Recommended Applications for Heat Treated Glass
Glass Informational Bulletin
GANA TD 07-0114
Recommended Applications for Heat-Treated Glass
The glass industry has been heat treating architectural glass to increase its strength since about
1930. The process of heat- treating glass involves uniformly heating glass close to its softening
temperature and then rapidly, and uniformly, cooling it. This process results in the development
of surface compressive stresses, thus increasing the strength of the glass. Heat-treated glass is
used in many of today’s architectural glazing applications where increased strength to resist glass
breakage is desired. Vision and spandrel areas of buildings as well as building entrances, glass
railings and balustrades, and other applications where public safety is a priority, are common
examples.
Types of Heat-Treated Glass
There are two types of heat-treated glass: heat-strengthened glass and fully tempered glass.
Both are produced in a similar fashion by heating and cooling the glass. It is the rate of cooling
that determines whether the final product is heat-strengthened or fully tempered. It is a generally
accepted rule of thumb that glass that has been heat-strengthened has approximately twice the
breakage strength of annealed glass of the same thickness and that fully tempered glass has
approximately four times the breakage strength of annealed glass.
The North American industry standard specification for heat-treated glass is ASTM C1048
Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass.
 Heat-strengthened glass is defined as having a surface compression of 3,500 to 7,500
psi (24 to 52 MPa); no requirement for edge compression is specified.
 Fully tempered glass is defined as having a minimum surface compression of 10,000
psi (69 MPa), an edge compression of not less than 9,700 psi (67 MPa), or meeting
the American National Standards Institute (ANSI) Standard Z97.1 - Safety
Performance Specifications and Methods of Test for Safety Glazing Materials Used in
Building1or the Consumer Product Safety Commission (CPSC) Standard 16 CFR
1201 - Safety Standard for Architectural Glazing Materials2Category II
Because of the high internal stresses in both glass types, all fabrication including cutting, holedrilling, notching, or edge treatment must be performed prior to the glass being heat-treated.
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Reasons for Heat- Treating
The reasons for heat-treating glass are two-fold: increase the glass strength to resist externally
applied loads and, for fully tempered glass, also to meet safety glazing requirements defined by
applicable codes or federal standards. External loads include wind and snow loads as well as
thermal stresses induced by the sun’s radiant energy. In many instances, combinations of these
loads must be considered by the design professional during the design process. Commonly
available software tools are available for the responsible design professional to evaluate the
breakage performance characteristics of both heat-strengthened and fully tempered glass, and
compliance with ASTM E1300 Standard Practice for Determining Load Resistance of Glass in
Buildings. Both heat-strengthened and fully tempered glass can significantly reduce the potential
for glass breakage due to uniformly applied wind and snow loads as well as from thermal
stresses.
Fully tempered glass that complies with ANSI and CPSC is recognized as a safety glazing
material. Federal, state, and local laws, as well as national building codes, require safety glazing
wherever human impact is probable. When broken by impact, fully tempered glass breaks into
small particles, thereby reducing the potential for serious personal injury. Building codes require
safety glazing in specific hazardous locations where human impact is likely. It is important to
note that heat-strengthened glass is not a safety glazing material.
Glass strength is significantly reduced by glass edge damage or poor edge quality. For heattreated glass, glass edge damage and/or poor edge quality can offset or negate the entire benefit
of heat-treating the glass; therefore, care must be taken through all steps of the fabrication and
installation processes to keep the glass free of damage.
Pros and Cons of Heat–Treated Glass
The design professional must carefully consider the performance and breakage characteristics of
heat-strengthened and fully tempered glass before selecting or specifying either glass type.
Heat-Strengthened Glass:
Pros:
 Increased resistance to wind and snow loads
 Increased resistance to thermal stress
 When broken, tends to remain in the frame until removed
 Virtually eliminates risk of spontaneous breakage
Cons:
 Heat-strengthened glass is NOT A SAFETY GLAZING product unless laminated
 Increased level of optical distortion compared to annealed glass
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Fully Tempered Glass:
Pros:
 May be fabricated to meet safety glazing requirements
 Increased resistance to wind and snow loads
 Increased resistance to thermal stress
Cons:
 If broken, small particles, or clusters of particles, or even the majority of the lite, may
easily fall out of the frame
 Increased level of optical distortion compared to annealed glass
 Possibility of spontaneous breakage
Alternatives
In some instances, increasing the thickness of annealed glass may be sufficient to meet certain
uniform wind or snow load requirements without the need to heat- treat the glass. It is strongly
recommended that a comprehensive wind/snow load analysis and thermal stress analysis be
conducted before making a final design decision. In addition, monolithic lites of annealed glass
of any thickness will not meet safety glazing requirements and may not provide adequate thermal
stress breakage protection.
As an alternative to fully tempered glass for safety glazing applications, laminated glass is a
consideration. Laminated glass is a requirement for overhead glazing and should be considered
for applications where glass fallout is a concern.
