15.1 MB
of the National Institute of Building Sciences
Homebuilders’ Guide to
Earthquake-Resistant
Design and Construction
FEMA 232 ─ June 2006
Prepared by the
Building Seismic Safety Council
for the
Federal Emergency Management Agency
of the Department of Homeland Security
National Institute of Building Sciences Washington, D.C. 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.
The opinions expressed herein regarding the requirements of the International Residential Code do not
necessarily reflect the official opinion of the International Code Council. The building official in a
jurisdiction has the authority to render interpretation of the code.
This report was prepared under Contract EMW-1998-CO-0419 between the Federal Emergency
Management Agency and the National Institute of Building Sciences.
For further information on the Building Seismic Safety Council, see the Council’s website -www.bssconline.org -- or 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]
In Memoriam
Edwin G. Zacher 1920-2005
The Federal Emergency Management Agency and
the Building Seismic Safety Council dedicate this guide to
Edwin G. Zacher in recognition of his significant contributions
to the profession of structural engineering and the nation’s
codes and standards development efforts. The residential built
environment and all those who work to create it and use it have
benefited tremendously from his work.
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PREFACE
The Federal Emergency Management Agency (FEMA), which is part of the Department of
Homeland Security, works to reduce the ever-increasing cost that disasters inflict on the nation.
Preventing losses before they occur by designing and constructing buildings and their
components to withstand anticipated forces from various hazards is one of the key components
of mitigation and is one of the most effective ways of reducing the cost of future disasters.
The National Earthquake Hazards Reduction Program (NEHRP) is the federal program
established to address the nation's earthquake threat. NEHRP seeks to resolve two basic issues:
how will earthquakes affect us and how do we best apply our resources to reduce their impact on
our nation. The program was established by Congress under the Earthquake Hazards Reduction
Act of 1977 (Public Law 95-124) and was the result of years of examination of the earthquake
hazard and possible mitigation measures. Under the NEHRP, FEMA is responsible for
supporting program implementation activities, including the development, publication, and
dissemination of technical design and construction guidance documents.
Generally, there has not been much technical guidance addressing residential buildings unless
they are located in areas of high seismicity or exceed a certain size or height. This is because
most residential buildings were thought to perform fairly well in earthquakes due to their low
mass and simple construction. While buildings may not normally experience catastrophic
collapse, they can still suffer significant amounts of damage, rendering them uninhabitable. This
is especially true when construction techniques are less than adequate. What is particularly
important from FEMA's point of view is that, given the sheer number of this type of building,
even minor damage represents a significant loss potential and temporary housing demand that
will need to be addressed after an earthquake by all levels of government.
After the San Fernando earthquake in 1971, a study of residential buildings and the damage they
suffered was conducted by a team of experts under funding from the Department of Housing and
Urban Development (HUD) and the National Science Foundation (NSF). HUD utilized these
data to develop a non-engineering document entitled Home Builder's Guide to Earthquake
Design. This manual, originally published in 1980, provided easy-to-follow information to the
average homebuilder on steps for reducing potential earthquake damage. In July 1992 it was
reprinted as a joint FEMA-HUD document, also known as FEMA 232. The manual was
subsequently updated for FEMA and the revised publication, Home Builders Guide to Seismic
Resistant Construction (FEMA 232), was published in August 1998.
Since that time, there have been several significant changes that needed to be incorporated into
this document to keep it current. The first and most important change was the completion of the
FEMA-funded Consortium of Universities for Research in Earthquake Engineering (CUREE)Caltech Woodframe Project. This project was funded using FEMA Hazard Mitigation Grant
Program funds available after the Northridge earthquake and was designed to address the
unexpected amount of damage suffered by wood frame residential structures. Similar to the
successful FEMA/SAC Steel Moment Frame Buildings Project, this effort combined academic
research and testing of wood frame buildings and components with the development of
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engineering-based design guidance for future construction. The project yielded some interesting
findings that needed to be captured in a guidance document.
A second change was the development and publication of the 2000 International Residential
Code (IRC) by the International Code Council. This model residential building code replaced
the Council of American Building Officials (CABO) One- and Two-Family Dwelling Code,
which did not adequately address earthquake loads. The IRC reflects on the NEHRP
Recommended Provisions and is intended to adequately address the earthquake hazard.
This publication presents seismic design and construction guidance for one- and two-family
houses in a manner that can be utilized by homebuilders, knowledgeable homeowners, and other
non-engineers. It incorporates and references the prescriptive provisions of the 2003
International Residential Code as well as the results of the FEMA-funded CUREE-Caltech
Woodframe Project. The manual includes prescriptive building detail plans based on state-of
the-art earthquake-resistant design for use by homebuilders and others in the construction of a
non-engineered residential structure. Further, the manual also uses the results of recent loss
investigations as well as current research and analysis results to identify a number of specific
above-code measures for improved earthquake performance along with their associated costs. A
typical modern house is used to illustrate the application and benefits of above-code measures.
This manual replaces the Home Builders Guide to Seismic Resistant Construction (FEMA 232)
published by FEMA in August 1998 as well as earlier FEMA and HUD versions.
Finally, FEMA wishes to express its deepest gratitude for the significant efforts of primary
authors J. Daniel Dolan, Kelly Cobeen, and James Russell; the members of the Project Team; the
many reviewers and workshop attendees; and the BSSC Board of Direction and staff. Their
dedication and hard work made this document possible.
Department of Homeland Security/Federal Emergency Management Agency
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ACKNOWLEDGEMENTS The Building Seismic Safety Council is grateful to all those involved in developing this updated
guide to earthquake-resistant home design and construction. As chairman of the BSSC Board of
Direction, it is my pleasure to express appreciation to the members of the team developing this
guide:
J. Daniel Dolan, PhD, PE, Professor, Washington State University, Wood Materials and
Engineering Laboratory, Pullman (team leader)
Kelly Cobeen, Structural Engineer, Cobeen and Associates Structural Engineering,
Lafayette, California
James E. Russell, Building Codes Consultant, Concord, California
Special thanks also go to Gerald Jones, retired building official, and Douglas Smits, the chief
building official of Charleston, South Carolina, who served with me and the writers on the
committee overseeing this project.
The BSSC also is grateful to those who participated in a workshop discussion (Appendix F) of an
early draft of this guide and willingly shared their experiences with respect to earthquakeresistant home building. Special thanks go to those individuals who reviewed and commented
on subsequent drafts of this document; their input has made this a much more useful guide.
I also wish to thank the members of the BSSC Board of Direction who have recognized the
importance of this effort and provided sage advice throughout the project. Special thanks also
are due to the BSSC staff who worked untiringly behind the scenes to coordinate the project
resulting in this report.
Finally the BSSC is grateful to Michael Mahoney, FEMA project officer, for his insight and
ongoing support, to the International Code Council for their work in reviewing and disseminating
this guide, and to the National Association of Home Builders for their assistance in
disseminating this guide.
Jim. W. Sealy
Chairman, BSSC Board of Direction
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vi TABLE OF CONTENTS Preface........................................................................................................................................... iii Acknowledgements ........................................................................................................................v Executive Summary ......................................................................................................................1 Chapter 1 Introduction..................................................................................................................3 1.1 Background..........................................................................................................................3 1.2 Above-code Recommendations ...........................................................................................4 1.3 The International Residential Code.....................................................................................5 1.4 IRC Seismic Design Categories...........................................................................................5 1.5 Building Response .............................................................................................................10 1.6 Important Concepts Concerning Homesite........................................................................12 1.7 Model House Used for Guide Analysis .............................................................................14 1.8 Other Hazards ....................................................................................................................17 Chapter 2 Earthquake-Resistance Requirements.....................................................................21 2.1 IRC General Earthquake Limitations.................................................................................21 2.2 Load Path ...........................................................................................................................22 2.3 House Configuration Irregularities ....................................................................................29 Chapter 3 Foundations and Foundation Walls .........................................................................49 3.1 General Foundation Requirements ....................................................................................49 3.2 Concrete Foundations ........................................................................................................53 3.3 Masonry Foundations ........................................................................................................56 3.4 Footing Width....................................................................................................................57 3.5 Special Soil Conditions .....................................................................................................58 3.6 Foundation Resistance to Sliding from Lateral Loads ......................................................59 3.7 Special Considerations for Cut and Fill Sites ....................................................................61 3.8 Foundation Walls...............................................................................................................62 3.9 Foundation Wall Thickness, Height, and Required Reinforcing.......................................63 3.10 Wood-Framed Wall Bottom Plate and Foundation Sill Plate Anchorage .......................64 3.11 Required Location for Anchor Bolts along Exterior Walls .............................................65 3.12 Required Anchorage along Interior Braced Walls...........................................................65 3.13 Anchoring of Interior Braced Walls in SDCs D1 and D2.................................................67 Chapter 4 Floor Construction.....................................................................................................69 4.1 General Floor Construction Requirements ........................................................................69 4.2 Wood-Framed Floor Systems ............................................................................................70 4.3 Cantilevered Floors............................................................................................................71 4.4 Requirements for Blocking................................................................................................73 4.5 Connection of Floor Joists to Wall Top Plate or Foundation Sill Plate Below .................75 4.6 Floor Sheathing..................................................................................................................77 vii
4.7 Lateral Capacity Issues for Wood Framed Floors Using Wood Structural Panels............77 4.8 Concrete Slab-on-Grade Floors .........................................................................................81 Chapter 5 Walls............................................................................................................................83
5.1 Wood Light-Frame Construction.......................................................................................83 5.2 Stone and Masonry Veneer..............................................................................................103 5.3 Cold-formed Steel Houses...............................................................................................107
5.4 Masonry Wall Houses .....................................................................................................110 5.5 Insulating Concrete Form (ICF) Wall Houses.................................................................115 Chapter 6 Roof-Ceiling Systems...............................................................................................121
6.1 General Roof-Ceiling Requirements ...............................................................................121 6.2 Special Framing Considerations ......................................................................................122 6.3 Blocking and Lateral Load Paths for Roof Systems........................................................124 6.4 Connection of Ceiling Joists and Rafters to Walls Below...............................................126 6.5 Roof Sheathing ................................................................................................................126 6.6 Lateral Capacity Issues for Wood Framed Roofs............................................................128 6.7 Quality Control ................................................................................................................129 Chapter 7 Chimneys, Fireplaces, Balconies, and Decks.........................................................131
7.1 Chimneys and Fireplaces.................................................................................................131 7.2 Balconies and Decks........................................................................................................137 Chapter 8 Anchorage of Home Contents.................................................................................141
8.1 General.............................................................................................................................141 8.2 Water Heater Anchorage .................................................................................................141
8.3 Securing Other Items .......................................................................................................144 Chapter 9 Existing Houses ........................................................................................................151
9.1 Additions and Alterations ................................................................................................151 9.2 Earthquake Upgrade Measures ........................................................................................154 APPENDICES
Appendix A Analysis of Model House Used in This Guide....................................................167 A1 Model House....................................................................................................................167 A2 Analysis Using Standard Engineering Design Methods..................................................171 A3 Analysis Using Nonlinear Methods.................................................................................171 A4 Analysis Results...............................................................................................................174 Appendix B Earthquake Provisions Checklist for Builders and Designers .........................179 Appendix C Earthquake Provisions Checklist for Designers and Plan Checkers...............187 Appendix D Significant Changes for the 2006 International Residential Code ....................193 viii
D1 Revised Seismic Design Maps.........................................................................................193 D2 Addition of Seismic Design Category D0 ........................................................................193 D3 Change in Applicability of Irregular Building Requirements .........................................199 D4 Clarification and Addition of Requirement for Masonry Veneer....................................199 Appendix E References and Additional Resources.................................................................201 Appendix F Homebuilders’ Guide Project Participants ........................................................207 LIST OF FIGURES
Figure 1-1 Seismic Design Categories......................................................................................... 6-9 Figure 1-2 Forces induced in a house due to earthquake ground motion ......................................10 Figure 1-3 Concept of actual vs. code seismic forces....................................................................11 Figure 1-4 Concept of ductility......................................................................................................11 Figure 1-5 Definition of drift .........................................................................................................12 Figure 1-6 A house damaged due to fault movement ....................................................................13 Figure 1-7 Houses damaged due to a landslide at the site .............................................................13 Figure 1-8 Elevation of model house used as an example for this guide ......................................14 Figure 1-9 Model house floor plan ................................................................................................15 Figure 2-1 Chain illustrating load path concept.............................................................................22 Figure 2-2 Lateral loads induced in a building due to wind or earthquakes..................................23 Figure 2-3 Loading and deflection of roof, ceiling, and floor systems..........................................24 Figure 2-4 Loading and deflection of bracing wall systems..........................................................25 Figure 2-5 Load transfer between components in a building ........................................................26 Figure 2-6 Horizontal load path connections and deformations....................................................27 Figure 2-7 Overturning load path connections and deformations .................................................28 Figure 2-8 Depiction of ideal building configuration for earthquake resistance ...........................30 Figure 2-9 Irregular building configuration with open front .........................................................31 Figure 2-10 Rotational response and resistance to torsion ............................................................31 Figure 2-11 Concentration of loads and possible damage and failure due to irregular plan .........32 Figure 2-12 Roof-level damage to building with a T-shaped plan ................................................33 Figure 2-13 Deformation pattern due to soft-story behavior .........................................................34 Figure 2-14 Change in deformation patterns associated with soft-story irregularity ....................34 Figure 2-15 House experiencing soft- and weak-story behavior ..................................................35 Figure 2-16 Apartment building experiencing soft- and weak-story behavior..............................35 Figure 2-17 Detailing required for stepped foundation walls........................................................36 Figure 3-1 Perimeter foundation with separately placed footing and stem wall ...........................49 Figure 3-2 Sliding action resisted by foundation...........................................................................50 Figure 3-3 Overturning action resisted by foundation...................................................................51 Figure 3-4 Interior braced wall and floor framing .........................................................................52 Figure 3-5 Foundation requirements for interior braced wall line on slab-on-grade construction52 Figure 3-6 Recommended minimum reinforcement for concrete footings and stem walls...........54 ix
Figure 3-7 Above-code horizontal reinforcing lap at corners and intersections............................54 Figure 3-8 Above-code use of vertical dowels to connect a slab-on-grade to a separately poured footing ............................................................................................................................................55 Figure 3-9 Inverted-T footing dimensions.....................................................................................58
Figure 3-10 Lateral resistance provided by foundation .................................................................59 Figure 3-11 Above-code stepped foundation reinforcing detail ....................................................60 Figure 3-12 Foundation supported on rock and fill .......................................................................61 Figure 3-13 Example of damage caused in building on cut and fill site........................................62 Figure 3-14 Interior braced wall on slab-on-grade ........................................................................66 Figure 4-1 An unblocked floor diaphragm ....................................................................................69 Figure 4-2 Slab-on-grade and perimeter footing transfer loads into soil.......................................70 Figure 4-3 Cantilevered floor restrictions......................................................................................71 Figure 4-4 Cantilevered joist at braced wall above .......................................................................73 Figure 4-5 Interior bearing line......................................................................................................74 Figure 4-6 Blocking below interior braced wall ............................................................................74 Figure 4-7 Blocking floor joists spaced apart for piping in floor ..................................................75 Figure 4-8 Toe nail configuration requirements ............................................................................76 Figure 4-9 Blocked diaphragm configuration................................................................................79 Figure 4-10 Diaphragm loads on long and short sides ..................................................................79 Figure 4-11 Reinforcing straps at large diaphragm openings........................................................80 Figure 5-1 Sliding, overturning, and racking action resisted by walls and foundation .................84 Figure 5-2 Exploded view illustrating load paths ..........................................................................85 Figure 5-3 Wall action for resisting lateral loads...........................................................................86
Figure 5-4 Exploded view of typical residential wall segment......................................................86 Figure 5-5 Detailing differences for three options when using wood structural panel sheathed walls ..............................................................................................................................................87 Figure 5-6a Plan view of crawl space for model house in SDC C ................................................92 Figure 5-6b Plan view of first floor for model house in SDC C....................................................93 Figure 5-6c Plan view of second floor for model house in SDC C ...............................................94 Figure 5-7a Plan view of crawl space for model house in SDC D2 ..............................................95 Figure 5-7b Plan view of first floor for model house in SDC D2 ..................................................96 Figure 5-7c Plan view of second floor for model house in SDC D2 .............................................97 Figure 5-8 Properly and overdriven nails ......................................................................................98 Figure 5-9 Illustration of concept of equal numbers of fasteners in line for symmetric nailing schedule........................................................................................................................................100 Figure 5-10 Sheathing detail for extending the sheathing over the band joist.............................102 Figure 5-11 Applications of masonry and stone veneer ..............................................................103 Figure 5-12 Stone veneer damage during the earthquake in Northridge, California...................104 Figure 5-13 Cold-formed steel house under construction............................................................107 Figure 5-14 House constructed with masonry walls....................................................................111 Figure 5-15 Symmetric layout of walls to distribute loads uniformly and thereby prevent torsion ..........................................................................................................................................114 Figure 5-16 Insulated concrete form house under construction...................................................116 Figure 5-17 Illustration of cross-grain bending of wood ledger ..................................................118 x
Figure 6-1 Typical light-frame roof-ceiling system.....................................................................121 Figure 6-2 Ridge board for 12:12 pitch roof................................................................................123 Figure 6-3 Gable end wall or gable truss bracing ........................................................................124 Figure 6-4 Blocking at rafters to exterior wall.............................................................................125
Figure 6-5 Typical roof diaphragm sheathing nailing when wood structural panels are used ....127 Figure 7-1 Chimney damage........................................................................................................131 Figure 7-2 Chimney damage in Northridge earthquake ..............................................................132 Figure 7-3 Locations for earthquake anchorage of masonry chimney at exterior house wall.....133 Figure 7-4 Chimney section showing earthquake anchorage .....................................................134 Figure 7-5 Anchorage detail for framing parallel to exterior wall...............................................134 Figure 7-6 Anchorage detail for framing perpendicular to exterior wall.....................................135 Figure 7-7 Factory-built flue and light-frame enclosure..............................................................136 Figure 7-8 Collapsed factory-built chimney, light-frame enclosure, and deck ...........................136 Figure 7-9 Decks and balconies in residential contruction..........................................................138 Figure 7-10 Hold-down device providing positive connection of deck framing to house framing. .......139 Figures 8-1a & 8-1b Securing a water heater with wall bracing .................................................142 Figures 8-2a & 8-2b Securing a water heater with corner bracing ..............................................143 Figures 8-3 a-f Reproductions of pages from FEMA 74 on securing household goods...... 145-150 Figure 9-1 Alterations to existing house......................................................................................152 Figure 9-2 Horizontal addition and vertical addition...................................................................153 Figure 9-3 Cape Mendocino earthquake damage to cripple wall house with slab-on-grade additions.......................................................................................................................................154 Figure 9-4 Common anchorage configurations ...........................................................................156 Figure 9-5a House with collapsed cripple walls ..........................................................................157 Figure 9-5b A house with severe damage due to cripple wall collapse.......................................157 Figure 9-6 Cripple wall bracing...................................................................................................158 Figure 9-7 Weak and soft story bracing.......................................................................................159 Figure 9-8 Common open-front occurrences in one- and two-family detached houses ..............160 Figure 9-9 Detailing of narrow bracing wall piers at open fronts................................................161 Figure 9-10 House located on a hillside site damaged during Northridge, California, earthquake ....................................................................................................................................162 Figure 9-11 Anchorage of floor framing to the uphill foundation...............................................163 Figure 9-12 Split-level ties for floor framing ..............................................................................164
Figure 9-13 Anchorage of concrete or masonry walls to floor, roof, or ceiling framing ............165 Figure A1 Slab-on-grade base .....................................................................................................168 Figure A2 Level cripple wall base ...............................................................................................168 Figure A3 Hillside base ...............................................................................................................168 Figure A4 Basement ....................................................................................................................168 Figure A5 Analysis model ...........................................................................................................172 Figure A6 Hyseteric parameters for model..................................................................................172 xi
Figure D1 Seismic Design Categories, Site Class D ........................................................... 194-198 LIST OF TABLES
Table 2-1 Load Path Connections for Horizontal Sliding ...................................................... 38-40 Table 2-2 Load Path Connections for Overturning................................................................. 41-43 Table 2-3 IRC House Configuration Irregularities ................................................................. 44-47 Table 3-1 Summary of 2003 IRC Continuous Foundation and Anchor Bolt Requirements for Braced Wall Lines in One- and Two-family Houses ................................68 Table 3-2 Summary of 2003 IRC Continuous Foundation and Anchor Bolt Requirements for Bearing Walls in One- and Two-family Houses .......................................68 Table 5-1 Braced Wall Panel Construction Methods (Materials) Recognized by the IRC .........88 Table 5-2 IRC Sheathing Requirements for Seismic Design Categories C, D1, and D2 ..............90 Table A-1 Example Wall Bracing per 2003 IRC, Slab-on-grade Base Condition......................170 Table A-2 Hysteretic Parameters Used for Nonlinear Dynamic Model .....................................173 Table A-3 Selected Results for IRC Bracing Provisions, Slab-on-grade Base Condition..........175 Table A-4 Selected Results for Above-Code Measures, Slab-on-grade Base Condition ...........176 xii
EXECUTIVE SUMMARY This guide provides information on current best practices for earthquake-resistant house
design and construction for use by builders, designers, code enforcement personnel, and
potential homeowners. It incorporates lessons learned from the 1989 Loma Prieta and
1994 Northridge earthquakes as well as knowledge gained from the FEMA-funded
CUREE-Caltech Woodframe Project. It also introduces and explains the effects of
earthquake loads on one- and two-family detached houses and identifies the requirements
of the 2003 International Residential Code (IRC) intended to resist these loads. The
stated purpose of the IRC is to provide:
. . . minimum requirements to safeguard the public safety, health, and general welfare,
through affordability, structural strength, means of egress facilities, stability, sanitation,
light and ventilation, energy conservation and safety to life and property from fire and
other hazards attributed to the built environment.
Because the building code requirements are minimums, a house and its contents still may
be damaged in an earthquake even if it was designed and built to comply with the code.
Research has shown, however, that earthquake damage to a house can be reduced for a
relatively small increase in construction cost. This guide identifies above-code
techniques for improving earthquake performance and presents an estimate of their cost.
Note that the information presented in this guide is not intended to replace the IRC or any
applicable state or local building code, and the reader is urged to consult with the local
building department before applying any of the guidance presented in this document.
The information presented in this guide applies only to one- and two-family detached
houses constructed using the nonengineered prescriptive construction provisions of the
IRC. Applicable IRC limits on building configuration and construction are described.
A typical model house is used to illustrate the concepts discussed and to identify
approximate deflections under earthquake loading, which permits performance to be
compared for various building configurations using the minimum code requirements and
the above-code techniques. The above-code recommendations are based on an analysis
of the model house as well as comparative tests performed by various researchers and the
lessons learned from investigation of residential building performance in past
earthquakes. A nonlinear time-history analysis was performed for the model building
using the SAWS computer program developed as part of the CUREE-Caltech
Woodframe Project (Folz and Filiatrault, 2002). Details of the analysis are presented in
Appendix A.
Additional appendices feature checklists for builders, designers, and plan checkers;
explain significant differences between the 2003 and 2006 editions of the IRC; and
present a list of reference materials.
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2
Chapter 1 INTRODUCTION 1.1 BACKGROUND
After the 1971 earthquake in San Fernando, California, a study of residential buildings and the
types of damage they suffered was conducted by a team of experts with funding from the
Department of Housing and Urban Development (HUD) and the National Science Foundation
(NSF). Subsequently, HUD utilized the results of that study to develop a nonengineering
guidance document entitled Home Builder's Guide to Earthquake Design. This manual was
originally published in June 1980 to provide homebuilders with easy-to-follow guidance for
reducing potential earthquake damage.
In July 1992, the Federal Emergency Management Agency (FEMA) reissued the HUD manual
with some updated material as a joint FEMA/HUD publication identified as FEMA 232. By the
mid-1990s, it was apparent that this publication was in need of updating, especially to take into
account some of the early findings from the 1994 Northridge earthquake. This update was
prepared for FEMA by SOHA Engineers and was published by FEMA in August 1998 as the
Home Builders Guide to Seismic Resistant Construction, again as FEMA 232.
Since that time, significant events have occurred warranting another updating of the guide. First
was completion of the FEMA-funded CUREE-Caltech Woodframe Project. This project, funded
under the FEMA Hazard Mitigation Grant Program, addressed the unexpected damage suffered
by woodframe residential structures during the Northridge earthquake. Project testing of
complete woodframe buildings and individual components that resist or transmit earthquake
loads yielded some interesting findings that needed to be captured in a guidance document.
Another significant event occurred in 1994 when the International Code Council was established
to develop a single set of comprehensive and coordinated national model construction codes.
Prior to this time, the three organizations that founded the ICC – the Building Officials and Code
Administrators International, Inc. (BOCAI), the International Conference of Building Officials
(ICBO), and Southern Building Code Congress International, Inc. (SBCCI) – each published a
set of model building codes that generally were used in distinct regions of the nation. The initial
editions of the ICC’s International Building Code (IBC) and International Residential Code
(IRC) were published in 2000 and updates were issued in 2003 and 2006. The IBC replaced
BOCAI’s National Building Code, ICBO’s Uniform Building Code, and SBCCI’s Standard
Building Code. For prescriptive residential construction, the IRC replaced the One- and TwoFamily Dwelling Code of the Council of American Building Officials (CABO). (Note that the
IBC contains prescriptive and engineering provisions for light-frame wood construction. In
certain cases, the IBC prescriptive provisions are different from those in the IRC. The National
Fire Protection Association’s NFPA 5000 Building Construction and Safety Code does not
contain provisions for prescriptive residential construction.)
3
FEMA 232, Homebuilders’ Guide
In order to address these and other changes, FEMA initiated a complete rewrite of the 1998
document; this guide, which retains the FEMA 232 designation, is the result. One- and twofamily detached houses of light-frame wood construction are addressed; however, the discussion
is relevant to other materials of construction likely to be used for houses including light-frame
cold-formed steel and insulated concrete form. Explained in this guide are:
•
•
•
•
The basic principles of earthquake-resistant design,
The specific prescriptive seismic provisions of the 2003 International Residential Code,
The results of recent research and analysis, and
Measures exceeding code requirements that are expected to reduce the amount of
damage from an earthquake (see Section 1.2 below).
The guide also includes limited guidance on applying the principles of earthquake resistance to
house additions and alterations and on anchoring typical house furnishings and equipment such
as hot water heaters. Appendices describe the analyses performed in developing this guide,
present checklists for builders, designers, and plan checkers; explain significant differences
between the 2003 and 2006 editions of the IRC; and identify reference materials and participants
in the project resulting in this guide.
1.2 ABOVE-CODE RECOMMENDATIONS
The above-code recommendations included in this guide describe details that, when incorporated
into a house, can be expected to result in improved performance above that expected from a
house designed and constructed following the minimum requirements of the IRC. The abovecode techniques reduce the deformations of the house during an earthquake and therefore reduce
the amount of damage. Above-code recommendations are printed in boldface type in this guide
and appear, with associated discussion, in boxes. While the above-code recommendations are
expected to improve the performance of a house in an earthquake and thereby reduce damage,
many will involve some added costs. The costs associated with above-code recommendations
presented in this guide are based on an estimate prepared by a homebuilder in the Seattle area
and are cited as a percentage of the basic framing cost for the model house analyzed during
development of this guide. Presenting the cost increase for the various above-code
recommendations in these terms permits homebuilders in any part of the nation to easily
determine what the associated added cost will be in his or her area, thus allowing builders and
potential homeowners to make reasonable cost-benefit decisions regarding implementation of the
recommendations.
4
Chapter 1, Introduction
1.3 THE INTERNATIONAL RESIDENTIAL CODE
As already indicated, this guide focuses on the ICC’s 2003 International Residential Code,
which provides a comprehensive collection of requirements for prescriptive (nonengineered)
residential construction. The IRC’s stated purpose is to provide:
. . . minimum requirements to safeguard the public safety, health, and general welfare, through
affordability, structural strength, means of egress facilities, stability, sanitation, light and
ventilation, energy conservation and safety to life and property from fire and other hazards
attributed to the built environment.
The IRC addresses other natural hazards in addition to earthquakes (up to limits described in the
IRC scoping provisions). When considering above-code recommendations, construction details
intended to reduce the risk from one hazard may be slightly different from those needed to resist
another. Thus, care should be taken to consider all natural hazards that present a risk to a
specific site and to formulate an appropriate mitigation strategy in accordance with the
jurisdiction’s building code. Additional guidance is provided in Section 1.8 of this guide.
1.4 IRC SEISMIC DESIGN CATEGORIES
The IRC designates the level of potential seismic hazard for dwellings by assigning a house to a
Seismic Design Category (SDC) based on its location. The IRC SDCs are A, B, C, D1, D2 and E,
with A representing the lowest level of seismic risk applicable to residential construction and E,
the highest. All residential buildings (detached houses and townhouses) in regions with SDC
designations A and B, the lowest levels of seismic risk, are exempt from the seismic
requirements of the IRC. SDC E regions have such a high level of seismic risk that, with a few
exceptions, houses in these regions fall outside the scope of the IRC and must be designed using
engineering principles following the International Building Code or NFPA 5000.
Whether or not required by the IRC and across all SDCs from A to E, many of the recommen
dations in this guide will improve the resistance of a dwelling to seismic forces, wind forces, and
possibly the effects of other natural hazards. The discussion and examples presented in this
guide focus on houses located in SDCs C, D1, and D2.
All U.S. model building codes provide maps identifying the seismic hazard. The 2003 IRC
seismic design maps (IRC Figure R301.1(2) shown in Figure 1-1 designate the Seismic Design
Categories for the nation and U.S. territories. It is a simplified version of the maps referenced by
the IBC and NFPA 5000 for all building types. The legend correlates the Seismic Design
Category with the acceleration expected at a location in terms of gravity (g). A value of 100% g
is equal to the vertical acceleration effects of gravity on Earth. More detailed information can be
found for a particular building site using a CD-ROM available with the building codes and from
FEMA; the CD allows the user to input the latitude and longitude coordinates of the site or the
zip code. Zip codes should be used with caution because they may not reflect the highest
possible SDC in the area covered by the postal zip code. Similar information is available on a
U.S. Geological Survey (USGS) website – http://earthquake.usgs.gov/research/hazmaps (click on
“seismic design values for buildings”).
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FEMA 232, Homebuilders’ Guide
Figure 1-1 Seismic Design Categories – Site Class D.
6
Chapter 1, Introduction
Figure 1-1 Seismic Design Categories – Site Class D (continued).
7
FEMA 232, Homebuilders’ Guide
Figure 1-1 Seismic Design Categories – Site Class D (continued).
8
Chapter 1, Introduction
Figure 1-1 Seismic Design Categories – Site Class D (continued).
9
FEMA 232, Homebuilders’ Guide
When using the seismic maps associated with the building codes, the reader should be aware that
local soil conditions have a major impact on the seismic hazard for each particular site. The IRC
map incorporates an assumed Site Class of D, which is stiff alluvial soil. If the soil conditions
are different from stiff soil (e.g., bedrock or the soft alluvial soils often found in valleys), the
local building department may have established special seismic requirements for these locations.
Check with the local building department to determine if any such special regulations apply to a
particular building site.
1.5 BUILDING RESPONSE
The series of drawings in Figure 1-2
illustrates how a house responds to
earthquake ground motion. Before
the earthquake occurs, the house is
stationary, resisting only the vertical
gravity loads associated with the
weight of the house and its
occupants and contents. When the
ground starts to move during an
earthquake, the foundation of the
house moves sideways but the roof
and upper stories try to remain
stationary due to the inertia of the
house. By the time the roof starts to
move in the direction of the
foundation, the foundation is already
moving back towards its initial
position and the roof and foundation
are moving in opposite directions.
This cycle repeats until the
earthquake ground motion stops and
the building movements slow and
then stop. If the shaking has been
severe enough, the house may be
damaged and have a residual tilt or
displacement.
Figure 1-2 Forces induced in a
house due to earthquake ground
motion.
To perform adequately in an earthquake, a house must have enough strength to resist the forces
generated without failure. Actual earthquakes can generate forces considerably higher than those
used for code-prescribed design (Figure 1-3). Nevertheless, design for the lower code forces
generally has prevented life loss and therefore satisfies the purpose of the code. Remember that
10
Chapter 1, Introduction
the primary goal of the building code is to prevent loss of life; the prevention of damage is only
incidental. It is for this reason that this guide presents above-code recommendations that
describe techniques intended to improve the performance of a house during an earthquake and
result in less damage.
Figure 1-3 Concept of
actual vs. code seismic
forces.
The reasons one- and two-family houses tend to perform adequately in earthquakes even when
designed to the minimum code forces are because houses often are stronger than recognized in
code-level design and because they often are constructed with ductile earthquake-resisting
systems. In residential construction, the finish materials and nonstructural partitions often add
significantly to the strength provided by required bracing materials. Houses constructed from
ductile earthquake-resisting systems generally will perform well during an earthquake because
they can deform without breaking. An example of ductility is given in Figure 1-4. The ductile
metal spoon simply bends while the brittle plastic spoon breaks.
Figure 1-4 Concept of ductility.
Stiffness also is important. The stiffer the house, the less it will move or deflect during an
earthquake, which will reduce the amount of damage to finish materials and, therefore, repair
costs. Stiffness is measured in buildings in terms of drift (horizontal deflection) and is usually
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FEMA 232, Homebuilders’ Guide
discussed as the horizontal deflection in a particular story in terms of either the amount of drift in
inches or the percent story drift, which is the deflection in inches divided by the floor to floor
story heights in inches (Figure 1-5). Unfortunately, increased stiffness also generally results in
increased seismic forces; therefore, the above-code recommendations made in this guide
simultaneously increase strength and stiffness.
Figure 1-5 Definition of drift.
One way to understand how the strength and stiffness of a house affect its earthquake
performance is to remember that the forces exerted by earthquake ground motion are resisted by
a house’s strength while the drift (deflection) is resisted by the house’s stiffness. An analogy
that can be used for this is a fishing pole. The amount the pole bends is dependent on how stiff
the pole is; whether the pole breaks or not is dependent on how strong the pole is. A strong
house will not fall down, but a house with relatively little stiffness will sustain considerable
damage.
1.6 IMPORTANT CONCEPTS CONCERNING HOMESITE
Site characteristics also affect how a house will perform during an earthquake:
• Certain types of soil amplify earthquake ground motions;
• Some types of soil slide, liquefy, and/or settle as a result of earthquake ground motions,
all of which will result in loss of vertical support for the house; and
• Fault rupture on a house site can result in both horizontal and vertical offsets of the
supporting ground.
It is most desirable to build on sites with stable, solid geologic formations. Deep and unbroken
rock formations, referred to as bedrock, generally will minimize earthquake damage whereas
deep soft sedimentary soils will result in the maximum seismic forces and displacements being
transferred to the house.
Sites located over known faults and in landslide-prone sites warrant additional attention. No
matter how well designed, a house cannot accommodate earthquake ground motions at a site
directly on top of an earthquake fault where the ground on either side of the fault moves in
opposite directions. Check with the local building department to determine where known faults
12
Chapter 1, Introduction
are located. Houses should not be built within 50 feet of a known fault and, even at that distance,
damage will still be significant. A house damaged by direct fault movement is shown in Figure
1-6.
Figure 1-6 A house damaged due to fault movement. The fault passed under
this house near Wright's Station, on the Southern Pacific Railroad, in Santa
Cruz County, California. The house was severed by fault movement during the
earthquake. Photo credit: R.L. Humphrey, U.S. Geological Survey.
Sites where landslides are likely to occur also should be avoided. An example of damage due to
a hillside collapsing in a landslide is shown in Figure 1-7. Engage a geotechnical or geological
engineer to inspect the site and/or check with the local building department to minimize the risk
of building on a landslide-prone site.
Figure 1-7 Houses damaged due to a landslide at the site. Photo Courtesy National Information Service for Earthquake Engineering, University of California, Berkeley
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FEMA 232, Homebuilders’ Guide
1.7 MODEL HOUSE USED FOR GUIDE ANALYSIS
A typical model house is used in this guide to illustrate various concepts and techniques (Figure
1-8). An analysis of the model house identified approximate deflections under seismic loading,
which permitted performance to be compared for various configurations using the minimum
code requirements and the above-code techniques.
Figure 1-8 Elevations of the model house used as an example for this guide.
The floor plan of the model house is shown in Figure 1-9. IRC-conforming seismic bracing
configurations were developed for SDCs C, D1, and D2. Gypsum wallboard was used to meet
IRC seismic bracing requirements where possible because the wallboard would already be
provided for wall finish. Where IRC bracing requirements could not be met with gypsum
wallboard, wood structural panel (oriented strand board or OSB) wall bracing was added.
Modifications to the basic floor plan included changing the foundation from a slab-on-grade to a
crawl space (cripple wall or hillside condition) to illustrate the differences resulting from these
changes.
14
Chapter 1, Introduction
Figure 1-9 Model house floor plan.
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FEMA 232, Homebuilders’ Guide
After the basic model house construction was analyzed, the following above-code construction
techniques were evaluated:
• A completely sheathed shear wall system in which the exterior walls were fully sheathed
(around door and window openings) with OSB and hold-down devices were provided at
building corners,
• A shear wall system with hold-down devices provided at each end of each OSB bracing
wall, and
• An oversize sheathing panel system with each OSB sheet lapping vertically onto and
edge-nailed to the rim joist at each floor level.
Approximate comparisons of the performance of the basic configuration of the model house with
the above-code configurations were made using a two-part process. First, each house
configuration was analyzed using a computer model to determine maximum drift or deformation
during earthquake loading.
Second, in order to translate the analytical results into an approximation of house performance,
three ranges of peak transient wall drift and assumed approximate descriptions of performance
were developed. The choice of drift ranges and performance descriptions are based on
component and full-building test results and the judgment of those participating in the
development of this guide.
The approximate performance categories and corresponding story drift ranges are:
• Minor damage potential – Less than or equal to 0.5% story drift
The house is assumed to suffer minor nonstructural damage such as
cracking of plaster or gypsum wallboard and hopefully would be “green
tagged” (occupancy not limited) by inspectors after an earthquake,
which would permit immediate occupancy. Some repairs should still be
anticipated.
• Moderate damage potential – Above 0.5% to 1.5% story drift
The house is assumed to suffer moderate damage including possible
significant damage to materials and associated structural damage, but the
building is assumed to have some reserve capacity in terms of strength and
displacement capacity. The house hopefully would be “green-tagged” or,
more likely, “yellow-tagged” (limited occupancy) by inspectors after an
earthquake and may or may not be habitable. Significant repairs should be
anticipated.
16
Chapter 1, Introduction
•
Significant damage potential – Greater than 1.5% story drift
The house is assumed to have significant structural and nonstructural
damage that could result in its being “red-tagged” (occupancy prohibited)
by inspectors after an earthquake. Significant repairs to most components
of the building should be anticipated, and it may be more economical to
replace the house rather than repair it.
Comparisons of performance based on this approach are discussed in a number of sections in this
guide. See Appendix A for a detailed description of the analysis.
1.8 OTHER HAZARDS
Although this guide has been written primarily to address the earthquake risk to a house,
homebuilders and homeowners need to be aware of many other natural hazards. These hazards
can also affect how a house should be designed and constructed as well where it should be sited
and even whether the site should be used at all.
1.8.1 Wind
Since wind acts on a house by imparting horizontal loads similar to those of an earthquake, the
seismic design and construction criteria contained in the IRC and further detailed in this guide
will provide a significant level of protection to a house’s structural system against wind loads.
