Windows on the World
- By Julie Sturgeon
- January 1st, 2004
It's nothing short of amazing what windows can stand up to these days. Even before 9/11, manufacturers were hard at work in their research labs seeking ways to strengthen glass against a hurricane's powerful pounding and a terrorist's bomb. And while the final products today are different, the common link is typically laminated glass, says Howell Cornell, director of Security Products for TRACO in Cranberry Township, Pa., just outside of Pittsburgh. "School markets, to this point, have vacillated between using tamper-proof or conventional resistance glazing products," he reports. But over time, these materials scratch and yellow. Laminated glass - basically two pieces of glass with a polymeric material sandwiched between, like a car windshield - solves both of these problems.
The difference stems from the impact each can handle. Hurricane-proof glass, for instance, needs
to withstand a nine-pound, two-by-four plank shot out of an air cannon at various distances and speeds. Once the window passes that requirement, TRACO puts it through a cycle test that simulates the onset and demise of a hurricane, starting at low pressure with 4,500 cycles in a positive direction followed by 4,500 cycles in a negative direction. Other tests load up steel shot or ball bearings to simulate wind blowing gravel off of nearby roofs into the glass. The goal, Cornell says, is not to keep the window intact,
but to make sure that, when it does break, the shards don't scatter into the building.
"If you can recall images from Hurricane Andrew, most of the small homes in Homestead, Fla., had their roofs blown off, because as soon as the pressure is breached with a broken window, it changes dynamically and actually lifts the roofs off," he describes.
Motivated by newer code requirements, colleges and universities along the coasts are starting to inquire about hurricane-proof glass, says Thomas Mifflin, research and product development manager for Wausau Window and Wall Systems in Wausau, Wis. On the other hand, the newer blast hazard mitigation systems have received no campus interest to date, both companies report.
For starters, this category offers no standard specifications, as of yet; the Government Services Administration and Department of Defense (DoD) offer only guidelines for their federal building requirements. DoD's is the least stringent, as its criteria is for designing or fortifying military bases against terrorism, an attempt to harden an already protected base. Here, experts test the window for
over pressure (the pressure generated by the bomb) and impulse, the duration of pressure applied. Over pressure is rated in terms of pounds per square inch (PSI) and impulse is rated in pounds per square inch, milliseconds. DoD applications only ask for a one-to-four rating.
(For comparison, Cornell reminds users that hurricane-proof glass is rated in terms of pounds per square foot (PSF), with numbers ranging from 30 to 90 PSF. One PSI on the blast hazard mitigation glass side can withstand 144 PSF.)
The GSA offers two levels: Level C at four PSI for 28 PSI-milliseconds, and Level D at 10 PSI for 80 PSI-milliseconds. Cornell even participated in a Department of State prototype window in early 2003, built to withstand 42 PSI for 296 PSI-millisecond.
But don't assume you can toss in these specs into your next remodeling project, warns Mifflin. The glass is but one component of an entire system. Both blast-resistant and hurricane-proof windows offer glass designed to move the load into the framing system, so it must act like a rubber band. "If you have a very high load placed at a very short period of time, you want that framing system to flex, bend and absorb as much of that load as possible," says Mifflin. "Otherwise, the window would just pop right off the building." But bullet-proof glass, for instance, has a steel-filled framing system to avoid allowing a
sniper to merely reach his target by shooting under the glass window.
"There's a world of difference between designing a system for blast hazard mitigation, as opposed
to someone throwing a full beer can at the window," agrees Ken Brenden, chief designer for Wausau Window. As one of the experts involved at the government level to establish official guidelines,
he says one of their concerns is manufacturing's tendency to offer a product in a catalog to solve a problem. "That's a false sense of security, if someone thinks that," he points out. "These disaster-proof windows are custom jobs involving the window's reaction, the glass, even the building's footings." Indeed, these aren't exactly ideal renovation
project materials because they often require elements stemming beyond what the older building can provide.
Disaster-proof glass also exceeds the average commercial window costs "by far," as Cornell phrases it. "The research, materials and installation effort make them more pricey, and that's just the nature of the beast," he says. Nor do insurance companies offer incentives like premium reductions to install disaster-proof glass, although he's hopeful that boost lies down the road.
"You have to analyze the possibility of an event ever occurring at a university," Brenden sums up. "It's like anything else: until a tragedy actually happens, decisions will be based on cost justification."