Heat-Strengthened Laminated Glass:
Pros:
 Increased resistance to wind and snow loads
 Increased resistance to thermal stress
 If broken, tends to remain in the frame until removed
 Virtually eliminates risk of spontaneous breakage
 Reduced sound transmission
 Meets safety glazing requirements
Cons:
 Increased level of optical distortion due to multiple lites of heat-treated glass
 Exposed edges will show layers versus a clean monolithic edge
Fully Tempered Laminated Glass:
Pros:
 Increased resistance to wind and snow loads
 Increased resistance to thermal stress
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 Meets safety glazing requirements
 Improved glass retention
 Reduced sound transmission
Cons:
 Possibility of spontaneous breakage
 Increased level of optical distortion due to multiple lites of heat-treated glass
 Exposed edges will show layers versus a clean monolithic edge
 If both lites break, there is a possibility the entire laminate will fall from the opening.
Guidelines for Typical Architectural Glazing Applications
Architectural glazing applications use glass as part of an overall finished product or glazing
system. Whether that product is a wall system, overhead or sloped glazing system, railing
system, impact rated system, etc., it is very important that all of the components including the
glass itself, are designed with the system in mind so that the finished product works as intended.
There is no substitute for proper glass design, physical testing, proper fabrication and proper
installation. All of these systems must take into account both the structural performance and
aesthetic characteristics of the glass. Prior to making final design decisions, the structural
integrity of the system needs to be verified by a design professional. The guidelines offered
below are general in nature and are not intended to replace specific and appropriate structural
design/engineering practices.
Vertical Glazing:
For vision and spandrel applications above the ground floor, heat–strengthened glass is typically
recommended for the exterior lite of an insulating glass unit if annealed glass is not strong
enough to resist wind load and/or thermal stresses.
For Fireman knock-out (fire break-out) panels or other smoke evacuation applications, fully
tempered glass is recommended or required.
For any safety glazing application, fully tempered or laminated glass must be used.
Overhead or Sloped Glazing:
When specifying insulating glass for sloped or overhead glazing applications, heat-strengthened
laminated glass is typically recommended for the interior lite facing the public space. Heatstrengthened or fully tempered glass may be used for the exterior, outward facing lite.
Doors, Side Lites, Entrances, or other Safety Glazing Applications:
Fully tempered or laminated glass that meets ANSI or CPSC specifications is required.
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Balcony railings or other applications where fallout is a concern (i.e. where there are
unprotected walking surfaces below):
Laminated glass is recommended. Heat-strengthened laminated glass or fully tempered
laminated glass is recommended to provide increased resistance to wind loads, thermal stresses,
and impact. According to the 2012 IBC, the only exception is: “Single fully tempered glass
complying with Category II of CPSC 16 CFR Part 1201 or Class A of ANSI Z97.1 may be used
in handrails and guardrails if there is no walking surface beneath them or the walking surface is
permanently protected from the risk of falling glass.”
Special Building Code Regions (windborne debris areas)
A safety glazing and impact resistant glass is required to meet the applicable building code
requirements.
Summary
Heat-treated glass plays an important role in the long-term performance and safety of today’s
modern architecture. Heat-treated glass, including both heat-strengthened and fully tempered,
increases the strength of annealed glass to meet design loads and stresses throughout the lifetime
of the building. Glass tempered to ASTM C1048 and ANSI or CPSC standards can meet the
safety glazing requirements of the building codes.
Depending on the specific building application, there are alternatives to monolithic heat-treated
glass that should be considered, especially where glass fallout is a safety concern. Laminated
glass is required for overhead glazing applications and, when constructed with either heatstrengthened or fully tempered glass, will provide an increased level of strength compared to
annealed laminated glass.
For any glass application, all design aspects, structural requirements, and building and safety
code concerns must be carefully and thoroughly considered before selecting the appropriate
product.
Additional resources related to heat-treated glass:
ANSI Z97.1
Safety Glazing Materials Used in Buildings - Safety Performance
Specifications and Methods of Test
ASTM C 1036
Standard Specification for Flat Glass
ASTM C 1048
Standard Specification for Heat-Strengthened and Fully Tempered Flat
Glass
ASTM C1279
Standard Test Method for Non-Destructive Photoelastic Measurement of
Edge and Surface Stresses in Annealed, Heat-Strengthened, and Fully
Tempered Flat Glass
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ASTM C1651
Standard Test Method for Measurement of Roll Wave Optical Distortion
in Heat Treated Flat Glass
ASTM C1652
Standard Test Method for Measuring Optical Distortion in Flat Glass
Products Using Digital Photography of Grids
ASTM E1300
Standard Practice for Determining Load Resistance of Glass in Buildings
CPSC 16 CFR 1201 Federal Safety Standard for Architectural Glazing Materials
GANA Engineering Standards Manual
GANA Glazing Manual
Consult the Tech Center section of the Glass Association of North America (GANA) website
(www.glasswebsite.com) for additional Glass Informational Bulletins and flat glass industry
reference resources.
The Glass Association of North America (GANA) has produced this Glass Informational Bulletin solely to provide
information regarding recommended application for heat-treated glass. This bulletin makes no attempt to provide
all information or considerations in the recommended application for heat-treated glass. The user of this Bulletin
has the responsibility to ensure their awareness of the recommended application for heat-treated glass. GANA
disclaims any responsibility for any specific results related to the use of this Bulletin, for any errors or omissions
contained in the Bulletin, and for any liability for loss or damage of any kind arising out of the use of this Bulletin.
This bulletin was developed by the GANA Tempering Division and approved by the membership and the GANA Board of
Directors. This is the original version of the document as approved and published in January 2014.
1
2
Available from the American National Standards Institute, 25 W. 43rd St., 4th Floor, New York, NY 10036.
Available from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402.
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