However, wind loads also act on the home’s cladding, walls, and roof coverings, and the wind
provisions of the IRC should be followed carefully (due to the differences in the load path for
wind versus earthquake loads). While the IRC wind provisions are adequate, additional guidance
is available and should be considered. This guidance includes the American Forest and Paper
Association (AF&PA) Wood Frame Construction Manual and the Southern Building Code
Congress International (SBCCI) Standard for Hurricane-Resistant Residential Construction
(SSTD 10).
1.8.2 Flooding
Care should be taken to ensure that the home site is not in a flood hazard area. If there is any
doubt in this regard, the local building code enforcement or floodplain management office should
be consulted. As part of the National Flood Insurance Program (NFIP), FEMA publishes local
flood hazard maps that delineate the extent of the flood hazard, using the Base Flood Elevation
or BFE, which is flood elevation that has a 1 percent chance of occurring in any given year. As
part of its agreement to participate in the NFIP in order to make federally-backed flood insurance
available to its residents, a community adopts and enforces a series of flood design and
construction requirements. Generally, these requirements mean that a home located in a flood
hazard area must have its lowest floor and utilities elevated to or above the BFE.
17
FEMA 232, Homebuilders’ Guide
FEMA has available a series of publications on protecting homes from flood damage. Some of
these publications are:
•
•
•
Design Guidelines for Flood Damage Reduction (FEMA 15)
Elevated Residential Structures (FEMA 54)
Above the Flood: Elevating Your Flood-prone House (FEMA 347)
For flood protection of existing houses, the reader is referred to Homeowner's Guide to
Retrofitting: Six Ways to Protect Your House from Flooding (FEMA 312). All of the above
publications are available from the FEMA Publications Center at 1-800-480-2520. Additional
information is also available online at www.fema.gov.
For guidance on the flood resistant construction provisions of the IRC, the reader is referred to
the document Reducing Flood Losses Through the International Codes: Meeting the
Requirements of the National Flood Insurance Program. This publication is available online at
www.fema.gov as well as for a modest fee from the International Code Council
(www.iccsafe.org) at (800) 786-4452.
Above-code Recommendations: If a house is to be located in a flood hazard area
and must be elevated to the BFE, it is recommended that the lowest floor of the
house actually be elevated 2 or 3 feet above the BFE. This will result in a
significantly lower insurance premium for the homeowner and will provide a
significantly higher level of protection for a minimal additional cost.
If a house is to be located just outside of a flood hazard area and is not required to
meet a community’s flood regulations, it is recommended that the home still meet
the community’s flood ordinances, such as not using a basement foundation, to
provide an additional level of protection. Further, the homeowner is still
encouraged to purchase flood insurance. Even if a home is just outside of a flood
hazard area, it can still be subject to flooding as higher floods than the design flood can
and have occurred. As a further measure of protection, flood insurance is available
outside of the flood hazard area, usually for a very reasonable premium.
1.8.3 Coastal Hazards
Homesites along the coast are subject to two different hazards from hurricanes and other coastal
storms such as Northeasters — high winds and flooding from storm surge. Storm surge is
caused by the storm’s low pressure and winds pushing the water onto land. In the recent
Hurricane Katrina, storm surge heights of almost 30 feet were measured along the coast of
Mississippi. Coastal flood hazard maps similar to the flood maps described above are prepared
by FEMA; however, the coastal flood hazard area is divided into two zones, the coastal high
hazard zone or “V-Zone” which has significant wave heights and the regular flood hazard zone
or “A-Zone” which does not.
18
Chapter 1, Introduction
Coastal homes must be designed and built to withstand both wind and storm surge loads. This
requires elevating the house above mapped storm surge elevations and ensuring that the house
can resist the loads associated with hurricane force winds. Further, since storm surge waters can
flow with considerable velocity which generates substantial hydrodynamic loads on the area
below the elevated floor, this area must be built to allow water to flow through without placing
additional loads on the structure. For this reason, any walls below the lowest floor must be made
of “breakaway” construction.
Note also that the coastline is a dynamic and ever-changing environment and is often subject to
erosion. In choosing a home site, consider whether the property is subject to erosion, which
could easily undermine and destroy the house in a relatively short period of time. Erosion maps
showing the rate of change of the shoreline may be available locally.
Above-code Recommendations: For additional protection of coastal homes, the
following is recommended:
• Houses should be elevated 2 or 3 feet above the storm surge flood elevation
provided on local maps.
• The area below the lowest floor should be kept open and free of as many obstructions as possible. • Houses located in A-Zones but still near the coast (also known as Coastal AZones) may still be subject to waves and storm surge and should be
constructed to V-Zone criteria.
• Houses should be designed to resist damage from wind and debris. This
includes limiting the size and amount of glazing and actually installing the
storm shutters before an event.
• Building sites should be investigated for erosion potential prior to purchase
and sites subject to erosion should be avoided. Houses built where erosion is
possible should be located as far from the shoreline as possible.
• Natural features such a sand dunes and vegetation should be used as much as
possible to protect houses from coastal forces.
All of the above recommendations will provide better protection than currently offered by
the IRC with little additional cost.
The reader is encouraged to consult the FEMA Home Builders Guide to Coastal Construction:
Technical Fact Sheet Series (FEMA 499). These fact sheets contain design and construction
suggestions based on lessons learned in FEMA’s investigations of past hurricanes and how they
impacted residential construction. This document is available from the FEMA Publications
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FEMA 232, Homebuilders’ Guide
Center as well as online at www.fema.gov. For more detailed engineering design information,
the reader may wish to consult the Coastal Construction Manual: Principles and Practices of
Planning, Siting, Designing, Constructing, and Maintaining Residential Buildings in Coastal
Areas (FEMA 55). This is a large document (500 pages) and is available from the FEMA
Publications Center in paper or on a CD-ROM.
1.8.4 Tornado
Building codes generally do not contain tornado provisions primarily because the probability of a
specific house being hit by a tornado powerful enough to do damage are sufficiently remote that
it is not thought to be economically justifiable to design for one. Therefore, further guidance on
resisting the loads from a tornado does not exist. If protection of a home’s occupants is a
concern, the reader is referred to Taking Shelter from the Storm: Building a Safe Room Inside
Your House (FEMA 320). This document is available from the FEMA Publications Center as
well as online at www.fema.gov.
20
Chapter 2 EARTHQUAKE-RESISTANCE REQUIREMENTS This chapter explains the International Residential Code’s (IRC’s) general earthquake-resistance
requirements as well as specific IRC requirements concerning load path and house configuration
irregularities. One- and two-family detached houses of wood light-frame construction are
addressed; however, the discussion is relevant to other materials of construction likely to be used
for detached houses including light-frame cold-formed steel.
2.1 IRC GENERAL EARTHQUAKE LIMITATIONS
The variety of configurations used for houses is very wide and they are constructed of an equally
wide variety of materials. IRC Section R301.2.2 imposes some limits on configuration and
materials of construction for one- and two-family detached houses in Seismic Design Categories
(SDCs) D1 and D2. These IRC limitations reflect the desire to provide equal earthquake
performance for houses designed using the prescriptive IRC provisions and for those with an
engineered design. Application of the prescriptive IRC requirements to houses that do not
comply with the limitations can be expected to result in inadequate performance.
IRC Section R301.2.2 earthquake limitations do not apply to one- and two-family detached
houses in SDCs A, B, and C; however, townhouses in SDC C are required to comply. In
general, design using the IRC is prohibited in SDC E, and the designer is referred to the
engineered design requirements of the International Building Code (IBC) or other adopted code.
There are, however, two methods by which houses designated as SDC E in IRC Figure
R301.2(2) may be designed in accordance with IRC provisions: (1) if site soil conditions are
known, IRC Section R301.2.2.1.1 permits determination of the Seismic Design Category in
accordance with the IBC, which may result in a reduced SDC, and (2) if the restrictions of IRC
Section R301.2.2.1.2 are met, houses located in SDC E may be reclassified to SDC D2 and
designed using the IRC provisions. IRC Section R301.2.2.1.2 requirements dictate a regular
house configuration.
The limitations imposed by IRC Section R301.2.2 are as follows:
• Weight Limitations – For houses in SDCs D1 and D2, IRC Section R301.2.2.2.1 specifies
maximum weights for the floor, roof-ceiling, and wall assemblies or systems. Because
earthquake loads are proportional to the weight of the house, an upper bound on assembly
weight provides an upper bound on earthquake loads. The specified maximum assembly
weights relate directly to the weights considered in developing the IRC earthquake
bracing provisions. The effect of the maximum weights is the exclusion of heavier finish
materials when using the IRC provisions. Where heavier finish materials are to be used,
an engineered design must be provided.
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FEMA 232, Homebuilders’ Guide
• House Configuration Limitations – For houses in SDCs D1 and D2, IRC Section R301.2.2.2.2 places limits on house configuration irregularities. These limits are discussed in detail in Section 2.3 of this guide. • House System Limitations – Another scope limitation for houses in SDCs D1 and D2 is
given in the combined requirements of IRC Sections R301.2.2.3 and R301.2.2.4. These
sections provide limits for number of stories based on building system and limits for
anchored stone and masonry veneer and masonry and concrete wall construction. Some
of these requirements are addressed in Chapter 5 of this guide.
• Story Height Limitation – IRC Section R301.3 provides a scope limitation that is not
related solely to earthquake loads but rather applies in all SDCs. This section limits story
height by limiting the wall clear height and the height of the floor assembly. This limits
both the lateral earthquake and wind loads and the resulting overturning loads.
The IRC requires design in accordance with accepted engineering practice when the general
earthquake limitations discussed above are not met (weight limitations, house configuration
limitations, building system limitations, and story height limitations). Engineered design is
addressed in Section R301.1.3. This section permits design to be limited to just the elements that
do not conform to the IRC limitations. Increased assembly weight and story height will globally
increase seismic and wind loads, generally making engineered design of the entire house
necessary. Design of portions of the house is particularly applicable when an irregularity such as
a cantilever, setback, or open front occurs. The extent of design is left to the judgment of the
designer and building code official. When multiple irregularities occur, engineered design of the
entire house may become necessary in order to provide adequate performance. The IRC requires
that engineering design methods be used but does not specify whether this must be done by a
registered design professional. State or local law governs who can perform the design; the reader
is advised to check with the local building department for requirements.
2.2 LOAD PATH
For a house to remain stable, a load applied at any point on the
structure must have a path allowing load transfer through each
building part down to the building foundation and supporting soils.
The term “load path” is used to describe this transfer of load through
the building systems (floors, roof-ceilings, bracing walls).
Basic Concept — To understand the concept of a load path, a house
can be represented by the chain shown in Figure 2-1. The chain is
pulled at the top and the load is transferred from one link to the next
until it is transferred to the ground. If any link is weak or missing,
the chain will not adequately transfer the load to the ground and
failure will result.
Figure 2-1 Chain
illustrating the load path
concept.
22
Chapter 2, Earthquake-Resistance Requirements
Likewise, houses must have complete and adequate load paths to successfully transfer
earthquake loads and other imposed loads to the supporting soils.
Load Path for Earthquake and Wind Loads — The example house in Figure 2-2 will be used to
discuss load path. The arrows provide a simplified depiction of earthquake or wind loads
pushing horizontally on the house. Although wind and earthquake loads can occur in any
horizontal direction, design procedures generally apply the loads in each of the two principal
building directions (i.e., longitudinal and transverse), one at a time, and this discussion of
loading will utilize that convention.
Internally, the house has to convey loads from the upper portions of the structure to the
foundation. For the example house, this is accomplished by transferring the loads through:
•
•
•
•
•
The roof-ceiling system and its connections to the second-story bracing wall system,
The second-story bracing wall system and its connections to the floor-ceiling system,
The floor-ceiling system and its connections to the first-floor bracing wall system, and
The first-story bracing wall system and its connections to the foundation, and
The foundation to the supporting soil.
The following discussion focuses primarily on the connections between the various building
systems; the systems themselves are addressed only briefly here but are discussed in detail in
later chapters of this guide.
Figure 2-2 Lateral loads induced
in a building due to wind or
earthquakes.
Roof-ceiling and Floor Systems — In the example house, the roof-ceiling system will resist
horizontal earthquake loads proportional to the weight of the roof, ceiling, and top half of the
second-story walls. The series of arrows at the right of Figure 2-3a depicts this load. The roofceiling system deflects horizontally under the load and transfers the load to the supporting walls
at both ends. The single arrows at the roof-ceiling system ends depict the reaction loads to the
supporting walls. Within the roof-ceiling system, the load is carried primarily by the roof
sheathing and its fastening (see Chapter 6 of this guide for discussion of roof-ceiling systems).
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FEMA 232, Homebuilder’s Guide
Figure 2-3 Loading and deflection of roof-ceiling and floor systems.
Similarly, the floor system will resist horizontal earthquake loads proportional to its weight and
the weight of walls above and below. As shown in Figure 2-4b, it will deflect and transfer load
to the supporting walls in much the same way as the roof-ceiling system. Again, the loading is
carried by the floor sheathing and its fastening (see Chapter 4 of this guide for discussion of floor
systems).
Bracing Wall Systems – The roof-ceiling reaction load is transferred into the second-story
bracing wall system as depicted by the arrow at the top of the wall in Figure 2-4a. The wall
deflects under this load and transmits the load to the wall base and through the floor system to
the first-story wall. Resistance to the wall load is provided by the wall sheathing and its
fastening.
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Chapter 2, Earthquake-Resistance Requirements
Figure 2-4 Loading and deflection of bracing wall systems.
The first-story bracing wall system resists loads from both the second-story wall and the secondstory floor system as depicted by the arrow at the top of the wall in Figure 2-4b. The wall
deflects under this load and transmits the load to the wall base and the foundation. Again,
resistance to the wall load is provided by sheathing and its fastening. Figure 2-5 provides an
exploded view of the example house that illustrates the combination of roof-ceiling, floor, and
wall systems and their connection to the foundation below.
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FEMA 232, Homebuilder’s Guide
Figure 2-5 Load transfer between components in a building.
Requirements for Connections Between Systems – As previously noted, a complete load path for
earthquake loads requires not only adequate roof-ceiling, floor, and bracing wall systems but also
adequate connection between these systems. Connections between systems must resist two
primary types of loads: horizontal sliding loads and overturning loads.
Load Path Connection for Horizontal Sliding – Figure 2-6 depicts the end wall at the left side of
the house illustrated in Figures 2-2 through 2-5 and provides a detailed illustration of one
possible path for horizontal loads from the roof assembly to the foundation. The left-hand
portion of the figure shows a section through the end wall in which each of the “links” in the
load path is given a number, H1 through H11, corresponding to a connection or mechanism used
to transfer the loads. The right-hand side of the figure shows an elevation of the same wall and
illustrates the deformation that will occur if adequate connection is not made. Table 2-1 at the
end of this chapter provides a detailed summary of the load path connections for the Figure 2-6
wall.
26
Chapter 2, Earthquake-Resistance Requirements
Figure 2-6 Horizontal load path connections and deformations.
Load Path Connection for Overturning – Because the horizontal loads are applied high on the
house and resisted at the foundation, overturning loads develop in the bracing walls. Figure 2-7
illustrates one possible load path for overturning loads. The left-hand side of the figure shows a
wall elevation in which each of the “links” in the overturning load path is given a number, OV1
to OV8, corresponding to locations with uplift or downward loads due to overturning. The righthand side of the diagram shows an elevation of the same wall that illustrates overturning
deformations that will occur with earthquake loading from the left to the right. Uplift or tension
occurs at one end of a wall simultaneously with downward force or compression at the other end.
The IRC only specifies connections (hold-down straps or brackets) to resist overturning loads for
a limited number of bracing alternatives. The connectors used to resist horizontal loads in most
IRC designs will be required to resist overturning loads as well as horizontal loads. This is a
major difference between prescriptive design and engineered design in which resistance to
overturning loads must be explicitly provided. The IRC requires use of hold-down devices in
Section R602.10.6 for alternative braced wall panels; in Section R602.10.11, Exception 2, for
SDCs D1 and D2 when braced wall panels are not located at the end of braced wall lines; and in
Section R703.7 when stone or masonry veneer is used. Table 2-2 at the end of this chapter
provides a detailed summary of the load path connections for the Figure 2-7 wall.
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FEMA 232, Homebuilder’s Guide
Figure 2-7 graphically depicts use of straps to resist overturning loads; however, overturning can
be resisted by connections employing such other devices as bolts, nails, or hold-down brackets.
Because different device types may deform differently under load, it is preferable to use the same
type of device for an entire story level. Variations in connector type from story to story are
acceptable. When considering overturning in an engineered design, it is customary to include the
effect of dead load (the actual weight of the house) in reducing uplift and overturning loads;
however, this level of calculation detail is beyond the scope of the IRC provisions. Hold-downs
should be provided wherever they are required by the IRC or recommended by this guide,
irrespective of dead load.
Figure 2-7 Overturning load path connections and deformations.
System and Connection Strength and Stiffness Requirements – The roof-ceiling, floor, and
bracing wall systems are the basic members resisting earthquake loads. Adequate earthquake
performance of a house relies on:
•
•
•
•
Adequate strength of roof-ceiling, floor, and bracing wall systems,
Adequate stiffness of roof-ceiling, floor, and bracing wall systems to limit deformation,
Adequate connection between systems to provide a functional load path, and
Adequate connection to the foundation.
For most houses, it is generally anticipated that bracing wall system behavior will have more
influence on the behavior of the house than the roof-ceiling or floor systems. Further, it is
anticipated that wall behavior at lower stories will generally be more critical than at upper stories
due to larger earthquake loads.
Accumulation of Loads in Systems and Connections – Wind and earthquake loads increase or
accumulate towards the bottom of the house. This is particularly applicable to loads in the
bracing wall systems and their connections for horizontal loads and uplift and downward loads
28
Chapter 2, Earthquake-Resistance Requirements
due to wall overturning. For example, overturning connections must be sized to resist all of the
loads generated above the connection location. In a two-story house, the second-floor uplift
connection, such as OV1 in Figure 2-7, will need to resist loads from the second story. The firststory uplift connection, such as OV5 in Figure 2-7, will have to resist the uplift loads from the
second story plus the additional uplift from the first story. It can generally be expected that OV5
will need to resist a load two to three times that resisted by OV1. Downward loads at the
opposite ends of walls and horizontal loads accumulate similarly. Although this accumulation of
load often is missed in design for wind and earthquake loads, it is important and should be
explicitly considered when connections are being selected.
Above-code Recommendation: For overturning loads, the use of hold-down
connectors (brackets, straps, etc.) is recommended as an above-code measure in order
to improve load transfer and thereby decrease damage. For most of the basic IRC wall
bracing types, the provisions rely on building weight and the fasteners resisting horizontal
loads to also resist overturning. Wall component testing, however, indicates that this
method is not as reliable in resisting overturning as the use of positive anchorage devices.
2.3 HOUSE CONFIGURATION IRREGULARITIES
A house’s configuration (shape) significantly affects its response to wind and earthquake loads.
This section discusses the concepts and the IRC provisions for irregular house configurations.
Ideal Earthquake-Resistant House Configuration – For earthquake resistance, the ideal house
would have:
•
•
•
•
•
•
A simple rectangular shape,
Bracing walls distributed uniformly and symmetrically through the house,
No large concentrations of weight,
Bracing walls at upper stories located immediately above walls in stories below,
Wall bracing lengths that increase in lower story levels compared to the story above,
No split-levels or other floor level offsets.
A version of this ideal house is shown in Figure 2-8. This ideal configuration results in loads
and deformations being uniformly distributed throughout the house, which permits resisting
elements to contribute equally to earthquake resistance. With good distribution of bracing walls,
earthquake loads can be resisted very close to where they are generated (by house weight), which
reduces the need for transfer of earthquake loads through floor and roof systems to other portions
of the house. This helps reduce the poor performance that often results when such transfers are
required.
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FEMA 232, Homebuilder’s Guide
Figure 2-8 Ideal building
configuration for earthquake
resistance.
Deviations from Ideal Configuration – Deviations from the ideal configuration are called
configuration irregularities. As houses deviate from the ideal configuration, loads and
deformations are concentrated, which causes localized damage that can result in premature local
or even complete failure of the house. While the ideal configuration is attractive from the
standpoint of earthquake resistance, houses with irregularities are much more popular and
common than those without. Large open great rooms and walls of nothing but windows are
examples of typical irregular configurations.
House Irregularity Concepts – House irregularities often are divided into two types: plan
irregularities and vertical irregularities.
Plan irregularities concentrate earthquake load and deformation in a particular area of a house. A
common cause is a center of mass (building weight) at a location different from the center of the
resisting elements (bracing walls). This can be due to non-uniform mass distribution, non­
uniform distribution of bracing walls, or an irregular house plan. Two common examples are a
house with one exterior wall completely filled with windows with no wall bracing provided
(Figure 2-9) and a house with a large masonry chimney at one end. Houses with plan
irregularities generally experience rotation in addition to the expected horizontal deformation.
House rotation, as illustrated in Figure 2-10, magnifies the displacements, resulting in increased
damage.
30
Chapter 2, Earthquake-Resistance Requirements
Figure 2-9 Irregular building configuration with open front (no second-story bracing walls
supporting roof on right-hand portion).
Figure 2-10 Rotational response and resistance to torsion.
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FEMA 232, Homebuilder’s Guide
Other common plan irregularities occur in T- and L-shaped houses that concentrate loads at the
corners where the different wings of the house connect. Figure 2-11 illustrates the concentration
of loads in an L-shaped house. The noted location of load concentration is where damage would
be anticipated. Adequate interconnection of the house wings is required for good performance.
Figure 2-12 illustrates damage due to inadequate interconnections. Without additional
consideration, poor performance and additional damage or failure would be expected for
structures with such plan irregularities.
Figure 2-11 Concentration of loads and possible damage and failure due to irregular plan.
32
Chapter 2, Earthquake-Resistance Requirements
Figure 2-12 Roof-level damage to building with a T-shaped plan. The damage at the roof
corner (right-hand side) is due to stress concentrations where the wings of the building connect.
Vertical Irregularities – Vertical irregularities concentrate damage in one story of a multistory
house. This occurs when the stiffness or strength of any one story is significantly lower than that
of adjacent stories. When the stiffness of a story is significantly lower, the deformations
associated with the earthquake loads tend to be concentrated at that “soft” story as illustrated in
Figure 2-13. The configuration on the left (normal) provides a uniform stiffness in each story,
while the configuration in the middle illustrates a soft story with the deflections being
concentrated in that story. If the displacements get large enough, they can cause complete failure
of the soft story as illustrated in the configuration on the right.
The CUREE-Caltech Woodframe Project found that many two-story light-frame residential
houses exhibit soft-story behavior to some extent because the first stories feature relatively large
window and door openings and fewer partitions (less bracing) than the second stories where the
bedrooms and bathrooms are located. Soft first-story behavior also was observed in the analysis
of the model house used for this guide. Figure 2-14 provides an exaggerated illustration of
deformation concentrated in the first story of the model house. Cripple walls around the
perimeter of a crawlspace also can result in soft-story behavior. A weak story can cause damage
or failure to be concentrated in that story if earthquake loading approaches the story strength.
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FEMA 232, Homebuilder’s Guide
drift
normal
drift
soft story
overstress
Figure 2-13 Deformation pattern due to soft-story behavior.
Deformation is concentrated in the first story.
Figure 2-14 Change in deformation patterns associated with soft-story irregularity.
Soft and weak stories, although technically different, can occur at the same time. Combined
soft- and weak-story house irregularities have been the primary cause of story failure or collapse
and earthquake fatalities in wood light-frame houses in the United States. To date, story failure
has only been observed in houses that would not meet the current IRC bracing requirements or
would fall outside the scope of the IRC. Figure 2-15 illustrates soft- and weak-story behavior in
34
Chapter 2, Earthquake-Resistance Requirements
a single-family house. Figure 2-16 illustrates the loss of a soft and weak story in an apartment
building.
Figure 2-15 House experiencing soft- and
weak-story behavior in the Cape Mendocino
earthquake. Deformed shape of garage door
opening shows approximately 6 inches of
horizontal movement in the first story.
Figure 2-16 Apartment building
experiencing soft- and weakstory behavior in the Northridge
earthquake. The two balcony
rails with little separation at the
center of the photo illustrate the
collapse of the lowest story.
IRC Approach to House Configuration Irregularities – The IRC incorporates two approaches to
limitation of irregularities. The first and more explicit approach is found in IRC Section
R301.2.2.2.2, which directly limits a series of irregular house configurations. Specific
exceptions allow inclusion of less significant irregularities within IRC prescriptive designs. For
one- and two-family detached houses, these provisions are applicable in SDCs D1 and D2,. The
IRC limitations on irregularities are derived from those required for engineered buildings by the
IBC and NFPA 5000. Table 2-3 at the end of this chapter provides detailed discussion of the IRC
Section R301.2.2.2.2 limits. Additional discussion may be found in the commentary to the 2003
NEHRP Recommended Provisions.
35
FEMA 232, Homebuilder’s Guide
The second and less obvious approach is the IRC requirement for distribution of wall bracing.
Along with braced wall lines at exterior walls, interior braced wall lines must be added so that
the spacing between wall lines does not exceed 35 feet per IRC Section R602.10.1.1 (adjustable
to 50 feet by an exception) or 25 feet in SDC D1 or D2 per Section R602.10.11. Maximum
spacing between braced walls in a braced wall line is also regulated. Houses perform better if
the walls are distributed throughout, rather than concentrated in limited portions of the structure.
This allows the earthquake load to be resisted local to the area where it is developed. Higher
earthquake loads develop when loading must be transmitted to bracing walls in another portion
of the house. Good distribution of bracing walls helps to mitigate the adverse effects of many
irregularities.
Stepped Cripple Walls – IRC Section R602.11.3 and Figure R602.11.3 provide specific detailing
to reduce the effect of a concentration of load due to stepped concrete or masonry foundation
walls. As illustrated in Figure 2-17, a direct tie to the tallest foundation segment provides
uniform stiffness along the wall line.
Figure 2-17 Detailing required for stepped foundation walls (ICC, 2003).
36
Chapter 2, Earthquake-Resistance Requirements
Above-code Recommendations: The concentration of damage as a result of irregular
building configuration is equally applicable to earthquake loading in all SDCs and to wind
loading. To date, the IRC has limited application to areas of high seismic risk. Use of the
irregularity limitations in SDCs A, B, and C and for wind loading will contribute to
better building performance and is recommended as an above-code measure.
Experimental and analytical results show that typical residential buildings are prone to softstory irregularities. Stiffening and strengthening of soft first stories is recommended as
an above-code measure to reduce deformation and resulting damage. Approaches to
increasing first-story strength and stiffness include: increasing the length of wood structural
panel sheathing; decreasing center-to-center spacing on sheathing nailing (although spacing
of less than 4 inches is not recommended due to the possibility of splitting the framing and
because other weak links could develop); using perforated shear wall construction (also called
“continuously sheathed wall”); using segmented shear wall construction (also called “wood
structural panel sheathed with overturning anchors”); or providing wood structural panel
sheathing on both faces of bracing walls. See Chapter 6 of this guide for detailed descriptions
of these measures.
Another common location of soft or weak stories is cripple walls. This is particularly true
where perimeter cripple walls have no inside face sheathing and under-floor basement areas
have few or no interior walls. Although the IRC has increased bracing wall lengths beyond
those required by earlier codes, further increasing length and providing interior braced wall
lines will help to limit deformation and damage. Additional length of wood structural
panel sheathing at the building perimeter and interior is recommended as an above-code
measure to limit cripple wall deformation.
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FEMA 232, Homebuilders’ Guide
Table 2-1 Load Path Connections for Horizontal Sliding
Minimum Fastening per
IRC Table R602.3(1) and Discussion
Item
Illustration
Sheathinga
Nailingb
5/16” to ½” 8d common @ 6”
19/32” to 1” 8d common @ 6”
11/8” to 11/4” 10d common @ 6”
H1
•
•
•
Resists roof sheathing sliding with respect to blocking below.
Six-inch nail spacing applies to supported sheathing edges
and blocking. Twelve-inch spacing applies at other panel
supports.
Rafter blocking is not always required by IRC; however,
sheathing should be nailed to blocking where blocking is
provided.
Three 8d box (0.113”x21/2”) or three 8d common (0.131x21/2”) toenails
each block.
H2
•
•
•
H3
&
H4
Resists rafter blocking sliding with respect to wall top plate.
Use of angle clips in lieu of toenails is a recommended
above-code measure.
Rafter blocking is not always required by IRC; however, it
should be fastened where provided.
Sheathinga
Nailingb
5/16” to ½” 6d common @ 6”
19/32” to 1” 8d common @ 6”
11/8” to 11/4” 10d common @ 6”
•
•
Provides wall racking resistance.
Six-inch nail spacing applies to sheathing edges. Twelve-inch
spacing applies at other studs.
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Chapter 2, Earthquake-Resistance Requirements
Minimum Fastening per
IRC Table R602.3(1) and Discussion
Item
Illustration
At Braced Wall Panels
Three 16d box (0.135”x31/2”) or three 16d sinker (0.148x31/4”) face
nails each 16 inches on center (space evenly).
H5
Between Braced Wall Panels
One 16d box (0.135”x31/2”) or one 16d sinker (0.148x31/4”) face nail at
16 inches on center.
• Resists wall sole plate sliding with respect to sheathing and
blocking or rim joist below.
Sheathinga
Nailingb
5/16” to ½” 6d common @ 6”
19/32” to 1” 8d common @ 6”
11/8” to 11/4” 10d common @ 6”
H6
•
•
Resists floor sheathing sliding with respect to blocking below.
Six-inch nail spacing applies to supported sheathing edges
and blocking. Twelve-inch spacing applies at other panel
supports.
Three 8d box (0.113”x21/2”) or three 8d common (0.131x21/2”) toenails
each block.
•
H7
•
Resists joist blocking sliding with respect to wall top plate.
Use of angle clips in lieu of toenails is a recommended
above-code measure.
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FEMA 232, Homebuilder’s Guide
Item
H8
&
H9
H10
Minimum Fastening (IRC Table R602.3(1) U.O.N.) & Discussion
Sheathinga
Nailingb
5/16” to ½” 6d common @ 6”
19/32” to 1” 8d common @ 6”
11/8” to 11/4” 10d common @ 6”
•
•
Provides wall racking resistance.
Six-inch nail spacing applies to sheathing edges. Twelve-inch
spacing applies at other studs.
Anchor bolts in accordance with IRC Sections R403.1.6 and
R403.1.6.1. Steel plate washers in accordance with R602.11.1.
Requirements vary by SDC. See Chapter 4 of this guide for further
discussion.
•
Resists foundation sill plate sliding with respect to slab-on­
grade or other foundation.
Foundation embedment in accordance with IRC Section 403.1.4
provides for development of lateral bearing and friction, which permits
transfer of loads between the foundation and supporting soil.
H11
a.
b.
40
•
Resists foundation sliding relative to soil (grade).
Wood structural panel sheathing; see IRC Table R602.3(1) for other sheathing materials.
Common nail diameter and length: 6d: 0.113”x2”, 8d: 0.131”x2-1/2”, 10d: 0.148”x3”.
Illustration
FEMA 232, Homebuilders’ Guide
Table 2-2 Load Path Connections for Overturning
Item
Overturning Load Path Description and Discussion
OV1
When Wall 1 is loaded from left to right, the wall tries to overturn causing the lower left corner to
uplift. This illustration shows a hold-down strap restraining this uplift. The hold-down strap carries
tension from an end post or studs in the second-story wall to an end post or studs in the first-story
wall, which in turn must be anchored to the foundation (OV5)
Illustration
When Wall 1 is loaded from left to right and an uplift load occurs at OV1, an approximately equal
downward load occurs at OV2. This load will be in the post or studs at the end of the wall and will
push down on the floor framing and first-story wall. This load will be transmitted through a firststory post to the foundation.
OV2
OV3
When Wall 1 is loaded from right to left, there is an uplift load in the hold-down strap at OV2.
Because this end of the wall is not aligned with a wall end in the first story, attention is needed to
make sure that a post is added in the first-story wall for strap nailing. The first-story post can have
edge nailing to the wall sheathing and transfer the uplift into the first-story wall or, alternatively, can
be anchored directly to the foundation with an additional hold-down anchor. One or the other of
these anchorage methods is needed to complete the load path.
When Wall 2 is loaded from left to right, the wall tries to overturn causing the lower left corner to
uplift. The location of OV3 over a first-story header makes the load path more complex than OV1.
Hold-down strap OV3a carries the uplift load from the Wall 2 end post to the first-story header.
Because the uplift load can be larger than the minimum load on the header, straps OV3b and OV3c
are shown tying the header down to the first-story posts or studs. If this is not done, it might be
possible for the header to pull up.
When Wall 2 is loaded from right to left, a downward load occurs at OV3a. This downward load
adds to the load already in the header and the studs supporting the header. When Wall 2 extends
more than a foot over the header, the condition is considered an irregularity and is subject to
limitations in SDC D1 and D2.
OV4
When Wall 2 is loaded from left to right and an uplift load occurs at OV3a, an approximately equal
downward load occurs at OV4. This load will be in the post or studs at the end of the wall and will
push down on the floor framing and first-story wall. This load will be transmitted through a firststory post to the foundation.
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FEMA 232, Homebuilder’s Guide
Item
Overturning Load Path Description and Discussion
OV5
When Wall 3 is loaded from left to right, the wall tries to overturn causing the lower left corner to
uplift. This illustration shows a hold-down strap restraining this uplift. The hold-down strap carries
tension from an end post or studs in the first-story wall to the foundation. The uplift load in the
first-story end post or studs is a combination of the second-story uplift load from OV1 and the uplift
load accumulated over the height of the first story.
Hold-downs anchored to the foundation should be used only where substantial continuous
foundations are provided. Hold-downs anchored to existing foundations that are weak or that do not
meet current dimensional requirements require engineering guidance. Hold-downs anchored to
isolated footings require engineering guidance.
OV6
OV7
When Wall 3 is loaded from left to right and an uplift load occurs at OV5, an approximately equal
downward load occurs at OV6. This load will be in the post or studs at the end of the wall and will
push down on the foundation. This load will be a combination of the downward load OV2 from
Wall 1 and the load accumulated over the height of Wall 3. An exact engineering calculation would
adjust this downward load based on the narrower width of Wall 2 and the uplift from the hold-down
at OV3b.
When Wall 4 is loaded from left to right, the wall tries to overturn causing the lower left corner to
uplift. This illustration shows a hold-down strap restraining this uplift. The hold-down strap carries
tension from an end post or studs in the first-story wall to the foundation. The uplift load in the
first-story end post or studs is a combination of the second-story uplift load from OV3c and the
uplift load accumulated over the height of the first story.
When Wall 4 is loaded from left to right and an uplift load occurs at OV7, an approximately equal
downward load occurs at OV8. This load will be in the post or studs at the end of the wall and will
push down on the foundation. This load will be a combination of the downward load OV4 from
Wall 2 and the load accumulated over the height of Wall 4.
OV8
42
Illustration
Chapter 2, Earthquake-Resistance Requirements
Details
43
FEMA 232, Homebuilder’s Guide
Table 2-3 IRC House Configuration Irregularities.
Irregularity
Number
1
2
44
IRC Section
R301.2.2.2
Description
Discussion
When exterior
shear wall lines
or braced wall
panels are not in
one plane
vertically from
the foundation to
the uppermost
story in which
they are
required.
IRC May Be Used If: The out of plane offset does not
exceed four times the nominal depth of the floor joists
and the detailing requirements of the exception are met.
When a section
of floor or roof is
not laterally
supported by
shear walls or
braced wall lines
on all edges.
Also called an
“Open Front”
irregularity.
IRC May Be Used If: A section of floor or roof extends
not more than six feet beyond the braced wall line, and
the cantilever does not support a braced wall panel.
Engineered Design Is Required If: The out-of-plane
offset exceeds that permitted or the detailing provisions
are not followed.
Discussion: Braced wall panels are intended to be
stacked in order to maintain uniform strength and
stiffness at each story and to aid in continuity of the load
path. Support of braced wall panels on cantilevers and set
backs can reduce their strength, stiffness and continuity.
Engineered Design Is Required If: The floor or roof
extension is greater than six feet or is supporting a braced
wall panel.
Discussion: Placement of bracing walls at the edge of
each floor or roof section aids in the uniform distribution
of bracing and aids in ensuring continuity of the load
path. Where bracing walls are not provided at all edges,
rotational behavior (plan irregularity) may result.
Illustration
Chapter 2, Earthquake-Resistance Requirements
Irregularity
Number
3
4
IRC Section
R301.2.2.2
Description
Discussion
When the end of
a braced wall
panel occurs
over an opening
in the wall below
and ends at a
horizontal
distance greater
than 1 foot from
the edge of the
opening. This
provision is
applicable to
shear walls and
braced wall
panels offset out
of plane as
permitted by the
exception to Item
1 above.
IRC May Be Used If: The braced wall panel does not
extend more than one foot over the header below, or if
the header meets the requirements of the exception.
When an
opening in a
floor or roof
exceeds the
lesser of 12 feet
or 50 percent of
the least floor or
roof dimension.
IRC May Be Used If: Floor and roof openings are kept
to minimum size, such as standard stair openings.
Illustration
Engineered Design is Required If: The braced wall
panel extends more than 1 foot over a header and the
header is not selected in accordance with exception
requirements, or if the entire braced wall panel falls on
the clear span of the header.
Discussion: When earthquake loads are applied to braced
wall panels, large downward loads develop at the panel
ends due to overturning (see Table 2). If the downward
load falls on a header that is not strong or stiff enough,
the effectiveness of the bracing is reduced and localized
damage may occur.
Engineered Design Is Required If: Stair openings are
enlarged, such as to create entry foyers or two story great
rooms, or accommodate large skylights.
Discussion: Large floor and roof openings can affect the
uniform distribution of earthquake loads to bracing walls,
resulting in increased deformations and concentration of
damage in the floor, roof and bracing wall systems.
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FEMA 232, Homebuilder’s Guide
Irregularity
Number
5
IRC Section
R301.2.2.2
Description
When portions of
a floor level are
vertically offset.
Also called
“Split Level”
irregularity.
Discussion
IRC May Be Used If: Floor framing on either side of a
common wall is close enough in elevation so that straps
or other similar devices can provide a direct tension tie
between framing members on each side of the wall.
Engineered Design Is Required If: Floor framing on
either side of a common wall cannot be directly tied
together.
Discussion: This irregularity results from observed
earthquake damage in which one of two floor or roof
levels pulled away from a common wall, resulting in
local collapse. The direct tie limits the distance that either
floor system can pull away, reducing likelihood of losing
vertical support.
6
When shear
walls and braced
wall lines do not
occur in two
perpendicular
directions.
IRC May Be Used If: Required braced wall panels are
oriented in the house longitudinal and transverse
directions.
Engineered Design Is Required If: Required bracing
walls fall at angles other than longitudinal and transverse.
Discussion: It has become somewhat common for houses
to have walls that fall at an angle to the main transverse
and longitudinal directions (often at 45 degrees). Where
angled walls are not required for bracing, this is not a
concern. Walls used for bracing must be aligned in the
longitudinal or transverse direction. When the angle of
bracing walls varies, the earthquake loads in the walls
vary from those assumed in developing the IRC
provisions. Non-standard load path detailing may also be
required.
46
Illustration
Chapter 2, Earthquake-Resistance Requirements
Irregularity
Number
7
IRC Section
R301.2.2.2
Description
Discussion
When stories
above grade
partially or
completely
braced by wood
wall framing in
accordance with
Section R602 or
steel wall
framing in
accordance with
Section R603
include masonry
or concrete
construction.
IRC May Be Used If: Concrete or masonry construction
within a light-frame house is limited to those items listed
in the exception (fireplaces, chimneys and veneer).
Illustration
Engineered Design Is Required If: Other concrete or
masonry construction is mixed with light-frame walls in
any story above grade.
Discussion: The IRC wall bracing requirements for wood
or steel light-frame walls are proportioned to resist
earthquake loads from light-frame wall systems only.
