave consensus approval to the proposed goals, objectives, charter, and
membership of the Council and accepted NIBS as the most likely candidate to serve as the home organization
of the NMMC.
At its November 1997 meeting, the NIBS Board of Directors reviewed the goals/objectives and activities
statements and charter for the NMMC as discussed at the forum. They accepted the charter with some
356
changes. The new council, to be called the Multihazard Mitigation Council (MMC), will now be a sister
council to the BSSC and other NIBS councils.
EMI Multihazard Building Design Summer Institute
In 1994, NIBS, at the request of FEMA's Emergency Management Institute (EMI), entered into a contract for
BSSC to provide support for the of the EMI Multihazard Building Design Summer Institute (MBDSI) for
university and college professors of engineering and architecture. The 1995 MBDSI, conducted in July 1995,
consisted of four one-week courses structured to encourage widespread use of mitigation techniques in
designing/rehabilitating structures to withstand forces generated by both natural and technological hazards by
providing the attending academics with instructional tools for use in creating/updating building design
courses.
357
BSSC MEMBER ORGANIZATIONS
AFL-CIO Building and Construction Trades
Department
AISC Marketing, Inc.
American Concrete Institute
American Consulting Engineers Council
American Forest and Paper Association
American Institute of Architects
American Institute of Steel Construction
American Insurance Services Group, Inc.
American Iron and Steel Institute
American Plywood Association
American Society of Civil Engineers
American Society of Civil Engineers--Kansas City
Chapter
American Society of Heating, Refrigeration, and AirConditioning Engineers
American Society of Mechanical Engineers
American Welding Society
Applied Technology Council
Associated General Contractors of America
Association of Engineering Geologists
Association of Major City Building Officials
Bay Area Structural, Inc.*
Brick Institute of America
Building Officials and Code Administrators
International
Building Owners and Managers Association
International
Building Technology, Incorporated*
California Geotechnical Engineers Association
California Division of the State Architect, Office
of Regulation Services
Canadian National Committee on Earthquake
Engineering
Concrete Masonry Association of California and
Nevada
Concrete Reinforcing Steel Institute
Earthquake Engineering Research Institute
General Reinsurance Corporation*
Hawaii State Earthquake Advisory Board
Insulating Concrete Form Association
Institute for Business and Home Safety
Interagency Committee on Seismic Safety in
Construction
International Conference of Building Officials
*
International Masonry Institute
Masonry Institute of America
Metal Building Manufacturers Association
National Association of Home Builders
National Concrete Masonry Association
National Conference of States on Building Codes
and Standards
National Council of Structural Engineers
Associations
National Elevator Industry, Inc.
National Fire Sprinkler Association
National Institute of Building Sciences
National Ready Mixed Concrete Association
Permanent Commission for Structural Safety of
Buildings*
Portland Cement Association
Precast/Prestressed Concrete Institute
Rack Manufacturers Institute
Seismic Safety Commission (California)
Southern Building Code Congress International
Southern California Gas Company*
Steel Deck Institute, Inc.
Steel Joist Institute*
Steven Winter Associates, Inc.*
Structural Engineers Association of Arizona
Structural Engineers Association of California
Structural Engineers Association of Central
California
Structural Engineers Association of Colorado
Structural Engineers Association of Illinois
Structural Engineers Association of Northern
California
Structural Engineers Association of Oregon
Structural Engineers Association of San Diego
Structural Engineers Association of Southern
California
Structural Engineers Association of Utah
Structural Engineers Association of Washington
The Masonry Society
U. S. Postal Service*
Western States Clay Products Association
Western States Council Structural Engineers
Association
Westinghouse Electric Corporation*
Wire Reinforcement Institute, Inc.
Affiliate (non-voting) members.
