STRUCTURAL GLASS FACADES - USC School of Architecture

glass above). In fact, perhaps owing largely to liability concerns, glass fabricators in North
America are cautioning against the use of tempered glass unless required for reasons of
safety or strength. Viracon’s website includes the following statement:
“Although the incidence of tempered glass breakage due to these inclusions is rare, greater
publicity of their occurrence has resulted in an increased awareness of this phenomenon. In fact,
limiting the use of tempered glass in commercial building applications has become the
recommendation of a number of glass suppliers, including Viracon.” (Viracon 2008c)
2.4.2.6 Heat-soaking
Heat-soaking is a process devised in response to the nickel sulfide and spontaneous
breakage problem. In this process, glass is heated to a specified temperature, usually about
290°C, held there for some specified time, usually several hours, and occasionally even
subjected to several cycles of this heating and cooling. The practice is somewhat
controversial in its effectiveness, and adds to the cost of tempered glass product, but has
become a standard practice for many structural glass producers and users.
On specification for heat-soaking is the European Din standard requiring a minimum 12 hour
cycle at a temperature of 290C.
2.4.2.7 Heat Strengthening (HS)
Partially tempered, partially toughened, or heat strengthened are equivalent terms for a heattreatment
of glass yielding a material with strength properties between that of annealed and
fully tempered glass. Heat strengthened glass is two to three times stronger than annealed
glass, whereas tempered is four to five times stronger. Heat-strengthened glass has a
surface compression between 3,500 and 7,500 psi and conforms to the requirements for
ASTM C 1048, kind HS. Heat strengthened glass has improved resistance to thermal stress,
but has a break behavior closer to annealed glass, so cannot be used in safety glass
applications. HS does not meet the requirements of the American National Standards
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