Introduction of concrete or masonry will increase
earthquake loads beyond the wall bracing capacity. In
addition, introduction of concrete or masonry walls will
likely effect the distribution of wall stiffness, causing a
plan irregularity.
47
FEMA 232, Homebuilders’ Guide
48
Chapter 3 FOUNDATIONS AND FOUNDATION WALLS This chapter discusses foundations and foundation walls constructed using the two most common
foundation materials – concrete and masonry. Although the IRC permits the use of treated wood
for foundations and foundation walls and insulating concrete form (ICF) for foundation walls,
this guide does not cover those materials other than to inform the reader that wood foundations in
Seismic Design Categories (SDCs) D1 and D2 require engineering design and many of the
reinforcement requirements for concrete also apply to ICF (see Chapter 4 of the IRC for more
information on the use of these materials). The subject of frost protection of foundations also is
not discussed in this guide but, where required by the code (see IRC Section R403.1.4.1) or local
regulations, foundations must either extend below the frost line or be protected from frost using
approved methods.
3.1 GENERAL FOUNDATION REQUIREMENTS
Foundations are the interface between a house and the supporting soils. Many issues must be
considered when selecting a foundation system including site topography, soils conditions,
retaining requirements, loading from the house above, frost depth, and termite and decay
exposure. Foundations primarily provide support for vertical gravity loads from the weight of a
house and its contents, but they also provide resistance to horizontal sliding resulting from
earthquake ground motions and must resist vertical loads at the ends of braced walls.
Regardless of Seismic Design Category, all houses require a continuous foundation extending at
least 12 inches below undisturbed soil along all exterior walls as shown in Figure 3-1.
Figure 3-1 Perimeter foundation with separately placed footing and stem wall.
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FEMA 232, Homebuilders’ Guide
When earthquake ground motion occurs, the resulting ground movements, velocities, and
accelerations are imparted to the foundation and, in turn, transferred to a house or other building.
How well the house performs during an earthquake depends on the foundation being able to
provide:
• Continued vertical support,
• Friction and passive bearing at the soil-to-foundation interface to minimize movement
and damage,
• Anchorage at the foundation-to-house interface to minimize movement and damage, and
• Strength and stiffness sufficient to resist both horizontal loads and vertical loads resulting
from racking and overturning of bracing walls within the house.
The foundation of the house must resist the sliding and overturning actions associated with an
earthquake. These two actions are illustrated in Figures 3-2 and 3-3. The soil surrounding a
foundation can resist sliding using a combination of friction along the bottom and bearing along
the sides of the foundation; therefore, a wider and deeper foundation provides greater friction
and greater bearing resistance than a shallow and narrow foundation. The whole overturning
action illustrated in Figure 3-3 is resisted at the foundation in two ways. The portion of the
foundation being pushed downward will bear against the soil below so a wider footing will
provide more surface area to resist that downward load. At the uplift end of the foundation, the
weight of any soil located above a footing helps to resist the loads trying to pull the foundation
out of the ground; therefore, a deep inverted T-shaped foundation will provide greater resistance
to uplift than a shallow footing or than a foundation having a shape that avoids having any soil
above the top of the footing.
Figure 3-2 Sliding action resisted by foundation.
50
Chapter 3, Foundations and Foundation Walls
Figure 3-3 Overturning action resisted by foundation.
The IRC discusses foundations (footings and stem walls) and foundation walls separately and
contains requirements for those elements based on the materials used for their construction. This
guide chapter is similarly organized and addresses foundations placed monolithically with a slab
on grade and foundations having a combination footing and stem wall as illustrated in Figure
R403.1(1) of the IRC. Foundation systems such as pilings, drilled piers, and grade beams require
the involvement of a licensed design professional and are not discussed in this guide.
IRC foundation wall provisions have evolved from similar provisions in the CABO One- and
Two-Family Dwelling Code and Standard Building Code. Neither the IRC nor the other building
codes give definitive guidance on when to use the foundation wall provisions of IRC Section
R404 rather than the footing stem wall provisions of IRC Section R403. The 2000 IRC
Commentary notes that the foundation wall provisions are primarily for masonry and concrete
basement walls. The provisions of IRC Section R404 become mandatory for wall heights of 5
feet and greater and for walls retaining unbalance fill of 4 feet or greater. For wall heights and
unbalanced fill heights less than this, there are few practical differences between the foundation
and foundation wall provisions.
IRC Section R403.1.2 contains a general rule applicable to buildings located in SDCs D1 and D2
that requires interior braced wall lines to be supported on a continuous foundation when the
spacing between parallel exterior wall lines exceeds 50 feet. However, IRC Section R602.10.9
contains a slightly more restrictive requirement. For a two-story house in SDC D2, a continuous
foundation is required below all interior braced walls, even when the distance between exterior
walls does not exceed 50 feet, unless three additional conditions can be met. Those conditions
are:
• The distance between braced wall lines does not exceed twice the building width measured parallel to the interior braced wall line; • In houses having either a crawl space or basement, cripple walls cannot exceed 4 feet in
height; and
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FEMA 232, Homebuilders’ Guide
•
In houses having a crawl space or basement, first-floor interior braced walls are
supported on double joists, beams, or blocking as shown in Figure 3-4.
Figure 3-4 Interior braced wall
on floor framing.
Above-code Recommendation: The main consequence of not providing a continuous
foundation below a first-story interior braced wall line is that the floor must be strong enough
to transfer earthquake loads from that interior location to a parallel perimeter foundation.
This transfer of lateral loads can be accomplished using a wood-framed and -sheathed floor
system, but this solution will definitely impart additional stresses in the floor and into cripple
walls at the perimeter that would not occur if a foundation was provided below the interior
braced wall line. Vertical loads due to the overturning loads in the braced wall segments also
must be transferred to the perimeter foundation through bending action. While the doubling
of the floor joist is an improvement, supporting these walls directly on a continuous
foundation is recommended to achieve above-code performance.
For slab-on-grade construction in SDCs D1 and D2, when the conditions described above require
an interior braced wall line to have a foundation, the foundation depth along that interior wall
must be at least 18 inches below the top of the slab as shown in Figure 3-5.
Figure 3-5 Foundation requirements for interior braced wall line on slab-on-grade construction.
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3.2 CONCRETE FOUNDATIONS
Regardless of Seismic Design Category, the minimum specified concrete strength for
foundations (and foundation walls) is 2,500 pounds per square inch (psi) with higher strength
necessary when a foundation is exposed to the weather and the house is located in a moderate or
severe weathering probability area as shown in IRC Figure R301.2(3). Specifying 2,500 psi
refers to a measure of the concrete’s compressive strength. To enable a concrete foundation to
resist all of the possible loads to which it may be exposed, compressive strength needs to be
complimented with tension capacity. Since concrete is unable to resist tension stresses without
cracking, steel reinforcing bars are added to resist tension. Reinforcing is particularly valuable
when resisting cyclic earthquake loads because, within the span of a few seconds, the loads may
start by causing compression and then reverse to cause tension in the same location.
IRC Section R403.1.3 specifies minimum reinforcement of concrete footings located in SDCs D1
and D2. Separate subsections within IRC Section R403.1.3 address reinforcing of foundations
consisting of a footing and a stem wall and reinforcing of the footing along the perimeter of a
slab-on-grade. IRC Section R403.1.3 also contains an exception that allows omitting the
longitudinal reinforcing in concrete footings for houses that are three stories or less in height and
constructed with stud walls, regardless of the Seismic Design Category.
Above-code Recommendation: To obtain above-code performance in SDC C, it is
recommended that the minimum foundation reinforcing requirements for SDCs D1 and
D2 be used. This added reinforcing will provide better footing performance whether it is
resisting earthquake loads or loads induced by differential soil settlement, expansive soils, or
frost heave.
Typically, the bottom portion of a concrete footing must have one horizontal No. 4 reinforcing
bar located 3 inches up from the bottom of the concrete (clear from the soil along the bottom of
the footing). When a foundation consists of both a footing (horizontal foundation segment) and
short stem wall (vertical foundation segment), two No. 4 continuous horizontal reinforcing bars
are required – one in the bottom of the footing and one near the top of the stem wall as shown in
Figure 3-6.
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Figure 3-6 Recommended minimum reinforcement for concrete footings and stem walls.
Above-code Recommendations: When horizontal reinforcing bars are used in foundations,
they need to be continuous to perform their intended function. The IRC does not provide any
specific guidance on reinforcing continuity; therefore, the following above-code
recommendation is derived from the basic standard for concrete construction (ACI 318-02,
2003). Where two or more pieces of reinforcing steel are used to provide continuous
horizontal reinforcing, the ends of the bars should be lapped to provide continuity. The
minimum recommended lap for No. 4 bars is 24 inches and for No. 5 bars is 30 inches.
As shown in Figure 3-7, horizontal bars terminating at corners of perimeter foundations
and where an interior foundation intersects a perimeter foundation should have a
standard 90-degree hook of 8 inches for No. 4 bars or 10 inches for No. 5 bars.
Figure 3-7 Above-code horizontal reinforcing lap at corners and intersections.
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Chapter 3, Foundations and Foundation Walls
In SDCs D1 and D2, No. 4 vertical reinforcing is required at 48-inch maximum spacing if a
construction joint (also called a “cold joint”) occurs between the footing and the stem wall. This
often is referred to as a “two-pour” foundation because the concrete for the footing and the
concrete for the wall are poured at separate times. These vertical bars (also called “dowels”)
must extend a minimum of 14 inches into the stem wall and be hooked at the bottom into the
footing as shown in Figure 3-6. These dowels provide a very important connection because,
without them, earthquake loads can cause sliding to occur along the joint between the two
separate concrete placements. Sliding along a similar construction joint between a slab-on-grade
and a separately poured footing below the slab edge occurred during the 1994 Northridge
earthquake and caused severe damage in houses located in Simi Valley, California.
When a house has a concrete slab-on-grade with a thickened edge forming its perimeter
foundation (also called a “turned-down slab edge”), one No. 4 horizontal reinforcing bar is
required in the top and bottom of this footing as shown in Figure 3-8. The exception to this is
when the slab and footing are poured at the same time; in this case, a single No. 5 bar or two No.
4 bars located in the middle third of the combined slab and footing depth may be used. For slabon-grade construction in SDCs D1 and D2, interior bearing walls and interior braced walls
required to have a continuous foundation must have the concrete slab thickened to 18 inches to
form a foundation as shown in Figure 3-5.
Above-code Recommendation: Although the IRC is basically silent on how to reinforce
these interior thickened slab foundations, it is recommended that they should be
reinforced as described above for the foundation along the slab perimeter. Horizontal
reinforcing in slab-on-grade foundations should be continuous as described earlier.
Figure 3-8 Above-code use of vertical dowels to connect a slab-on-grade to a
separately poured footing.
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Above-code Recommendation: When a slab-on-grade is poured separately from the footing
below, the IRC does not specify any vertical reinforcing across this joint. When this
condition occurs, however, the possibility of earthquake loads causing sliding along that joint
is very real. The basic provisions of ACI 318-02, Sec. 11.7.9, for concrete require that all
construction joints be provided with a mechanism for transferring loads through the joint.
Therefore, it is recommended that the same amount of vertical reinforcing required for
a separately poured stem wall and footing condition (No. 4 at 48 inches on center) be
installed to connect a slab to a separately poured footing as shown in Figure 3-8. In
addition the surface of the construction joint should be cleaned to remove loose debris
prior to placing the concrete slab.
Above-code Recommendation: Anchor bolts connecting a wood sill plate to a foundation
also must be installed in the foundation or stem wall. Although it is possible to install
anchor bolts during the placement of the foundation concrete before it has hardened
(also called “wet setting”), this is not recommended. Anchor bolts should be installed
and secured so that they will not move prior to placing the concrete. The reason is that
a wet setting can create a void in the concrete adjacent to the bolt. Anchor bolts, whether
they have heads or hooks on the embedded end, will either make a hole or carve a slot as
they are pushed into the wet concrete. That hole or slot forms a void over the entire length
of the bolt’s embedment. This void prevents the bolt from completely bearing against the
surrounding concrete and could result in movement of the bolt when subjected to
earthquake loads. Additional anchor bolt requirements are provided at the end of this
chapter.
3.3 MASONRY FOUNDATIONS
Masonry foundation requirements are generally more dependent upon the Seismic Design
Category of the site than concrete foundation requirements. Solid clay masonry or fully grouted
concrete masonry and rubble stone masonry may be used for foundations in SDCs A, B, and C;
however, rubble stone masonry is not allowed in SDCs D1 and D2 due to its relatively low
strength and stability.
Above-code Recommendation: Because of their relatively low strength and stability,
rubble stone foundations are not recommended for use in SDC C as an above-code
measure.
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Chapter 3, Foundations and Foundation Walls
Above-code Recommendation: Like concrete, a masonry foundation with a stem wall must
have at least one No. 4 horizontal bar in the bottom of the footing and one No. 4 bar near the
top of the stem wall. This horizontal reinforcing needs to be continuous just as it does in
concrete construction. Therefore, the above-code recommendations for lap splices are 24
inches for No. 4 bars and 30 inches for No. 5 bars with hook extensions at corners as
shown in Figure 3-7.
In SDCs D1 and D2, masonry stem walls also must have vertical reinforcing and must be grouted
as required by IRC Sections 606.11.3 and 606.12. (See Chapter 6 of this guide for a more
detailed discussion of grouting in masonry wall construction.) For a masonry stem wall
supported on a concrete footing, the vertical reinforcing must be one No. 4 bar at 48-inches
maximum spacing, extending into the footing with a standard hook at the bottom, similar to what
is shown in Figure 3-6. This is necessary to prevent sliding along the bottom mortar joint
between the concrete footing and the masonry stem wall.
Above-code Recommendation: In order to prevent sliding along the bottom mortar
joint between the concrete footing and the masonry stem wall, it is recommended that
masonry stem walls in SDC C also use the vertical reinforcing required in SDCs D1 and
D2.
3.4 FOOTING WIDTH
Footing width is not dependent upon Seismic Design Category but instead is solely based on
vertical load considerations. The minimum width for a concrete or masonry footing is dependent
on the load bearing capacity of the soil measured in pounds per square foot (psf) and the number
of stories and the weight of the wall it supports. For instance, when brick veneer is installed, this
added weight requires a wider footing.
The minimum soil bearing capacity considered by the IRC is 1,500 psf. In this case, the
minimum footing width is 12 inches but it can increase to as much as 32 inches for a three-story
house with brick veneer as shown in Figure 3-9.
Bearing capacity is determined based on the soil classification determined for the site. Soil
classifications and corresponding bearing capacities are listed in IRC Table R401.4.1 at the
beginning of IRC Chapter 4. The soil classification system is described in IRC Table R405.1.
There are four distinct groups of soils that comprise a total of 15 separate soil classifications
ranging from well graded gravels as the best and peat as the worst. Accurate determination of
soil bearing capacity requires correct classification of the soil at the building site and may require
the expertise of a soils engineer or geologist. However, most building departments have
determined the soil classification for most of the sites within their jurisdiction. Accurate
determination of the correct site soil classification is important not just for determining the
minimum footing width but also for determining the minimum reinforcing required for concrete
or masonry foundation walls.
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When a footing is constructed with a width at the bottom that is greater than the thickness of its
stem wall or a width greater than the thickness required for a foundation wall (e.g., an inverted-T
shape), the minimum thickness of the footing is 6 inches. IRC Section R403.1.1 also specifies
both a minimum and maximum width for the footing portion of this inverted-T foundation. As a
minimum, the projection of the vertical face of the footing must be at least 2 inches beyond each
vertical face of the stem wall or foundation wall. The maximum projection may not exceed the
thickness of the footing. These dimensional requirements for an inverted-T foundation are
illustrated in Figure 3-9. These minimum and maximum projection limits of the footing beyond
the stem wall are consistent with ACI 318 design of plain concrete footings. Based on those
requirements, an L-shaped footing should not be permitted unless its dimensions and reinforcing
are designed to account for the eccentricity of the vertical load on the footing.
Figure 3-9 Inverted -T footing dimensions.
3.5 SPECIAL SOIL CONDITIONS
On some sites, such as those in marshy areas or bogs, the bearing capacity of the soil may be
known or suspected to be less than 1,500 psf. In such a case, the IRC requires a soil
investigation and report to determine the actual bearing capacity of the soil and to define an
appropriate footing width and depth. Soil testing also is necessary when it is likely that existing
soil has shifting, expansive, compressive, or other unknown characteristics. Local officials often
are aware of such conditions in locations where previous buildings have been constructed and
may have maps identifying such areas and/or special rules that apply to foundation construction
in such areas. When existing soil data are not available, such as on previously undeveloped sites,
it is always prudent to obtain tests to validate bearing capacity and to determine if expansive
soils are present.
When expansive soils that exhibit large changes in volume (usually in response to changes in
moisture content) are encountered, foundations must be designed in accordance with IBC
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Chapter 3, Foundations and Foundation Walls
Chapter 18 because the prescriptive requirements of the IRC assume no such special conditions
are present. Methods to address expansive soil include foundation designs to resist the stresses
caused by the soil volume changes that are likely to occur, isolation of the foundation, removal
of the expansive soil, or stabilization of the soil by chemical, dewatering, presaturation, or other
methods. Failure to identify and adequately compensate for differential movements caused by
expansive soils can result in excessive stresses on the foundation causing cracking of even
reinforced concrete or masonry foundations. Foundation movement induced by expansive soil
also can result in differential movement of the house’s wood framed walls that can crack brick,
gypsum wallboard, and stucco finishes. Movement of the walls can create stresses that loosen
the nailed connections of wall sheathing used to provide lateral bracing. These effects can, in
turn, weaken a building’s earthquake resistance so it is very important to address expansive or
other special soil conditions to limit differential foundation movement.
3.6 FOUNDATION RESISTANCE TO SLIDING FROM LATERAL LOADS
Foundations are the final link in the load path within a building to transfer the earthquake loads
to the ground. At the foundation level, the combined lateral loads from the entire building are
attempting to push the building laterally. To resist movement, the foundation pushes against the
soil that surrounds it. Consequently, footing width and depth are factors that determine the
resistance that can be provided by the foundation to earthquake ground motions. This is because
footing width determines the horizontal surface area of the bottom of a foundation in contact
with the ground and the depth determines the vertical surface area bearing against the soil on
either side. These surface areas provide resistance to sliding through a combination of friction
and bearing against the soil as shown in Figure 3-10. Therefore, a wider or deeper foundation
will be capable of resisting greater lateral loads than a narrower or shallower foundation.
Similarly, the sliding resistance of a slab-on-grade house will be greater than that of one having
only a perimeter foundation due to the added frictional surface area provided by the underside of
the slab.
Figure 3-10 Lateral resistance provided by foundation.
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Above-code Recommendation: For houses with the first floor located above a crawl space
rather than a basement, IRC Section 408.5 allows finished grade under the floor to be located
at the bottom of the footing except where the groundwater table is high, surface drainage is
poor, or the area is prone to flooding. However, when the finished grade is located at the
bottom of the footing, the vertical face of the footing on that side does not bear against soil.
Therefore, as an above-code measure, in SDCs D1 and D2, it is recommended that crawl
space perimeter footings have their entire depth below the finished grade of the crawl
space. Embedment along both vertical faces of the footing provides additional bearing
surface area to resist sliding loads perpendicular to the exterior walls.
Above-code Recommendation: To aid in providing sliding resistance, the bottom of
footings should be level. The IRC does allow the bottom of footings to be sloped at a rate of
not more than 1 foot vertical in 10 feet horizontal. When this maximum slope along the
bottom of the footing cannot be met, the bottom must be stepped. Similarly, the top of
foundations must be level but can also be stepped. As an above-code measure when steps
are used, the horizontal reinforcing should be bent to extend through the steps as shown
in Figure 3-11.
Figure 3-11 Above-code stepped foundation reinforcing detail.
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Chapter 3, Foundations and Foundation Walls
Above-code Recommendation: Special consideration also must be given to sites where the
ground slopes upward or downward beyond the limits of the perimeter foundation to ensure
that adequate resistance to foundation sliding and settlement are provided and to protect
against the effects of drainage, erosion, and shallow failures of the sloping surface.
Prescriptive rules for setbacks from either an ascending or descending slope are given in IRC
Chapter 4. Smaller (or greater) setbacks may be approved (or required) based on a soil
investigation report prepared by a qualified engineer. Because landslides can be triggered
by earthquakes, any building site having natural or man-made sloping terrain above or
below should be thoroughly evaluated for landslide potential even when the prescriptive
setbacks of the code are met.
3.7 SPECIAL CONSIDERATIONS FOR CUT AND FILL SITES
A hillside site can result in foundations being supported on soils having very different bearing
capacities. Figure 3-12 shows a situation where a portion of the foundation is supported on rock
and the other side is supported on a fill that extends above the existing rock grade. This
condition often occurs when soil is removed (cut) from the high side of a lot and the lower side
has fill material installed to create a level building pad. Although the IRC requires all fill soils to
be designed, installed and tested in accordance with accepted engineering practice, there is no
specific guidance given in the IRC regarding what should be addressed. Site-specific guidance
on the design and placement of fill material is a particularly important concern in high seismic
areas.
Figure 3-12 Foundation supported on rock and fill.
Studies of damage to houses located on cut and fill sites as a result of the 1971 San Fernando
(McClure, 1973, Slosson, 1975) and the 1994 Northridge (Stewart, et al., 1994 and 1995)
earthquakes document the consequences of improperly installed and compacted fill. Figure 3-13
shows an example of the type of damage that occurred to slab-on-grade houses during the
Northridge earthquake. Generally, these sites experienced settlement and extension of the fill
portion of the site and vertical displacement along the line of transition from cut to fill.
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Above-code Recommendation: To meet the intent of the IRC requirement for all fill
soils to be designed, installed and tested in accordance with accepted engineering
practice, it is recommended that the reporting requirements conform to those specified
in IBC Section 1803.5.
Figure 3-13 Example of damage
caused in building on cut and fill
site.
Above-code Recommendation: Even with proper installation and compaction of engineered
fills, earthquakes are expected to result in some differential settlement and consequent
damage when a house has a portion of its foundation on a cut pad and other portions on fill.
Therefore, in SDCs C, D1 and D2, sites that require a cut and fill approach should be
avoided, particularly if a slab-on-grade foundation is used. Where cut and fill grading
of a site is unavoidable, increased levels of quality control should be used to ensure the
optimum installation of the fill, and foundations should be designed to either
accommodate or resist the expected settlement.
3.8 FOUNDATION WALLS
Foundation walls are typically basement walls but also include foundation stem walls that extend
from the top of a footing to the bottom of a wood framed floor and enclose a crawl space as
shown in Figure 3-6. Depending on the difference in ground level on each side of a foundation
wall, certain minimum requirements for reinforcing and wall thickness will apply. Foundation
wall construction is dependent to some extent on Seismic Design Category, particularly for
SDCs D1 and D2 sites.
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Foundation walls can be constructed of concrete, masonry, or preservatively treated wood or by
using insulating concrete form (ICF) systems. As mentioned earlier, treated wood systems are
not discussed in this guide. ICF foundation walls are likewise not discussed because IRC Section
R404.4.1 limits use of ICF foundation walls to SDCs A, B and C; however, future IRC editions
may allow adequately reinforced ICF foundation walls in SDCs D1 and D2 if proposed code
changes on this subject are approved. Rubble stone masonry foundation walls also are not
permitted in SDCs D1 and D2 for reasons similar to those that prohibit this material’s use as a
footing in SDC D1 and D2.
Regardless of Seismic Design Category, all foundation walls must extend a minimum of 6 inches
above the adjacent grade or, when brick veneer is used on a wall, the foundation wall must
extend a minimum of 4 inches above the adjacent grade.
Concrete and masonry foundation walls must conform to the prescriptive requirements of the
IRC or may be based on other recognized structural standards such as ACI 318 for concrete or
either NCMA TR68-A or ACI 530/ASCE 5/TMS 402 for masonry. When concrete or masonry
walls are subject to hydrostatic pressure from groundwater or support more than 48 inches of
unbalanced fill, without permanent lateral support at the top and bottom, they must be designed
using accepted engineering practice. The discussion below relates only to the IRC prescriptive
methods for concrete or masonry foundation walls.
Several terms used in this discussion warrant definition. “Plain concrete” and “plain masonry”
are not necessarily devoid of all reinforcing; however, they have less reinforcing than is required
to be officially designated as being “reinforced.” “Unbalanced backfill” is defined as the
difference in height between the exterior and interior finish ground levels except that, when an
interior concrete slab floor is present, the height is measured from the top of the slab to the
exterior finished ground level.
3.9 FOUNDATION WALL THICKNESS, HEIGHT, AND REQUIRED REINFORCING
The minimum thickness of plain concrete and plain clay or concrete masonry foundation walls
ranges from 6 inches to 12 inches depending on several variables. These variables include:
•
•
•
The height of the wall,
The site’s soil classification, and
The height of any unbalanced backfill.
Foundation wall thickness also must be at least equal to the width of the supported wall.
For plain masonry foundation walls, the minimum thickness also is dependent on the use of solid
masonry units or hollow units that can be either grouted or ungrouted. Generally, the minimum
thicknesses for solid masonry and grouted hollow masonry are identical whereas the minimum
thickness for ungrouted hollow masonry is greater. Minimum wall thickness for plain concrete
and plain masonry walls, based on specific combinations of these variables, are listed in IRC
Table R404.1.1(1).
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For SDCs D1 and D2, IRC Section R404.1.4 imposes the following additional limitations on plain
concrete and plain masonry foundation walls that are not specified in IRC Table R404.1.1(1):
•
•
•
•
•
Wall height is limited to 8 feet.
Unbalanced fill height is limited to 4 feet.
A single horizontal No. 4 reinforcing bar shall occur in the upper 12 inches of the wall.
Plain masonry walls shall be a minimum of 8 inches thick.
Plain concrete walls shall be a minimum of 7.5 inches thick except that a 6-inch
minimum thickness is permitted when the wall height does not exceed 4 feet 6 inches.
• Vertical reinforcing of masonry stem walls shall be tied to horizontal reinforcement
located in the footing.
In SDCs D1 and D2, when the foundation exceeds 8 feet in height or supports more than 4 feet of
unbalanced fill, additional minimum reinforcing requirements apply. First, two No. 4 horizontal
reinforcing bars are required at the top of the wall rather than just one. In addition, a concrete or
masonry wall’s vertical reinforcing is required to meet additional prescriptive minimums. The
vertical reinforcing size and maximum spacing are dependent on wall thickness, wall height,
unbalanced fill height, and soil classification. Three IRC Tables — Tables R404.1.1(2), (3) and
(4) – specify the minimum vertical reinforcing based on specific combinations of these variables.
The IRC also allows alternative sizes and spacing of reinforcing up to a maximum spacing of 6
feet; however, the alternate size and spacing must result in an equivalent cross-sectional area of
reinforcing per lineal foot of wall as prescribed in the tables.
Above-code Recommendation: As described earlier, reinforcing should always be
continuous; therefore, lap splices are needed where discontinuities occur in either
horizontal or vertical reinforcing. Lap lengths should not be less than 24 inches for
No. 4 bars or 30 inches for No. 5 bars.
Regardless of Seismic Design Category, all vertical reinforcing for these reinforced masonry or
concrete foundation walls must be at least ASTM Grade 60 (yield strength of 60,000 psi). This
is important to note because No. 4 bars are commonly available in Grade 40, which has a lower
yield strength. In addition, the distance from the soil side face of the wall to the centerline of the
vertical reinforcing must be 5 inches in an 8-inch-thick wall, 6.75 inches in a 10-inch-thick wall,
and 8.5 inches in a 12-inch-thick wall.
3.10 WOOD-FRAMED WALL BOTTOM PLATE AND FOUNDATION SILL PLATE
ANCHORAGE
For the purpose of the following discussion, the terms “wall bottom plate” and “foundation sill
plate” are used to distinguish between two different locations of wood members along a
foundation. A wall bottom (sole) plate either is directly supported by a slab-on-grade foundation
as shown in Figure 3-5 or is part of a cripple wall supported directly on a foundation when a slab
on grade does not occur. A foundation sill plate is different in that it occurs without wall studs
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Chapter 3, Foundations and Foundation Walls
framing on top of it; instead, a foundation sill plate is the bearing support between floor joists or
floor beams and the foundation as shown in Figure 3-6.
In all Seismic Design Categories, where anchor bolts are required to connect a bottom plate or
foundation sill plate to a foundation, the bolts must have a minimum embedment of 7 inches into
a concrete or masonry foundation, and they must have a nut and washer tightened on each bolt.
Typically, anchor bolts must be a minimum of 1/2-inch diameter and be spaced not to exceed 6
feet on center. However, in SDCs D1 and D2, the maximum bolt spacing is limited to 4 feet for
buildings with more than two stories. Because the IRC prescribes bracing only for houses in
SDC D2 up to a maximum of two stories, the closer spacing is actually only applicable in SDC
D1 where three stories is permitted. This decreased bolt spacing provides 50 percent more shear
capacity at the sill-plate connection in recognition that earthquake loads generated in a threestory building will be greater than those in a one- or two-story building.
Some builders may choose to use 5/8-inch diameter bolts at the standard 6 feet on center spacing
in place of using 1/2-inch diameter bolts at a required 4 feet on center spacing. Based on bolt
capacities listed in 2001 NDS Table 11-E, for most species of wood, 5/8-inch bolts at 6 feet on
center spacing provide approximately 95 percent of the capacity of 1/2-inch bolts at 4 feet on
center spacing. However, the approval of local officials should always be obtained prior to
making any such substitution.
In SDCs D1 and D2, the washer to be installed under the anchor bolt nut must be a 3-inch by 3­
inch square plate with a minimum thickness of 1/4 inch. This increased washer size limits the
potential for splitting of bottom plates.
Above-code Recommendation: Splitting of sill plates can occur when the ends of braced
wall segments are lifted vertically during the rocking motion braced walls undergo while
resisting earthquake loads. Cyclic testing of shear walls has shown that a standard round
cut washer is too small to provide any appreciable resistance to this uplift and allows sill
plates to split but, when larger square washers are used, walls can sustain much higher
lateral loads before sill plate splitting will occur. For this reason, the use of plate
washers is also recommended on anchor bolts connecting to the foundation in SDC C.
3.11 REQUIRED LOCATIONS FOR ANCHOR BOLTS ALONG EXTERIOR WALLS
Independent of Seismic Design Category, anchor bolts are required to connect a wall bottom
(sole) plate or foundation sill plate to foundations located along all exterior wall lines. A
minimum of two anchor bolts are required in each individual length of plate, with one bolt
located not more than 12 inches nor less than 7 bolt diameters from each end.
3.12 REQUIRED ANCHORAGE ALONG INTERIOR BRACED WALLS
The anchorage of wall bottom plates along interior wall lines is a bit more complex than it is for
exterior walls. The general rule is that when interior braced walls are supported directly on a
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continuous foundation, the bottom plate of the wall must be anchored using the same bolting
pattern as required for exterior walls. However, the IRC currently requires a continuous
foundation along interior braced wall lines in only a few situations. Consequently, bolting of the
sole plate of an interior braced wall to a foundation occurs only rarely because a foundation is
not normally required.
When an interior braced wall frames onto a raised wood framed floor, the code is clear that the
anchorage connection of the braced wall sole plate uses nails. However, the IRC is silent
regarding exactly what kind of anchorage is required for connecting an interior braced wall to a
slab-on-grade when a continuous foundation is not required and not provided. One common
example is a slab-on-grade house that has its exterior walls less than 50 feet apart and therefore
does not require its interior braced walls be supported on a continuous foundation. In such a
case, the slab usually will not have the thickness necessary to allow installation of anchor bolts
with the standard 7 inches of embedment. However, it is reasonable to assume that the braced
wall bottom-plate-to-slab connection should provide lateral load resistance at least equivalent to
the typical anchorage to a foundation using bolts. For the 1/2-inch diameter bolts at 6 feet on
center, the bolt shear capacity from NDS Table 11E, adjusted by 1.6 for earthquake loads, ranges
from 152 plf in Spruce-Pine-Fir lumber, to 176 plf in Southern Pine lumber. Therefore, the
connection to be provided to the slab should provide at least the same capacity as the 1/2-inch
anchor bolts. For 5/8-inch bolts at 6 feet on center, the capacity ranges from 219 plf in SprucePine Fir to 248 plf in Southern Pine.
When anchor bolts cannot be used because of limited slab-on-grade thickness, interior wall
bottom plates often are connected to a slab-on-grade with powder-driven nails or pins as shown
in Figure 3-14. However, because of their small diameter and shallow embedment length into
the slab, each powder-driven nail or pin has significantly less lateral capacity than a single 1/2­
inch diameter bolt. The nails or pins would need to be spaced at a much closer center-to-center
spacing to be equivalent to a 1/2-inch diameter bolt at 6 feet on center. The actual spacing
needed to achieve equivalence with anchor bolts generally depends on the diameter and length of
the nail or pin.
Figure 3-14 Interior braced wall on slab-on-grade.
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Chapter 3, Foundations and Foundation Walls
Many manufacturers of powder-driven nails or pins have published shear and tension capacity
values for their specific products when connecting wood members to concrete. However, it
should be noted that powder-driven nails or pins used for this purpose generally have very little
embedment depth into the concrete and, as a result, have very little tension capacity to resist the
uplift that can occur at the ends of braced walls. Because of this limited tension capacity, they
may not perform nearly as well as bolts. In addition, when pins are used, there is no way to
install the square plate washers required on anchor bolts in SDCs D1 and D2 and recommended
for use in SDC C.
Above-code Recommendation: In SDCs C, D1 and D2, when interior braced walls use
wood structural panel sheathing, it is recommended that bottom plates be anchored to a
thickened slab-on-grade using bolts as shown in Figure 3-5. Instead of the 18-inch
thickness shown in Figure 3-5, only a minimum 10-inch thickness is necessary for the slabon-grade to allow for 7-inch minimum embedment of the bolt and 3 inches of additional
clearance from the end of the bolt to the bottom of the thickened slab.
3.13 ANCHORING OF INTERIOR BRACED WALLS IN SDCs D1 AND D2
In SDCs D1 and D2, anchor bolts are required for interior first-story walls when:
• The first-story interior wall is a bearing wall (which may or may not be a braced wall
line) and it is supported on a continuous foundation.
• The interior first story-wall is a braced wall line and that wall line is required to be
supported on a continuous foundation.
When applying the two rules listed above, one must differentiate between interior walls that are
bearing walls and those that are braced walls. Although interior bearing walls can be braced
walls, this is not always the case. Therefore, to apply the rules it must be determined that:
• An interior wall is only a bearing wall and not a braced wall,
• An interior wall is a braced wall but is not a bearing wall, or
• The interior wall is both a bearing wall and braced wall.
Summaries of the 2003 IRC minimum requirements for continuous foundations and for installing
anchor bolts to a foundation along braced wall lines and bearing walls are presented in Tables 3­
1 and 3-2, respectively. When an interior wall is both a bearing wall and a braced wall, the most
restrictive requirement from Tables 3-1 and 3-2 applies.
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Table 3-1 Summary of 2003 IRC Continuous Foundation and Anchor Bolt Requirements
for Braced Wall Lines in One- and Two-family Houses
Seismic Design
Category
Continuous Foundation Required At
Braced Wall Lines
Anchor Bolts
Required
A, B and C
A, B and C
D1
YES along Exterior Braced Walls
NO along Interior Braced Walls
YES along Exterior Braced Walls
YES
NO
YES1
D1
NO along Interior Braced Walls
Unless wall lines spaced > 50 feet
YES along Exterior Braced Walls
NO along Interior Braced Walls
of 1 Story unless spaced > 50 feet
YES along Interior Braced Walls
of Two-Story2
NO
D2
D2
D2
Anchor Bolt
Diameter
Spacing
½”
@ 6’-0”
N/A
½” @ 6’-0”
½” @ 4’-0” >2 story
N/A
YES1
NO
½”
@ 6’-0”
N/A
YES1, 2
½”
@
3
6’-0” 3
1
Requires a square plate washer 3 x 3 x ¼ inch on each bolt. A continuous foundation is NOT required for interior braced wall lines of a two story building in SDC-D2, provided that all of the following conditions are met: A) The spacing between continuous foundations does not exceed 50 feet, B) cripple walls (if provided) do not exceed 4 feet in height, C) first story braced walls are supported on beams double joists or blocking, and D) braced wall line spacing does not exceed twice the building width measured parallel to the braced wall line. 3
Buildings are limited to two-story in SDC D2, therefore anchor bolts at 4’-0” on center spacing do not apply.
2
Table 3-2 Summary of 2003 IRC Continuous Foundation and Anchor Bolt Requirements
for Bearing Walls in One- and Two-family Houses
Seismic Design
Category
Continuous Foundation Required At
Bearing Walls
Anchor Bolts
Required
A, B and C
A, B and C
D1
YES along Exterior Walls
NO along Interior Bearing Walls
YES along Exterior Walls
YES
NO
YES1
D1
YES along Interior Bearing Walls
supported on a slab-on-grade
NO along Interior Bearing Walls
supported on a raised floor
YES along Exterior Walls
YES along Interior Bearing Walls
supported on a slab-on-grade
NO along Interior Bearing Walls
supported on a raised floor
YES1
D1
D2
D2
D2
1
2
NO
YES1
YES1
NO
Anchor Bolt
Diameter
Spacing
½”
@ 6’-0”
N/A
½” @ 6’-0”
½” @ 4’-0” >2 story
½” @ 6’-0”
½” @ 4’-0” >2 story
N/A
½”
@ 6’-0” 2
N/A
N/A
Requires a square plate washer 3” x 3” x ¼” on each bolt.
Buildings are limited to two-story in SDC D2, therefore anchor bolts at 4’-0” on center spacing do not apply. 68
Chapter 4 FLOOR CONSTRUCTION
Woodframe floor systems and concrete slab-on-grade floors are discussed in this chapter.
Although cold-formed steel framing for floor systems also is permitted by the IRC, it is not
covered here; rather, the reader is referred to the AISI Standard for Cold-Formed Steel Framing
– Prescriptive Method for One- and Two-Family Dwellings (AISI, 2001) for guidance. Also
permitted but not discussed here are pressure-treated wood floor systems on ground; information
on the use of these systems is provided in IRC Chapter 5.
4.1 GENERAL FLOOR CONSTRUCTION REQUIREMENTS
Woodframe floor systems form a horizontal diaphragm at each level where they occur and
transfer earthquake lateral loads to braced walls below that floor level or directly to the
foundation when the lowest floor is supported on a foundation. When a floor supports walls
above and is supported on walls below as shown in Figure 4-1, the lateral loads in the floor
system are based on the mass of the floor itself and a portion of the mass of all the walls in the
stories immediately above and below the floor. (See Chapter 2 of this guide for a discussion of
the complete load path.)
Figure 4-1 An unblocked floor diaphragm.
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Concrete slab-on-grade floors typically are constructed with a concrete perimeter foundation and
together these elements form the base of the building. Lateral forces from exterior and interior
braced wall lines are transferred to a slab-on-grade via connections between the bottom plate of a
braced wall and the slab. In turn, the concrete slab and foundation transfer those forces directly
to the ground as shown in Figure 4-2. (For more information on anchorage of braced walls to
slab-on-grade construction, see Chapter 3 of this guide.)
Figure 4-2 Slab-on-grade and perimeter footing transfer loads into soil.
4.2 WOODFRAME FLOOR SYSTEMS
Woodframe floors typically consist of repetitive joists or trusses, at a prescribed spacing,
sheathed with either boards or wood structural panels attached to the top surface. Finish
materials such as gypsum board typically are applied to the bottom surface where it serves as the
ceiling for a room below. Blocking between joists or trusses is used at the ends of the floor joists
or trusses (or a continuous band joist can be used at the ends) and where walls occur above or
below. Floor systems also include beams, girders, or headers where needed to support joists.
