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JWF, Sec. Fim Facts, Earthquake Protection
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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.
Building codes today focus mainly on wind pressure with little consideration to the pressures caused by an earthquake. This is surprising considering the property damage and loss of life in recent earthquakes in the U.S., Central America, Asia and elsewhere around the world. However, the 1985 earthquake in Mexico City revealed a number of critical finding in regards to glazing system performance:
• Irregular building designs caused twice as much structural and glass breakage compared to “traditional” buildings
• Large glass areas received more glass breakage vs. smaller glass systems
• Flexible glazing systems (e.g. metal frames) received more glass breakage
• Vertical glass shapes encountered more damage than horizontal designs
• Single story commercial buildings with glass storefronts are extremely vulnerable
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Existing Earthquake Tests
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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