"The Structural Power of Glass," by C.C. Sullivan and Barbara Horwitz-Bennett, Building Design and Construction
Historically regarded for aesthetic qualities, the introduction of daylight indoors, and ability to link building interiors with the outside world, glass building systems have traditionally played a prominent role in commercial and institutional buildings. In recent years, glass systems have been shown to support building concepts in even more direct ways...
Steve Thomas, PE, a senior staff member in building technology in the Los Angeles office of Simpson Gumpertz & Heger, explains how the process works: "Quenching induces permanent compressive stresses on the glass surfaces that must be overcome by the applied load before the glass experiences tensile stresses." The other advantage offered by fully tempered glass is that when it does break, it shatters into relatively safe cubes, rather than large, sharp shards. As for heat-strengthened glass, "Depending on the level of pre-stress [applied], it may break into large shards similar to annealed glass or into much smaller and more rounded pieces similar to fully tempered glass," says Thomas.
While such benefits are appealing, Thomas and his colleague, Russ Davies, PE, senior project manager in SGH's New York City office, point out that both heat-strengthened and fully tempered glass are susceptible to roller-wave distortions, which can create undesirable reflectivity patterns. In addition, says Davies, "Heat-treated glass is susceptible to fracture due to edge impacts that penetrate the compressive stress zone, and fully tempered glass is susceptible to spontaneous breakage as a result of nickel-sulfide material impurities within certain production runs." One way to mitigate this is through heat-soaking-returning the glass to a high temperature after tempering-by inducing nickel-sulfide breakage before the glass leaves the fabrication plant.