Better Blast Resistance Coming Soon to Facade
In security design, windows are a major area of concern. Nearly three-quarters of the injuries from the 1995 bombing of Oklahoma City's Alfred P. Murrah Building were caused by flying glass. To help win the fight, manufacturers are devoting unprecedented resources to research and blast-test a new generation of heat-hardened and chemically laminated glass.
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| STATE OF ART Las Vegas courthouse curtain wall mock-ups were tested in New Mexico. (Photo courtesy of Enclos Corp.) |
Glass manufacturers and structural engineers are testing not only new glass interlayers, but also new window systems, including mullions, frames and anchors. New anchor systems; cheaper, thinner laminates; and novel blast-resistant curtain walls will soon make their way to the commercial market, say industry sources. "We have stuff in development…test results were astounding," says Jeff Granato, architectural market manager for Wilmington-based DuPont's performance materials division.
To many, the nearly two-year-old U.S. courthouse in Las Vegas, with a glass curtain wall designed, fabricated and installed by Harmon Ltd., represents the state-of-the-art in blast resistance (ENR 7/10/00 p. 26). Harmon, now named Enclos Corp., currently is at work on courthouses in Seattle and in the Bronx and Brooklyn in New York City. Each pushes the envelope even further, says John Walker, vice president of engineering for Enclos, Minneapolis.
For Las Vegas, Harmon fabricated and installed a 1-in.-thick insulated, blast- resistant unit. It is composed of standard, annealed exterior glass; a 1/2-in. air space; and, on the interior surface, a laminate of polyvinyl butyral between two sheets of 1/8-in.-thick annealed glass held in place by aluminum frames. "We like the glass to be only as strong as is required for wind and thermal loads," says Walker. "In the past, the response was brute force. Now we are trying to create a flexible element that will absorb the force of a blast."
Full-scale tests were conducted at White Sands Missile Range in New Mexico. There, Harmon tested two curtain wall systems, one using the aluminum frame eventually utilized in construction; the other, in which glass was attached to a steel truss frame. "The computer model told us the system would work," says Kevin Cole, Enclos design manager. "The actual results exceeded our expectations."
Fully tempered glass must have a surface compression of 10,000 psi, four times more resistant to breakage than non-tempered glass. But a majority of post-World Trade Center curtain wall systems eschew tempered glass in favor of more ductile systems capable of "absorbing" the impact of an explosion. The blast pressures felt on Murrah's facade were on the order of 4,000 psi, says Tod Rittenhouse, a principal who specializes in blast resistance at Weidlinger Associates Consulting Engineers, New York City.
Seattle structural engineer Skilling Ward Magnusson Barkshire Inc. and Seele, a German curtain wall designer that opened its first U.S. office a year ago, are partners at the Seattle courthouse. The firms are working on a blast-resistant curtain wall that will absorb and dissipate the impact of a blast "like bubble gum, or like a spider web," says Terry Palmer, SWMB principal and director of SWMB's airport group. He hopes to have a system ready for full-scale testing within six months. It would be similar to a cable-supported system Seele installed recently at the University of Bremen.
"A 767 is not a credible threat" for most buildings, says Rittenhouse. Collateral damage to buildings surrounding a target building, whether attacked by a plane or a bomb, is more realistic, he adds.
Rittenhouse says that in Oklahoma City, more than 250 buildings suffered collateral damage–much of it broken windows. At the WTC, the number was even greater. "You can't protect against an aircraft, but we can protect against the collateral damage in the area," he says. Blast-resistant glass is a way to do that.