Joists can be sawn lumber, end jointed lumber, or a variety of prefabricated (engineered)
members. Examples of engineered lumber include wood I-joists, trusses, and solid rectangular
structural composite members such as parallel strand lumber (PSL), laminated veneer lumber
(LVL), and laminated strand lumber (LSL). Beams, girders, or headers and blocking also can be
either sawn lumber or engineered lumber.
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Chapter 4, Floor Construction
The primary design consideration in choosing the minimum size and the maximum span and
spacing of floor joists, trusses, beams, girders, and headers is adequate support for dead and live
vertical loads as prescribed by the code depending on the uses that a floor must support. Vertical
deflection of a floor is another design consideration that can limit the maximum span of floor
members. Tables in IRC Chapter 5 and similar tables in other documents such as those
published by the American Forest and Paper Association (AF&PA) or engineered lumber
manufacturers are available for use in selecting the proper combination of minimum size and
maximum span and spacing of floor framing members.
4.3 CANTILEVERED FLOORS
When floor joists cantilever beyond a support, joist size and spacing are limited by prescriptive
tables in IRC Chapter 5. IRC Table R502.3.3(1) addresses support of a roof and one story of
wall for roof spans up to 40 feet and snow loads up to 70 psf. IRC Table R502.3.3(2) addresses
cantilever joists supporting an exterior balcony. When a floor is supporting more than a roof and
one story of wall, the maximum prescriptive cantilever distance is limited by the IRC to the
depth of the joist. If longer cantilevers are desired, a registered design professional must design
that portion of the floor system.
In Seismic Design Categories D1 and D2, when cantilevered floor joists support braced wall
panels in the story above, the cantilevered floor is limited by several additional prescriptive
requirements in IRC Chapter 3. This is because the braced wall above and braced wall below are
offset out-of-plane. When a floor cantilever supporting a braced wall does not meet the IRC
limits, that portion of the house is defined as having an irregularity that prevents the use of
prescriptive wall bracing where the irregularity occurs. In such a case, engineering must be
applied to resolve the out-of-plane offset of the braced walls located in the stories above and
below that floor. The maximum permitted cantilever of a second floor supporting a braced wall
and roof is illustrated in Figure 4-3. (Also see the discussion of load path in Chapter 2 of this
guide.)
Figure 4-3 Cantilevered floor restrictions.
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The specific limits and requirements in IRC Chapter 3 for cantilevered floors in SDCs D1 and D2
that support braced walls are not particularly difficult to meet and appear to omit addressing the
uplift restraint that may be necessary at the back span support of cantilever joists. In SDCs D1
and D2, cantilever floor joists supporting a braced wall panel may not extend more than four
times the nominal depth of the joist when the following set of rules is met:
• Joists must be 2x10 nominal or larger at 16-inch maximum spacing.
• The back span of the cantilever joist must be at least twice the cantilever distance.
• Joists must be doubled at the ends of the braced wall panel above.
• A continuous rim joist is connected to the end of each cantilevered joist. If that rim joist
is spliced along its length, the splice must be made with either: (a) a 16-gage strap
having 6 – 16d common nails on each side of the splice or (b) by using wood blocking
having the same size as the rim joist, installed between the cantilevered joists, and nailed
to the rim with 8 –16d common nails on each side of the splice.
• The cantilever end of the joist is limited to supporting uniform loads from a roof and wall
above and, if supporting a header above, the header span is limited to 8 feet.
These rules are illustrated in the framing plan shown in Figure 4-4. What is not mentioned in
this list of rules is the need for connections to resist uplift at the back-span (interior) end of a
cantilever joist as noted in Figure 4-4. In IRC Table R502.3.3(1) for cantilever joists supporting
a roof and wall, the uplift is determined using a back-span distance that is three times the
cantilever distance (3:1). Because the minimum back span specified in the IRC Chapter 3 (see
Item 2 above) is only twice the cantilever distance (2:1), the uplift values in IRC Table
R502.3.3(1) would need to be increased by a factor of 1.5 just to address the gravity loads.
When the downward earthquake overturning load from the ends of a braced wall panel supported
by cantilever joists are considered in addition to gravity loads, the uplift load at the back-span
end of the joist obviously will increase. Therefore, depending on the actual back-span-to
cantilever-length ratio, the back-span end of the double cantilever joists supporting the ends of a
braced wall may need to provide uplift restraint as much as twice than that listed in IRC Table
R502.3.3(1). However, because the magnitude of the uplift load at the back-span end of a
cantilevered joist reduces as the back-span length increases, it is possible that a cantilever joist
that is continuous over its interior support will result in zero uplift at the back-span end. When
cantilever joists are continuous over an interior support, the back span increases and the uplift at
the end of the joist is greatly reduced. Therefore, the specific cantilever floor joist layout and
ratio of the length of the back span to the cantilever will determine if and how much uplift may
need to be resisted.
Above-code Recommendation: In SDCs C, D1 and D2, when a braced wall is supported
at the ends of cantilever joists, the back-span uplift connection capacity should be
determined using engineering principals for the specific back-span and cantilever
distances involved.
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Chapter 4, Floor Construction
Figure 4-4 Cantilever joist at braced wall above.
4.4 REQUIREMENTS FOR BLOCKING
It is important in floor framing construction to prevent joists (or trusses) from rotating or
displacing laterally from their intended vertical position. Rotation loads occur because, when
floor sheathing is resisting lateral loads oriented perpendicular to the joist, those lateral loads are
actually trying to move the top edge of the joist sideways.
Preventing rotation is often accomplished by installing full-depth solid blocking at the ends of
joists. The ends of joists also can be restrained by attaching the joist to a continuous rim or band
joist or a header or, in balloon framed walls, by attaching the joist to the side of a stud. In SDCs
D1 and D2, additional solid blocking between joists (or trusses) is necessary at each intermediate
support even when that location is not at the end of the joist. For example, intermediate support
should be located at an interior girder or bearing wall where joists are continuous over that
support. Blocking installed between joists supported by an interior floor girder is illustrated in
Figure 4-5.
Blocking also is required below an interior braced wall line in all Seismic Design Categories
when joists are perpendicular to the braced wall. Although the IRC is silent regarding minimum
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FEMA 232, Homebuilders’ Guide
depth and width for these blocks, the intent of this added blocking is to provide a nailing surface
for the 16d common nails used to connect the bottom plate of the braced wall to the floor. This
nailing is an important part of the lateral load path; therefore, the blocking should be of a depth
sufficient to allow full embedment of the 16d common nails and of sufficient width to prevent
the nails from missing the block.
Assuming floor sheathing is at least 1/2- inch thick, the minimum depth of the blocking should
be 1-1/2 inches. Therefore, a flat 2-inch by 4-inch block as shown in Figure 4-6 can provide
sufficient depth and, when accurately placed below a wall, can provide a width that greatly
reduces the potential for bottom plate nails missing the block.
Figure 4-5 Interior bearing line.
Figure 4-6 Blocking below interior braced wall where floor joists are perpendicular to wall.
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Chapter 4, Floor Construction
When an interior braced wall also is a bearing wall and joists below the wall are parallel to the
wall, a double joist or a beam typically is provided in the floor below the wall. Occasionally this
pair of joists may be spaced apart to allow for piping or vents passing vertically from the wall
above through the floor. When this occurs, the double joists cannot be located directly below the
wall’s bottom plate. To provide a nailing surface for the bottom-plate connection of the braced
wall above, 2x flat blocking should be installed in line with the braced wall’s bottom plate
between and parallel to these spaced joists as shown in Figure 4-7.
Figure 4-7 Blocking for floor joists spaced apart for piping in floor.
4.5 CONNECTION OF FLOOR JOISTS TO WALL TOP PLATE OR FOUNDATION
SILL PLATE BELOW
Floor joists (or trusses) are required to be connected to the top plate of supporting walls or to a
foundation sill plate as specified in IRC Table R602.3(1). Each of these connections provides a
load path to transfer loads from the floor diaphragm into the braced walls or the foundation
below. Nailed connections must meet the following minimum requirements:
• Rim or band joists parallel to a wall or foundation require a toe-nailed connection to the
wall top plate or foundation sill plate using 8d box or common nails at 6 inch spacing.
• Floor joists perpendicular to a wall top plate or foundation sill plate require a toe-nailed
connection using three 8d box or common nails.
• When blocking is installed between the floor joists, the blocking requires a toe-nailed
connection to the wall top plate or foundation sill plate using a minimum of three 8d box
or common nails in each block.
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Where toe nailing is used, it must be done correctly so that it can transfer the intended loads and
so that the nails do not split the wood when being installed. Toe-nailed connections prescribed
by the IRC should be acceptable when connecting joists to wall top plates or foundation sills that
are perpendicular to the joists because these connections are not highly loaded by lateral loads.
The primary lateral load transfer in a floor system occurs through the rim or band joists and
through blocking that is parallel to braced walls or foundation sill plates.
Information on proper toe-nail installation is presented in Figure 4-8; however, that idealized
picture of nail inclination and location is difficult to achieve in actual construction.
Consequently, many toe-nailed connections that must transfer lateral loads may not actually
perform very well.
Above-code Recommendation: In SDCs C, D1 and D2, connections between joists or
blocking and wall top plates or foundation sill plates that are parallel to the joist or
blocking should use commercially available light-gage steel angles and nails of the
correct diameter and length for the product. Many of the toe-nailed connections specified
in the IRC also can be made using light-gage steel angles through which face nails are driven
into the two wood framing members being connected. Although light-gage angles may
require more time to install than toe nails, the angle connections should reduce splitting of the
wood and can provide a more reliable connection capacity for lateral loads compared to toe
nails.
Figure 4-8. Toe nail configuration requirements.
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Chapter 4, Floor Construction
4.6 FLOOR SHEATHING
Floor sheathing can be either wood boards installed perpendicular or diagonally to the joists or
wood structural panels (as subfloor or combination subfloor-underlayment) installed with the
long direction of the panel perpendicular to the joists. The minimum thickness for wood board
sheathing depends on joist spacing and the orientation of the boards to the joist (e.g., perpen
dicular or diagonal). For wood structural panels, the minimum thickness is based on joist
spacing and the grade of sheathing panels selected. IRC Chapter 5 contains tables to use in
determining the required minimum thickness for sheathing materials based on a variety of joist
spacings.
Above-code Recommendation: Wood boards are rarely used in modern house construction
unless the underside of the floor is intended to be visible to the interior space below to
achieve a specific architectural effect. As an above-code measure, wood boards installed
perpendicular to joists should not be used in SDC C, D1, or D2 unless wood structural
panels are installed over the boards because the wood board sheathing alone provides
little resistance to lateral loads. In contrast, diagonally placed wood boards provide greater
lateral capacity and should be acceptable for small rectangular-shaped floor areas.
For modern construction, floor sheathing typically will be wood structural panels (OSB or
plywood). These panels are fastened to the joists based on a schedule prescribed in tables in IRC
Chapter 6.
4.7 LATERAL CAPACITY ISSUES FOR WOOD FRAMED FLOORS USING WOOD
STRUCTURAL PANELS
The lateral capacity of a floor diaphragm sheathed with wood structural panels is based on five
factors:
•
•
•
•
•
Sheathing thickness,
Sheathing fastener type and size,
Fastener spacing along supported sheathing edges,
Presence or absence of blocking along all edges of each piece of sheathing, and
Layout of the sheathing joints with respect to direction of lateral loading.
Below is a discussion of how differences in lateral capacity can result depending on how each of
these is applied to the construction of a floor.
Sheathing thickness usually is selected based on the spacing of joists and, for floors, will never
be less than 7/16 inch but normally is at least 5/8 inch. Generally, thicker sheathing will provide
a more comfortable floor for the occupants to walk on and will have a greater lateral capacity
compared to thinner sheathing using the same fastener size and spacing.
The most common sheathing fasteners used are nails with a minimum size of 6d common (0.113
inch x 2 inches) for a floor sheathing thickness of up to 1/2 inch. The minimum fastener size
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FEMA 232, Homebuilders’ Guide
increases with increasing sheathing thickness to a minimum of 10d common nails (0.148 inch x 3
inches) for sheathing that is 1-1/8 inches thick. Larger diameter nails will provide greater lateral
capacity than smaller nails in the same thickness of sheathing because the lateral capacity of a
nail is directly proportional to its diameter. Therefore, using box nails or gun nails that have a
smaller diameter than common nails will reduce the lateral capacity of a floor diaphragm.
Staples also can be used to fasten sheathing to framing members. Although not commonly used,
IRC Table R602.3 (2) has information for specifying alternative sheathing fasteners including
staples. Generally, staples of either 15 or 16 gage can be used in place of most nails at the same
spacing as those nails. However, when using staples, it is important to understand that they must
be installed with the crown parallel to the framing member below the sheathing edge being
fastened.
Fastener spacing for floor sheathing is typically 6 inches along continuously supported panel
edges and 12 inches along supporting members not located at panel edges. Greater lateral
capacity can be obtained when fastener spacing along supported edges is reduced from 6 inches
to 4 or 3 inches.
The IRC only requires floor diaphragms to be fastened along continuously supported panel
edges. This includes where panel edges are located parallel to and over a joist and at the floor
framing members forming the perimeter of the floor. The unsupported panel edges that are
spanning perpendicular to the joists only need to be fastened at each joist. In engineering terms,
this is called an unblocked diaphragm. See Figure 4-1 for the sheathing layout and nailing
pattern for a portion of an unblocked diaphragm.
In contrast, a fully blocked floor diaphragm means that all edges of each sheathing panel that are
not located on a joist are supported on and fastened to blocking. A blocked diaphragm will have
significantly greater lateral capacity than an unblocked diaphragm having the same thickness of
sheathing and attached with identical fasteners because the extra fasteners along the blocked
edges provide additional capacity. Figure 4-9 shows the layout of sheathing and nailing of a
portion of a blocked floor diaphragm. Fastening the sheathing to joists or blocking along all
panel edges allows the shear loads being carried in the sheathing to be transferred from one panel
to the next much more effectively. This, in turn, ties the floor together better and allows the
braced walls below that floor to resist an earthquake more as a system than as individual walls.
Above-code Recommendation: The IRC requires that wood structural panel sheathing be
installed with the long dimension of the panel perpendicular to joists, but it does not specify
staggering of panel joints along the short direction of the panels. Although not specifically
required by the IRC, sheathing should be installed as shown in Figures 4-1 and 4-9 to
achieve the greatest capacity. This staggered sheathing layout pattern causes the individual
sheathing panels to interlock and makes the whole floor act as a unit.
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Chapter 4, Floor Construction
Figure 4-9 Blocked diaphragm configuration.
Lateral loads in a floor diaphragm also are affected by the distance between braced wall lines or
between foundations located below the floor. The loads increase with increasing distance
between lines of parallel braced walls or foundations. Therefore, a long and narrow floor
diaphragm as shown in Figure 4-10 will have to transfer a greater load per foot along its short
sides than along its long sides. In order to limit the maximum load along a short side, IRC
Chapter 6 places limits on the maximum spacing between braced wall lines or foundations.
Figure 4-10 Diaphragm loads on long and short sides.
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The size and location of floor openings such as for stairs or a two-story entry foyer can create
concentrations of lateral loads in a floor diaphragm. To address this, IRC Chapter 3 limits
openings through a floor or roof to the lesser of either 12 feet maximum or 50 percent of the least
dimension of the floor. When openings exceed these limits, engineering of the floor or roof
diaphragm is required.
Above-code Recommendation: In SDCs C, D1 and D2 , when floor openings exceed 50
percent of the IRC prescriptive opening size limits, it is recommended that 16-gage
straps be installed along the edges perpendicular to the joists and extended beyond the
opening by at elast 2 feet at each end as shown in Figure 4-11. The straps can be nailed
with 10d nails into the framing members forming the perimeter of the opening and into
blocking beyond the corners. The straps and additional nailing act to reinforce the diaphragm
and provide a dedicated path for lateral loads to be transferred around the opening to the
portions of the floor beyond. Smaller openings such as those for chimneys or duct shafts do
not require any special reinforcing.
Figure 4-11 Reinforcing straps at large diaphragm openings.
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Chapter 4, Floor Construction
4.8 CONCRETE SLAB-ON-GRADE FLOORS
A concrete slab-on-grade can be used as the base of a first-floor level or of a basement level.
The minimum thickness for a concrete slab-on-grade is 3-1/2 inches except where expansive soil
is present. Where expansive soils are encountered, a design for the slab-on-grade must conform
to IBC Chapter 18 (see Chapter 3 of this guide for a discussion of the effects of expansive soil).
Concrete Strength Requirements – The minimum concrete compressive strength is 2,500 pounds
per square inch (psi). Floor slabs having an exterior surface exposed to the weather in areas of
moderate to severe concrete weathering must have higher compressive strength as specified in
IRC Table R402.2. A map in IRC Chapter 3 identifies locations where moderate and severe
weathering of concrete is expected to occur.
Reinforcing of Concrete Slab-on-Grade Floors – In the absence of expansive soils, the IRC does
not require reinforcing of concrete slabs. Reinforcing typically is used to provide tension
capacity in concrete and thereby reduce cracking caused by a variety of loads including
temperature variations. Concrete alone has very good compression capacity but has a very low
capacity for tension. Therefore, adding reinforcing bars to a slab-on-grade will provide much
greater resistance to tension loads originating from earthquake loads and other soil conditions
that could induce tension stress in the slab.
Reinforcing is required only where a slab-on-grade is thickened along its perimeter edge or
below an interior bearing wall in SDCs D1 and D2. When exterior braced walls are spaced more
than 50 feet apart, an interior braced wall also needs a foundation as part of the slab-on-grade in
SDCs D1 and D2. (See Chapter 3 of this guide for information on where and how much
reinforcing is needed in foundations provided with a slab-on-grade.)
Above-code Recommendation: When a slab-on-grade is placed separately from the
perimeter footing below, the IRC does not specify any vertical reinforcing across this joint.
When this condition occurs, the possibility of earthquake loads causing sliding along that
joint is very real. Therefore, as an above-code measure in SDCs C, D1 and D2, when the
footing and the slab concrete are separately placed, it is recommended that vertical
reinforcing dowels be placed across the joint between the slab and footing. These
vertical dowels should be No. 4 steel reinforcing bars at 48-inch maximum spacing.
(More information on this subject is presented in Chapter 3 of this guide.)
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82 Chapter 5 WALLS
5.1 WOOD LIGHT-FRAME CONSTRUCTION
5.1.1 General Components
In residential construction, the walls provide the primary lateral resistance to wind and
earthquake loads. Even in frame type houses (e.g., post-beam construction), the exterior walls
provide most of the lateral stability to the house. Although this guide focuses on wood lightframe construction, alternatives such as cold-formed steel, masonry, and concrete construction
are used in many regions of the country. The reader is referred to the sections on masonry and
concrete construction later in this chapter for some guidance on the use of these materials. For
cold-formed steel construction, the reader is referred to the American Iron and Steel Institute’s
(AISI) industry standard for prescriptive cold-formed steel construction, Standard for Coldformed Steel Framing Prescriptive Method for One- and Two-Family Dwellings (2001).
Light-frame walls provide resistance to sliding, overturning, and racking loads induced in the
house by an earthquake as illustrated in Figure 5-1. The walls are the principle element for
transmitting the loads from the upper stories and roof to the foundation. The concept of how
these loads are transferred between the major components of the house is illustrated in Figure 5
2, and the action of the individual wall segments resisting the lateral loads is illustrated in Figure
5-3. Wood light-frame walls typically consist of the lumber framing covered by sheathing
material that is attached to the wood framing with nails, staples, or screws. Figure 5-4 illustrates
the components of a wall that is sheathed with wood structural panels (OSB or plywood) on the
outside and gypsum wallboard on the inside. The figure also shows the addition of hold-down
connectors to the framing, which are required by the IRC for some specific bracing
configurations. When used, hold-down connectors increase the strength and stiffness of the wall
segment.
Four different bracing wall configurations and eight methods (materials) are recognized by the
IRC. The bracing wall configurations include:
•
•
•
•
IRC Section R602.10.3 braced wall panels (Figure 5-5a),
IRC Section R602.10.5 continuous (wood) structural panel sheathing (Figure 5-5b),
IRC Section 602.10.6 alternate braced wall panels (similar to Figure 5-5c), and
Wood structural panel sheathed walls with hold-down connections as required by the
exceptions in IRC Section R703.7 when stone or masonry veneer is used (Figure 5-5c).
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FEMA 232, Homebuilders’ Guide
Figure 5-1 Sliding, overturning, and racking action resisted by walls and foundation.
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Chapter 5, Walls
Figure 5-2 Exploded view of house illustrating load paths.
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FEMA 232, Homebuilders’ Guide
Figure 5-3 Wall action for resisting lateral loads
Figure 5-4 Exploded view of typical residential wall segment.
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Chapter 5, Walls
Differences in these bracing wall configurations include sheathing materials, minimum bracing
length, extent of sheathing, and anchorage at the wall base. Differences in overturning
anchorage for walls are shown in Figure 5-5.
Figure 5-5 Detailing differences for three options when using
wood structural panel sheathed walls.
The braced wall panel (IRC Section R602.10.3) is the most commonly used approach. Eight
different methods (materials) are recognized by the IRC as acceptable bracing. These are called
“braced wall panel construction methods” in the IRC and are listed in Table 5-1. Method 1, letin bracing, is not allowed to be used in regions of high earthquake hazard because it often will
fail as the walls are racked during an earthquake; therefore, this method is not discussed further
in this guide.
Of the acceptable braced wall panel materials, wood structural panels and diagonal lumber
sheathing are known to perform better than others (i.e., withstand higher deformations while
supporting higher loads). Wall panel bracing is required by IRC Section R602.10.4 to be
provided in 4-foot minimum lengths sheathed on one face for other than Method 5 and either 4
foot lengths sheathed on both faces or 8-foot lengths sheathed on one face for Method 5. Other
than with Method 5, this bracing often is provided in 4-foot-long isolated segments along the
wall length.
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Table 5-1 Braced Wall Panel Construction Methods (Materials) Recognized by the IRC
Construction Method Designation
Sheathing Material
1
Nominal 1x4 inch continuous let-in bracing
2
5/8-inch minimum thickness boards applied diagonally to
studs
3
Wood structural panels (OSB or plywood) 5/16-inch
minimum thickness
4
1/2- or 25/32-inch thick structural fiberboard
5
1/2-inch gypsum wallboard
6
Particleboard sheathing
7
Portland cement plaster
8
Hardboard panel siding
Where braced wall panels use wood structural panel (Method 3) or diagonal lumber (Method 2)
sheathing, the panel base anchorage to the supporting floor framing or foundation limits the
bracing strength and stiffness. The braced wall panel anchorage includes two critical weak links
for uplift: the panel end stud connection and the bottom plate (sole plate) connection. Braced
wall panel bottom plates are specified by IRC Table R602.3(1) to be attached to the floor
platform using three 16d common (0.162 x 3.5 inch) or 16d box (0.148 x 3.5 inch) nails every 16
inches or are specified by IRC Section R403.1.6 to be anchored to the foundation with 1/2-inch
diameter anchor bolts at not more than 6 feet on center. Together, these two weak links cause
the wall to fail along the bottom of the wall under relatively low loads (these walls have a
capacity of approximately 150 to 400 plf, which translates to an allowable design value of about
50 to 140 plf maximum).
IRC Section R602.10.5, continuous structural panel sheathing, requires that all exterior wall
surfaces on a given story level other than door and window openings be sheathed with wood
structural panel sheathing (Figure 5-5b). This bracing wall configuration has greater strength
and stiffness than braced wall panels with minimum wall base anchorage. The increased
strength and stiffness are due in part to the continuity provided by additional sheathing above
and below windows and doors (which makes the wall-base connection capacity less critical) and
increased overturning capacity due to required corner framing details or hold-downs at wall ends.
In recognition of the improved strength and stiffness provided with continuous structural panel
sheathing, IRC Section R602.10.5 permits the minimum length of bracing to be reduced and IRC
Table R602.10.5 permits use of individual wall bracing panels that are more slender than would
otherwise be permitted.
Wood structural panel wall bracing with uplift anchorage provided at each end, per IRC Sections
R602.10.6 and R703.7, is the strongest and stiffest option for resisting lateral loads. This wall
configuration is illustrated in Figure 5-5c. Although sheathing, fastening, and hold-down loads
for these walls are prescribed, the walls are essentially equivalent to engineered shear walls. The
alternate braced wall panel provisions of IRC Section R602.10.6 were developed to allow use of
braced wall panels narrower than the 4-foot minimum required by IRC Section R602.10.4 (e.g.,
at the side of garage doors); however, the IRC permits an alternate braced panel to be substituted
for each 4 feet of bracing throughout the house. The provisions of IRC Section R703.7 use
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increased wall strength and stiffness to compensate for the increased earthquake loads that occur
due to the weight of stone or masonry veneer.
As discussed above, where wood structural panel bracing is used, the strength and stiffness of the
wall bracing is very dependent on the extent of sheathing and anchorage at the wall base. The
configuration shown in Figure 5-5a, using IRC minimum braced wall panel anchorage, is the
weakest and least stiff of the wood structural panel bracing configurations. The configuration
shown in Figure 5-5b adds strength and stiffness by providing continuous structural panel
sheathing and added detailing or tie-downs at wall ends. The strongest and stiffest configuration
is illustrated in Figure 5-5c where overturning anchors are provided at each end of each wall
segment.
Above-code Recommendation: Use of the configurations shown in Figures 5-5b or 5-5c
can significantly increase the strength and stiffness of braced wall panels sheathed with
wood structural panel or diagonal lumber sheathing and may be used to provide
improved performance whether or not specifically required by the IRC. Use with
sheathing materials other than Methods 2 or 3 would provide less benefit and is not
recommended.
When the wood structural panel wall bracing option is used in an engineered design, the walls
are designed according to empirical tables that provide allowable design loads in pounds per foot
of wall depending on the thickness and grade of the sheathing and the size and spacing of the
sheathing nails. These walls rely on hold-down anchor connections to resist the overturning
loads and substantial connections (nails, lag screws, bolts, etc.) along the top and bottom plates
to transmit the lateral loads between the wall framing and the floor platform or foundation. This
type of wall can resist allowable design loads up to 870 plf when sheathed on one face with
wood structural panels.
5.1.2 Design Requirements
The 2003 IRC requires that wall bracing be provided both at exterior walls and at interior braced
wall lines. IRC Section R602.10.1.1 specifies that interior braced wall lines must be added such
that the distance between braced wall lines does not exceed 35 feet; however, allowance is made
for spacing up to 50 feet. IRC Section R602.10.11 reduces the maximum spacing to 25 feet in
SDCs D1 and D2. Within each braced wall line, IRC Table R602.10.1 specifies the minimum
bracing as a percentage of the length of the wall line based on the wind and earthquake exposure
and the story level under consideration (the lower in the house, the more sheathing is required to
resist the higher loads). Some interpretations of the IRC would allow a house located in SDC C
to be braced according to the amounts required for SDCs A and B due to the exemption for
detached houses in SDC C from all earthquake requirements.
Above-code Recommendation: Houses in SDC C should be braced according to the
requirements of SDC D1. Use of the required percentage of wall bracing in each 25-foot
length of wall will provide a distributed resistance system of walls rather than a concentrated
wall system. Experience has shown that a distributed wall system performs better in an
earthquake than concentrated walls in only a few locations.
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Table 5-2 excerpts a portion of this bracing information. As previously noted, the amount of
bracing may be modified in accordance with IRC Section R602.10.5 when continuous wood
structural panel sheathing is provided. In addition, an alternate braced wall panel (per IRC
Section R602.10.6) can be substituted for each 4 feet of braced wall panel. The sheathing must
be located at the ends of each braced wall line at least 25 feet on center. The idea is to provide a
distributed resisting system rather than to concentrate the resistance in a few highly loaded wall
segments. Overall, this provides a more robust structure that can resist the loads induced into the
house close to the source of the load and not require the floor and roof diaphragms to transmit
the loads long distances. Let-in bracing is allowed only for the top story of houses in SDC C due
to its tendency to fail at relatively low lateral load levels.
Table 5-2 IRC Sheathing Requirements for Seismic Design Categories C, D1, and D2
Seismic
Design
Category
Floor Level
Sheathing Requirements
16% of wall line for wood structural panel
Top story
and 25% for all other sheathing types
First story of 2-story or
30% of wall line for wood structural panel
C
second story of 3-story
and 45% for all other sheathing types
45% of wall line for wood structural panel
First story of 3-story
and 60% for all other sheathing types
20% of wall line for wood structural panel
Top story
and 30% for all other sheathing types
First story of 2-story or
45% of wall line for wood structural panel
D1
second story of 3-story
and 60% for all other sheathing types
60% of wall line for wood structural panel
First story of 3-story
and 85% for all other sheathing types
25% of wall line for wood structural panel
Top story
and 40% for all other sheathing types
First story of 2-story or
55% of wall line for wood structural panel
second story of 3-story
and 75% for all other sheathing types
D2
Not allowed prescriptively must use IBC
First story of 3-story
design methods
75% of wall line using wood structural
Cripple walls
panel sheathing only
Although the basic IRC bracing concept is reasonably straightforward, a number of other
adjustments may modify the required length of bracing. Close attention is required to ensure that
the IRC requirements are met. IRC Section R301.2.2.2.1 limits the dead load of assemblies for
houses in SDCs D1 and D2. The maximum permitted for roof plus ceiling dead load is 15 psf
(typical asphalt shingle roofs with gypsum ceilings); however, IRC Table R301.2.2.2.1 permits
this assembly weight to be increased to 25 psf (for heavier roofing materials) provided that the
length of wall bracing is increasd as specified. Footnote d to IRC Table R602.10.1 notes that the
earthquake bracing requirements are based on 15 psf (exterior) wall dead load and permits the
required earthquake bracing length to be multiplied by 0.85 for walls with dead loads of 8 psf or
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less, provided that the length is not less than required by IRC Section R602.10.4 (4 feet for all
but let-in bracing and gypsum board, which require a length of 8 feet) nor less than required for
wind loading.
In addition to the adjustments to required bracing length, the second paragraph of IRC Section
R602.10.11 (errata, second printing) permits the wood structural panel wall sheathing to begin up
to 8 feet from the corner in SDCs D1 and D2 provided that:
•
•
A 2-foot braced panel is applied in each direction at the house corner or
Tie-downs are provided at the end of the braced wall panel closest to the corner.
This is more stringent than for low SDCs where braced wall panels are permitted to be located
up to 12 feet 6 inches from wall corners without providing tiedowns or sheathed corners and
farther from corners if collectors are provided to carry loads to the braced wall panels. However,
a house that is tied together at the corners will resist the loads expected from an earthquake much
better than if the corners are not connected well. When using bracing other than wood structural
panels in SDCs D1 and D2, braced wall panels must be located at each end of braced wall lines.
Above-code Recommendation: Braced wall panels should extend to every corner.
IRC Section R703.7 presents additional bracing modifications where exterior veneer is used.
These requirements are discussed in Section 5.2 of this guide. For the guide’s model house,
Figures 5-6a through 5-6c provide plan views identifying the bracing required when the house is
in SDC C, has a light finish system such as vinyl siding, and has a crawl space. The bracing
requirements for the same house located in SDC D2 are shown in Figures 5-7a through 5-7c.
Notice the significant increase in designated wall bracing due to the higher level loads expected
in SDC D2.
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Figure 5-6a Plan view of crawl space for model house with light-weight finish material located in SDC C.
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Figure 5-6b Plan view of first floor for model house with light-weight finish material located in SDC C.
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Figure 5-6c Plan view of second floor for model house with light-weight finish material located in SDC C.
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Figure 5-7a Plan view of crawl space for model house with light-weight finish material located in SDC D2.
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Figure 5-7b Plan view of first floor for model house with light-weight finish material located in SDC D2.
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Figure 5-7c Plan view of second floor for model house with light-weight finish material located in SDC D2.
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5.1.3 Cripple Walls
Cripple walls are short frame walls that extend from the foundation to the bottom of the first
floor. They are most often found in the western United States. These walls often enclose a crawl
space or serve as walls for a stepped foundation. Historically, these walls have been the cause of
significant failures in residential construction during earthquakes primarily due to inadequate inplane strength or inadequate anchorage to the foundation. These walls are the most highly
loaded of all the light-frame walls in a house because they have to resist the entire load from the
house above. For cripple walls, IRC Section R602.10.2.1 specifies the length of bracing as 1.15
times the bracing required for the story above and indicates that the spacing should be 18 feet
instead of 25 feet. This is not applicable in SDC D2, however, where the IRC requires that the
cripple wall bracing be a minimum of 75 percent of the wall length and be constructed using
wood structural panels. When interior braced wall lines are not supported on a continuous
foundation, cripple wall bracing lengths in SDCs D1 and D2 must be modified to increase the
sheathing length by 50 percent at exterior braced wall lines or to decrease the nail spacing along
sheathing edges to 4 inches on center (per IRC Section R602.10.11.1).
5.1.4 Quality Assurance
Quality assurance for the typical light-frame wall is really quite simple. There are essentially
three areas to inspect:
•
•
•
The sheathing nails,
The anchorage of the framing to the floor framing or foundation below, and
The anchorage of the wall framing to the roof or floor framing above.
The most common problem that adversely affects the performance of all wall types is the
overdriving of the nails attaching the sheathing to the studs; this is especially a problem when
pneumatic or power-driven nail guns are used. All of the nails used to attach the sheathing to the
wall framing should be driven only to where the nail head is flush with the surface of the
sheathing as shown in Figure 5-8; an improperly driven (overdriven) nail also is shown in Figure
5-8.
Figure 5-8 Properly driven and overdriven nails.
Figure 5-8 Properly and overdriven nails.
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The most important nails are those around the perimeter of each panel of sheathing. These nails
govern the strength and stiffness of the panel. If the nails are overdriven (as in Figure 5-8), the
strength of the connection is severely compromised. For instance, if 3/8-inch wood structural
panel sheathing is used and the nails are overdriven 1/8 inch (typical for many pneumatic nail
tools), the strength of the wall is reduced as much as 40 to 50 percent. It is therefore imperative
that the sheathing nails be inspected to ensure that they are properly driven. Many pneumatic
nail tools have default driving pins that overdrive the nails but changing the driving pin costs
little (less than 10 percent of the original cost of the tool). Many tool manufacturers now provide
an adjustment on the tool to allow the user to change the depth of the driving pin without
replacing the part. (It is recommended that where nail heads occasionally are more than 1/16
inch below the surface, an additional nail should be provided between existing nails. If a
substantial number of nails are overdriven, the sheathing should be removed and the framing
checked for splitting before replacing the sheathing with proper nailing.)
Nails often are located too close to the edge of the sheathing panel, which can result in the nails
tearing out the side of the panel and weakening the wall. Therefore, the minimum edge distance
for nailing sheathing is 3/8 inch. The larger the edge distance is, the stronger the wall will be.
This is especially true for the row of nails at the bottom of the wall. If the bottom row of nails is
located at the mid-height of the bottom plate for the wall, the displacement capacity of the wall is
almost double that when the nails are spaced at 3/8 inch. When the edges of two sheathing
panels meet on a common stud (2x nominal), the maximum edge distance should be 3/8 inch to
prevent the nail from splitting the edge of the stud behind the sheathing.
Another common problem is that the nails are driven through the sheathing but miss the stud
behind. Although it is obvious that nails in the air provide no strength or stiffness, this problem
is so common it merits emphasizing that sheathing nails must be driven into the studs and top
and bottom plates of the wall and not miss the framing. The use of pneumatic nail guns makes it
very difficult for the operator to “feel” whether or not the nail has been driven into the stud
behind; therefore, all such nailing must be visually inspected from behind to ensure that the nails
did not miss the studs.
Finally, the sheathing nailing around the perimeter of each sheet of sheathing should be
symmetric about the center of the sheathing panel. This means that there should be
approximately the same number of nails along each parallel side of the panel as illustrated in
Figure 5-9.
The second area to inspect for quality assurance is the anchorage at the bottom of the wall. For
prescriptive walls, the nails must pass through the bottom plate into the floor framing. If the
nails miss the floor joists, they have no capacity to resist withdrawal and sliding. If the wall is
attached directly to the foundation, 1/2-inch-diameter anchor bolts should be spaced at a
maximum of 6 feet on center, and the end bolts should be not more than 12 inches nor closer
than 7 bolt diameters from the end of the plate. If the house is located in SDC D1 or D2, the
anchor bolts are required to have a steel plate washer between the wood sill plate and the nut.
(See Section 3.10 of this guide for more discussion of the use of plate washers.) If the walls are
segmented (engineered), the hold-down connector needs to be attached to the end stud (usually a
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double stud) and the bolt or threaded rod holding the connector to the foundation or story below
needs to be tightened to a snug one-quarter turn condition. Tightening the nut more does not
provide higher reliability due to the fact that wood is viscoelastic and the tension in the bolt will
not be sustained over time. A common practice to prevent the nuts from loosening during an
earthquake is to use a double nut or slack take-up device. The hold-down should connect the end
stud either directly to the foundation or to the top of a stud in a wall of the story below in order to
effectively transmit the overturning loads into an active resisting element.
Figure 5-9 Illustration of concept of equal numbers of fasteners in line
for symmetric nailing schedule.
Finally, the connection between the top plate and the roof framing or floor framing for the story
above should be checked. The lateral loads are transmitted through nails or other fasteners
connecting the top plate of the wall to the floor or ceiling joists above. (For vaulted ceilings, the
connection might be between the top plate and the rafters.) If there is a wall on the story above
that has an overturning anchor attached to the end stud, there should be an equivalent connection
near the top of a stud in the story below that is upside down from the one on the wall above. A
threaded rod should connect the two hold-down anchors to transmit the loads between them. If a
stud is being used to transmit overturning loads from the story above, it will need to have an
overturning anchor at its base to transmit the loads to the next story or foundation below.
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Above-code Recommendations: Improving the detailing of the braced wall system is the
most effective way to obtain earthquake performance levels higher than the code minimums.
It is also most effective to concentrate on the lower stories of the house since this is the area
that typically has the fewest walls and experiences the highest loads during an earthquake.
Use of continuous structural panel sheathing is recommended as an above-code
measure (see Section 5.1.1 and Figure 5-5b of this guide). The analyses of the model house
used in this guide indicated that use of continuous wood structural panel sheathing with
overturning anchors at corners significantly reduced the drift in all SDCs and improved the
performance category from “significant damage” to “moderate damage” in SDC D2. The
cost of making this change would be approximately 9 to 10 percent of the cost of the
structural portion of the model house. The percentage of the total cost of the house would
depend on the level of finishes, fixtures, windows, etc., and the cost of the land relative to
the cost of the structural portion of the house. The structural portion of house costs ranges
from 15 to 25 percent of total cost; therefore, the cost of fully sheathing a typical house
would be approximately 1.5 to 2.5 percent of its total cost.
Use of wood structural panel sheathing extending over and nailed to the floor rim joist
or blocking is recommended as an above-code measure. This is illustrated in Figure 5-10
and can be accomplished either by sheathing the wall with oversized panels (9-foot panels
on an 8-foot wall) or by cutting and blocking standard size sheets. It is important to leave a
vertical gap between sheathing panels at the mid-height of the band joist (approximately 3/4
inch for green solid sawn floor framing) to allow for shrinkage of the wood floor member
without causing the sheathing to buckle. Analytical studies of the model house used in this
guide indicated that use of sheathing spliced on the rim joist increased the approximate
performance category from “significant damage” to “moderate damage” in SDC D2.
Improvements identified in the analytical study were less significant in SDCs C and D1. For
the model house, the cost of implementing this improvement would be approximately 0.5
percent of the cost of the structural portion of the house, essentially a no-cost item when the
total cost of the house is considered.
Use of hold-down anchors at each end of each wood structural panel wall segment is
recommended as an above-code measure (see Section 5.1.1 and Figure 5-5c of this guide).
