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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.
t
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.
Alternatives
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
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Architects’ Guide to Glass & Metal