[quote]Testing tries to ensure that glass structures don't court disaster
John King, Chronicle Urban Design Writer
Monday, November 26, 2007
Look at a glass-clad building, and it's hard not to think the worst: What if there's a major earthquake or some other disaster?
The same question has occurred to architects, developers and building inspectors - which is why
the wall systems for towers are tested extensively before construction begins.
"There's always the anticipation something can go wrong," said Ben Zelazny, a project manager at Benson Industries. The company fabricates curtain wall systems, the panels attached to the structural bones of high-rises. Benson is installing the curtain wall at 555 Mission St., a 33-story tower now on the rise.
For 555 Mission, testing was done at Construction Consulting Laboratory in Ontario (San Bernardino County), one of the nation's handful of accredited testing centers. Benson shipped the pieces for a two-story, 40-foot-wide set of panels; it also sent workers to assemble the mock-up and attach it to a metal frame inside a pressurized chamber.
The inspections begin with a check for gaps in how the panels are sealed, flaws that would allow heat or cooled air to leak from the building. Then comes the first hard-core test: a "rain rack" - a metal scaffold that's like an enormous sprinkler - is rolled into place. It drenches the wall for 15 minutes, followed by an inspection for leaks.
Next, an airplane engine is placed behind the rain rack and turned on, blasting the panels to see if their design can withstand the maximum wind forces, based on environmental studies of its height and location, and not leak.
The most important test - at least from California's perspective - involves what is called "seismic racking."
The frame to which the wall section is attached begins to move - the horizontal beam in the middle of the frame shifting back and forth, in and out, up and down.
The first round simulates a moderate earthquake, followed by another shot of sprinklers and a close inspection. Everything should slide back into place as if nothing had happened.
When the test resumes, the racking picks up force. Gyrations push several inches in each direction: The force is supposed to simulate an earthquake that's 150 percent greater than the maximum that seismologists anticipate in a given location.
This time, the panels aren't expected to stay airtight.
The gauge of success is whether they stay in one piece.
"Nothing can fall off the building - that's the easiest way to describe it," Zelazny said. "Joints can come apart, you can see through the mullions, but everything should still be attached. ... A panel should fail the way you want it to fail."
And if glass does break, whether the culprit is an earthquake or a crane from an errant window-washing unit? Towers have heat-strengthened glass, so it won't shatter into sharp shards; either there's a spiderweb effect with the pieces holding in place, or it crumples in a manner similar to an automobile window.
From assembly to certification, the testing process can take two weeks - "and that's if the testing is pretty successful," Zelazny said.
E-mail John King at
jking@sfchronicle.com.[/quote
http://sfgate.com/cgi-bin/article.cg...MNRTT6N06.DTL]
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