Analytical studies of the model house used in this guide indicated that the addition of holddown anchors significantly reduced drift in all SDCs and the approximate performance
category was increased from “significant damage” to “moderate damage” in SDC D2. For
the model house, the cost of this improvement would be 18 percent of the structural cost of
the house or 2.5 to 4.5 percent of its total cost. This option provides a house with the best
connectivity and is the basis for engineered shear wall construction. Further, of all the
options, it has the greatest effect on the stiffness and strength of the wall.
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Above-code Recommendation: Although not addressed in the analytical studies, use of
the above-code measures in combination is thought to have a cumulative effect and is
recommended. This level of connectivity will further improve earthquake performance by
stiffening and strengthening the walls. Additional above-code options for increasing
strength and stiffness include spacing sheathing nails closer than the standard 6 inches
on center (additional detailing requirements may be applicable at 3-inch or closer
spacing; see the IBC) and placing wood structural panel sheathing on both faces of
walls. Use of these above-code recommendations will be most effective where the highest
loads are present, such as lower stories and cripple wall levels.
Figure 5-10 Sheathing detail for extending the sheathing over the band joist.
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5.2 STONE AND MASONRY VENEER
Masonry and stone veneer are popular exterior finish materials for residential construction
(Figure 5-11). Veneer provides a durable finish for the house but, unfortunately, the added
weight of these veneers increases the loads experienced during an earthquake. The IRC permits
use of stone and masonry veneer installed over concrete or masonry walls and over a backing of
wood light-frame or cold-formed steel construction.
Figure 5-11 Applications of masonry and stone veneer.
This guide section discusses general principles of earthquake-resistant design for houses with
veneer, specific IRC requirements important to earthquake performance, and above-code
measures for improved earthquake performance.
Stone and masonry veneer is addressed in IRC Sections R702.1 (interior) and R703.7 (general
and exterior). For earthquake-resistant construction using veneer, there are two major areas of
concern:
•
•
The increased earthquake loading on the house due to the weight of the veneer and
Adequate anchorage of the veneer.
The weight of stone and masonry veneer permitted under the IRC provisions can vary from as
little as 20 pounds per square foot installed for adhered veneer to as much as 70 psf installed for
a 5-inch-thick anchored veneer with 1 inch of grout. The weight of stone and masonry veneer
greatly increases the overall weight of a light-frame house and, as a result, the earthquake loads.
The IRC provisions rely exclusively on the strength and stiffness of the light-frame bracing
systems to resist wind and earthquake loads, discounting any strength and stiffness that might be
provided by the veneer. This is primarily because there is only limited understanding of the
ability of the veneer to resist cyclic earthquake loading while acting in combination with the
light-frame bracing systems and because the veneer cracks and breaks at smaller displacements
than required for the light-frame system to achieve its capacity. As a result, additional
requirements apply to light-frame systems with veneer in areas with higher earthquake risk.
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Masonry and stone veneers have been damaged in historic earthquakes. Figure 5-12 illustrates
damage that occurred during the Northridge earthquake that appears to be due to inadequate
connection of the wall studs to the top and bottom plates. In failing, the stone veneer pulled the
wall sheathing and many of the studs out of the wall perpendicular to the pictured wall.
Figure 5-12 Stone veneer damaged during the earthquake in Northridge, California.
5.2.1 IRC Earthquake Requirements
The IRC permits use of veneer above the first story above grade in SDCs D1 and D2 only for oneand two-family detached houses of wood light-frame construction without cripple walls.
If a house does not meet these requirements, a design professional should design the house
following the requirements of the International Building Code or NFPA 5000. For veneer
installed over a light-frame system, requirements are incrementally more restrictive for higher
SDCs (IRC Section R703.7). References to the veneer provisions for SDCs C, D1, and D2 appear
in IRC Sections R301.2.2.3.1 and R301.2.2.4.2.
For light-frame systems, the general rule is that stone and masonry veneer not exceeding 5 inches
in thickness is permitted in the first story above grade across all SDCs without any further
requirements (IRC Section R703.7).
Although this veneer will have some impact on earthquake performance, the increase in
earthquake load close to the house base is not judged to have significant life-safety implications.
In SDCs A and B, veneer on wood light-frame or cold-formed steel houses can be up to 5 inches
thick and extend up to 30 feet above a noncombustible foundation with an additional 8 feet
permitted for gable end walls. The 30-foot limit corresponds to the height at which additional
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vertical support of the veneer would be required in an engineered design. Veneer in SDCs A and
B has no imposed earthquake limitations, leaving wind provisions to control.
In SDC C, veneer on wood light-frame or cold-formed steel houses can be up to 5 inches thick
and extend up to 30 feet above a noncombustible foundation with an additional 8 feet permitted
for gable end walls. It is required, however, that the length of bracing walls in other than the
topmost story be 1.5 times the length required if veneer is not used. (Note that this provision is
applicable to all houses in SDC C.)
In SDC D1, wood light-frame houses are permitted to have veneer up to 4 inches in thickness for
a height of 20 feet above a noncombustible foundation with an additional 8 feet allowed for
gable end walls or up to 30 feet where the bottom 10 feet (first story) is anchored to a concrete or
masonry wall. However, IRC Section R301.2.2.2.1 assembly weight limits for concrete and
masonry leave little allowance for veneer weight. Where assembly weight limits cannot be met,
engineered design of the concrete or masonry walls may be required.
Veneer on wood light-frame walls in SDC D1 (see IRC Section R703.7) is dependent on the use
of wood structural panel sheathing with specified nail size and spacing, increased bracing
lengths, use of hold-downs with specified capacities at each end of each braced wall panel, and
the prohibition of cripple walls as previously mentioned. The minimum sheathing must be at
least 7/16 inch thick and it must be fastened with at least 0.131 x 2.5 inch (8d common) nails that
are spaced no more than 4 inches on center at the panel edges. The braced wall length for the top
story of the house must be at least 45 percent of the total wall length. Finally, the first two
stories must have hold-down connectors installed. The hold-down connectors for the transfer of
loads from the second to the first story must have a minimum capacity of 2100 lb and from the
first floor to foundation must have a minimum capacity of 3700 lb. Load path requirements with
hold-down devices are discussed in Chapter 2 of this guide.
In SDC D2, veneer provisions are similar to those for SDC D1, except for a couple of small
changes:
• The maximum thickness of the veneer is limited to 3 inches,
• The top story must be sheathed to 55 percent of the total wall length,
• The second-to-first story overturning anchors must have a capacity of 2300 lb, and
• The first-story-to-foundation overturning anchors must have a minimum capacity of 3900 lb. Anchorage requirements for anchored stone or masonry veneer are given in IRC Section
R703.7.4. Use of galvanized corrugated sheet metal ties or metal strand wire ties is required.
The minimum gage required is No. 9 U.S. gage wire for strand wire ties and No. 22 U.S. gage by
7/8 inch corrugated sheet metal ties. The maximum supported veneer area is 2-2/3 square feet
per tie for SDCs A, B, and C. The supported area is reduced to 2 square feet per tie for houses in
SDC D1 and D2.
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The intent of using ties for masonry veneer is not to prevent the veneer from cracking but rather
to prevent the veneer from pulling away from the supporting wall system. In earthquakes, the
desire is to prevent damaged veneer from becoming a falling hazard and the use of ties should
accomplish this by holding the cracked veneer to the supporting wall.
Above-code Recommendations: The sheet metal ties or wires used to fasten veneers
should be corrosion resistant, should penetrate the house paper and sheathing, and
should be embedded into the wall studs. Fasteners that do not penetrate the studs (i.e., only
the sheathing is penetrated) have low withdrawal resistance and significantly reduce the tie’s
ability to hold the veneer to the wall.
Where veneer is limited to the first story above grade, increase the length of wood
structural panel bracing and use hold-down devices on the braced wall panels in the
first story to increase both the strength and stiffness of the first story above grade.
These measures will help make the deformation behavior of the light-frame system more
compatible with the veneer and reduce cracking of the veneer.
5.2.2 Quality Control
All mortar joints should be completely filled and well tooled for water tightness. This can have a
significant effect on the strength and durability of the masonry walls and veneers. To augment
the tooling of joints for improving moisture control (which directly affects the longer-term
strength of the wall), flashing and weep holes need to be placed at the bottom of the veneer so
that any water trapped between the veneer and the structural wall behind can be directed to the
outside of the house. The flashing and weep holes frequently are omitted in residential
construction, resulting in expensive repairs to the structural walls of the house.
Veneer should be placed such that a 1-inch cavity is maintained between the veneer and the
supporting wall. This cavity provides a drainage plane for channeling the moisture that will
migrate from the outside surface of the veneer to the inside and then to the bottom of the wall
where it can escape to the outside through the weep holes. This drainage plane is very important
to prevent the wall sheathing from being constantly wet, which will result in mold, mildew, and
decay.
Anchor ties that hold the veneer to the wall need to be placed at the proper spacing to ensure that
the area of veneer attributed to each tie does not exceed the maximum allowed for the particular
Seismic Design Category. For SDCs D1 and D2, the spacing should be such that each tie is
supporting no more than 2 square feet of veneer. The ties also need to be placed such that the
nail holding the tie to the wall is embedded into a stud and not just the sheathing material.
Finally, if horizontal reinforcing wire is used, it should be a minimum size of W1.7 and should
be placed at a maximum vertical spacing of 18 inches according to the 2002 Masonry Standards
Joint Committee Code (ACI 530/ASCE 5/TMS 402, Section 6.2.2.10.2.4.). This wire is
embedded in the horizontal mortar joints between the courses of brick. The brick ties that hold
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the veneer onto the wall behind it should be bent around the wire to improve the effectiveness of
the tie in preventing the veneer from falling off the wall during an earthquake.
5.3 COLD-FORMED STEEL HOUSES
The IRC permits cold-formed steel framing prescriptive construction. This guide section
discusses the general principles of earthquake-resistant design for cold-formed steel houses,
specific IRC requirements important to earthquake performance, and above-code measures for
improved earthquake performance.
Cold-formed steel construction is, in most respects, like wood light-frame construction. The IRC
contains provisions similar to the wood light-frame provisions for cold-formed steel floors, roofs,
and walls. The weights of cold-formed steel floor, roof, and wall assemblies are the same or
slightly lower than those with wood light-frame construction and result in very similar
earthquake loads. The system resisting wind and earthquake loads most commonly consists of
floor and roof assemblies acting as horizontal beams carrying loads to shear walls. Cold-formed
steel shear walls carry wind and earthquake loads to the foundation. The IRC provides extensive
illustrations of minimum construction requirements for cold-formed steel framing. One
significant difference between steel and wood light-frame construction is that steel construction
assumes that the framing is in-line – that is, that the rafters, studs, and floor joists line up so that
the top channels of the walls and other elements are not subjected to compression and bending
loads. Figure 5-13 shows a cold-formed steel house under construction.
Figure 5-13 Cold-formed steel house under construction. Photo Courtesy of Dietrich Metal Framing -- a Worthington Industries Company 107
FEMA 232, Homebuilders’ Guide
5.3.1 Scope Limitations
The IRC limits the scope of cold-formed steel houses permitted under the prescriptive provisions.
Some scope limitations are found in IRC Chapter 3 while others are found in IRC Section R603.
For all SDCs, steel light-frame houses are limited to two stories above grade; the maximum
permitted plan dimensions are 60 feet perpendicular to truss or joist span and 36 feet parallel to
truss or joist span (IRC Sections R505.1.1, R603.1.1, R804.1.1). For all SDCs, story height is
limited to a 10 foot stud clear height plus a height of floor framing not to exceed 16 inches (IRC
Section R301.3). For houses in SDCs D1 and D2, IRC Section R301.2.2.2.1 limits cold-formed
steel wall assembly weights to 14 psf for exterior walls and 5 psf for interior walls. Floor and
roof plus ceiling assembly weights are also limited to 15 psf by IRC Section R301.2.2.2.1.
Although some interpretations of the IRC assume the code does not directly require that the
irregularity limits of IRC Section R301.2.2.2.2 be followed, the same limits are imposed by
AISI’s Standard for Cold-formed Steel Framing – Prescriptive Method for One- and TwoFamily Dwellings (2001) for SDCs D1 and D2. Attention to the issues associated with
irregularities is important to prevent torsional response overloading of critical elements in the
wall.
As for wood houses, the design of cold-formed steel houses is typically controlled by wind rather
than earthquake loading through SDC C; as a result, there are no added IRC earthquake load
requirements for SDC C. Per IRC Section R301.2.2.4.5, houses in SDCs D1 and D2 must
conform to the requirements of the 2001 AISI Standard for Cold-Formed Steel Framing –
Prescriptive Method for One- and Two-Family Dwellings (AISI/COFS/PM).
5.3.2 IRC Wind and Earthquake Requirements
Unlike wood light-frame systems, the cold-formed steel provisions rely exclusively on bracing at
exterior walls for wind and earthquake load resistance. IRC Section R603.7 requires that the
exterior walls be fully sheathed. IRC Table R603.7 specifies minimum percentages of fullheight sheathing as a function of wind speed and exposure, roof slope, and story being braced.
Per IRC Section R603.7, the minimum braced length is not permitted to be less than 20 percent
of braced wall length and bracing panels must be a minimum of 4 feet in length to be counted
towards the required percent with a minimum 4-foot-long bracing panel required at each corner
of each exterior wall. The minimum wood structural panel sheathing thickness is specified as
7/16 inch for oriented strand board and 15/32 inch for plywood. The minimum edge fastening is
spaced 6 inches on center using No. 8 screws. All wood structural panel edges are to be blocked.
A number of modifying factors are required to be applied to the tabulated percentage of fullheight sheathing. The tabulated bracing percentages are based on walls with an 8-foot clear
height and must be multiplied by 1.10 for a 9-foot clear height and 1.20 for a 10-foot clear height
(IRC Section R603.7). The required sheathing percentage is permitted to be multiplied by 0.6
where hold-down devices having 4,300 lb capacity are provided at each end of each exterior wall
(IRC Section R603.7.2). The minimum percentages of sheathing are based on a house aspect
ratio (ratio of greater plan dimension to lesser plan dimension) of 1:1. The minimum
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percentages for the shorter walls must be multiplied by 1.5 and 2.0 for house aspect ratios of
1.5:1 and 2:1 respectively (IRC Table R603.7, footnote c). The minimum percentages are
permitted to be multiplied by 0.95 or 0.90 for hipped roofs, depending on roof slope (IRC Table
R603.7, Footnote d).
AISI/COFS/PM provisions for bracing in SDCs D1 and D2 are significantly different from IRC
requirements for areas of less earthquake risk with respect to bracing approach, percentage of
full-height sheathing, and detailing. The IRC basic bracing provisions (SDCs A, B, and C) do
not require use of hold-downs and permit reduction of the percentage of full-height sheathing if
hold-downs are provided. In contrast, AISI/COFS/PM basic bracing provisions for SDCs D1 and
D2 require use of hold-downs at each end of each required full-height sheathing segment. An
alternative approach allows hold-downs to only occur at each end of a braced wall line provided
that the length of full-height sheathing is increased (up to 3x length otherwise required). This
alternative approach is similar to the continuous structural panel sheathing (perforated shear
wall) method used in wood light-frame construction. The user is referred to the AISI/COFS/PM
document for full details of bracing requirements.
IRC Section R505 addresses floor construction using cold-formed steel framing. The use of
wood structural panel floor sheathing is implied by details and fastening requirements. IRC
Section R503 controls selection of floor sheathing depending on the spacing of joists and
material used. IRC Table R505.3.1(2) requires a minimum fastening of No. 8 screws at 6 inches
on center on the perimeter of the sheathing panels and 10 inches on center in the field. This is
assumed to be at supported edges with no requirements for blocking at other edges. Similarly,
IRC Section R804 implies wood structural panel roof framing, while IRC Table R804.3 requires
minimum No. 8 screws at 6 inches on center at edges, 12 inches on center in the field, and 6
inches on center at gable end trusses.
It is very important to use appropriate framing materials and fasteners for the performance of the
wall bracing and floor and roof systems to be acceptable. IRC Sections R505.2, R603.2, and
R804.2 specify material grade, corrosion protection, and identification of the materials required.
IRC Sections R505.2.4, R603.2.4, and R804.2.4 specify fastener requirements. Steel to steel and
wood structural panel sheathing to steel connections are required to be made with self-drilling
tapping screws conforming to SAE J78 with a Type II coating in accordance with ASTM B633.
Screws are required to extend through the steel a minimum of three exposed threads. Steel to
steel connections are to be made with self-drilling tapping screws at a minimum edge and centerto-center distances of 0.5 inches. Connections are typically specified with No. 8 screws, but
adjustments are given for use of No. 10 and No. 12 screws. Wood structural panel sheathing to
steel framing connections are to be made with No. 8 self-drilling tapping screws at minimum
head diameter of 0.292 inches, countersunk heads, and a minimum edge distance of 3/8-inches.
Gypsum wallboard ceilings are to be attached with minimum No. 6 screws conforming to ASTM
C 954.
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Above-code Recommendations: There are no IRC maximum spacing limitations between
lines of cold-formed steel bracing walls corresponding to the wood light-frame braced wall
line spacing limits of 25 and 35 feet. With wood light-frame construction, these spacing
limitations are intended to help distribute the bracing walls in proportion to the house mass
(dead load) as well as to limit the loading to the floor and roof. This helps to reduce concerns
regarding house rotation due to torsional irregularities and concerns regarding irregular floor
and roof system shapes. For cold-formed steel houses, the maximum 60-foot house
dimension provides some limit for earthquake loads in the floor and roof; however, for
improved earthquake performance, it is recommended that interior cold-formed steel
braced wall lines be added such that the distance between braced wall lines does not
exceed 35 feet. This will help lower the loads within any given wall segment and better
distribute the earthquake loads throughout the house. The cost associated with adding
additional bracing walls would be consistent with the percentage increase in wall length.
For cold-formed steel houses in all SDCs, the irregularity limitations developed for
wood light-frame houses should be applied.
5.3.3 Quality Control
Quality control for cold-formed steel construction can play a major role in the satisfactory
performance of the house. Important steps in steel construction include ensuring proper load
path connections. Just as in wood light-frame construction, the connections between the end
studs and the floor or foundation become critical for the strength and stiffness to be the highest
possible. Strong connections ensure the load path will function properly.
Adequate lateral bracing of all floor, roof, and wall framing members is important to prevent the
individual members (joists or studs) from buckling. Cold-formed members have to be braced
well before they can support significant compression loads. Attaching sheathing to the stud or
joist typically will provide this lateral support.
Screws that attach the sheathing to the framing should be inspected to ensure that they are not
overdriven (the resulting loss in capacity will be similar to that experienced in wood light-frame
construction when nails are overdriven). Just as in wood framing, the power tools used to install
the screws can easily over drill the screws, leaving the head of the screw below the surface of the
sheathing material and significantly increasing the chances of pull through.
5.4 MASONRY WALL HOUSES
While many houses use masonry walls for foundations, some houses are built using masonry for
the walls above grade. Figure 5-14 shows a house constructed using masonry for the walls. The
IRC permits construction of houses with masonry walls in accordance with prescriptive
provisions for concrete, brick, and stone masonry. General provisions for masonry construction
appear in IRC Section R606. Additional details of construction for concrete unit masonry, brick
masonry, and masonry grouting appear in IRC Sections R607 through R609. However, it is
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important that all masonry cells with reinforcement be grouted, and care should be taken to
ensure that the grout is consolidated. Discussed below are the general principles of earthquakeresistant design for masonry wall houses, specific IRC requirements important to earthquake
performance, and above-code measures for improved earthquake performance.
Figure 5-14 House constructed with masonry walls.
Masonry wall construction is significantly heavier than light-frame wall construction; however,
in most respects, the same principles of earthquake-resistant construction apply. The system
resisting wind and earthquake loads most commonly consists of floor and roof assemblies acting
as horizontal beams carrying loads to bracing walls. Masonry bracing walls carry wind and
earthquake loads down to the foundation with masonry walls primarily resisting loads acting in
their strong direction – that is, parallel to the wall. Because masonry walls are heavier than stud
framing, it follows that the earthquake loads resisted by each wall element and connection will
be greater. As a result, the design of the connections is as important as the proportioning of the
floor and roof systems and bracing walls. Significant earthquake loads also develop
perpendicular to the wall surface due to the wall’s weight. These loads tend to pull the walls
away or push them toward the floor or roof, making wall to floor or roof anchorage critical.
Damage from insufficient anchorage or connectivity similar to that shown in Figure 5-12 can
result if these connections are not strong enough.
5.4.1 Scope Limitations
One of the primary approaches the IRC uses to deal with the increased earthquake loading
associated with masonry wall houses is to limit the scope of houses permitted under the
prescriptive provisions. IRC Section R301.2.2 introduces earthquake provisions applicable to
houses in SDCs D1 and D2. The first scope limitation is found in IRC Section R301.2.2.2.1,
Weights of Materials, which limits masonry walls to 8 inches thick and 80 pounds per square
foot, which essentially prohibits the use of rubble stone masonry (IRC Section R606.2.2) as well
as relatively thick concrete or brick masonry walls. IRC Section R301.2.2.4.3 triggers the
reinforcement and configuration requirements of IRC Section R606.11.3 for all houses in SDC
D1 and of Section R606.11.4 for all houses in SDC D2. In SDCs D1 and D2, the scope of
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prescriptive construction of houses with masonry walls is effectively limited to walls one story in
height with up to 9 feet between lateral supports; beyond this, engineered design is required. A
final scope limitation in IRC Section R301.3, Item 3, limits masonry house walls to a clear height
of 12 feet with a maximum floor framing depth of 16 inches. An additional 8 feet of masonry
wall height is permitted for gable end walls. It is also worth noting that IRC Section
R301.2.2.2.2, Item 7, prohibits the mixing of light-frame and masonry construction such that
light-frame walls would be required to support earthquake loads due to masonry wall
construction (e.g., mixing masonry and light-frame walls on the same story.)
5.4.2 Wall Lateral Support, Reinforcing, and Anchorage
General requirements for lateral support of masonry walls for all SDCs are provided in IRC
Section R606.8. Walls are permitted to be laterally supported by cross-walls, pilasters,
buttresses, or structural frame members where walls span horizontally between supports and by
floors and roofs where walls span vertically between supports. Minimum reinforcing
requirements apply only for masonry laid in stack rather than running bond and for interior nonload bearing walls where they intersect with other masonry walls. IRC Section R606.10 requires
anchorage of masonry walls to floor and roof systems in accordance with specific details.
However, in recent U.S. earthquakes, similar details have been observed to be susceptible to
damage. Performance of some of these details for supporting site-built masonry walls also is
thought to be problematic. Several details on how to comply with the IRC provisions are
provided in the code itself.
IRC Section R606.11.1.1 triggers additional requirements for floor and roof systems for houses
in SDCs D1 and D2. Floor and roof wood structural panel sheathing is required to have all edges
blocked and nailed at 6 inches on center. Where the floor or roof system is long and narrow
(more than twice as long as it is wide), the nailing at sheathing edges is required to be reduced
from 6 to 4 inches on center.
For houses in SDC D1, IRC Section R606.11.3 places further limits on masonry walls using
prescriptive provisions:
•
•
•
•
Masonry walls are limited to one-story,
Additional reinforcing is required,
Reinforcement detailing provisions for masonry columns apply, and
Type N mortar and masonry cement are prohibited.
Similar additional requirements are triggered by IRC Section R606.11.4 for SDC D2. SDCs D1
and D2 requirements are illustrated in IRC Figure R606.10(3). Again, specific design is required
for wall to framing anchorage per IRC Section R606.11.2.2.1; thus, the anchorage to roof
framing illustrated in IRC Figure R606.10(3) may not be the only alternative.
In the design of wall anchorage to the floor and roof, it is important that a direct tension tie be
provided from the wall to the floor or roof framing members if framing is perpendicular or to
blocking that is continued across the floor or roof if framing is parallel. Detailing in IRC Figures
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611.8(1) to (7) shows the concept of this connection when used with concrete walls, but they are
equally applicable to masonry construction.
5.4.3 Parapet walls
IRC Section R606.2.4 limits masonry thickness and height of parapets in all SDCs and sets the
requirements for reinforcing walls of houses in SDCs D1 and D2. Parapet walls are easily
damaged in an earthquake and attention to placement of reinforcement and bracing to stabilize
the parapet is important.
5.4.4
Problematic Gaps Between Prescriptive and Engineered Construction
The current IRC requirements for masonry construction reveal significant gaps between
prescriptive and engineered construction, creating a significant opportunity to employ abovecode measures to improve performance.
For areas of high earthquake risk, one notable omission is the lack of a limit on the amount of
opening in masonry walls. Light-frame provisions require a percentage of the wall to be solid to
provide bracing so that the bracing wall strength is in proportion to the earthquake load, but the
same requirement is not imposed for masonry walls. This is of particular concern when the
house layout results in front and back walls with a large percentage of openings for doors and
windows.
Above-code Recommendation: Each exterior wall and each interior bracing wall should
have at least one, and preferably two, sections of solid wall not less than 4 feet in length.
Further, sections of solid wall should not be spaced more than 40 feet on center and
should be placed as symmetrically as possible (Figure 5-15). The provisions of IRC
Section R611.7.4 for ICF walls illustrate a reasonable approach to regulation of minimum
wall length. The improvement requires only reasonable planning and should not result in
higher construction costs.
IRC Section R403.1 requires that exterior walls be supported on continuous concrete or masonry
footings and regulates minimum footing width and depth, but nothing in the IRC appears to
require that other masonry walls be supported on foundations.
Above-code Recommendation: It is vital that all masonry walls be supported on
substantial continuous footings extended to a depth that provides competent bearing. If
this is not done the walls have a high probability of being damaged due to uneven settlement.
There are no IRC maximum spacing limitations between lines of masonry bracing walls
corresponding to the light-frame limits of 25 and 35 feet. In light-frame construction, these
spacing limitations are intended to help distribute the bracing walls in proportion to the house
mass (dead load) as well as to limit the loading to the floor and roof, which helps prevent
damage to the house from rotation due to torsional irregularities caused by irregular floor and
roof system shapes.
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Figure 5-15 Symmetric layout of walls to distribute loads uniformly and
thereby prevent torsion.
Above-code Recommendations: Because prescriptive detailing is not available for the
addition of interior masonry braced wall lines, it is recommended that the distribution
of bracing walls be carefully balanced and that the floor and roof plans use simple
rectangular shapes without indentions, bump-outs, or openings. The cost associated with
this improvement is proportional to the length of wall added in relation to the initial total wall
length.
The IRC also does not regulate house irregularities for masonry construction. The concepts
behind IRC Section R301.2.2.2.2, Irregularities 1 through 6, should apply equally to a
masonry wall house. The exceptions to Irregularities 2 and 5 also can be applied;
however, the rest of the exceptions are not applicable.
Above-code Recommendation: Solid portions of wall should be stacked from floor to
floor and masonry walls should be continuous from the top of the structure to the
foundation. Masonry walls not directly supported on walls below require engineered
design for gravity load support. Design for earthquake and wind loads also should be
provided. The cost of this improvement is negligible; it requires only proper planning when
laying out the wall positions for the house.
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Above-code Recommendation: Running bond lay up of masonry units is inherently
much stronger than a stack bond lay up. For concrete masonry, use of open end units at
locations of vertical reinforcement and use of bond beam units for horizontal
reinforcement also help to increase the interlocking of the masonry construction,
thereby increasing strength. The cost associated with using running bond rather than
stacked bond masonry is minimal. The cost of bond beam masonry units for placing
horizontal reinforcement at the top of each wall segment would be significantly less than 1
percent of the cost of the original structural system but would dramatically improve the
performance of the wall and connection to the floor or roof framing.
Above-code Recommendation: Any of the many required or recommended measures
for areas of high earthquake risk would improve the performance of masonry wall
houses in areas of lower earthquake risk as well as in high-wind areas. Priorities
include provision for reinforcing such as that shown in IRC Figure R606.10(2), wall
anchorage using details developed to resist out-of-plane wall loads such as those shown
in IRC Figures R611.8(1) to (7) for walls, minimum length of bracing walls, and a
maximum spacing of bracing wall lines.
5.4.5 Quality Control
Quality control during masonry construction can play a major role in the satisfactory
performance of the masonry. The following should be monitored:
• Proper placement of reinforcing in masonry. Unless specifically designed otherwise,
reinforcing should be located as near the centerline of the masonry cavity as possible. In
no case should reinforcing be closer than 5/8 inch to a masonry unit wall.
• Anchor bolts should be secured into place before grout placement (not placed into grout
following the pour).
• Excess mortar and other obstructions should be cleaned from the cavity to allow free
placement of grout.
• Consolidation of the grout after it is placed into the cavity is necessary to eliminate voids.
5.5 INSULATING CONCRETE FORM (ICF) WALL HOUSES
The IRC permits construction of houses with concrete walls in accordance with the prescriptive
provisions of IRC Section R611 for ICF (insulating concrete form) walls and Section R612 for
conventionally formed walls. ICF walls are concrete that is cast into forms that remain in place
to serve as house insulation. The code covers three geometries of ICF forms: flat, waffle-grid,
and screen grid forms. Both the waffle and screen grid consist of concrete cast into a series of
interconnected horizontal and vertical cores within the insulation. Interior and exterior wall
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finishes are applied over the ICF wall form. ICF walls may be used for the full height of the
house or for the lower story only with light-frame walls above. The provisions of IRC Section
R611 are based on the use of light-frame floor, roof, and ceiling assemblies in ICF wall houses.
The light-frame construction can be either wood or steel light-frame; however, current detailing
developed for areas of high earthquake demand provides solutions for wood light-frame floors
and roofs. This section discusses general principles of earthquake-resistant design for ICF wall
houses, specific IRC requirements important to earthquake performance, and above-code
measures for improved earthquake performance. An example of ICF construction is shown in
Figure 5-16.
Figure 5-16 Insulated concrete form
house under construction.
ICF wall construction, like masonry, is significantly heavier than light-frame wall construction.
In most respects, however, the same principles of earthquake-resistant construction apply. The
system resisting wind and earthquake loads consists of floor and roof assemblies acting as
horizontal beams carrying loads to bracing walls. ICF bracing walls carry wind and earthquake
loads down to the foundation with ICF walls primarily resisting loads acting in their strong
direction, parallel to the wall (shear walls). Because ICF walls are heavier than light-frame
walls, the earthquake loads to be resisted by each wall element and connection will be greater.
As a result, proportioning of the floor and roof systems and bracing walls is important as are the
connections between the walls and the floor and roof framing. Significant earthquake loads also
develop perpendicular to the wall surface due to the wall weight. These loads tend to pull the
walls away or push them toward the floor or roof, making wall to floor and wall to roof
anchorage very important.
5.5.1 Scope Limitations
One of the primary approaches the IRC uses to address the increased earthquake loading with
ICF wall houses is to limit the scope of houses permitted under the prescriptive provisions. For
ICF, some scope limitations are found in Chapter 3 while others are found in IRC Section R611.
For all Seismic Design Categories, ICF wall houses are limited to two stories above grade. The
maximum permitted house plan dimension is 60 feet, and framing clear spans are limited to 32
feet for floors and 40 feet for roofs (IRC Section R611.2). For houses in SDCs D1 and D2, IRC
Section R301.2.2.2.1 limits the dead weight of concrete walls to 85 pounds per square foot for
walls that are 6 inches of solid concrete. IRC Section R611.7.4 limits the minimum nominal
thickness of the wall to 5-1/2 inches. Application of these criteria and the maximum unit
weights in IRC Table R611.2 permits use of 5-1/2-inch flat ICF walls, 6- and 8-inch waffle-grid
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walls, and 6-inch screen grid walls. The weight of interior and exterior wall finishes is limited to
8 psf (IRC Section R611.2). Floor and roof plus ceiling assembly weights also are limited by
IRC Section R301.2.2.2.1 as for the other materials.
For houses in SDCs D1 and D2, IRC Section 611.2 limits the scope to rectangular houses with a
maximum floor and roof aspect ratio (length to width ratio) of 2:1 and requires that the ICF walls
be aligned vertically (stacked) without cantilevers or setbacks. An additional limitation is that
the gable end portion of walls must be of light-frame rather than ICF construction. Further, the
top of the ICF wall must be anchored to a required attic floor as described in IRC Section
R611.9.
For houses in SDCs D1 and D2, IRC Section R301.2.2.2.2 requires engineered design for
irregular portions of houses. The irregularities are discussed in detail in an earlier section of this
guide. The exceptions to irregularities are mostly applicable to wood-frame construction and so
do not apply. The 2003 IRC as printed refers to irregularities in IRC Sections R301.2.2.7 and
R301.2.2.9, which do not exist; the intent was to reference IRC Section R301.2.2.2. IRC Section
R611.8.3.1 repeats the prohibition of vertical offsets in floor framing (such as split levels) in
SDCs D1 and D2.
IRC Section R301.2.2.4.4 triggers the requirements of IRC Sections R611 and R612 for all
houses in SDCs D1 and D2. These requirements include all of the reinforcement and detailing
provisions for concrete walls that provide the continuity and connectivity for the system to
function effectively under wind or earthquake loads.
5.5.2 Wall Reinforcing and Anchorage
Reinforcing steel is of primary importance for concrete and masonry house performance. The
reinforcing steel is what ties the components together and provides toughness to the concrete
when it cracks. General requirements for reinforcing of ICF walls are provided in IRC Sections
R611.3 through R611.5. Reinforcing is required for all ICF walls across all SDCs. IRC Section
R611.7.1.2 imposes additional minimum reinforcing size and spacing limits for houses in SDCs
D1 and D2. IRC Section R611.7.1.3 provides similar limits for horizontal reinforcing and
includes requirements to provide for continuity of reinforcing around house corners and
termination of horizontal reinforcement. Reinforcement is required to be doweled into the
foundation and extend for the full height of the ICF wall, including the parapet if applicable.
Lap splicing of reinforcing is permitted.
Like light-frame wall bracing provisions, ICF wall provisions require a minimum length or
percentage of the wall to be solid (without openings) so that the bracing wall strength is in
proportion to the earthquake load. The minimum required lengths of bracing wall for ICF
construction in each Seismic Design Category are defined in IRC Section R611.7.4. Minimum
bracing lengths must be provided for wind load parallel and perpendicular to the ridge per IRC
Tables 611.7(9A) through (10B). The required bracing length must be provided using wall
segments not less than 2 feet in width. For all houses in SDCs D1 and D2, the bracing length
requirements of IRC Table R611.7(11) also must be met using wall segments that are not less
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than 4 feet in width. The increase from 2 to 4 feet provides a wall that is stronger and much
better able to withstand cyclic earthquake loading.
Also of primary importance for house performance is the connection of the ICF walls to the
floors and roofs, both for bracing loads parallel to the wall and out-of-plane loads perpendicular
to the wall. The IRC addresses three types of connections to wood light-frame floors and roofs:
• Top bearing connections where the light-frame floor bears on the top of the ICF wall with
light-frame walls above (IRC Section R611.8.1),
• Ledger-bearing connections where the floor or roof is supported off the face of the ICF
wall by a wood ledger (IRC Section R611.8.2), and
• Top-bearing connections where the light-frame roof bears on top of the ICF wall (IRC
Section R611.9).
It is important that a direct tension tie be provided from the wall to the floor or roof framing or
blocking to resist loads perpendicular to the wall (this connection keeps the wall from pulling
away from the floor or roof framing). This does not necessarily occur for lower Seismic Design
Categories. For top bearing connections, sill plate to framing connections rely on minimum
prescriptive fastening at framing perpendicular to the wall and no provision exists for a tension
connection at framing parallel to the wall. For ledger-bearing connections, the direct tension ties
shown in IRC Figures R611.8(2) through R611.8(5) are not required for lower SDCs, leaving the
remaining ledger connection susceptible to cross-grain tension failure as illustrated in Figure 5
17. Very specific prescriptive anchorage requirements capable of resisting perpendicular-to-wall
loads are given for sill or ledger to wood framing connections for houses in SDCs D1 and D2.
Included are angle clips from sill to joist or blocking, continuous straps tying blocking together,
and direct tension ties at ledger-bearing connections. Detailing is illustrated in IRC Figures
R611.8(1) to 611.8(7) and R611.9. The specification of a maximum bolt size of 3/8-inch
diameter for top-bearing connections is intended to favor bending of the bolt as a failure method
over a brittle failure of the concrete or splitting of the wood. Larger bolt sizes should not be
substituted.
Figure 5-17 Illustration of cross-grain bending of wood ledger.
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IRC Section R611.8.3 addresses requirements for floor and roof systems. The floor and roof
systems act as horizontal beams carrying wind and earthquake loads to the bracing walls. The
increased weight of the ICF walls results in increased earthquake load in the floor and roof
systems. For SDCs A through C, use of wood structural panel sheathing is required, and the IRC
minimum fastening schedules of 6 inches around the perimeter of each sheet and 12 inches along
intermediate supports are referenced. IRC Section R611.8.3.1 triggers additional requirements
for houses in SDCs D1 and D2. Minimum panel thicknesses are specified for floor and roof
wood structural panel sheathing, minimum nail diameter and penetration are specified, all panel
edges are required to be blocked, and the panel edge nailing is required to be 4 inches on center
in SDC D1 and 3 inches on center in SDC D2.
IRC Section R611.9.1 has similar requirements for roof construction but also contains two
notable variations. First, the requirements apply to houses in SDCs D1 and D2. Second, where
gable end wall conditions occur, it is required that a wood structural panel sheathed attic be
provided to support the gable end wall (for truss roofs, the wood structural panel sheathing can
be applied to the bottom of the truss between the truss chord and the ceiling finish material) and
that the wall be anchored in accordance with requirements for anchorage to floor and roof
diaphragms. IRC Section R611.2 requires that the gable-end portion of this wall be light-framed.
The writers of the ICF provisions preferred this solution over allowing increased ICF wall
heights at the gable end due to the increased loads associated with concrete and the additional
bracing requirements.
IRC Section R403.1 requires that exterior walls be supported on continuous concrete or masonry
footings and regulates minimum footing width and depth. If interior ICF walls are provided,
these too should be supported on substantial continuous footings extended to a depth that
provides competent bearing.
5.5.3 Problematic Gaps Between Prescriptive and Engineered Construction
The current IRC requirements for ICF wall construction reveal some gaps between prescriptive
and engineered construction creating an opportunity for above-code measures to improve
performance in areas of high earthquake risk.
There are no IRC maximum spacing limitations between lines of ICF bracing walls
corresponding to the light-frame limits of 25 and 35 feet. In light-frame construction, these
spacing limitations are intended to help distribute the bracing walls in proportion to the house
mass (dead load) as well as to limit the loading to the floor and roof. This helps to reduce
concerns regarding house rotation due to torsional irregularities and concerns regarding irregular
floor and roof system shapes. For ICF wall houses, the scope limitation to rectangular houses
reduces the likelihood of rotational behavior.
Above-code Recommendation: Careful balancing of bracing walls around the house
perimeter is recommended to further limit torsional behavior. The maximum 60-foot
house dimension will provide some limit for earthquake loads in the floor and roof. The cost
of distributing the wall segments around the perimeter of the house should not result in any
increased cost for construction.
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For ICF wall houses, some of the irregularity limitations developed for light-frame houses have
been made applicable. The concepts behind IRC Section 301.2.2.2.2, Irregularities 1 through 6,
apply equally to an ICF wall house.
Above-code Recommendation: Any of the many required or recommended measures
for areas of high earthquake risk would improve performance of ICF wall houses in
areas of lower earthquake risk as well as in high-wind areas. Most of the
recommendations would improve house performance in high-wind events. Priorities
include wall anchorage using details developed to resist out-of-plane wall loads such as
those shown in IRC Figures R611.8(2) to R611.8(7).