(January 1998)
359
BUILDING SEISMIC SAFETY COUNCIL
PUBLICATIONS
Available free from the Federal Emergency Management Agency at 1-800-480-2520
(order by FEMA Publication Number)
For detailed information about the BSSC and its projects, contact:
BSSC, 1090 Vermont Avenue, N.W., Suite 700, Washington, D.C. 20005
Phone 202-289-7800; Fax 202-289-1092; e-mail [email protected]ORG
NEW BUILDINGS PUBLICATIONS
The NEHRP (National Earthquake Hazards Reduction Program) Recommended Provisions for Seismic
Regulations for New Buildings, 1997 Edition, 2 volumes and maps (FEMA Publication 302 and
303)—printed copies expected to be available in early 1998.
The NEHRP (National Earthquake Hazards Reduction Program) Recommended Provisions for Seismic
Regulations for New Buildings, 1994 Edition, 2 volumes and maps (FEMA Publications 222A and 223A).
The NEHRP (National Earthquake Hazards Reduction Program) Recommended Provisions for the
Development of Seismic Regulations for New Buildings, 1991 Edition, 2 volumes and maps (FEMA
Publications 222 and 223) — limited to existing supply.
Guide to Application of the 1991 Edition of the NEHRP Recommended Provisions in Earthquake Resistant
Building Design, Revised Edition, 1995 (FEMA Publication 140)
A Nontechnical Explanation of the NEHRP Recommended Provisions, Revised Edition, 1995 (FEMA
Publication 99)
Seismic Considerations for Communities at Risk, Revised Edition, 1995 (FEMA Publication 83)
Seismic Considerations: Apartment Buildings, Revised Edition, 1996 (FEMA Publication 152)
Seismic Considerations: Elementary and Secondary Schools, Revised Edition, 1990 (FEMA Publication
149)
Seismic Considerations: Health Care Facilities, Revised Edition, 1990 (FEMA Publication 150)
Seismic Considerations: Hotels and Motels, Revised Edition, 1990 (FEMA Publication 151)
Seismic Considerations: Office Buildings, Revised Edition, 1996 (FEMA Publication 153)
Societal Implications: Selected Readings, 1985 (FEMA Publications 84)
EXISTING BUILDINGS PUBLICATIONS
NEHRP Guidelines for the Seismic Rehabilitation of Buildings, 1997 (FEMA Publication 273)
NEHRP Guidelines for the Seismic Rehabilitation of Buildings: Commentary, 1997 (FEMA Publication
274)
Planning for Seismic Rehabilitation: Societal Issues, 1998
Example Applications of the NEHRP Guidelines for the Seismic Rehabilitation of Buildings, to be available
in mid-1998 (FEMA Publication 276)
361
NEHRP Handbook of Techniques for the Seismic Rehabilitation of Existing Buildings, 1992 (FEMA
Publication 172)
NEHRP Handbook for the Seismic Evaluation of Existing Buildings, 1992 (FEMA Publication 178)
An Action Plan for Reducing Earthquake Hazards of Existing Buildings, 1985 (FEMA Publication 90)
MULTIHAZARD PUBLICATIONS
An Integrated Approach to Natural Hazard Risk Mitigation, 1995 (FEMA Publication 261/2-95)
LIFELINES PUBLICATIONS
Abatement of Seismic Hazards to Lifelines: An Action Plan, 1987 (FEMA Publication 142)
Abatement of Seismic Hazards to Lifelines: Proceedings of a Workshop on Development of An Action
Plan, 6 volumes:
Papers on Water and Sewer Lifelines, 1987 (FEMA Publication 135)
Papers on Transportation Lifelines, 1987 (FEMA Publication 136)
Papers on Communication Lifelines, 1987 (FEMA Publication 137)
Papers on Power Lifelines, 1987 (FEMA Publication 138)
Papers on Gas and Liquid Fuel Lifelines, 1987 (FEMA Publication 139)
Papers on Political, Economic, Social, Legal, and Regulatory Issues and General Workshop Presentations, 1987 (FEMA Publication 143)
362
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