Experiments and guidance on glass breaking out in fires
The earliest guidance to be found in the literature on the question of when glass breaks out in fires comes from the Russian researcher Roytman who notes that a room gas temperature of around 300°C is needed to lead to glass breakage. The research base for this conclusion is unclear, however.
The Gaussian fit that can correlate this data corresponds to a mean temperature rise of 340°C, and a standard deviation of 50°C. Thus, the BRI results are somewhere in between the two earlier values.
For radiant exposure, Cohen and Wilson reported on an interesting series of experiments simulating flame exposure from a wildland fire. They examined small (0.61 x 0.61 m) and large (0.91 x 1.5 m) panes, single- and double-glazed. They also repeated the tests with tempered glass and with double-glazed windows. For the small windows, at their lowest heat flux, 9.3 kW m2, all windows cracked, but no glass fell out. Even at the highest flux of 17.7 kW m2 panes did not fall out. For the larger size panes, at fluxes of 16.2 to 50.3 kW m2, at least one out of 3 test specimens exhibited fall-out. Tempered glass, by contrast, showed no cracking at tests up to 29.2 kW m2 in the larger size. The authors also did tests on double-glazed windows, which showed better performance. In experiments with large-size double-glazed windows (non-tempered), they found that fluxes between 20 and 30 kW m2 were required to cause fall-out in both panes.
Additional data are available from the NRCC study, where heat-strengthened and tempered glass (unspecified thickness) was found not to break at an irradiance of 43 kW m-2. The latter heat flux corresponded to 350ºC on the exposed face and 300ºC on the unexposed face. Thus, this appears to extend Cohen's data point of 29.2 kW m-2 for non-breakage to 43 kW m-2, without actually determining the point at which breakage and fall-out do occur.
Other types of glass
Effect of window frame type
Actual fall-out of glass from windows is also influenced by the window-frame material. Mowrer  found that vinyl-frame windows tended to show a failure of the frame (e.g., the whole assembly collapsing) before significant fall-out of glazing. Vinyl frame failures were observed when heat fluxes came up to the range of 8 to 16 kW m-2. By contrast, McArthur found that glass in aluminum-framed windows showed a tendency to survive longer than did glass in conventional wood-frame windows.
In terms of external fires, at a heat flux of 9 kW m2 some experimental results on ordinary glass showed the possibility of significant fallout. Other studies found that higher heat fluxes were needed, thus 9 kW m-2 appears to be a conservative, but realistic lower bound. Double-glazed windows can resist approximately 25 kW m2 without fall-out. Tempered glass is able to resist fluxes of 43 kW m2, at least under some conditions.
Factors such as window size, frame type, glass thickness, glass defects, and vertical temperature gradient may all be expected to have an effect on glass fall-out. Over-pressure due to gas explosions is an obvious glass failure mechanism. Yet, normal fires do show pressure variations and these could potentially affect the failure of glass panes. All of these factors deserve some more study to obtain useful, quantitative guidance.
The above review has dealt only with the role of glass breakage in fire ventilation. A number of other aspects of glass breakage are important to fire investigators; these have been well presented by Schudel.
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