5.5.4 Quality Control
Quality control for ICF wall construction can play a major role in the satisfactory performance of
the house. Important steps in ICF construction include:
• Reinforcing should be placed properly. Unless specifically designed otherwise,
reinforcing should be located as near the centerline of the ICF cavity as possible but at
least within the middle third of the wall.
• Anchor bolts should be secured into place before concrete placement (not placed into
concrete following placement of the concrete) to prevent air pockets from forming around
the bolt which reduces its strength.
• Concrete should be consolidated as it is placed into the forms to prevent voids from
forming.
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Chapter 6 ROOF-CEILING SYSTEMS
Woodframe roof-ceiling systems are the focus of this chapter. Cold-formed steel framing for a
roof-ceiling system also is permitted by the IRC but will not be discussed; rather, the reader is
referred to the AISI Standard for Cold-Formed Steel Framing – Prescriptive Method for Oneand Two-Family Dwellings (2001) for guidance. Most of the recommendations for improving
the earthquake performance of woodframe roof-ceiling systems also apply to cold-formed steel
construction since the systems are very similar.
6.1 GENERAL ROOF-CEILING REQUIREMENTS
Woodframe roof-ceiling systems, regardless of the pitch of the roof, form a roof diaphragm that
transfers earthquake lateral loads to braced walls in the story level immediately below the roof in
the same manner that floors transfer loads from interior portions of the floor to the braced wall
lines of the story below. The lateral loads in the roof-ceiling are based on the mass of the roofceiling assembly and a portion of the mass of the walls in the story immediately below the roof.
Woodframe roof-ceilings typically consist of repetitive rafters and ceiling joists or prefabricated
(engineered) trusses at a prescribed spacing. They are sheathed with either spaced solid wood
boards or with wood structural panels attached to the top surface of the rafter or truss. Figure 6-1
illustrates this type of roof and ceiling framing system. Roof members also can consist of
repetitive beams spaced further apart than rafters, either with or without ceiling joists.
Figure 6-1 Typical light-frame roof-ceiling system.
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Depending on the roof shape chosen for a house, hip and valley beam members may be needed
where intersecting rafters change the direction of their span. Depending on the slope of the roof,
hip and valley rafters can experience very high loads when supporting long-span rafters.
Therefore, purlins are sometimes provided below and perpendicular to rafters. The purlins, in
turn, are supported by a stud or post attached to a wall or beam below. Ridge boards also are
often provided at the peak of a roof where sloping rafters meet. Blocking between rafters (or
trusses) is used at the bearing points of rafters and ceiling joists to prevent lateral movement or
rolling of the rafter. Finish materials such as gypsum board are typically applied to the bottom
surface of ceiling joists or the bottom chord of a truss if the space below is to be occupied.
Rafters, purlins, ridge boards, and hip or valley members can be sawn lumber, end-jointed
lumber, or any one of a variety of prefabricated (engineered) members. Examples of engineered
lumber include wood I-joists or solid rectangular structural composite members such as parallel
strand lumber (PSL), laminated veneer lumber (LVL), or laminated strand lumber (LSL). Roof
beams and blocking can be either sawn lumber or engineered lumber.
The minimum required size and maximum span and spacing of roof rafters or beams, ceiling
joists, and trusses is based on providing adequate support for dead and live vertical loads
prescribed by the code. Snow loads must be considered for rafters, and attic storage must be
taken into account for ceiling joists. Vertical deflection of rafters and ceiling joists is another
design consideration that may limit the maximum span of these members. Rafter spans listed in
prescriptive tables are based on the horizontal projection of the rafter rather than being measured
along the slope, which would be a greater distance.
Tables in IRC Chapter 8 and similar tables in other documents such as those published by the
American Forest and Paper Association (AF&PA) or engineered lumber manufacturers are
available for use in selecting the proper combination of size, span, and spacing of most roofceiling framing members. Depending on the roof pitch, certain roof members require
engineering to determine their size. For a roof pitch less than 3:12, the size of ridge boards and
hip or valley members must be individually determined based on their spans and the span of the
rafters they support.
6.2 SPECIAL FRAMING CONSIDERATIONS
Roof rafters must either be tied together at the ridge by a gusset plate or be framed to a ridge
board. Ridge boards in roofs having a pitch of 3:12 or greater must be at least 1x nominal
thickness and at least the same depth as the cut end of the intersecting rafters. Valley and hip
members in a roof having a pitch of 3:12 or greater must be at least 2x nominal thickness and at
least the same depth as the cut end of the rafters. Because the cut end depth of a rafter increases
with increasing roof pitch, a 2x8 rafter will need a 1x10 nominal ridge board and a 2x10 nominal
hip or valley member for a pitch up to 9:12. At a pitch exceeding 9:12, a 1x12 or 2x12 will be
needed because the cut end of a 2x8 rafter will be greater than the 9-1/4 inch actual depth of a
1x10 or 2x10 nominal member. Figure 6-2 shows a 1x12 ridge board for a 12:12 pitch condition
with the dimension for the cut end of a 2x8 rafter.
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Figure 6-2 Ridge board for 12:12 pitch roof.
One additional consideration when using nominal 1x ridge boards is that ceiling joists or rafter
ties are needed at the top plate of the supporting walls to prevent the walls from spreading
outward. The ceiling joists or rafter ties act as a brace to resist the outward thrust of the rafters at
the wall support ends of rafters. In the absence of ceiling joists or ties at the wall top plate,
rafters must be supported at the ridge by a beam designed for the support of rafter loads. In
addition, where a nominal 1x ridge is used and a 2x valley or hip member intersects the ridge, the
valley or hip member must be supported by a stud or post attached to a bearing wall below to
transfer the high loads associated with hip and valley rafters.
Above-code Recommendations: A special condition can occur at the gable ends of a roof.
When the exterior wall at the gable end has its double top plate level with the low ends of the
roof framing, the wall studs are not continuous to the roof sheathing along the gable edge. A
section view of this condition is shown in Figure 6-3. To provide above-code performance,
the framing extending above the top plate to the level of the sloping gable end roof
sheathing should be braced at regular intervals of not more than 4 feet on center at both
the wall top plate level and along the top edge of the sloping roof edge. This bracing
permits the top of the exterior wall and the framing extending above to resist the lateral loads
that are acting perpendicular to the wall. Without this bracing, the framing above the double
top plate could be easily displaced because a hinge can form where the gable end wall
framing attaches to the top plate.
Another above-code alternative to the framing shown in Figure 6-3 would be to provide
wall studs that are continuous to a sloping double top plate located just below the roof
sheathing (balloon framing). When continuous studs are provided, there is no weak
location for a hinge to form. However, when this method is used, the required stud size,
spacing, and maximum height must comply with IRC Table R602.3.1 or the wall studs will
need to be engineered by a design professional. Bracing a gable end wall also is important for
providing resistance to high winds, especially for roofs with a steep pitch.
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Figure 6-3 Gable end wall or gable truss bracing.
6.3 BLOCKING AND LATERAL LOAD PATHS FOR ROOF SYSTEMS
Rafters and ceiling joists having a nominal depth-to-thickness ratio exceeding 5:1 (e.g., 2x10)
need blocking at their points of bearing to prevent them from rotating or displacing laterally from
their intended position. Rotation loads on rafters occur when the roof sheathing is resisting
lateral loads perpendicular to the rafter because these loads are actually trying to move the top
edge of the rafter sideways. Preventing rotation is typically accomplished by installing fulldepth solid blocking along wall top plates between rafters and ceiling joists. Figure 6-4
illustrates blocking installed between adjacent pairs of a ceiling joist and rafter that are bearing
on an exterior wall.
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Figure 6-4 Blocking at rafters to exterior wall.
However, when the nominal size of a ceiling joist or rafter is 2x10 or smaller, blocking over the
exterior wall may be omitted. This would be likely to occur where the roof overhang is enclosed
by a soffit. Without an enclosed soffit, blocking is typically provided between rafters along the
exterior wall line regardless of the rafter size to serve as a barrier between the outside and the
attic space.
Because attics (or spaces between rafters where ceiling finish is attached to the bottom of the
rafter) require ventilation openings, the spaces between rafters along exterior walls are typically
used to provide some portion (if not all) of the required attic ventilation opening. When blocking
is installed, ventilation openings often are provided by drilling holes in the blocking; when
blocking is not required, ventilation can be provided through the entire opening space between
rafters.
Although blocking may not be required for 2x10 and smaller rafters, there still must be a load
path for lateral loads in the roof sheathing to reach the exterior braced walls immediately below
the roof. The most direct load path is for the roof sheathing to be edge nailed to blocking
between each rafter. That blocking is then nailed to the wall top plate with three 8d nails per
rafter space as prescribed in IRC Table 602.3(1). This load path is shown in Figure 6-4.
Although alternate load paths are certainly possible, the one shown is the most direct and is
essentially the same load path the IRC provides between a floor and the braced walls below that
floor.
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Above-code Recommendation: In Seismic Design Categories D1 and D2, blocking is
recommended between rafters along exterior wall lines to provide a surface for the edge
nailing of roof sheathing and to provide a very direct load path to the top plate of the
exterior braced walls. When this blocking is provided, it also is necessary to ensure that
minimum attic ventilation opening requirements are met. Depending on the attic area being
ventilated, this can be accomplished by drilling holes in the blocking and, when more opening
area is necessary, by adding gable end wall openings or ridge vents.
6.4 CONNECTION OF CEILING JOISTS AND RAFTERS TO WALLS BELOW
Ceiling joists and rafters (or trusses) are required to be connected to the top plate of supporting
walls as specified in IRC Table R602.3(1). These connections also provide a portion of the load
path that transfers loads from the roof diaphragm into the braced walls below. Ceiling joists
require a toe-nailed connection to the top plate using three 8d box or common nails. Rafters also
require a toe-nailed connection to the top plate using two 16d box or common nails. Blocking
installed between the rafters or ceiling joists requires a toe-nailed connection to the top plate
using a minimum of three 8d box or common nails in each block. Toe nailing must be done
correctly if the needed transfer of loads is to occur; therefore, ensure that the nails do not split the
wood. In high-wind areas, light-gage steel connectors often are used in place of these toe-nailed
connections. The use of commercially available light gage steel connectors in place of toe nails
can reduce wood splitting and provide more reliable load transfer. Additional information on
proper toe-nailing installation is illustrated in Figure 4-8.
6.5 ROOF SHEATHING
Wood boards installed either perpendicular or at an angle to the rafters (or trusses) or wood
structural panels can be used as roof sheathing. IRC Table R803.1 specifies the minimum
thickness for wood board roof sheathing for various spacings between roof rafters, trusses, or
beams.
Above-code Recommendation: Solid wood board roof sheathing is rarely used in modern
housing construction except perhaps along roof eave overhangs. When wood boards are used
and are installed perpendicular to rafters they provide a very weak diaphragm with little
stiffness. As a result, solid wood board sheathing installed perpendicular to rafters
should not be used in Seismic Design Categories C, D1, and D2. Wood boards installed
diagonally would provide much better diaphragm capacity but are very rarely used in modern
housing construction.
Wood structural panel roof sheathing is the most common roof sheathing used in current
construction. The minimum thickness is based on rafter (or truss) spacing and the grade of
sheathing panels selected. IRC Table R503.2.1.1(1) is used to determine the minimum required
thickness for wood structural panel roof sheathing materials for a variety of rafter spacings. For
roofs, the short direction panel joints between wood structural panels can be either staggered or
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not staggered. Typical wood structural panel roof sheathing installation using staggered joints is
illustrated in Figure 6-5.
Figure 6-5 Typical roof diaphragm sheathing nailing when wood structural panels are used.
When the roof covering consists of wood shingles or shakes, IRC Chapter 9 permits the use of
spaced wood boards as roof sheathing. However, spaced wood board sheathing is not permitted
in Seismic Design Category D2. This limitation in SDC D2 is intended to provide a stiffer and
stronger roof diaphragm that will resist the larger lateral loads anticipated in SDC D2. When
wood boards are used, they typically are nailed to each rafter with only two 8d box or common
nails. Wood structural panel sheathing results in the stiffest roof diaphragms, and the panels
typically are fastened to the rafters with 8d box or common nails spaced at 6 inches along
supported edges and 12 inches in the field of the panel. When spaced wood boards are used,
even fewer nails are provided than with solid wood board sheathing and even less lateral
resistance results.
Above-code Recommendation: Spaced wood board sheathing is not recommended for
use in Seismic Design Categories C and D1.
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6.6 LATERAL CAPACITY ISSUES FOR WOOD FRAMED ROOFS
The lateral capacity of a roof diaphragm sheathed with wood structural panels is based upon the
same five factors as floor diaphragms. (See the discussion in Section 4.7 of this guide for more
information on the effects of sheathing thickness, fastener size and spacing, use of blocking, and
layout of wood structural panel sheathing.)
The effects of distance between braced wall lines below the roof and the effects of large roof
openings (e.g., skylights) are also similar to those discussed for floors. The recommendations
for reinforcing floor diaphragms with large openings also apply when roof openings exceed the
code maximum limits. (See Section 4-7 for more information.)
Although roof-ceiling systems typically are subjected to the smallest loads when a house
experiences earthquake ground motion, several simple things can be done to improve the
performance of the system.
Above-code Recommendations:
Reinforce the framing around skylights to provide positive connections to transfer the
diaphragm loads around the opening. Strapping and blocking as illustrated in Figure 4-11
can be used to strengthen the roof around large openings and the additional cost for the
blocking and strapping for a typical skylight opening would add less than 0.4 percent to the
cost of the structural portion of the project.
Make sure all rafters and ceiling joists are blocked at all locations where they are in
contact with a top plate of a wall below (alternately a rim joist may be used). For the
exterior walls of the model house, this would cost 0.8 percent of the structural portion of the
project. If interior walls were also blocked, an additional cost of 0.7 percent of the structural
portion of the project would be incurred.
Glue the sheathing to the roof framing in the same way most floor sheathing is installed
and then block the roof as well. Construction adhesives are typically used to prevent
squeaky floors but they also strengthen and stiffen. The cost of adding adhesive to the roof
system for the model house in this guide would be 1.8 percent of the structural system, and
adding blocking to the roof system would add another 3.4 percent to the cost of the structural
system.
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6.7 QUALITY CONTROL
The most important item to monitor for quality of the roof diaphragm system is to ensure that the
nails used to attach the sheathing to the framing are driven flush with the top of the sheathing and
not overdriven and counter sunk into the sheathing materials. Overdriving sheathing nails has
been shown to reduce the strength of shear walls and diaphragms. Evidence of a 40 to 60
percent loss in strength of the shear wall and diaphragm has been observed in laboratory tests of
assemblies with overdriven nails. (See Section 5.1.4 of this Guide for more discussion on the
effects of overdriven nails and how pneumatic tools can be altered to correct for this error.)
To ensure the quality of the roof-ceiling system, make sure blocking is installed correctly so that
the wood framing is not split by the toe nailing.
If adhesives are used to attach the sheathing, attention needs to be paid to the time between the
application of the adhesive and when the nails are driven to hold the sheathing in place.
Especially in hot weather, the adhesive tends to skin over or cure on the surface quickly, which
reduces the adhesion between the glue and the sheathing. Check the time allowed for each
specific product used.
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130 Chapter 7 CHIMNEYS, FIREPLACES, BALCONIES, AND DECKS This chapter provides an overview of the IRC provisions for earthquake-resistant design and
construction of chimneys, fireplaces, balconies, and decks in houses. Above-code
recommendations for improved earthquake performance are provided.
7.1 CHIMNEYS AND FIREPLACES
IRC Chapter 10 presents requirements for masonry fireplaces and chimneys and for factory-built
fireplaces and chimneys enclosed in framing. The provisions of the IRC are intended for the
moderately sized fireplaces and chimneys commonly found in houses.
Above-code Recommendation: Where fireplaces or chimneys are large or oddly
configured, an engineered design is encouraged in order to fully address the design of
the chimney and fireplace and their influence on the house.
7.1.1 Masonry Chimneys and Fireplaces
Although the IRC permits construction of masonry fireplaces and chimneys in earthquake-prone
regions, masonry chimneys are particularly vulnerable to earthquake damage, and such damage
has occurred in most moderate to severe U.S. earthquakes (Figures 7-1 and 7-2). Masonry
fireplaces and chimneys can be heavy and rigid, and many chimneys in existing houses also are
brittle. The movement of the fireplace and chimney in response to earthquake ground motions
can be significantly different from the movement of the light-frame house itself, creating the
potential for damage to both the chimney and the house.
Figure 7-1 Chimney damage.
Photo Courtesy National
Information Service for Earthquake Engineering, University of California, Berkeley 131
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Figure 7-2 Chimney damage in Northridge
earthquake.
The IRC triggers requirements for masonry fireplace and chimney reinforcing steel and
anchorage to floors, roofs, and ceilings for houses in SDCs D1 and D2. Although these
requirements cannot completely eliminate the possibility of damage to the fireplace and chimney
in an earthquake, their use permits a chimney to better withstand earthquake loads and should
lessen the falling hazard posed by a damaged chimney. Although it may be possible with
systematic engineering design to mitigate the damage often seen in masonry chimneys and
fireplaces, the lower weight and greater flexibility of factory-built fireplaces and chimneys make
them the better choice for light-frame houses in earthquake-prone areas.
A substantial footing is necessary if a fireplace and chimney is to perform well under any type of
loading. The footing should extend to a depth not less than that of surrounding footings. IRC
Section R1003 contains minimum footing requirements.
IRC Section R1003.3 provides requirements for masonry chimney and fireplace reinforcing steel.
The minimum amount of vertical reinforcing steel is four No. 4 bars for a chimney up to 40
inches wide (a depth of approximately 24 inches is common). An additional two No. 4 vertical
bars are required for each additional flue or each additional 40 inches of width. Where the
reinforcing bars cannot run full height, a lap splice of not less than 24 inches is needed. Grout,
continuous from the footing to the top of the chimney, must surround the reinforcing steel. For
horizontal reinforcing, a minimum of 1/4-inch ties at not more than 18 inches on center is
required in the mortar joints. IRC Section R1003.3 also cites the Section R609 requirements for
grouted masonry discussed in Chapter 5 of this guide. Proper grouting and consolidation around
the reinforcing steel is needed in order for the reinforcing and anchorage to contribute to
earthquake resistance. Lack of grout and poorly consolidated grout are common contributors to
earthquake damage. It is also important to note that IRC Section R609 prohibits the use of Type
N masonry mortar in SDCs D1 and D2.
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Anchorage of the masonry chimney to the framing at each above grade floor, roof, and ceiling
level is necessary. IRC Section R1003.4 provides anchorage requirements applicable in SDCs
D1 and D2. Steel straps not less than 3/16-inch by 1-inch are required to extend a minimum of 12
inches into the chimney masonry, hook around outer reinforcing bars, and extend not less than 6
inches beyond the hook. Chimney anchorage locations are illustrated in Figures 7-3 and 7-4.
The IRC provisions specify anchorage to a minimum of four ceiling or roof joists with not less
than two 1/2-inch bolts. This description does not give details of the intended configuration and
does not address framing parallel to the chimney wall. Figures 7-5 and 7-6 illustrate
implementation of this anchorage provision with detailing consistent with industry
recommendations (MIA, 1995) and earlier Uniform Building Code provisions. Anchorage in
general conformance with Figures 7-5 and 7-6 should be consistent with the intent of the IRC.
Figure 7-3 Locations for earthquake
anchorage of masonry chimney at
exterior house wall.
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Figure 7-4 Chimney section showing earthquake anchorage.
Figure 7-5 Anchorage detail for framing parallel to exterior wall.
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Figure 7-6 Anchorage detail for framing perpendicular to
exterior wall.
To reduce the fire hazard with respect to the surrounding wood structure, IRC Section R1001.15
requires a clearance of 1 to 2 inches (depending on configuration) between combustible framing
materials and the masonry chimney. Detailing of earthquake-resistant anchorage at the floor,
ceiling, and roof levels needs to maintain this required clearance with only the steel straps
extending across this clearance gap.
Even when fireplaces and chimneys show no signs of damage after an earthquake, the masonry
or flue liner may have cracked, and inspection before reuse is recommended.
7.1.2
Factory-Built Fireplaces and Flues
Factory-built fireplaces and flues generally are installed within light-frame fireplace enclosures
and chimneys. During an earthquake, the deflections of the light-frame enclosure are compatible
with those of the house, which greatly reduces the potential for damage. Detailing of framing
anchorage, however, is important. At exterior walls, as shown in Figure 7-7, the framing for the
chimney disrupts the typical wall and roof framing. Wall top plates often are discontinued and
studs are balloon framed to the top of the light-frame chimney. Measures should be taken to
restore the continuity of the top plates and to anchor the fireplace/chimney wall framing to the
floor and roof. Figure 7-8 shows how one light-frame chimney without sufficient connections
behaved. No specific requirements for this construction currently exist in the IRC. Clearances to
combustible wood framing remain important with factory-built fireplaces and chimneys and are
typically addressed in the installation instructions.
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Figure 7-8 Collapsed factory-built chimney, light-frame
enclosure, and deck after the San Simeon earthquake.
Figure 7-7 Factory-built flue
and light-frame enclosure.
Photo courtesy of Josh Marrow, Simpson Gumpertz and Heger
Inc.
Above-code Recommendations: Factory-built fireplaces and flues typically are
installed within light-frame fireplace enclosures and chimneys that generally perform
well during earthquakes; therefore, their use in the higher Seismic Design Categories is
recommended. However, special attention should be given to the detailing of the
framing anchorage and to compliance with clearances to combustible wood framing
addressed in installation instructions.
Use of stone or masonry veneer increases the weight of the light-frame chimney, thereby
increasing earthquake loads proportionately. Particular care should be taken to tie the wood
framing into the floor and roof when veneer is used. Veneer attachment to the framing should be
in accordance with IRC Chapter 7. See Section 5.2 of this guide for a discussion of veneer
attachment.
Above-code Recommendation: Use of reinforcing steel and chimney anchorage are
recommended to improve the performance of fireplaces and chimneys across all Seismic
Design Categories and particularly in SDC C. Adding reinforcement and anchorage for
the chimney on the model house used in this guide would increase the cost of the structural
portion of the house by approximately 2 percent, which is approximately 0.5 percent of the
total cost.
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Above-code Recommendation: Both masonry and factory-built fireplaces and chimneys
result in increased weight and earthquake loading as well as discontinuities in house
configuration. Additional bracing walls in both directions in the vicinity of the fireplace
are recommended to resist the additional earthquake load. This will result in a reduction
in the amount of window opening available, but the cost of adding the wall will likely be
offset by the reduced cost for windows.
7.2 BALCONIES AND DECKS
Balconies and decks often are prominent features of modern residential construction and, for
many people, add much desired living space (see Figure 7-9). Although the IRC contains some
provisions addressing balconies and decks, the earthquake resistance of balconies and decks has
not been systematically considered in the development of the IRC provisions. This section
addresses three aspects of balconies and decks that need consideration:
• The effect of added floor area beyond braced wall lines,
• Anchorage for earthquake loads, and
• Vertical support.
7.2.1
Added Floor Area Beyond Braced Wall Lines
The bracing provisions of the IRC reflect the need to support the floor area within the exterior
braced wall lines. The addition of balconies and decks creates additional weight and increases
earthquake loads, a fact that was not envisioned when required bracing lengths were determined.
Although the addition of a small balcony or deck is not likely to greatly affect the earthquake
performance of a house, the addition of a large balcony or deck may. Further, balconies and
decks tend to concentrate the added earthquake load on one side of the house and one braced
wall line. This can contribute to rotational behavior (see the discussion of plan irregularities in
Section 2.3 of this guide) and concentrations of damage.
Two IRC sections limit balcony and deck size:
• IRC Section R502.3.3 addresses floor framing cantilevers, including exterior balconies.
IRC Table R502.3.3(2) specifies permitted cantilevers as a function of framing size and
spacing and live load. Permissible cantilevers range up to a maximum of 6 feet (see
additional discussion of back-span connections in Section 4.3 of this guide).
• For SDCs D1 and D2, IRC Section R301.2.2.2.2, Item 2, requires braced wall lines on all
edges of a floor or roof. The exception to Item 2 permits floors that do not support
braced wall panels to extend up to 6 feet beyond a braced wall line.
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Figure 7-9 Decks and balconies in residential construction.
Balconies extending more than 6 feet beyond the exterior house wall in all Seismic Design
Categories fall outside of the framing provisions of the IRC and require engineered design.
Where balconies or decks extend more that 6 feet beyond the house exterior, additional lines of
bracing for earthquake and wind loads are sometimes provided. When this approach is taken, it
is important that loading in both the longitudinal and transverse directions be considered. In
addition, when both the house and supplemental bracing are used to support the balcony or deck,
the load-deformation behavior of the bracing system must be compatible with that of the house.
Where this is not the case, it is best to completely separate the balcony or deck from the house
and provide a gap large enough to permit independent movement.
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7.2.2 Anchorage for Earthquake Loads
Where a balcony or deck is laterally supported by the house, adequate connection to the house is
key to good earthquake performance. The IRC includes two provisions that address anchorage to
the house:
• IRC Section R311.2.1 requires that the connections used to attach exterior balconies,
stairs, and similar exit facilities to the rest of the structure provide for resistance to both
vertical and lateral loads, but the magnitude of loads to be resisted is not specified. Use
of toe nails or nails subject to withdrawal is prohibited.
• IRC Section R502.2.1 provides similar requirements for decks.
Because balcony framing generally has a back span that extends into the interior of the house,
adequate connection generally is not an issue; however, care should be taken to ensure that the
back span is adequately fastened to the floor sheathing or to lapped floor framing and that
blocking is provided where the cantilever bears on the exterior wall.
Above-code Recommendation: Deck framing, on the other hand, does not generally have
inherent continuity into the house. Attachment only to the floor band (rim) joist is not
sufficient and will result in failures between the band joist and the rest of the floor system.
Engineered design of connections is recommended as is use of a positive connection,
such as the hold-down device shown in Figure 7-10.
Figure 7-10 Hold-down device providing positive connection of deck framing to house framing.
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7.2.3 Vertical Support Issues
Although beyond the scope of earthquake resistance, two issues – vertical load connections and
moisture and decay – related to vertical support of decks and balconies are of enough
significance to warrant discussion.
Deck construction is notoriously problematic and typically at least one deck collapse occurs
somewhere in the United States every week. Inadequate connection between the deck and the
house for vertical loads is the biggest problem. Where an engineered design is not provided,
connections for vertical loads very often are inadequate. Inadequate connection for lateral
loading also can be a contributor. Attention to adequacy of vertical load connections is
imperative for safety.
Above code Recommendation: Providing a line of vertical support (posts and beams)
alongside the exterior house wall can help to reduce the load on the deck-to-house
connection.
The interface between a balcony or deck and the exterior house wall is critical for the
waterproofing system. Penetration of moisture at this interface can endanger not only the
capacity of the connection but also the interior and exterior framing members. IRC Section
R319.1.2 requires that the joints of exterior balconies be designed such that moisture will not
collect in the connection area or that the connection be otherwise protected from moisture.
Above-code Recommendation: The joints of exterior decks should be designed such that
moisture will not collect in the connection area or that the connection is otherwise
protected from moisture.
The resource list provided in Appendix E includes references addressing deck connections and
moisture and decay issues.
Above-code Recommendation: As noted, balconies and decks often are subjected to
significant lateral loads during an earthquake. As an above-code measure for houses
located in SDCs C, D1 and D2, the connections used to attach the deck to the house
should be designed by a registered design professional to ensure that the loads acting on
the deck are properly transferred to the framing of the house. In addition, the lateral
bracing of the deck for the sides not attached to the house and for all sides of free-standing
decks must be sufficient to prevent a torsional response.
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Chapter 8 ANCHORAGE OF HOME CONTENTS 8.1 GENERAL
Anchorage of home contents can greatly reduce the risk of injury, property loss, and interruption
of home use as a result of an earthquake. Anchorage is particularly recommended for large and
heavy items such as water heaters, bookcases, and file cabinets and for items that could cause
injury if they fell (e.g., items on shelves above a bed). Other priority items to anchor or
otherwise restrain include wood stoves, similar heating appliances, and outside fuel tanks, all of
which pose a fire risk.
Anchorage of other home contents will further reduce disruption following an earthquake.
Measures to anchor computers and televisions range from very simple home fixes to specialized
restraint systems available from a number of manufacturers. Locks on kitchen and china
cabinets can help reduce the spilling and breakage of contents during an earthquake. Measures
are available to secure a wide range of other home contents including pictures, mirrors, fragile
objects, and fire extinguishers.
Section 8.2 provides guidance on the anchorage of water heaters. Section 8.3 addresses
anchorage of a number of items using excerpts from FEMA 74, Reducing the Risks of
Nonstructural Earthquake Damage (FEMA, 1994).
8.2 WATER HEATER ANCHORAGE
When not properly anchored, water heaters can fall over, resulting in a fire hazard and water
damage. Bracing is required for new water heater installations and is recommended as a top
priority for existing installations. IRC Chapters 20, 24, and 28 contain some provisions for the
installation of water heaters but do not specifically address anchorage. Kits for bracing water
heaters are available at many hardware stores. As an alternative to kits, Figures 8-1 and 8-2
provide example details for anchorage using plumber’s tape (24-gage by 3/4-inch minimum steel
straps) and electrical conduit. These figures are adapted from Guidelines for Earthquake
Bracing of Residential Water Heaters (California Division of the State Architect, 2002); consult
this publication for detailed installation instructions and additional cautions and limitations.
Note that Figures 8-1 and 8-2 are applicable only to water heaters with a maximum capacity of
52 gallons with two strap locations and with a maximum capacity of 75 gallons with three.
Some jurisdictions place additional limits on water heater bracing details. Where water heaters
are installed on a platform, the water heater base should be attached to the platform and the
platform should be anchored to the floor. Placing water heaters in metal pans to retain any
spilled water is a possible precaution in addition to bracing. The IRC requires that water heaters
be placed in pans where loss of water would cause damage. Required clearances to walls and
combustible construction need to be maintained; the water heater UL listing and local
jurisdiction requirements should be verified prior to moving or installing a water heater.
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Figure 8-1a and 8-1b Securing a water heater with wall bracing.
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Figure 8-2a and 8-2b Securing a water heater with corner bracing.
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8.3 SECURING OTHER ITEMS
The examples included here show representative details for protecting common items from
earthquake damage. Two different types of details are discussed:
• Do-it-yourself methods, which are simple generic methods for securing typical nonstructural
items found in the home. Enough information is provided to permit a handyman with
common tools and readily available materials to complete an installation.
• Engineered methods, which are schematic details showing common solutions for the items
in question. These sketches do not contain enough information for installation; they are
provided here primarily as an illustration of the scope of work required. The designation
“Engineering Required” has been used for items where do-it-yourself installation is likely to
be ineffective. FEMA 74 recommends that design professionals be retained to evaluate the
vulnerability of these items and design appropriate anchorage or restraint solutions,
particularly where safety is an issue.
Cost estimates are provided with the details in this section as a rough guide for planning or
budgeting purposes. The values are intended to cover the cost of materials and labor. They do
not include allowance for architectural or engineering fees, permits, special inspection, etc.
These estimates represent a professional opinion based on information available at the time
FEMA 74 was published in 1994 so current actual construction costs may vary significantly,
depending on the timing of construction, changes in conditions, the availability of materials,
regional cost variations, and other factors.
FEMA 74 provides several pages of installation notes that will be of assistance to users
interested in selection and installation of fasteners appropriate to construction materials of the
house (www.fema.gov/hazards/earthquakes/nehrp/fema-74.shtm). Reproductions of FEMA 74
pages are shown in Figures 8-3a through 8-3f.
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Figure 8-3a Reproduction of FEMA 74 suggestions for securing drawer and cabinet latches.
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Figure 8-3b Reproduction of FEMA 74 suggestions for securing a freestanding wood stove.
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Figure 8-3c Reproduction of FEMA 74 suggestions for securing desktop computers
and office equipment.
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Figure 8-3d Reproduction of FEMA 74 suggestions for securing freestanding walls or fences.
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Figure 8-3e Reproduction of FEMA 74 suggestions for securing compressed-gas cylinders.
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Figure 8-3f Reproduction of FEMA 74 suggestions for securing miscellaneous furniture.
150
Chapter 9 EXISTING HOUSES Additions or alterations can reduce the earthquake resistance of an existing house. With proper
consideration, however, earthquake resistance can be maintained or even increased as part of an
addition or alteration. This chapter discusses the earthquake-resistance implications of additions
and alterations and provides recommendations and references for earthquake upgrades.
9.1 ADDITIONS AND ALTERATIONS
Additions and alterations modify the load-resisting systems of existing houses. Generally, both
the systems supporting gravity loads and those supporting lateral (wind and earthquake) loads
are affected. For additions and alterations, IRC Section R102.7.1 requires that any new work
conform to the IRC, but existing construction is allowed to remain unless it is made unsafe or
will adversely affect the performance of the house. This wording provides significant
opportunity for interpretation by the user and building official. IBC Section 3402 provides more
specific guidance for acceptable reduction in strength or increase in loading, which may be
appropriate to some additions and alterations. The following discussion of additions and
alterations highlights issues and concerns that should be considered when interpreting IRC
requirements.
9.1.1
Alterations
Alterations to existing houses often involve modification or removal of existing bracing walls
and portions of floors and roofs. Figure 9-1 shows two alterations that remove exterior bracing
walls from a house and disrupt the roof. Interior remodels often remove interior walls that
provide bracing for earthquake and wind loads.
Where existing bracing walls are removed or reduced due to alterations, the remaining bracing
walls should be checked for conformance with the bracing location, length, and bracing type
requirements of the IRC provisions. The primary focus should be on bracing in the immediate
vicinity of the alteration. If bracing deficiencies occur in other portions of a house, upgrade of
those areas is encouraged.
When skylights, dormer windows, or similar openings are added to existing roofs, the openings
should be checked for conformance with IRC requirements. For earthquake loading, this would
include checking the opening size against permitted maximum sizes in the irregularities
provisions and checking detailing against IRC requirements. The framing around the opening
also should be checked for gravity load requirements such as doubled rafters and headers. If a
significant rebuilding of the roof is occurring, a broader range of IRC provisions require
checking as does the completeness of the load path for gravity and lateral loads.
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Figure 9-1 Alterations to existing house: bracing wall and roof modifications
(left) and modification for the addition of sun room (right).
9.1.2
Additions
An addition to an existing house often results in both the removal of some existing bracing wall,
roof, and floor areas and the addition of weight and, therefore, increased earthquake loading.
Most additions can be categorized as horizontal additions, vertical additions, or a combination of
the two. Horizontal additions generally are built along the side of an existing house and often
require reframing of the roof. Vertical additions generally involve the addition of an upper story.
Figure 9-2 illustrates horizontal and vertical additions.
Horizontal additions may create irregularities or make existing irregularities worse. Thus, the
IRC building configuration irregularity provisions should be reviewed to assess the post-addition
configuration.
Horizontal additions include the construction of new bracing walls at the new exterior of the
house (and sometimes on the interior). A significant reconfiguration of bracing walls at the
interface of existing and new construction also often occurs. All bracing walls in the addition
and interface should be checked for conformance with the IRC as should any portions of the
existing house where framing and bracing modifications have been made.
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Figure 9-2 Horizontal addition (above) and vertical addition (below).
Finally, it is very important that the addition and the existing house be tied together very well.
Ideally the level of interconnection should be the same as would occur if they had been built at
the same time, but this generally cannot be practically achieved. Top plates and sill plates should
be strapped between the new and existing construction to provide continuity.
Sheathing should be continuous and fastened to the same or interconnected framing members
where possible. Where not possible, strapping of framing members should occur at a regular
interval. Figure 9-3 shows the earthquake behavior of a house with failed cripple walls and
adjoining (very small) slab-on-grade addition; a combination of cripple wall upgrade and
strapping between the house and the addition would have greatly improved the performance of
this house.
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Figure 9-3 Cape Mendocino earthquake damage to cripple
wall house with slab-on-grade additions.
A vertical addition demands significantly greater consideration of both gravity and lateral
(earthquake and wind) loads. This is because the story that is added often will more than double
both the gravity and lateral loads on the existing lower story. Thus, when adding an upper story,
the entire house, including the lower story, should be brought into conformance with current
code requirements. Although it should be possible to meet IRC requirements, use of the
engineered design requirements of the IBC or NFPA 5000 may result in a more practical design.
With any addition or alteration, it is important that the gravity and lateral load paths be checked
in detail to ensure that they are complete and meet the requirements of the IRC, IBC, or NFPA
5000. Additions and alterations often create nontypical load path details, and it is important that
these details result in a complete load path with load-carrying capacity that is not less than would
have resulted had typical details been used. In some cases, the new detailing deviates enough
from that which is typical that engineered design should be employed.
Adding to or altering an existing house offers a clear opportunity to voluntarily upgrade existing
portions of the house to better resist earthquake forces. The following section describes where
such upgrades might be employed.
9.2 EARTHQUAKE UPGRADE MEASURES
As noted elsewhere in this guide, the life-safety performance of houses in past earthquakes has
been good with only a few exceptions. There are, however, certain conditions or portions of
houses that have repeatedly resulted in earthquake damage, loss, and, in some cases, life loss or
injury. Among these are:
• Missing or inadequate bolting to the foundation,
• Inadequate cripple wall bracing,
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•
•
•
•
•
Damage to bracing and finish materials,
Excessive drift at garage fronts,
Partial or complete collapse of hillside houses,
Separation and loss of vertical support at “split-level” floor offsets, and
Damage and collapse of masonry chimneys.
Because existing houses vary widely in configuration and construction based on age, region,
siting, etc., it is necessary to identify upgrade measures appropriate to the particular house. In
deciding on voluntary upgrade measures, take into account the configuration of the house and the
potential benefit of the upgrade. Based on the principles discussed in this guide, some simple
upgrade measures for existing houses are described below. This discussion addresses when the
various upgrade measures are appropriate and suggests approximate levels of priority. Note,
however, that the primary objective of most published upgrade measures for houses is reduction
of hazard to life.
If an upgrade is being undertaken on a voluntary basis, the house generally will not be required
to conform to all of the code requirements for new construction. The building official or
authority having jurisdiction should be consulted regarding minimum requirements. It is
recommended that a basis for the upgrade work (i.e., published prescriptive method or
earthquake load level) be established and clearly documented. When a house is being remodeled
or extensively renovated, a systematic upgrade to meet the IRC requirements for new
construction may be reasonable and may be required by the authority having jurisdiction.
The remainder of this section provides an overview of common upgrade measures. Those
interested in implementing specific measures are referred to the list of references and resources
in Appendix E for further information on implementation.
9.2.1 Foundation Bolting
Many houses constructed in California before the 1950s and even later in other parts of the
United States do not have anchor bolts that attach the wall bottom plate or foundation sill plate to
the concrete or masonry foundation. During an earthquake, this can allow the wood framing to
slide off the foundation, causing loss of vertical support and sometimes cripple wall or partial
basement story collapse. Where the headroom and foundation configuration permit, this
situation can be remedied by adding new anchor bolts. Common anchorage configurations are
illustrated in Figure 9-4. Expansion-type anchors can be used in strong concrete and masonry
foundations. Adhesive anchors are recommended for use with unreinforced masonry or weaker
concrete foundations but can be used with all foundation types. When anchor bolts are added,
use of steel plate washers in accordance with the IRC is recommended.
If there is insufficient overhead room or the foundation configuration does not permit the use of
anchor bolts, a wide variety of proprietary anchors are available from manufacturers. The
primary purpose of this anchorage is to transmit horizontal earthquake loads acting parallel to the
foundation from the foundation sill plate to the foundation. Where used, proprietary anchors
should be designed for this use and loading direction by the manufacturer.
