According to a recent engineering study, the "sacrificial ply" glazing system is the safest, most economical way to save lives and protect property where architectural glass windows are employed in windstorm-prone areas.
The study team, consisting of Richard Behr, professor and head of architectural engineering at Penn State; Paul Kremer, research associate in architectural engineering at Penn State; and Joseph Minor, research professor at the University of Missouri-Rolla's Graduate Center for Materials Research, came to its conclusion after examining buildings in areas devastated by hurricanes and conducting extensive laboratory tests.
In the "sacrificial ply" glazing system, two plies of glass are laminated to a thin, clear plastic sheet to produce a laminated glass window unit. During a hurricane or tornado, the outer, exterior-facing glass ply is sacrificed to the impact of flying debris, while the inner glass ply and weather seal around the glass perimeter is preserved, thus protecting the building, its occupants, and its contents. There are many manufacturers that currently make laminated glass and the sacrificial ply laminated glass can be installed in many commercially available window frames.
"We concluded that the majority of damage didn't come from high wind pressures as many building designers originally thought, but from windborne debris impacts," say Behr and Minor. "It's costly to repair buildings. We saw entire building facades without glass, which allowed wind and rain to ruin the entire building interior. What's worse is that some of those businesses had to shut down for months. It was a huge economic loss in terms of productivity."
In addition to examining damage in the wake of hurricanes, including 1992's Hurricane Andrew, the team developed computer models and tested laminated glass to check models and prove the sacrificial ply design concept. They simulated flying debris by launching wooden 2x4s (representing large objects) and steel ball bearings (representing small objects) at samples of laminated glass, using compressed air cannons.
"We started at low velocities and kept incrementing until we reached failure in the inner glass ply," Kremer explains. "We're using the data and developing design tables and curves that supplement the current procedure designers use when choosing glass to handle wind loads. Now they will have quantitative information that lets them take into account windborne debris in the design process."
Minor says, "What you have to do is keep the glass in the opening to stop the wind and rain from coming in, one method is to use shutters or boards in front of the window. The second method is to use laminated glass that breaks but stays in the opening with an anchor seal around the glass perimeter. Both of these methods are very expensive.
"The third method is the sacrificial ply. We `sacrifice' the outer ply of laminated glass to the missile and preserve the inner ply. We save money by not using the expensive special extrusions for window frames or anchor seals needed in the second method and instead use a conventional means of holding the glass panel in the window frame opening."
Although it is not currently mandated by building codes, Kremer says the concept offers a promising option for building designers. "We are very optimistic that our sacrificial ply research will significantly enhance the hurricane resistance of windows in buildings," says Behr.
The researchers' paper on experimental sacrificial ply laminated glass data was published in the Journal of Architectural Engineering in March 2000. The paper is titled, "Impact Resistance of Laminate Glass Using Sacrificial Ply: Design Concept." The authors are Nathan Kaiser, graduate student, University of Missouri-Rolla; Richard Behr, professor and head of architectural engineering, Penn State; Joseph Minor, research professor at the University of Missouri-Rolla; Lokeswarappa Dharami, professor of engineering mechanics and aerospace engineering, University of Missouri-Rolla; and Paul Kremer, research associate in architectural engineering, Penn State. .
The research was supported by a grant from the National Science Foundation and partial matching funds from the Missouri Department of Economic Development through the Manufacturing Research and Training Center, E. I. duPont de Nemours and Co., and Solutia, Inc
Journal
Journal of Architectural Engineering