Glazing systems face another potential threat in areas where earthquakes occur. Recently, much of the attention has been centered on wind-borne debris from hurricanes, but earthquakes can have an equally disastrous affect on the integrity of a window unit. As more and more commercial buildings are designed with glass curtain wall systems (exterior walls mostly of glass), steps must be taken to improve their performance when faced with seismic earthquake loadings.
In 1992, the University of Missouri-Rolla began a project to investigate glazing system performance during earthquake conditions. A series of “crescendo tests”tests utilizing various volume frequencies were performed on glass storefront systems as well as commercial building curtain walls. The goal was to measure two different effects on glass:
1. Glass Breakage called serviceability drift limit (SDL). It defines the force causing glass to be cracked or crushed.
2. Glass Fallout called ultimate drift limit (UDL). It defines the force causing the entire glass to completely fall out.
The test results indicate that certain glass products do a better job at preventing breakage or fallout during earthquake forces:
• Heat-treated monolithic glass & laminated glass perform better against cracking
• Laminated glass provides the highest glass fallout resistance
Unfortunately, the crescendo tests were conducted to determine the effects primarily on glass. No security film was used, and only a 4 mil. safety film on an unanchored system was involved. Safety film was found to have helped provide additional resistance to glass breakage and fallout when applied to annealed monolithic glass.
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