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A high priority is suggested for addition of foundation bolting to houses that are not bolted based
on the relatively low upgrade cost and generally high benefit. Applicable references include the
International Residential Code (ICC, 2003a), the International Existing Building Code Appendix
Chapter A3 (ICC, 2003c), Training Materials for Seismic Retrofit of Wood-Frame Homes
(ABAG), Homeowner’s Handbook (City of San Leandro, California), and Project Impact (City
of Seattle).
Figure 9-4 Common anchorage configurations.
9.2.2
Cripple Wall Bracing
Cripple walls are partial-height wood light-frame walls that extend from the top of the
foundation to the first framed floor. Cripple walls are very susceptible to damage during an
earthquake but are also one of the easiest portions of a house to upgrade for improved earthquake
performance. Many of the cripple walls of existing houses (especially those constructed prior to
1960) have inadequate bracing capacity due to the type of sheathing used, inadequate attachment
of the sheathing, inadequate attachment of the framing to the foundation and first floor, or decay
of the system. Figure 9-5a shows a house with collapsed cripple walls (note that the porch floor
is still at the original house floor elevation). As a minimum, this house will have to be jacked up
to be repositioned on the foundation and have utilities reconnected. The house in Figure 9-5b
suffered more severe damage due to cripple wall collapse.
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Figure 9-5a House with collapsed
cripple walls. Photo Courtesy of
National Information Service for
Earthquake Engineering, University of
California, Berkeley
Figure 9-5b A house with
severe damage due to cripple
wall collapse. Photo Courtesy
of FEMA.
Prior to an upgrade, the existing cripple wall system should be inspected and any sections of the
framing that show signs of decay should be replaced. Framing materials in areas where moisture
is present or in contact with the foundation should be replaced with preservative treated or
decay-resistant materials. The upgrade should include anchorage of the foundation sill plate to
the foundation, framing anchorage to the first-floor framing, and sheathing of the cripple walls
with wood structural panel sheathing applied to either the exterior or interior face of the crawl
space walls. Sheathing and connections may be installed to meet the requirements of the IRC or
in accordance with other provisions developed specifically for upgrades. The basic elements of
cripple wall bracing are illustrated in Figure 9-6.
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Figure 9-6 Cripple wall bracing.
Upgrade of interior cripple walls also is recommended where a crawl space is large. Where
bracing is to be installed only at the perimeter of a large crawl space, performance can be
improved by providing additional bracing length, reducing nail spacing from 6 inches to 4
inches, or providing sheathing on both faces of the cripple wall. A high priority is suggested for
upgrading inadequately braced cripple walls based on the relatively low cost and generally high
benefit. Applicable references include the International Residential Code (ICC, 2003a), the
International Existing Building Code Appendix Chapter A3 (ICC, 2003c), Training Materials
For Seismic Retrofit of Wood-Frame Homes (ABAG), Homeowner’s Handbook (City of San
Leandro), and Project Impact (City of Seattle).
9.2.3 Weak- and Soft-Story Bracing
Earthquake damage often is concentrated in the first story of multistory houses because the first
story experiences higher loads while usually having the least amount of bracing. To reduce this
damage, the first-story walls can be upgraded to increase their strength and stiffness. One
method of accomplishing this is to remove the interior finish material (usually gypsum wall
board or interior plaster) at the bottom of the wall in the corners of the house and to add holddown anchors for overturning resistance. The anchors should be attached to the end studs (or
other studs that have sheathing edge nailing) and the vertical rod or bolt should be attached to the
foundation below (see Figure 9-7). This upgrade does not require that the entire interior finish
be removed but only a section in each corner that is one stud spacing in width and several feet in
height. This is an effective upgrade measure where continuous reinforced foundations exist.
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Hold-down anchorage to isolated footings or unreinforced masonry footings is likely to be much
less effective and engineering review is recommended.
Figure 9-7 Weak- and soft-story bracing.
If the interior finish material is being removed for other reasons, then additional upgrade
opportunities present themselves. These include additional anchorage of the top of the wall to
the floor framing above, attachment of the bottom plate of the wall to the floor framing below,
and the addition of blocking at the floor framing for the story above if not present at all locations
that bear on walls. In addition, before gypsum wallboard or another finish material is attached to
the walls, wood structural panel sheathing can be applied to the interior of the walls using 4-inch
nail spacing around the perimeter of each sheet of sheathing and to the stud or post with the holddown attached. A moderate priority is suggested for soft- and weak-story bracing upgrades in
one- and two-family detached houses based on their moderate costs. The benefit can vary widely
depending on the configuration of the existing house.
9.2.4 Open-Front Bracing
An open-front configuration occurs when bracing walls are omitted (or of grossly inadequate
length) along one edge of a floor or roof. This is applicable to stories braced by light-framed
walls. A number of apartment buildings with open front first stories were severely damaged or
collapsed in the Loma Prieta and Northridge earthquakes. In this apartment building type, called
“tuck-under parking,” significant lengths of first-story bracing wall were omitted in order to
provide access to under-building parking. One- and two-family houses with open-front
configurations also are vulnerable to earthquake damage. Two common occurrences of open
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front configurations in one- and two-family houses are the fronts of attached garages with
inadequate bracing length and window walls with no bracing. These conditions are illustrated in
Figure 9-8.
The open-front garage condition is of most concern when there is living space over the garage.
In newer houses, narrow wall segments at the side of garage door openings may contain pre
fabricated bracing wall systems or engineered bracing walls that can be identified by the use of
steel hold-down connectors or straps. Where there is no indication of such bracing systems, the
installation of bracing is recommended. In detached one- and two-family houses, wood
structural panel sheathing and anchorage connectors can be added in accordance with IRC
provisions using adhesive anchors to existing foundations (Figure 9-9). The performance of
narrow bracing walls with hold-down devices relies on the continuity of the existing foundation.
If the existing foundation is not continuous, shows signs of damage or is constructed of
unreinforced masonry or post-tensioned concrete, an engineering evaluation should be
undertaken. Steel moment frames or collectors transferring loads to other portions of the house
are alternative upgrade measures where use of bracing walls is not possible.
Figure 9-8 Common open-front occurrences in one- and two-family detached houses.
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Figure 9-9 Detailing of narrow bracing wall piers at open fronts.
A high priority is suggested for upgrade of open fronts in the first stories of multistory houses. A
moderate priority is suggested in single-story houses or the top story of multistory houses.
9.2.5 Hillside House Bracing
A number of houses on steep hillsides collapsed or were severely damaged in the Northridge
earthquake. Where damage occurred, the lot sloped downward from the street level, and the
main floor of the house was located at or near street level with either a stilt system or tall wood
light-frame walls between the house and grade. Many of the failures began with floor framing
pulling away from the uphill foundation or foundation wall. Typical damage to these types of
houses is shown in Figure 9-10. As a result of Northridge earthquake damage, the City of Los
Angeles has developed provisions for anchoring such houses to their uphill foundations to reduce
the risk of failure.
There are no prescriptive upgrade measures currently available for these hillside houses so
upgrade requires an engineered design. Upgrade measures to improve the response of the
structure and reduce the amount of damage that occurs during an earthquake include:
• Securely anchoring the floor framing to the uphill foundation. This will require that
anchors (i.e., hold-down connectors) be used to attach the floor joists to the foundation
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using adhesive anchors (see Figure 9-11). This detail should be repeated at each framed
floor level that attaches directly to an uphill foundation.
• Attaching the bottom plates of the framing of the stepped (or sloped) and downhill
cripple walls to the foundations. Supplemental anchorage is particularly important where
the top of the side foundations are sloped rather than stepped.
• Attaching the stepped or sloped side cripple wall top plates together at all splice joints
using strap connectors. These straps should be heavy and connected securely to both
sides of the splice, making the top plate of the stepped cripple wall act as though it were
one piece along the entire length of the wall.
• Continuously sheathing the stepped wall and the down-hill wall with wood structural
panel sheathing. Adequate shear transfer into and out of the cripple walls should be
provided.
These upgrade measures should reduce but will not necessarily eliminate earthquake damage. A
high priority is suggested for evaluation of hillside house vulnerability. The need for upgrade
should be determined based on an engineering evaluation. Applicable references are Voluntary
Earthquake Hazard Reduction in Existing Hillside Buildings (City of Los Angeles, 2002) and
Framing Earthquake Retrofitting Decisions: The Case of Hillside Homes in Los Angeles (von
Winterfeldt, Roselund and Kitsuse, 2000).
Figure 9-10 House located on a hillside site
damaged during the Northridge, California,
earthquake.
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Figure 9-11 Anchorage of floor framing to the uphill foundation.
9.2.6
Split-Level Floor Interconnection
Split-level houses experienced partial collapse and significant damage in the 1971 San Fernando
earthquake. These houses had vertical offsets in the floor framing elevation on either side of a
common wall or other support as shown in Figure 9-12. Earthquake damage occurred when
sections of floor and roof framing pulled away from the common wall. (See Section 2.3 of this
guide for additional discussion of irregularities.) The behavior of split-level configurations can
be improved by adequately anchoring floor framing on either side to the common wall. Where
offset floors are close enough in elevation that a direct tension tie can be provided between
levels, an upgrade can be accomplished with installation of steel straps; a strap spacing of not
more than 8 feet on center is recommended (Figure 9-12). Where direct tension ties are not
practical, ATC (1976) provides a variety of details for anchorage of framing to the supporting
wall. Finish removal will often be required in order to install connections, making this upgrade
most practical when remodel work is occurring. It is difficult to establish a priority for this
upgrade because significant damage was observed only in the San Fernando earthquake and
photos suggest that the houses damaged had little or no positive connection provided between
offset floor levels. An applicable reference is A Methodology for Seismic Design and
Construction of Single-Family Dwellings (ATC, 1976).
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Figure 9-12 Split-level ties for floor framing.
9.2.7
Anchorage of Masonry Chimneys
Fireplaces and chimneys in new construction were discussed in Chapter 7 where it was noted
that masonry and concrete fireplaces are heavy, rigid, brittle, and very susceptible to earthquake
damage. IRC requirements for new construction in SDCs D1 and D2 dictate use of horizontal and
vertical reinforcing and anchorage of the chimney to the framing at floors, ceilings, and roof.
Chimneys on existing houses generally are even more vulnerable than new chimneys because
they seldom have reinforcing or are anchored to the house. Common chimney failures range
from hairline fractures of masonry and flue liners to complete fracture (i.e., the top of the firebox
and at the roof line) permitting large sections of the chimney to fall away from or into the house,
shattering into a pile of rubble (see Chapter 7 for discussion and illustrations). Chimneys are
reported to have caused one fatality in the 1992 Landers earthquake and critically injured one
person in the 2000 Napa earthquake (Association of Bay Area Governments).
The upgrading of chimneys is very controversial within the earthquake engineering community.
Upgrading of existing masonry chimneys most often includes strapping the chimney to the house
at the roof, ceiling, and floor levels. Where the chimney extends a significant distance above the
roof line, braces from the top of the chimney down to the roof also may be added. Advocates of
chimney bracing believe that anchoring the chimney will reduce the hazard posed by falling
portions of the chimney. Opponents note that even with strapping, a chimney seeing significant
earthquake loading is likely to be damaged to the point that removal and reconstruction are
required. Opponents further point out that upgrading does not always improve chimney
performance. Both arguments deserve consideration and the reader is referred to the reference
list for additional discussion. Easy and practical approaches to reducing risk to life are provided
on the Association of Bay Area Governments (ABAG) website and include the suggestion that
occupants should not sleep in the area immediately surrounding a fireplace with an unreinforced
or unanchored chimney.
164
Chapter 9, Existing Houses
Where chimneys occur at the house exterior, steel straps similar to those discussed in Chapter 7
can be wrapped around the outside of the chimney and anchored to floor, ceiling, and roof
framing in much the same way as was illustrated for new construction. Because this strap will
have exterior exposure, heavy steel should be used and corrosion protection will need to be
maintained. It also is important that the IRC Section R1001.15 gap be maintained between the
chimney face and combustible framing. Although the addition of straps is not likely to keep a
chimney from being damaged, it may reduce the falling hazard if it is damaged. A recommended
alternative is removal of the chimney or fireplace and chimney and replacement with a factorybuilt fireplace and flue surrounded by light-frame walls.
Applicable references include Info on Chimney Safety and Earthquakes (ABAG) and
Reconstruction and Replacement of Earthquake Damaged Masonry Chimneys (City of Los
Angeles, 2001).
9.2.8
Anchorage of Concrete and Masonry Walls
Under earthquake loading, concrete and masonry walls can pull away from roof and floor
framing. This is primarily a concern where bolted ledgers support framing and no direct
anchorage of the wall to the framing exists. This condition can be effectively upgraded by
providing a tension connection between the wall and the floor and roof framing as shown in
Figure 9-13. The connection should be made to the joists when the joists run perpendicular to
the wall and should be made to blocking and extend at least 4 or 5 feet into the interior of the
floor system when the joists run perpendicular. An engineering evaluation of the existing
condition and engineered design of upgrade measures are recommended.
Applicable references include International Existing Building Code Appendix Chapter A2 (ICC,
2003c) and Guidelines for Seismic Evaluation and Rehabilitation of Tilt-up Buildings and Other
Rigid Wall / Flexible Diaphragm Structures (SEAONC, 2001).
Figure 9-13 Anchorage of concrete or masonry walls to floor, roof, or ceiling framing.
165
FEMA 232, Homebuilders’ Guide
166 Appendix A ANALYSIS OF THE MODEL HOUSE USED IN THIS GUIDE
A model single-family detached house was developed and analyzed in preparing this guide. The
house is described in Chapter 1, and the results of the analysis are referred to throughout the
guide. This appendix provides additional details concerning the model house, the analysis, and
the interpretation of analysis results.
The analysis of the model house provided an approximate comparison of performance for
varying wood light-frame house and bracing configurations permitted by the IRC and permitted
the assessment of improved performance resulting from application of the above-code
recommendations made in this guide. While the model house and the analysis performed cannot
represent all houses that may be constructed using IRC provisions, they do provide a specific
example of relative performance from which trends can been observed.
A1 MODEL HOUSE
The model house contained both one-story and two-story portions, three bedrooms, 2-1/2 baths,
and an area of approximately 2,500 square feet plus garage. The house design is intended to
reflect current configurations for wood light-frame construction but not necessarily any specific
region of the United States. Separate analytical models were developed for common variations
in the design including base conditions, exterior finishes, and earthquake bracing configurations.
The base conditions are slab-on-grade construction with turned-down footings (Figure A-1),
continuous exterior footings with level 2-foot-high cripple walls (Figure A-2), a hillside
condition with cripple walls of varying height (Figure A-3), and a full basement with concrete or
masonry walls (Figure A-4). Exterior finishes are categorized as light and veneer. The light
finish is intended to represent low-weight finishes such as vinyl or fiber-cement board siding.
The veneer is intended to represent a single-wythe anchored brick veneer used for the entire
house exterior. Bracing requirements were determined for each configuration and Seismic
Design Category (SDC) in accordance with the 2003 IRC. Chimneys of light-frame construction
were used for all house configurations.
167
FEMA 232, Homebuilders’ Guide
Figure A-1 Slab on grade base.
Figure A-3 Hillside base.
Figure A-2 Level cripple wall
base.
Figure A-4 Basement.
.
IRC prescriptive bracing requirements were determined for each combination of base condition,
exterior finish, and Seismic Design Category. Because use of veneer is not permitted on houses
with cripple walls in SDC D1 and D2 (IRC Section R703.7, Exceptions 3 and 4), both the level
and hillside cripple wall configurations with veneer were limited to SDC C. Table A-1 is an
example of one bracing spreadsheet. The remaining spreadsheets used in determining bracing
requirements for each of the designs are not included here due to their length; however, they and
other information used in the analysis are available upon request from the Building Seismic
Safety Council.
Because gypsum wallboard is used in almost every U.S. residential building, it was used for the
structural bracing wherever possible. Since it would be installed as a finish anyway, its use for
bracing has the least construction cost. Wood structural panel wall bracing was used where
length and percentage bracing requirements could not be met with gypsum wallboard.
Alternative braced wall panels conforming to IRC Section R602.10.6 were used for the slender
walls at the house front and garage front for slab-on-grade and basement base conditions. The
alternative braced wall panels require support directly on a continuous foundation; therefore,
they could not be used in combination with cripple walls. The IRC Section R602.10.5
168
Appendix A, Analysis of the Model House Used in This Guide
“continuous structural panel sheathing” modifications to bracing length and percent were not
used. See Figure 5-6a through 5-6b for an example bracing plan of a cripple wall base condition
in SDC C.
Several interpretations of IRC requirements were made in developing the bracing designs. First,
it was recognized that the roof-plus-ceiling assembly weight would fall just below the limit of 15
psf of IRC Section R301.2.2.2.1 if the roof assembly weight were considered based on the unit
weight on slope (12:12 roof slope) but would exceed 15 psf if the weight were adjusted to
horizontal projected area. Although adjustment to the horizontal projected area is common
practice in engineering calculations, it was decided that this calculation is not specifically noted
in the 2003 IRC provisions so the roof-plus-ceiling assembly weight was deemed to fall within
the 15 psf IRC limit.
The second interpretation related to the use of gypsum wallboard bracing (IRC Section
R602.10.3, Method 5). IRC Section R602.10.4 requires that gypsum wallboard braced wall
panels applied to one side of a wall be at least 8 feet in width. It was interpreted to mean that a
continuous length of full-height wall not less than 8 feet wide would have to be available in order
to use this bracing method. Interruption of the 8-foot length by perpendicular walls was
interpreted to mean that it was not permitted. Where an 8-foot length of full-height wall was not
available, wood structural panels were used as bracing instead. Based on this interpretation and
the configuration of the model house, wood structural panels rather than gypsum wallboard were
used for a significant portion of the exterior wall bracing. Where gypsum wallboard bracing can
be applied to both faces of a wall (such as at interior walls), the minimum required length of fullheight sheathing is reduced to 4 feet. While the perforated shear wall method that includes holddown anchorage at the ends of the wall line was used as an above-code option for the analysis,
the continuous sheathed option of IRC Section R602.10.5 that allows a 10 percent reduction in
the sheathing percentage was not used in the analysis.
The third interpretation relates to the bracing requirements used for the model house in SDC C.
IRC Table R602.10.1 specifically identifies sheathing length requirements for SDC C. Some
IRC users, however, interpret the IRC Section R301.2.2 exception to mean that the table bracing
length requirements for SDCs A and B can be used for houses in SDC C. Analysis of the model
house performed for this guide used the SDC C bracing length requirements.
The resulting bracing configurations are illustrated on a set of bracing plans and elevations for
each of the designs are available from the Building Seismic Safety Council on a CD-ROM. The
increased bracing length requirements for higher Seismic Design Categories can be observed to
have reduced allowable door and window openings.
169
Appendix A, Analysis of the Model House Used in This Guide
Table A-1 Example Wall Bracing per 2003 IRC, Slab-on-grade Base Condition
Adjustments
Seismic
Design
Category
Wall
Finish
Wall Line
Total
Story
Type 3
Wall Line Length
Stories Considered
Percent
(ft)
2
Type 3
Length
(ft)
Other
Type
Percent
Other
Type
Length Wall Line
Spacing
(ft)
R602.10.1.1
Other
Type 3
Type
Adjusted
Adjusted
Length (ft)
Length (ft)
2
2
2
2
B
E
1.7
5, 6
33.5
30
37
37
16
16
16
16
5.4
4.8
5.9
5.9
25
25
25
25
8.4
7.5
9.3
9.3
1.06
1.06
1.00
1.00
Wall Wt.
Table
R602.10.3
Footnote d
(1)
0.85
1.00
1.00
1.00
1
A
40
16
6.4
25
10.0
1.00
1.00
1.00
1.00
6.4
10.0
1
1
1
1
1
8
B
E
20
20
40
29
16
16
30
30
3.2
3.2
12.0
8.7
25
25
45
45
5.0
5.0
18.0
13.1
1.14
1.14
1.06
1.06
1.00
1.00
0.85
0.85
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
3.7
3.7
10.8
7.8
5.7
5.7
16.2
11.7
1
2
37
30
11.1
45
16.7
1.00
0.85
1.00
1.00
9.4
14.2
1
5, 6
37
30
11.1
45
16.7
1.00
0.85
1.00
1.00
9.4
14.2
2
2
2
2
B
E
1.7
5, 6
33.5
30
37
37
16
16
16
16
5.4
4.8
5.9
5.9
25
25
25
25
8.4
7.5
9.3
9.3
1.06
1.06
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
5.7
5.1
5.9
5.9
8.9
7.9
9.3
9.3
1
A
40
16
6.4
25
10.0
1.00
1.00
1.00
1.00
6.4
10.0
1
1
1
1
1
1
8
B
E
2
20
20
40
29
37
16
16
30
30
30
3.2
3.2
12.0
8.7
11.1
25
25
45
45
45
5.0
5.0
18.0
13.1
16.7
1.14
1.14
1.06
1.06
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.50
1.50
1.50
3.7
3.7
19.0
13.8
16.7
5.7
5.7
28.5
20.7
25.0
1
5, 6, 7
37
30
11.1
45
16.7
1.00
1.00
1.00
1.50
16.7
25.0
Roof + Ceil.
Table
R301.2.2.4
Veneer
R703.7
Exc. 2
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
4.8
5.1
5.9
5.9
7.5
7.9
9.3
9.3
1 (2)
C
light
2
2
1 (2)
C
veneer
2
1
2
Reduction cannot be applied to top-most story where resulting bracing length would be less than required for wind.
The garage and family room areas are treated as a one-story building attached to the two-story house.
170
FEMA 232, Homebuilders’ Guide
A2 ANALYSIS USING STANDARD ENGINEERED DESIGN METHODS
Prior to evaluation using other methods, earthquake forces and deformations were estimated
using the linear static methods commonly used in engineering design of new buildings. Included
were force calculations using the International Building Code (IBC) linear static method,
estimation of drift using the APA-The Engineered Wood Association four-term shear wall
deflection equations at strength level forces, and amplification to estimated drifts using IBC
amplification factors. This approach resulted in the APA shear wall deflection equations being
used outside of their intended range (based on force per nail limits included with nail slip
variables). This provided clearly unrealistic shear wall deflections amplified to unrealistic
estimated drifts (over 36 inch drifts in some cases). Thus, it was concluded that the use of these
engineered design estimates as predictors of performance for non-engineered buildings was not
realistic and it was not pursued. Likewise, use of other available deflection equations that
represent simplifications of the APA equations were not pursued. This issue should not occur
when using this standard deflection calculation method for engineered buildings.
A3 ANALYSIS USING NONLINEAR METHODS
Nonlinear time-history analysis using the Seismic Analysis of Woodframe Structures (SAWS)
analysis program was chosen as the best available method for estimating force and deformation
demands based on analytical studies that were verified against shake table results from the
FEMA-funded CUREE-Caltech Woodframe Project. Analysis models included both designated
bracing and finish materials. The Woodframe Project analytically predicted forces and
deflections compared favorably with shake-table results and were clearly differentiated from
analysis and testing results without finish materials (Folz and Filiatrault, 2002).
The SAWS analysis program uses rigid diaphragms to represent floor and roof diaphragms.
Walls are modeled as nonlinear springs with hysteretic parameters developed specifically to
describe the behavior of woodframe bracing systems. For the example house, rigid diaphragms
were used to represent the high roof, the low roof plus second floor, and, where appropriate, the
first floor. A simplified representation of the rigid diaphragms and wall springs for the model
house is presented in Figure A-5.
Ten sets of hysteretic parameters were developed from component testing data to describe wall
bracing and interior gypsum wallboard finishes. Figure A-6 illustrates the meaning of the
parameters, and a summary of analytical modeling parameter values is provided in Table A-2.
For each of the bracing materials (with the exception of No. 5 and 6), the hysteretic parameters
were determined for a 4-foot bracing length. Because widely varying lengths are used in the
house, the parameters were scaled for varying bracing lengths.
171
FEMA 232, Homebuilders’ Guide
Figure A-5 Analysis model.
Figure A-6 Hysteretic parameters for model ((Folz and Filiatrault, 2002).
Hysteretic parameters currently available from laboratory testing of wall components vary based
on wall boundary conditions, test set-up, and test protocol. Parameters chosen for the analysis of
172
Appendix A, Analysis of the Model House Used in This Guide
the model house tended towards lower bounds of strength and stiffness. Future analytical studies
should consider exploring upper and lower bounds.
In order to simplify interpretation of analysis results, the analysis model uses consistent
identification of bracing walls across all building configurations. Because of this modeling
approach, cripple walls have been included in the model for all building configurations; where
slab-on-grade construction occurs, the cripple walls are modeled as extremely rigid elements
(Property No. 9) resulting in negligible deflection. In addition, some wall elements occur only in
limited configurations; a bracing length of 0.1 foot is used where a bracing panel is intended to
have no effect.
Table A-2 Hysteretic Parameters Used for Nonlinear Dynamic Model
Property No.
Use
gypsum
wallboard
1
lower bound
4 ft OSB w/o
tie-down
3
4
2'-8" wall onesided
5
2'-8" wall twosided
2 ft high
1
cripple wall
1:3 stepped
1
cripple wall
zero height
cripple wall =
100 x No. 7
6
7
8
9
r3
r4
alpha
beta
∆CUREE
(in)
Fu
-0.050
1.000
0.030
0.40
1.10
0.80
0.47
0.096
-0.021
1.000
0.010
0.40
1.10
1.90
0.63
3.0
0.020
-0.100
1.000
0.024
0.87
1.10
2.00
0.62
2.70
6.0
0.020
-0.100
1.000
0.024
0.87
1.10
2.00
1.24
0.30
1.00
20.0
0.010
-0.050
1.000
0.010
0.87
1.10
1.00
3.20
3.30
0.60
1.00
23.0
0.090
-0.110
1.000
0.040
0.87
1.10
0.90
5.48
300.00
30.00
1.00
2000.0
0.010
-0.050
1.000
0.010
0.87
1.10
1.00
320.00
So (Ko)
r1
r2
(k/in)
Basic Model Properties
Fo (k)
FI (k)
∆u (in)
0.50
0.20
0.70
3.0
0.130
0.59
0.15
1.00
2.8
1.20
0.20
2.70
2.40
0.40
3.00
Code Plus Properties
10
11
Code Plus fully
sheathed
perforated
1
shear wall
4 ft OSB w/ tiedown
2.30
0.05
2.20
2.6
0.200
-1.000
1.000
0.020
0.87
1.10
1.50
3.00
1.70
0.50
3.00
6.8
0.040
-0.056
1.000
0.024
0.87
1.10
2.60
2.51
12
1
Values in table are for 4 ft length. Fo, Fi & Ko in model are scaled bylength of full height sheathing. See sepatate spreadsheets.
The strength and stiffness contribution of exterior wall finishes was not included in the analysis.
This approach was chosen because it would lead to a lower bound and, therefore, conservative
estimate of deformation demand. In addition, some exterior finish materials are believed to have
very little impact on building behavior (e.g., vinyl siding) and information was not available on
the contribution of some other finishes (e.g., brick veneer). Due to the judgment necessary to
select appropriate component testing and to derive parameters and the simplification of not
including exterior finishes, the resulting modeling must be qualified as being approximate.
Earthquake demand is represented using the larger horizontal acceleration record from Canoga
Park for the 1994 Northridge, California, earthquake. This record was chosen because it
corresponds well with the code design spectra over a range of building periods. The peak
acceleration was scaled for each Seismic Design Category by dividing the maximum SDS value
for each category by 2.5, resulting in peak accelerations of 0.2, 0.33 and 0.47g for SDCs C, D1,
and D2, respectively. For comparison, the recorded ground motion has a peak acceleration of
173 FEMA 232, Homebuilders’ Guide
0.42g and was scaled to 0.50g to represent Zone 4 anticipated ground motions in the CUREE
shake-table testing. The ground motion scaling used for this analysis represents the demand used
as a basis for code design. The demand from the maximum considered earthquake (MCE)
ground motion (MCE) would be approximately 50 percent greater.
Detailed assembly weights and building weights have been determined for each house
configuration. The analysis model spreads the resulting mass uniformly over a single rectangle
used to describe each above-ground diaphragm. The center of the mass rectangle is set at the
calculated center of mass of the building. This simplification, made necessary by analysis
limitations, should have a minor effect on results.
A4 ANALYSIS RESULTS
The selected ground motion was run once in the horizontal X-direction and once in the
horizontal Y-direction for each combination of base condition, exterior finish, and Seismic
Design Category as well as for a series of above-code recommendations. From the nonlinear
time-history analysis, peak drifts in each of the bracing wall lines and peak reactions to
supporting foundations were extracted and summarized in tables. These tables are not included
here for brevity but are available upon request from the Building Seismic Safety Council on the
analysis CD. The “controlling” value was the largest absolute value of the X- and Y-directions.
A4.1 Deformation Demand Relation to Performance
In order to translate the results of the analysis into an approximation of house performance, three
ranges of peak transient wall drift and associated approximate descriptions of building
performance were developed. The choice of range and description of performance are based on
component and full-building test results combined with the opinions of those participating in the
development of this guide.
The approximate performance categories and corresponding drift ranges are:
• Minor damage potential – Less than or equal to 0.5% story drift
The house is assumed to suffer minor nonstructural damage such as
cracked plaster or gypsum wallboard and hopefully would be “green
tagged” (occupancy not limited) by inspectors after an earthquake,
which would permit immediate occupancy. Some repairs should still be
anticipated.
• Moderate damage potential – Above 0.5% to 1.5% story drift
The house is assumed to suffer moderate damage including possible
significant damage to materials and associated structural damage, but the
building is assumed to have some reserve capacity in terms of strength and
displacement capacity. The house hopefully would be “green-tagged” or,
174
Appendix A, Analysis of the Model House Used in This Guide
more likely, “yellow-tagged” (limited occupancy) by inspectors after an
earthquake and may or may not be habitable. Significant repairs should be
anticipated.
• Significant damage potential – Greater than 1.5% story drift
The house is assumed to have significant structural and nonstructural
damage that could result in its being “red-tagged” (occupancy prohibited)
by inspectors after an earthquake. Significant repairs to most components
of the building should be anticipated, and it may be more economical to
replace the house rather than repair it.
Use of these three categories permits an approximate comparison of the relative performance of
different IRC bracing solutions and above-code recommendations.
A4.2 Discussion of Results
Selected results of peak drift values and approximate performance category are provided in
Tables A-3 and A-4. In most cases, the drift increased with increased SDC in spite of the
bracing requirements also having increased. The approximate performance often increased from
minor or moderate to significant as the SDC went from C to D2. The primary reason is the
inclusion of interior gypsum wallboard in the models for all Seismic Design Categories. As the
SDC increased, interior walls became required braced wall panels per IRC requirements rather
than simply nonstructural partition walls; however, the analytical model did not change because
the interior walls had already been included. The result was application of a higher demand to a
model with only nominal increases in resistance.
Table A-3 Selected Results for IRC Bracing Provisions, Slab-on-grade Base Condition Walls
Seismic Design
Category
Light
C
Light
D1
Light
D2
Veneer
C
Veneer
D1
Veneer
D2
1st Story Peak
Drift (in.) and
Approximate
Performance
0.46
Minor
1.02
Moderate
1.72
Significant
1.29
Moderate
1.34
Moderate
2.21
Significant
175
FEMA 232, Homebuilders’ Guide
Although the building mass increased significantly with the addition of brick veneer, the increase
in drift ranged from moderate to slight. This is due to the IRC requirement for wood structural
panel sheathing and hold-down devices for veneer in SDCs D1 and D2. The analysis model
differentiated between wood structural panel shear walls with and without hold-down devices so
the different strength, stiffness, and deformation capacity were accounted for. Because of this,
the IRC bracing required for brick veneer was seen to partially compensate for the increased
demand.
The above-code measures were applied to the slab-on-grade base condition. The measures were
seen to generally reduce the building drift, although drift increases were seen in a few walls due
to changes in diaphragm rotation. In SDC D2, the approximate performance was improved by
one category for all three above-code measures. In SDCs C and D1, significant decreases in drift
were seen within an approximate performance category.
Table A-4 Selected Results for Above-code Measures, Slab-on-grade Base Condition
1st Story Peak Drift (in.), Approximate Performance, and
Maximum 1st Story Drift Reduction
Above-code
Recommendation
Walls
SDC C
SDC D1
SDC D2
Original
Code Minimum
Light
0.5
Minor Damage
1.0
1.7
Moderate Damage Significant Damage
Above-code
Continuous
Sheathing
Above-code
Hold-downs
Light
Above-code
Lap on Rim Joist
Light
0.3
Minor Damage
39 percent
0.5
Minor Damage
28 percent
0.5
Minor Damage
6 percent
0.7
Moderate Damage
42 percent
0.9
Moderate Damage
36 percent
1.0
Moderate Damage
7 percent
Light
1.2
Moderate Damage
30 percent
1.0
Moderate Damage
47 percent
1.4
Moderate Damage
24 percent
The cost of implementing each above-code measure during construction of the house was
estimated in terms of percentage change to the construction cost for the basic house structure.
Comparison to total house cost was not made because variations in finishes and fixtures can
dramatically vary the house cost.
Use of continuous wood structural panel wall sheathing (fully sheathed) with overturning
anchors in the corners of the house significantly reduced the drift in all Seismic Design
Categories, and the approximate performance category was increased by one in SDC D2. The
cost of making this change was estimated to be 9 to 10 percent of the cost of the structural
portion of the model house used in this guide.
The addition of hold-down anchors at the ends of each full-height wall segment (at the corners
and edges of each door and window) significantly reduced the drift in all Seismic Design
176
Appendix A, Analysis of the Model House Used in This Guide
Categories, and the approximate performance category was increased by one step in SDC D2.
For the model house, the cost of implementing this improvement was estimated to be 18 percent
of the structural cost of the house.
Lapping wood structural panel wall sheathing over the band joist of the floors did not have a
significant effect in SDC C or D1 but did improve the approximate performance category by one
in SDC D2. The cost of implementing this improvement was estimated to be 0.5 percent of the
cost of the structural portion of the house. This above-code measure can be accomplished by
either sheathing the wall with oversized panels (9-foot panels on an 8-foot wall) or cutting and
blocking standard size sheets.
Use of the above-code measures in combination is thought to have a cumulative effect in
improving performance and so is encouraged.
177
FEMA 232, Homebuilders’ Guide
178 Appendix B EARTHQUAKE PROVISIONS CHECKLIST FOR BUILDERS AND DESIGNERS General Earthquake-Resistance Requirements
Load Path
C NC N/A Priority
HIGH Foundation anchor bolts. IRC Section R403.1.6 for all SDCs. IRC
Section R403.1.6.1 and R602.11.1 for foundation anchor bolts and plate
washers in SDCs D1 and D2 (and townhouses in SDC C).
Typical 1/2-inch diameter bolts at 6 feet along all exterior walls. Also,
1/2-inch bolts at 6 feet along interior braced walls and interior bearing
walls supported on a continuous foundation, in SDCs D1 and D2 (and
townhouses in SDC C); 1/2-inch at 4 feet along all exterior walls and
along all interior bearing walls and interior bracing walls supported on a
continuous foundation for three-story in SDC D1 (and three-story
townhouses in SDC C).
3x3x1/4-inch steel plate washers: On all required anchor bolts in SDCs D1
and D2 (and townhouses in SDC C).
First anchor bolt should be placed seven bolt diameters minimum (3-1/2
inches for 1/2-inch diameter, 4-1/2 inches for 5/8-inch diameter) and 12
inches maximum from each end of a foundation sill plate.
Sheathing and framing fasteners (except for 1/2-diameter or larger steel
bolts) used in pressure preservatively treated wood framing members must
have corrosion-resistant coating or be of corrosion-resistant material.
HIGH Overturning Anchorage. IRC Section R602.10.6 for alternate braced wall
panels, all SDCs. IRC Section R602.10.11, second paragraph, Exception
2, where braced wall panels are not located at corners in SDCs D1 and D2.
IRC Section R703.7, Exceptions 3 and 4, when veneer is used in SDCs D1
and D2. Overturning load path to foundation needed where hold-down
anchors are used.
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Load Path Above-code Recommendations:
•
Provide 4-foot anchor bolt spacing along all exterior and interior braced wall
lines for two-story houses in SDCs D1 and D2.
• Provide continuous foundation below interior braced walls with anchor bolts at
spacing of 6 feet of less in all SDCs.
• Provide corrosion-resistant coatings for anchor bolts installed through pressure
preservatively treated foundation sill plates in all SDCs.
• Do not “wet set” anchor bolts; securely place anchor bolts prior to placing
concrete.
• In SDC C, provide overturning anchorage as required in SDCs D1 and D2 for
braced wall panels not located at corners and for houses with masonry veneer.
• Add tie straps between first and second story corner studs to tie the walls
together in SDCs C, D1 and D2.
• Use oversized sheathing panels on exterior walls and lap over rim-joist. Nail
both into the plates (top and bottom) and the rim-joists in all SDCs.
MED Minimum fastening. All SDCs per IRC Table R602.3.
MED Designed collector members aligned with and connected to the top plate of
braced walls (continuous from the end of a braced wall line to the end of
the braced wall panel closest to the end of the wall line). In all SDCs
where the first braced wall panel begins more than 12 feet from the end of
a braced wall line. (Section IRC R602.10.1)
Also, in SDCs D1 and D2, where the braced wall panel uses wood
structural panel sheathing and is not located at the end of the wall line.
However, when either (a) a minimum 24-inch-wide panel is provided each
side of the wall corner or (b) the braced panel end closest to the corner is
provide with a hold-down, a designed collector is not required if the wood
structural panel braced wall panel is located 8 feet or less from the end of
the braced wall line. (IRC Section R602.10.11 last paragraph)
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Foundations and Foundation Walls
Concrete Foundations
C NC N/A Priority
HIGH Horizontal reinforcing. In SDCs D1 and D2, typical one No.4 in footing.
Additional No. 4 in concrete stem wall (if stem wall occurs). One No. 4
top and bottom in thickened slab footing, with alternate of one No. 5 or
two No. 4 in middle third of footing height for thickened slab footings cast
monolithically with slab. (IRC Sections R403.1.3 through R403.1.3.2)
HIGH Vertical reinforcing. In SDCs D1 and D2, one No. 4 at 48 inches
maximum spacing where a pour joint occurs between concrete footing and
concrete stem wall. (IRC Section R403.1.3)
MED Adequate support of reinforcing and anchor bolts. Reinforcing concrete
cover distances of 3 inches when cast against earth and 1-1/2 inches when
concrete will be exposed to weather.
HIGH Clean footing excavations before casting concrete. Proper concrete
consolidation. No water added to concrete mix at site.
MED Minimum concrete strength. 2500 psi, all SDCs. 3000 or 3500 psi in
moderate or severe weathering probability areas (IRC Section R402.2 and
Table R402.2).
MED Rebar lap splice length of 24-inches (straight lap) (IRC Section
R611.7.1.2). Rebar bend radius (outer) of 2 inches for No. 4 and 2-1/2
inches for No. 5. Hook at corners and intersections of 8 inches for No. 4
and 10 inches for No. 5.
Masonry Foundations
C NC N/A
HIGH Horizontal reinforcing.
HIGH Vertical reinforcing.
MED Rebar lap splice length of 24-inch (straight lap) (IRC Section
R606.11.2.2.3). Rebar bend radius (outer) of 2 inches for No. 4 and 2-1/2
inches for No. 5. Hook at corners and intersections of 8 inches for No. 4
and 10 inches for No. 5 (IRC Section R606.11.7.4).
Foundation Walls
C NC N/A
HIGH Wall thickness.
HIGH Horizontal reinforcing.
HIGH Vertical reinforcing.
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MED Rebar lap splice length of 24-inches (straight lap). Rebar bend radius
(outer) of 2 inches for No. 4 and 2-1/2 inches for No. 5. Hook at corners
and intersections of 8 inches for No. 4 and 10 inches for No. 5
Above-code Recommendations:
• Avoid construction of slab on grade homes on cut and fill sites where possible.
