G l a s s Te c h  
by Erin Fletcher  
he glass industry has offered heat-treated archi-  
can offset or negate the entire benefit of heat-treat-  
tectural glass since the 1930s. The process involves ment; therefore, care must be taken through all  
uniformly heating glass close to its softening tem-  
perature and then cooling it rapidly and uniform-  
steps of the fabrication and installation processes  
to keep the glass free of damage.  
ly. This results in the development of surface com-  
pressive 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.  
Pros and Cons  
The design professional must carefully consider  
the performance and breakage characteristics of  
heat-strengthened and fully tempered glass before  
selecting or specifying either type. While both  
increase resistance to wind loads, snow loads and  
thermal stress, each has its unique positives.  
Heat-strengthened glass tends to remain in the  
frame until removed and virtually eliminates the  
risk of spontaneous breakage. Fully tempered  
glass, meanwhile, may be fabricated to meet safety  
glazing requirements.  
One negative both types share is an increased  
level of optical distortion compared to annealed  
glass. A con unique to heat-strengthened glass is that  
it’s not a safety glazing product unless laminated.  
Fully tempered glass has a negative, too. If bro-  
Types of Heat-Treated Glass  
There are two types: heat-strengthened and fully  
tempered. Both are produced by heating and cool-  
ing the glass, but the rate of cooling determines  
whether the final product is heat-strengthened or  
fully tempered. A generally accepted rule is that  
heat-strengthened glass has approximately twice the  
breakage strength of annealed glass of the same  
thickness, and fully tempered glass has approximate- ken, small particles, or clusters of particles, or even  
ly four times the breakage strength of annealed  
glass. Because of the high internal stresses in both  
types, all fabrication including cutting, hole-drilling,  
notching or edge treatment must be performed  
prior to heat-treating the glass.  
the majority of the lite, may easily fall out of the  
frame. Spontaneous breakage is also possible.  
In some instances, increasing the thickness of  
annealed glass may be sufficient to meet certain uni-  
form windload or snowload requirements without  
Reasons for Heat-Treating  
Glass is heat-treated to increase strength to resist the need to heat-treat the glass. It is strongly recom-  
externally-applied loads. External loads include mended that a comprehensive wind/snowload analy-  
windloads and snowloads as well as thermal stresses sis and thermal stress analysis be conducted before  
induced by the sun’s radiant energy. In many  
instances, combinations of these loads must be  
considered during the design process.  
making a final design decision. In addition, mono-  
lithic lites of annealed glass of any thickness will not  
meet safety glazing requirements and may not pro-  
Fully tempered glass that complies with ANSI and vide adequate thermal stress breakage protection.  
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,  
reducing the potential for serious personal injury.  
Glass strength is reduced significantly by glass  
edge damage or poor edge quality. For heat-treated  
glass, glass edge damage and/or poor edge quality  
Laminated glass is an alternative. It is a require-  
ment for overhead glazing and should be considered  
for applications where glass fallout is a concern.  
Heat-strengthened laminated glass has a num-  
ber of advantages, including reduced sound trans-  
mission and compliance with safety glazing  
requirements. One con is an increased level of  
continued on page 8  
Architects’ Guide to Glass & Metal  
G l a s s Te c h  
optical distortion due to multiple lites of heat-  
treated glass. Also, exposed edges will show layers  
versus a clean monolithic edge.  
Fully tempered lamin ated glass, meanwhile,  
has improved glass retention and also reduces  
sound transmission. If both lites break, though,  
there is a possibility the entire laminate will fall  
from the opening.  
Guidelines for Typical Glazing Applications  
Architectural glazing applications use glass as  
part of an overall finished product or glazing sys-  
tem. Whether that is a wall system, overhead or  
sloped glazing system, railing system, impact-rated  
system, etc., it is important that all of the compo-  
nents, 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 fabri-  
cation and proper installation. All of these sys-  
tems must take into account both the structural  
performance and aesthetic characteristics. Prior to  
making final design decisions, structural integrity  
of the system needs to be verified by a design pro-  
fessional. These guidelines are general in nature.  
Vertical glazing: For vision and spandrel applica-  
tions above the ground floor, heat-strengthened glass  
typically is recommended for the exterior lite of an  
insulating glass unit if the annealed glass is not strong  
enough to resist wind load and/or thermal stresses.  
For fire break-out panels or other smoke evacuation  
applications, fully tempered glass is recommended or  
required. For any safety glazing application, fully tem-  
pered or laminated glass must be used.  
Overhead or sloped glazing: When specifying  
insulating glass for sloped or overhead glazing appli-  
cations, heat-strengthened laminated glass typically  
is recommended for the interior lite facing the pub-  
lic space. Heat-strengthened or fully tempered glass  
may be used for the exterior, outward facing lite.  
Doors, entrances or other safety glazing appli-  
cations: Fully tempered or laminated glass that  
meets ANSI or CPSC specifications is required.  
Balcony railings or other applications where  
fallout is a concern: Glass used in a handrail,  
guardrail or a guard section must be laminated.  
Heat-strengthened or fully tempered laminated  
glass is required to provide increased resistance to  
windloads, thermal stress and impact. According to  
the CPSC, the only exception is, “Single fully tem-  
pered glass complying with Category II of CPSC 16  
CFR Part 1201 or Class A of ANSI Z97.1 shall be  
permitted to be used in handrails and guardrails  
where there is no walking surface beneath them or  
the walking surface is permanently protected from  
the risk of falling glass.” AGG  
For new and retrofit projects, Mapes-R and Mapes-R+  
rabbet edge panels add to the aesthetic appeal of  
any building and have an R-value up to 27.79.  
00-228-2391 WWW.MAPES.COM  
Erin Fletcher is the director of marketing and  
communications for the Glass Association of North  
America in Topeka, Kan.  
Architects’ Guide to Glass & Metal