Heat Strengthened Glass in Chicago
Heat Strengthened Glass (HST) has a mechanical strength of about twice that of annealed (float) glass and if broken, breaks into large pieces from edge to edge and does not fracture into small fragments like tempered safety glass.
For glass to be considered HST, this compressive stress on the surface of the glass should be from 3,500 psi to 7,500 psi. This breakage characteristic allows the glass to remain in the window frame as the large pieces support each other in the opening
Heat Strengthened Glass also provides resistance to thermal breakage caused by heat build-up in spandrel situations and the probability of nickel sulfide inclusions inducing spontaneous breakages in heat strengthened glass is practically non-existent.