Where this condition cannot be avoided, provide additional quality control for
fill placement and compaction operations.
• Regardless of SDC, provide not less than one continuous horizontal No. 4
reinforcing bar in concrete footings. Provide a second No. 4 horizontal bar in
stem wall if occurs. This will provide tension and bending capacity to help
mitigate foundation damage due to earthquake, wind, soil movement and frost
heave.
• Regardless of SDC, remove lose debris in the construction joint between a
concrete footing and a separately cast slab-on-grade.
• In SDCs C, D1 and D2, provide not less than No. 4 at 4-feet vertical bars as
dowels between a concrete footing and a separately cast slab on grade.
• Regardless of SDC, provide not less than one continuous No. 4 reinforcing bar in
masonry foundations.
Floor Construction
C
NC N/A
HIGH Floor sheathing nailing. Floor sheathing should be edge nailed to blocking
above all braced wall lines, exterior and interior, as part of the load path
(IRC Table R602.3(1), Footnote i). Blocking with edge nailing needs to
have a load path to top of braced wall panels.
MED SDCs D1 and D2, blocking or lateral restraint. Required at intermediate
floor framing member supports. (IRC Section R502.7, Exception)
Light-Frame Wall Construction
HIGH Overdriven sheathing nails. For wood structural panel sheathing, nails are
to be driven so that the top of the head is flush with the face of the
sheathing. (It is recommended that where nail heads occasionally are
more than 1/16-inch below the surface, an additional nail should be
provided between existing nails. If a substantial number of nails are
overdriven, the sheathing should be removed and the framing checked for
splitting before replacing the sheathing with proper nails.)
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HIGH Sheathing nailing to hold-down posts and studs. Hold-downs cannot carry
any load unless the wall sheathing is edge-nailed to the hold-down post or
stud.
HIGH Threaded rods with properly attached nuts need to be in place before the
wall sheathing is attached to the second side of the walls.
Above-code Recommendations:
• Increase first-story strength and stiffness to mitigate weak-story irregularity.
Approaches include: (a) use of wood structural panel wall bracing and holddown connectors at each end of each full height wall segment, (b) fully
sheathing all exterior walls including below windows and above and below
doors and providing hold-down connectors at building corners, and (c)
providing more than the minimum braced wall panel length.
• Increase cripple wall strength and stiffness to mitigate weak-story irregularity
by sheathing full length of exterior cripple walls.
• Use oversized sheathing panels on exterior walls to increase wall stiffness and
strength. Lap the sheathing over the floor joists and nail to both the plates (top
and bottom) and the floor joists.
Roof Construction
C
NC N/A
HIGH Sheathing nailing at braced wall lines. Roof sheathing should be edge
nailed (to blocking where present) above all braced wall lines, exterior and
interior, as part of the load path (IRC Table R602.3(1), Footnote i).
Blocking with edge nailing needs to be nailed to the top of the braced
wall below to provide a complete load path.
Cold-formed Steel Construction
C
NC N/A
HIGH Load path connections. Connection of cold-formed steel framing
members is different than wood light-frame connection. The IRC
provisions include a significant number of specific connection details.
Attention to these details is important to the building performance for all
load types.
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Above-code Recommendations:
• Add interior cold-formed steel braced walls such that the distance between braced
wall lines does not exceed 35 feet. • In all SDCs, apply the irregularity limitations developed for wood light-frame houses
(IRC Section R301.2.2.2.2).
Masonry Wall Buildings
C
NC N/A
HIGH Construction quality control. Proper type of mortar for masonry being
used and proper mortar mixing. (Type N mortar is prohibited in higher
SDCs.) Proper placement of reinforcing. Adequate support and
attachment of reinforcing and anchor bolts. Cleaning out of grout space to
allow proper grout placement, including cleaning out excess mortar if
necessary. Provide cleanouts if necessary for adequate cleaning.
Consolidation of grout.
Above-code Recommendations:
• Each exterior wall and each interior braced wall should have one, and preferably two, sections of solid wall not less than 4 feet in length. • Sections of solid wall should be spaced no more than 40 feet on center and should be
placed as symmetrically as possible.
• All masonry walls should be supported on substantial continuous footings extended
to a depth that provides competent bearing.
• The distribution of interior masonry braced walls should be carefully balanced and
floor and roof plans should use simple rectangular shapes without jogs and openings.
• Apply the irregularity limitations developed for wood light-frame houses (IRC
Section R 301.2.2.2.2). The IRC exceptions to Irregularities 2 and 5 can be applied
but the rest of the exceptions are not applicable.
• Solid portions of wall should be stocked from floor to floor and masonry walls
should be continuous from the top of the structure to the foundation. Masonry walls
not directly supported on walls below require engineered design for gravity load
support and design for earthquake and wind loads should be provided.
• Running board lay up of masonry units should be used instead of stack bond lay up.
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Above-code Recommendations (continued):
• For concrete masonry, use open end units at locations of vertical reinforcement and
use wood beam units for horizontal reinforcing to increase the interlocking of
masonry construction.
• Apply the measures required or recommended for masonry construction in areas of
high earthquake risk, in areas of lower earthquake risk, and in high-wind areas.
Priorities include provisions for reinforcing (IRC Figure R606.10 (2)), wall
anchorage using details developed to resist out-of-plane wall loads (e.g., IRC Figures
R611.8 (1) through (7)), minimum length of bracing walls, and a spacing limit for
braced wall lines.
Concrete and Insulating Concrete Form Wall Buildings
C
NC N/A
HIGH Construction quality control. Proper placement of reinforcing. Adequate
support and attachment of reinforcing and anchor bolts. Cleaning out of
cells space to allow proper concrete placement. Consolidation of concrete.
Above-code Recommendations:
• Carefully balance bracing walls around the perimeter of the building.
• Apply the measures required or recommended for ICF houses in areas of high
earthquake risk, in areas of lower earthquake risk, and in high-wind areas.
Priorities include wall anchorage using details developed to resist out-of-place wall
loads (e.g., IRC Figures R611.8 (2) through (7)).
Stone and Masonry Veneer
C
NC N/A
HIGH Veneer thickness limited to 5-inch nominal thickness in SDCs A, B, and
C, 4-inch nominal thickness in SDC D1, and 3-inch actual thickness in
SDC D2 except that up to 5-inch nominal thickness can be used in SDC D1
and D2 if veneer only extends to first story above grade. (IRC Section
R703.7)
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Above-code Recommendations:
• Use corrosion-resistant sheet metal ties or wires to fasten veneer. The ties or wires
should penetrate the house paper and sheathing and should be embedded in the wall
studs.
• Where veneer can be used only on the first story above grade, increase the length of
the structural wood panel bracing and use hold-down devices on braced wall panels
in the first story.
Fireplaces and Chimneys
C
NC N/A
HIGH Masonry reinforcing. Vertical reinforcing of not less than four No. 4 bars
for chimneys up to 40 x 24 inches. Should extend from bottom of
foundation (3-inch minimum concrete cover) to top of chimney except that
splices of not less than 24 inches are acceptable. Must be placed such that
reinforcing can be surrounded in grout. Horizontal ties of 1/4-inch
minimum at 18 inches maximum on center in mortar joint. SDCs D1 and
D2. (IRC Section R1003.3)
186
Appendix C EARTHQUAKE PROVISIONS CHECKLIST FOR DESIGNERS AND PLAN CHECKERS General Earthquake-Resistance Requirements
General Earthquake Limitations
C
NC N/A
Seismic Design Category. Buildings in SDCs A through D2 may be designed per
the IRC; buildings in SDC E require engineered design unless the alternate
determination of Seismic Design Category provisions of IRC Sections
R301.2.2.1.1 or R301.2.2.1.2 are met. (IRC Section R301.2.2)
Assembly weight. Weight of roof plus ceiling, floor, interior wall and exterior
wall assemblies are limited in SDCs D1 and D2 and townhouses in SDC C (IRC
Section R301.2.2.2.1).
Number of stories. Wood light-frame buildings are limited to two stories plus
cripple walls in SDC D2 (IRC Section R301.2.2.4.1 and Table R602.10.1). Coldformed steel framed buildings are limited to two stories above grade in SDCs D1
and D2 (Section R301.2.2.41). Masonry walls are limited to one story and 9 feet
between lateral supports in SDCs D1 and D2 (IRC Section R606.11.3.1 and
R606.11.4).
Story height. All SDCs. Building story height is limited by the following limits
on bearing wall clear height plus a maximum of 16 inches for the floor framing
depth:
Wood light frame
Cold-formed steel
Masonry
ICF
12 ft (IRC Section R301.3, Item 1 Exception) 10 ft (IRC Section R301.3, Item 2) 12 ft plus 8 ft at gable ends (IRC Section R301.3, Item 3) 10 ft (IRC Section R301.3, Item 4, and Section 611) Load Path
C NC N/A
Minimum wood light frame fastening. All SDCs. (IRC Table R602.3)
Anchor bolts and plate washers. IRC Section R403.1.6 for all SDCs. IRC
Sections R403.1.6.1 and 602.11.1 for SDCs D1 and D2 and townhouses in SDC C.
Overturning Anchorage. IRC Section R602.10.6 for alternate braced wall panels,
all SDCs. IRC Section R602.10.11 Exception 2 ,where braced wall panels are not
located at corners for SDCs D1 and D2. IRC Section R703.7, Exceptions 3 and 4,
when veneer is used for SDCs D1 and D2.
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Designed collector members aligned with and connected to the top plate of braced
walls (continuous from the end of a braced wall line to the end of the braced wall
panel closest to the end of the wall line). In all SDCs IRC Section R602.10.1. In
SDC D1 and D2, IRC Section R602.10.11 last paragraph. IRC Section
R301.2.2.2.2 for SDCs D1 and D2 and townhouses in SDC C.
Irregularities
C NC N/A
Irregularity 1: Exterior braced wall panels not in one plane (stacked) from
foundation to top most story in which they are required.
Irregularity 2: Section of floor or roof not supported by braced wall lines on all
edges.
Irregularity 3: End of braced wall panel occurs over opening in wall below, and
extends more than one foot beyond the edge of the opening.
Irregularity 4: Opening in floor or roof exceeds lesser of 12 feet or 50% of least
floor or roof dimension. Figure 2-x.
Irregularity 5: Portions of floor level are vertically offset (split level).
Irregularity 6: Braced wall lines do not occur in two perpendicular directions.
Irregularity 7: Stories braced by light-frame walls include concrete or masonry
construction.
Above-code Recommendations:
•
Apply irregularities to all SDCs because they are also applicable for wind load.
•
Increase first-story strength and stiffness to mitigate weak-story irregularity.
•
Increase cripple wall strength and stiffness to mitigate weak-story irregularity.
Foundations and Foundation Walls
General
C NC N/A
Continuous perimeter foundations. All exterior walls are to be supported on
continuous perimeter foundations. All SDCs. (IRC Section R403.1)
Continuous interior foundations. At interior braced wall lines in buildings with
plan dimensions greater than 50 ft. SDCs D1 and D2 . (IRC Section R403.1.2).
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Appendix C, Earthquake Provisions Checklist for Designers and Plan Checkers
Special Soils Conditions
C NC N/A
Low bearing capacity. Soils investigation required when building official
determines that soil bearing capacities of less than 1500 psf might be present at
site. All SDCs. (IRC Table R401.4.1, footnote b)
Soil testing when expansive, compressible, or shifting soils are encountered or are
likely. (IRC Section R401.4)
Frost protection. Footings are to be below the frost line or adequate frost
protection should be provided. (IRC Section R403.1.4.1)
Concrete Foundations
C NC N/A
Minimum concrete strength. 2500 psi for all SDCs. 3000 or 3500 psi in
moderate or severe weathering probability areas. (IRC Section R402.2 and Table
R402.2)
Horizontal reinforcing. One No.4 in footing and second No. 4 in stem wall. No.
4 top and bottom in thickened slab footing with alternative of one No. 5 or two
No.4 in middle third of footing height for thickened slab footings cast
monolithically with slab. SDCs D1 and D2. (IRC Sections R403.1.3 and
R403.1.3.2)
Vertical reinforcing. No. 4 at 48 inches maximum spacing required where a pour
joint occurs between concrete footing and concrete stem wall. SDCs D1 and D2.
(IRC Section R403.1.3)
Masonry Foundations
C NC N/A
Masonry foundation type. Solid clay masonry and fully grouted concrete
masonry permitted in all SDCs (IRC Section R403.1). Rubble stone masonry
foundation walls limited to SDCs A through C (IRC Section R404.1.1).
Horizontal reinforcing. One No. 4 in footing and second No. 4 in stem wall.
SDCs D1 and D2. (IRC Section R403.1.3 and R403.1.3.1)
Vertical reinforcing. Minimum No.4 at 4 feet on center extending into footing
with standard hook. SDCs D1 and D2. (IRC Section R403.1.3)
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Foundation Walls
C NC N/A
Wall thickness. Six inches minimum up to 12 inches required based on soil type
at site. SDCs A through D2. (IRC Table R401.1.1(1)).
Horizontal reinforcing. Dependent upon all thickness and material. Minimum
No. 4 in upper 12 inches of wall. SDCs D1 and D2. (IRC Sections R404.1.4 and
R606.11).
Vertical reinforcing. Varies depending on wall height and soil type at site.
ASTM Grade 60 minimum. All SDCs. (IRC Tables R404.1.1(2)).
Above Code Recommendations:
Avoid construction of slab-on-grade homes on cut and fill sites where possible. Where
this condition cannot be avoided, provide additional quality control for fill placement
and compaction operations.
Regardless of SDC, provide not less than one continuous No. 4 reinforcing bar in
concrete footings. Provide a second No. 4 in stem wall if present. This will provide
tension and bending capacity to help mitigate foundation damage due to earthquake,
wind, soil movement, and frost heave.
In SDCs C, D1 and D2, provide not less than No. 4 vertical bars at 4 feet as dowels
between a concrete footing and separately cast slab-on-grade.
In concrete foundations, lap reinforcing bars not less than 24 inches. Bend radius
(outer) for No. 4 bar is 2 inches and 2-1/2 inches for No. 5. Hook at corners and
intersections of 8 inches for No.4 bars and 10 inches for No. 5 bars.
Regardless of SDC, provide not less than one continuous No. 4 reinforcing bar in
masonry foundation stem walls.
In masonry foundation walls and stem walls, lap reinforcing bars not less than 24
inches. Bend radius (outer) for No. 4 bar is 2 inches and 2-1/2 inches for No. 5. Hook at
corners and intersections of 8 inches for No. 4 and 10 inches for No. 5.
Floor Construction
C
NC N/A
Blocking or lateral restraint. Required at intermediate floor framing member
supports. SDCs D1 and D2. (IRC Section R502.7, Exception).
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Light-Frame Wall Construction
C
NC N/A
Braced wall length required for each 25 ft of wall length. (IRC Section R602.10.6)
Sheathing attachment spacing. (Various IRC sections)
Cold-formed Steel Construction
C
NC N/A
Cold-formed steel framing. Buildings in SDCs D1 and D2 need to comply with
the AISI Standard for Cold-Formed Steel Framing – Prescriptive Method for
One- and Two-Family Dwellings in addition to the requirements of the IRC
Section R301.2.2.4.5.
Masonry Wall Buildings
C
NC N/A
Limited to one story for SDCs D1 and D2. (IRC Section R606)
In SDCs D1 and D2, 9 feet between lateral supports. (IRC Section R606.8)
Light-frame restricted from supporting lateral loads from masonry. (IRC Section
R301.2.2.2.2)
Reinforcement detailing (IRC Section R606.11).
Concrete and Insulating Concrete Form Wall Buildings
C
NC N/A
Limited to two stories above grade. (IRC Section R611)
Minimum wall thickness of 5.5 inches for IFC and 6 inches for solid concrete.
(IRC Section R611.7.4)
Maximum plan dimension of 60 feet and aspect ratio of 2:1. (IRC Section
R611.2)
Reinforcement detailing. (IRC Sections R611.3 - R611.5 and R611.7.1.2 and
R611.7.1.3)
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Stone and Masonry Veneer
C
NC N/A
Veneer. In SDCs D1 and D2, veneer is not permitted on buildings with cripple
walls. (IRC Section R703.7, Exceptions 3 and 4)
Fireplaces and Chimneys
C
NC N/A
Vertical reinforcement requirements (four No. 4 Bars). (IRC Section R1003.3)
Type N mortar prohibited in SDCs D1 and D2. (IRC Section R609)
Anchorage requirements for SDCs D1 and D2. (IRC Section R1003.4)
192
Appendix D SIGNIFICANT CHANGES FOR THE 2006 INTERNATIONAL RESIDENTIAL CODE
During the 2006 cycle of technical updates for the International Residential Code (IRC), a
number of important technical changes were made that will have an impact on many houses
around the country. This appendix highlights the most significant of these changes so that the
designer can continue to use this document when the 2006 IRC is adopted by his or her
jurisdiction.
D1 REVISED SEISMIC DESIGN MAPS
In response to concerns over the perceived increases in earthquake design forces that were
implemented with the adoption of the 2000 editions of the International Building Code (IBC)
and IRC, the U.S. Geological Survey (USGS) conducted a detailed evaluation of earthquake risk
on a county-by-county basis for regions with a high probability of earthquake occurrence. The
project incorporated significant new information about local geological and geotechnical features
and local experts for the regions being investigated were consulted. The result of the project is
revised design maps that are incorporated into the 2006 editions of the IRC and IBC. In general,
the new maps reduce the amount of geographic area affected by the high seismic risk, but the
remaining area is also affected so some extent. This is primarily evident in the Charleston, South
Carolina, region where the Seismic Design Category was raised in the counties closest to
Charleston, but the rest of the state experiences a reduction in seismic risk level. The seismic
design map that was adopted for the 2006 IRC is shown in Figure D-1 on the following pages.
D2 ADDITION OF SEISMIC DESIGN CATEGORY D0
In addition to provide some relief for the construction of houses using heavier finish materials
such as masonry veneers and stucco, Seismic Design Category D1 was divided into two SDCs –
D0 and D1. Since design values must be set to the highest value in the range, dividing the
original D1 into two lowered the earthquake design load for the lower design category. Only the
brick masonry veneer industry has started to take advantage of this change to date. However, it
is expected that other materials also will propose changes in the future to take advantage of the
lower forces associated with Seismic Design Category D0. The geographic area associated with
the change can be seen in Figure D-1.
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Appendix D, Significant Changes for the 2006 International Residential Code
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Appendix D, Significant Changes for the 2006 International Residential Code
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Appendix D, Significant Changes for the 2006 International Residential Code
D3 CHANGE IN APPLICABILITY OF IRREGULAR BUILDING REQUIREMENTS
During the update cycle resulting in the 2006 IRC, it was noticed that the wording requiring that
the building conform to the requirements of IRC Section R301.2.2.2.2 only applied to wood and
ICF concrete construction. Cold-formed steel construction had to conform to these irregularity
requirements even through the referenced design and construction document (COFS/PM) had
less stringent requirements and application to masonry wall buildings was not specified. Thus, a
change was made to require all buildings to conform to the irregularity provisions of the IRC,
which limit the concentrations of loads and deformations that irregularities cause.
D4 CLARIFICATION AND ADDITION OF REQUIREMENTS FOR MASONRY
VENEER
A significant change was made to the requirements for the use of stone and masonry veneer in
areas of high earthquake risk. The changes clarify veneer weight limits and stories where veneer
is permitted. They also clarify and illustrate the required hold-down anchorage of walls and the
requirements for ties and other reinforcement and attachment of the veneer to the walls. The
change is too extensive to document here; rather, the reader is referred to the 2006 IRC for
details.
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200 Appendix E REFERENCES AND ADDITIONAL RESOURCES American Forest & Paper Association (AF&PA). 2001. Wood Frame Construction Manual for
One- and Two-Family Dwellings and Commentary. AF&PA, Washington, D.C.
American Forest & Paper Association. 2001. Allowable Stress Design (ASD) Manual for
Engineered Wood Construction. AF&PA, Washington, D.C.
American Forest & Paper Association. 1996. Load and Resistance Factor Design (LRFD)
Manual for Engineered Wood Construction. AF&PA, Washington, D.C.
American Forest & Paper Association. 2001. Details for Conventional Wood frame
Construction. AF&PA, Washington, D.C.
American Forest & Paper Association. 1993. Span Tables for Joists and Rafters. AF&PA,
Washington, D.C.
American Forest & Paper Association. 1987. Basic Requirements for Permanent Wood
Foundation System, Technical Report 7. AF&PA, Washington, D.C.
American Institute of Timber Construction (AITC). 2004. Timber Construction Manual – Fifth
Edition. AITC, Englewood, Colorado
American Iron and Steel Institute (AISI). 2001. Standard for Cold-Formed Steel Framing Prescriptive Method for One- and Two-Family Dwellings (AISI/COFS/PM 2001). AISI,
Washington, D.C.
Anderson, C., F. Woeste, and J. Loferski. 2005. Manual for the Inspection of Residential Wood
Decks and Balconies. ICC, Country Club Hills, Illinois, and Forest Products Society, Madison,
Wisconsin.
Applied Technology Council (ATC). 1976. A Methodology for Seismic Design and
Construction of Single-Family Dwellings, ATC 4. ATC, Redwood City, California.
Association of Bay Area Governments (ABAG). Training Materials for Seismic Retrofit of
Wood-Frame Homes, ABAG, Oakland, California.
www.abag.ca.gov/bayarea/eqmaps/fixit/training.html
Association of Bay Area Governments (ABAG). Info on Chimney Safety and Earthquakes.
ABAG, Oakland, California. www.abag.ca.gov/bayarea/eqmaps/fixit/chimneys.html.
Association of Bay Area Governments. Training Materials for Seismic Retrofit of Wood-Frame
Homes. Available online at http://www.abag.ca.gov/bayarea/eqmaps/fixit/training.html.
201
FEMA 232, Homebuilder’s Guide
Breyer, D. E., K. J. Fridley, D. G. Pollock, K. E. Cobeen. 2003. Design of Wood Structures –
ASD. McGraw-Hill Publishing, New York, New York.
Building Seismic Safety Council (BSSC). Website: http://www.bssconline.org. This site
provides links to the BSSC’s 60+ member organizations’ websites.
California Seismic Safety Commission. 1992. The Homeowner’s Guide to Earthquake Safety.
Seismic Safety Commission, Sacramento, California.
California Division of the State Architect (DSA). 2002. Guidelines for Earthquake Bracing of
Residential Water Heaters. DSA, Sacramento, California.
Canadian Wood Council (CWC). 1991. Wood Reference Handbook. CWC, Ottawa, Ontario,
Canada.
Canadian Wood Council. 1993. Wood Building Technology. CWC, Ottawa, Ontario, Canada.
Chen W.-F. and C. Scawthorn. 2004. Earthquake Engineering Handbook, Second Edition.
CRC Press, LLC, Boca Raton, Florida.
City of Los Angeles, California. 2001. Reconstruction and Replacement of Earthquake
Damaged Masonry Chimneys, Information Bulletin P/BC 2001-70.
City of Los Angeles, California. 2002. City of Los Angeles Building Code, Chapter 94
“Voluntary Earthquake Hazard Reduction in Existing Hillside Buildings.”
City of San Leandro, California. Homeowner’s Handbook and Home Strengthening Plan Set.
www.ci.san-leandro.ca.us/homehandbook.html.
City of Seattle, Washington. Project Impact Home Retrofit Series.
www.ci.seattle.wa.us/projectimpact/pages/publications/homeretrofitseries.htm.
Consortium of Universities for Research in Earthquake Engineering. CUREE-Caltech
Woodframe Project. A number of reports are available; consult the project website:
https://secure.curee.org/catalog/index.php?main_page=index&cPath=3.
Earthquake Engineering Research Institute (EERI)/International Conference of Building
Officials (ICBO). Resisting the Forces of Earthquakes (video).
Faherty, K. F., and T. G. Williamson. 1999. Wood Engineering and Construction Handbook –
Third Edition. McGraw-Hill Publishing, New York, New York
Federal Emergency Management Agency (FEMA). Website: http://www.fema.gov.
Federal Emergency Management Agency. Above the Flood: Elevating Your Flood-prone
House, FEMA 347. FEMA, Washington, D.C.
202
Appendix E, References and Additional Resources
Federal Emergency Management Agency. Coastal Construction Manual: Principles and
Practices of Planning, Siting, Designing, Constructing, and Maintaining Residential Buildings in
Coastal Areas, FEMA 55. FEMA, Washington, D.C.
Federal Emergency Management Agency. Design Guidelines for Flood Damage Reduction,
FEMA 15. FEMA, Washington, D.C.
Federal Emergency Management Agency. Elevated Residential Structures, FEMA 54. FEMA,
Washington, D.C.
Federal Emergency Management Agency. FEMA Home Builders Guide to Coastal
Construction: Technical Fact Sheet Series, FEMA 499. FEMA, Washington, D.C.
Federal Emergency Management Agency. Homeowners Guide to Retrofitting: Six Ways to
Protect Your House from Flooding, FEMA 312. FEMA, Washington, D.C.
Federal Emergency Management Agency. Reducing Flood Losses Through the International
Codes: Meeting the Requirements of the National Flood Insurance Program. FEMA,
Washington, D.C., and ICC, Country Club Hills, Illinois.
Federal Emergency Management Agency. Reducing the Risks of Nonstructural Earthquake
Damage, FEMA 74. FEMA, Washington, D.C.
Federal Emergency Management Agency. Taking Shelter from the Storm: Building a Safe
Room Inside Your House, FEMA 320. FEMA, Washington, D.C.
Folz, B., and A. Filiatrault. 2002. A Computer Program for Seismic Analysis of Woodframe
Structures, CUREE W-21. Consortium of Universities for Research in Earthquake Engineering,
Richmond, California.
Gurfinkel, G.. 1973. Wood Engineering. Southern Forest Products Association, New Orleans,
Louisiana.
International Code Council (ICC). Website: http://www.iccsafe.org.
International Code Council (ICC). 2003a. International Residential Code. ICC, Country Club
Hills, Illinois.
International Code Council (ICC). 2003b. International Building Code. ICC, Country Club
Hills, Illinois.
International Code Council (ICC). 2003c. International Existing Building Code (IEBC). ICC,
Country Club Hills, Illinois.
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International Code Council (ICC). 2003d. Conventional Construction Provisions of the 2003
IBC, An Illustrated Guide. ICC, Country Club Hills, Illinois.
Masonry Institute of America (MIA). 1995. Masonry Fireplace and Chimney Handbook. MIA,
Torrance, California.
International Code Council (ICC). 2005. International Existing Building Code Commentary.
ICC, Country Club Hills, Illinois.
Masonry Standards Joint Committee (MSJC). 2002. Masonry Standards Joint Committee Code,
Specification, and Commentaries. ACI International, Structural Engineering Institute, The
Masonry Society, Washington, D.C.
McClure, F. E. 1973. Performance of Single Family Dwellings in the San Fernando Earthquake
of February 9, 1971. U.S. Department of Commerce, National Oceanic and Atmospheric
Administration, Washington, D.C.
National Fire Protection Association (NFPA). 2006. Building Construction and Safety Code,
NFPA 5000. NFPA, Quincy, Massachusetts.
Porter, Keith A., Charles R. Scawthorn, and James L. Beck. 2006. “Cost-effectiveness of
Stronger Woodframe Buildings.” Earthquake Spectra, 22(February):1, 0pp 239-266.
Slosson, J. E. 1975. “Effects of the Earthquake on Residential Areas.” Chapter 19 of San
Fernando, California, Earthquake of 9 February, California Division of Mines and Geology,
Bulletin 196.
Southern Building Code Congress International. 1999. Standard for Huricane-Resistant
Residential Construction, SSTD 10. International Code Council, Country Club Hills, Illinois.
Stewart, J. P., J. D. Bray, R. B. Seed, and N. Sitar, Eds. 1994. Preliminary Report on the
Principal Geotechnical Aspects of the January 17, 1994, Northridge Earthquake, UCB/EERC
94/08. Earthquake Engineering Research Center, University of California, Berkeley.
Stewart, J. P., J. D. Bray, D. J. McMahon, and A. L. Kroop. 1995. “Seismic Performance of
Hillside Fills.” In Landslides under Static and Dynamic Conditions – Analysis Monitoring and
Mitigation, Geotechnical Special Publication 52, edited by C. L. Ho and D. K. Keefer. American
Society of Civil Engineers, Reston, Virginia.
Structural Engineers Association of California. 2003. Commentary on Chapter 3 Guidelines for
Seismic Retrofit of Existing Buildings, 1 October 2003 Version. Existing Buildings Committee,
SEAOC, Sacramento, California.
Structural Engineers Association of Northern California (SEAONC). 2001. Guidelines for
Seismic Evaluation and Rehabilitation of Tilt-up Buildings and Other Rigid Wall/Flexible
Diaphragm Structures. SEAONC, San Francisco, California.
204
Appendix E, References and Additional Resources
U.S. Department of Agriculture (USDA). 1999. Wood Handbook – Wood as an Engineering
Material, USDA Forest Products Laboratory General Technical Report FPL-GTR-113. USDA
Forest Products Laboratory, Madison, Wisconsin.
Von Winterfeldt, D., N. Roselund, and A. Kitsuse. 2000. Framing Earthquake Retrofitting
Decisions: The Case of Hillside Homes in Los Angeles, PEER Report 2000-03. Pacific
Engineering Research Center, Richmond, California.
Wood Truss Council of America (WTCA). 1997. Metal Plate Connected Wood Truss
Handbook – Third Edition. WTCA, Madison, Wisconsin.
205
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206 Appendix F HOMEBUILDERS’ GUIDE PROJECT PARTICIPANTS BSSC HOMEBUILDERS’ GUIDE PROJECT COMMITTEE
Chair – J. Daniel Dolan, PhD, PE, Professor, Washington State University, Wood Materials and
Engineering Laboratory, Pullman (writing team leader)
Members
Kelly Cobeen, Structural Engineer, Cobeen and Associates Structural Engineering, Lafayette,
California (co-author)
Gerald Jones, PE, Code Official, Retired, Kansas City, Missouri
James E. Russell, Building Codes Consultant, Concord, California (co-author)
Jim W. Sealy, FAIA, Architect/Consultant, Dallas, Texas
Douglas M. Smits, CBO, City of Charleston, South Carolina
FEMA Liaison Members
Robert D. Hanson, Federal Emergency Management Agency, Walnut Creek, California
Michael G. Mahoney, Federal Emergency Management Agency, Washington, D.C.
INVITEES/PARTICIPANTS IN HOMEBUILDERS’ GUIDE WORKSHOPS AND
REPORT REVIEWERS
Mamood Abolhoda, City of Freemont, California
Thomas Ahrens, City of Mill Valley, California
John C. Anderson, City of San Diego, California
Paul Armstrong, International Code Council, Whittier, California
Christopher Arnold, FAIA, RIBA, Building Systems Development Inc., Palo Alto, California
James Bela, Oregon Earthquake Awareness
Carol Bloom, State Farm Insurance, Rohnert Park, California
J. Gregg Borchelt, PE, Brick Industry Association, Reston, Virginia
Amanda Brady, City of San Diego, California
Patrick Bridges, Builder, Portland, Oregon
Mark Caldwell, Apex Engineers, Calvert City, Kentucky
Lee Campbell, Property Solutions, Inc., Irvine, California
Cathleen Carlisle, Federal Emergency Management Agency, Washington, D.C.
Tom Caulfield, Muller and Caulfield Architects, Oakland, California
Frank Chiu, City and County of San Francisco, California
Michael Christoffersen, Architectural Designs, Inc., Fort Wayne, Indiana
John Chrysler, Masonry Institute of America, Torrance, California
Glen Clapper, Southern Brick Institute, Conyers, Georgia
Nicolino G. Delli Quadri, SE, City of Los Angeles, California
Bradford K. Douglas, PE, American Forest and Paper Association, Washington, D.C.
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FEMA 232, Homebuilders’ Guide
Ali Fattah, City of San Diego, California
Jeffrey K. Feid, State Farm Insurance Company, Bloomington, Illinois
Art Garcia, City of San Diego, California
Robert S. George, Architect, South San Francisco, California
Frank Golon, Pulte Homes, Bernardsville, New Jersey
Rose Grant, State Farm Insurance, Bloomington, Illinois
Michael Gregor, SGM Architecture Interiors, Mount Pleasant, South Carolina
Cynthia Hammond, CPCU, State Farm Insurance, Rohnert Park, California
Isam Hasenn, City of San Diego, California
Perry A. Haviland, FAIA, Haviland Associates Architects, Oakland, California
John R. Henry, PE, International Code Council, Sacramento, California
Fred M. Herman, City of Palo Alto, California
George Horton, Oakland, California
Eric N. Johnson, Brick Association of the Carolinas, Charlotte, North Carolina
Stephan A. Kiefer, City of Livermore, California
Joseph Knarich, National Association of Home Builders, Washington, D.C.
Vladimir G. Kochkin, National Association of Home Builders Research, Upper Marlboro,
Maryland
Laurence Kornfield, City and County of San Francisco, California
Ray Kothe, Builder, Baton Rouge, Louisiana
Jay W. Larson, American Iron and Steel Institute, Bethlehem, Pennsylvania
Sheila Lee, City of Santa Clara, California
Adlai Leiby, Berkeley, California
Philip Line, American Forest and Paper Association, Washington, D.C.
Jeff Lusk, Federal Emergency Management Agency Region IX, Oakland, California
Joan O. MacQuarry, City of Berkeley, California
Bonnie Manley, National Fire Protection Association, Quincy, Massachusetts
Harry W. Martin, American Iron and Steel Institute, Auburn, California
Zeno A. Martin, PE, APA-The Engineered Wood Association, Tacoma, Washington
Joseph J. Messersmith, Jr., PE, Portland Cement Association, Rockville, Virginia
Greg Mulvey, City of San Diego, California
Steve Pfeiffer, City of Seattle, Seattle, Washington
David Pollock, Washington State University, Pullman
Steven E. Pryor, Simpson Strong-Tie, Dublin, California
Kevin Reinerston, HCD Division of Codes and Standards, Sacramento, California
Timothy A. Reinhold, PhD, PE, Institute for Business and Home Safety, Tampa, Florida
Dennis Richardson, San Jose, California
Alan Robinson, Tuan and Robinson Structural Engineers, Inc., San Francisco, California
William R. Schock, City of San Leandro, California
Roger Sharpe, City of Walnut Creek, Retired, Berkeley, California
Thomas D. Skaggs, PhD, PE, APA-The Engineered Wood Association, Tacoma, Washington
Steve Skalko, Portland Cement Association, Macon, Georgia
Charles A. Spitz, AIA, CSI, NCARB, Architect-Planner-Code Consultant, Wall, New Jersey
William W. Stewart, FAIA, Stewart-Schaberg/Architects, Chesterfield, Missouri
Jason Thompson, SE, National Concrete Masonry Association, Herndon, Virginia
Ray Tu, Hardy Frames, Inc., Ventura, California
208
Appendix F, Homebuilders’ Guide Project Participants
Fred M. Turner, Seismic Safety Commission, Sacramento, California
David P. Tyree, PE, CBO, American Forest and Paper Association, Colorado Springs, Colorado
Calvin N. Wong, PE, City of Oakland, California
Matt Zamani, City of San Diego, California
2005-2006 BSSC BOARD OF DIRECTION
Chair -- Jim W. Sealy, FAIA, Architect/Consultant, Dallas, TX
Vice Chair -- David Bonneville, Degenkolb Engineers, San Francisco, California
Secretary -- Jim Rinner, Project Manager II, Kitchell CEM, Sacramento, California
Ex-Officio Member -- Charles Thornton, Chairman/Principal, Thornton-Tomasetti Group, Inc.,
New York, New York
Members
Edwin Dean, Nishkian Dean, Portland, Oregon
Bradford K. Douglas, Director of Engineering, American Forest and Paper Association,
Washington, D.C.
Cynthia J. Duncan, Director of Specifications, American Institute of Steel Construction,
Chicago, Illinois
Henry Green, Executive Director, Bureau of Construction Codes and Fire Safety, State of
Michigan, Department of Labor and Economic Growth, Lansing, Michigan (representing
the National Institute of Building Sciences)
Jay W. Larson, American Iron and Steel Institute, Bethlehem, Pennsylvania
Joseph Messersmith, Coordinating Manager, Regional Code Services, Portland Cement
Association, Rockville, Virginia (representing the Portland Cement Association)
Ronald E. Piester, Assistant Director for Code Development, New York State, Department of
State, Kinderhook, New York
James Rossberg, Manager, Technical Activities for the Structural Engineering Institute,
American Society of Civil Engineers, Reston Virginia
W. Lee Shoemaker, Director, Engineering and Research, Metal Building Manufacturers
Association, Cleveland, Ohio
Howard Simpson, Simpson Gumpertz and Heger, Arlington, Massachusetts (representing
National Council of Structural Engineers Associations)
Shyam Sunder, Deputy Director, Building Fire Research Laboratory, National Institute of
Standards and Technology, Gaithersburg, Maryland (representing Interagency Committee
on Seismic Safety in Construction)
Charles A. Spitz, Architect/Planner/Code Consultant, Wall New Jersey (representing the
American Institute of Architects)
Robert D. Thomas, Vice President Engineering, National Concrete Masonry Association,
Herndon, Virginia
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FEMA 232, Homebuilders’ Guide
BSSC Staff
Claret M. Heider, Vice President for BSSC Programs
Bernard F. Murphy, PE, Director, Special Projects
Carita Tanner, Communications/Public Relations Manager
210
Appendix F, Homebuilders’ Guide Project Participants
THE BUILDING SEISMIC SAFETY COUNCIL The purpose of the Building Seismic Safety Council is to enhance the public's safety by providing a
national forum to foster improved seismic safety provisions for use by the building community. For the
purposes of the Council, the building community is taken to include all those involved in the planning,
design, construction, regulation, and utilization of buildings.
To achieve its purposes, the Council shall conduct activities and provide the leadership needed to:
• Promote development of seismic safety provisions suitable for use throughout the United States;
• Recommend, encourage, and promote adoption of appropriate seismic safety provisions in
voluntary standards and model codes;
• Assess implementation progress by federal, state, and local regulatory and construction agencies;
• Identify opportunities for the improvement of seismic regulations and practices and encourage
public and private organizations to effect such improvements;
• Promote 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.
• Provide advice to governmental bodies on their programs of research, development, and imple
mentation; and
• Periodically review and evaluate research findings, practice, and experience and make re
commendations for incorporation into seismic design practices. The scope of the Council's activities encompasses seismic safety of structures with explicit consideration
and assessment of the social, technical, administrative, political, legal, and economic implications of its
deliberations and recommendations.
Achievement of the Council's purpose is important to all in the public and private sectors. Council
activities will provide an opportunity for participation by those at interest, including local, State, and
Federal Government, voluntary organizations, business, industry, the design professions, the construction
industry, the research community and the public. Regional and local differences in the nature and
magnitude of potentially hazardous earthquake events require a flexible approach adaptable to the relative
risk, resources and capabilities of each community. The Council recognizes that appropriate earthquake
hazard reduction measures and initiatives should be adopted by existing organizations and institutions and
incorporated into their legislation, regulations, practices, rules, codes, relief procedures and loan require
ments, whenever possible, so that these measures and initiatives become part of established activities
rather than being superposed as separate and additional.
The Council is established as a voluntary advisory, facilitative council of the National Institute of Build
ing Sciences, a nonprofit corporation incorporated in the District of Columbia, under the authority given
the Institute by the Housing and Community Development Act of 1974, (Public Law 93-383), Title VIII,
in furtherance of the objectives of the Earthquake Hazards Reduction Act of 1977 (Public Law 95-124)
and in support of the President's National Earthquake Hazards Reduction Program, June 22, 1978.
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212 
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