Life Safety: Curtain Wall
The Glass Shield

By Eileen McMorrow

While dampers have been used for decades, mostly in seismic restraint, a new application for this technology emerged about two years ago: blast damper systems that make glass curtain wall safer. The idea came to Terry Palmer, principal of structural engineer Magnusson Klemencic, in Seattle, from looking at robust glass frames and deep-mullion systems. “There is limited capacity in those systems because they are too stiff. The glass panels, curtain wall, and mullions would fail in a bomb blast,” says Palmer.

Until two years ago most blast tests performed on windows used 5-foot-by-5-foot samples; results did not always translate into a robust full-scale curtain wall. Instead, tests have shown a flexible curtain wall performs best. With traditional curtain wall, mullions are large, glass transparency is limited, and the framework interferes with the building’s aesthetic.

Before testing of flexible systems was initiated, Palmer realized that everything was based on stiff
systems and there was a need to absorb the pressure of a potential blast. The answer was to apply friction through blast dampers, much like using a raster cable to stop a plane when it lands on an aircraft carrier. So, Magnusson Klemencic considered the possibility of a glass-absorbing cable-supported curtain wall.

Ready, willing, and cable

Seele, a German company that designs, fabricates, and installs glass walls, also thought a cable-supported wall would be viable. It tested outer and inner curtain walls that consisted of single- and double-layers of laminated glass. In spring 2002, Seele conducted blast testing of cable-stayed glass walls and no shards were released. The company’s report on blast-resistant glazed façades showed that, upon degradation of blast energy, the cable system would return to its original form. All outer toughened glass panes fractured. The panes of the central and upper laminated glass composites fractured completely. The inner laminated glass panes showed very few splinter fragment deposits. Basically, the outer glass bends, breaks, and absorbs energy while a glass interlayer holds the broken glass back.

The test also showed that the friction damper will slip at a predictable point, further ensuring that architects and engineers could design curtain wall to slip at a predictable blast pressure, Palmer expounds. The result? Architects can design curtain walls that are more visually transparent and yet can absorb a greater blast.

Now, using computational fluid dynamics, or CFD, designers can compute wind loads and engineer precise glass systems that can absorb the forces or load. “We can design glass facades to deal with the shock waves—actually soak them up,” explains Phil Khalil, engineering director of the façade-consulting firm Front Inc., in New York City.

Seele commissioned blast testing of its cable-stayed curtain wall system. Upon degradation of blast-energy, the system was seen to considerably return to its original form. All outer toughened glass panes fractured. The panes of the central and upper laminated glass composites fractured completely. In the lower laminated glass composite, only the inner pane fractured. There was good splinter fragment fixation as the inner laminated glass panes showed very few splinter fragment deposits. In the photo at right, the previously damaged glass element, without rectification of defects, was exposed to a second detonation. The lower laminated glass composites fractured.

Securing occupants & structures

With cable-wall systems, the reflection of the energy wave from an explosion is also contained. In the Oklahoma City bombing, hundreds of people were injured by glass projectiles blocks away. Experts say cable-wall systems benefit their building’s occupants as well as those in neighboring structures and pedestrians because of the wall’s capacity to absorb the shocks.

“Cable walls and steel-and-glass structures are interesting, but as we reduce the amount of load-bearing elements, we have to design and calculate everything differently,” says Hans Frey, vice president of in Seele’s Chicago office. The payoff in terms of blast protection is that it’s much more effective than a stiff system, as it can absorb ten times more blast pressure. “And from a cost perspective, it’s only a 10-to-20-percent premium over a steel-mullion system,” says Frey. “It’s very easy to use versus a traditional robust system.”

“The number-one priority is to make sure the laminated glass stays sufficiently intact to remain within its frame,” warns Kahlil. “The laminated interlayer captures the glass, so even if the glass has failed, the interlayer holds onto it. Glass loading lets materials deflect shock. As the load comes onto the cable, the cable deflects, strains, and soaks up the energy.” Kahlil cautions, however, that the cable technology is only appropriate for new construction, not retrofits. A cable-supported wall system needn’t be glass: Kevlar, fabric, perforated architectural materials, or woven wire screening all work.

Practical and pretty

“The backlash of 9/11 has settled down, and now a reasonable level of protection is being sought, the exception being government projects where there’s a high level of risk,” says Robert Heintges, principal, R.A. Heintges & Associates, a curtain wall consulting firm in New York City. His company presently has a project where the glass deflections will be very high and the lights are large. To prevent the glass panels from leaving the window opening and being disbursed, a cable restraint is being considered, which would employ a glass cable-stayed wall as a second wall or containment curtain behind the conventional curtain wall. In effect, the outer glass would absorb a certain percentage of the blast energy and upon failure the balancer would be borne by the inner wall. There are also projects where the cable within the mullion is also acting as a shock absorber or damper. In breaking, glass wants to try to fly out of the opening; but the cables allow the glass to absorb as much energy as possible before breaking, creating less of an impact.

“You have to introduce relatively high tension into the cables to prevent deflection, but as a result the client gets architecturally astounding glass façade,” says Kahlil, who notes there is momentum for adopting this novel approach. Curtain-wall blast dampers are being applied in military projects, courthouses, airports, and convention centers, with more commercial usage anticipated.