Shattering Myths About Glass

As architects and builders put more faith in the structural properties of glass, its use has expanded to all areas of design.

By Josephine Minutillo

Glass may be stronger than concrete, but you’re not likely to see too many glass columns holding up floor slabs. Nevertheless, more and more projects are beginning to embrace glass as a structural element to create innovative facades and interiors as well as bold urban spaces.

Though vast expanses of glass are not holding up huge sections of the soon-to-open Museum aan de Stroom (MAS) in Belgium, it certainly looks as if they are. The surprising building, designed by Dutch architects Neutelings Riedijk and located along Antwerp’s waterfront, contains a series of stacked boxes housing galleries, each twisted 90 degrees and connected by a spiraling staircase. Visitors traveling up the staircase have broad, unobstructed views of the harbor and city center thanks to the groundbreaking use of corrugated glass in the facade.

“If you used straight panels, the glass would have been enormously thick because the free span is 18 feet,” explains Rob Nijsse of ABT consulting engineers. “Since the corrugated glass is so much stronger in bending, we were able to use 1⁄2-inch-thick panels to take up the wind load for the large span.” Nijsse first used corrugated glass in the Casa da Música in Porto, Portugal, designed by Office for Metropolitan Architecture (OMA) and completed in 2005. There, three layers of 13-foot-high corrugated panels rest on top of each other to form nearly 40-foot-high window openings within the heavy concrete facade.

With MAS, the heavy elements of the facade seem to float above the glass, which wraps around the building. In reality, the concrete boxes cantilever out from a central core and are separated from the glass panels by a 2-inch-wide airspace. “We had to keep the glass clear of the cantilever because there’s a tendency for the concrete to deform slightly when it is loaded with people,” says Nijsse.

At the building corners, two layers of 18-foot-high panels are stacked on top of each other. The stacked glass panels in the Casa da Música incorporate steel I-beams to help support the wind load, but only a hollow steel tube, which is set back 1.5 feet behind the corrugated panels, is used for that purpose at MAS’s 36-foot-high spans. To connect the stacked panels, a steel U-profile is glued to the top of the lower panel and the underside of the upper panel, then bolted together with a corrugated steel plate in front and back. Structural silicone joins the individual vertical panels, each forming one S-shape 5.25 feet long.

The corrugated glass is fabricated in a similar fashion to curved glass, but goes one step further. A flat glass panel, cut to the appropriate dimensions, is placed in a furnace, where it melts over a mold. Since the glass gets its stiffness from its shape, the designers could use ordinary float glass rather than laminated or tempered glass, allowing significant cost savings.

According to Nijsse, the curved glass only produces slight deformation in views when standing at a distance from the panels. Its effect on the overall building, he says, is a “spectacular openness.”

Of course, the desire to create a spectacular openness makes glass the material of choice. Diller Scofidio + Renfro enlisted Dewhurst Macfarlane, engineers of the iconic, glass-themed Apple flagship stores, for the design of a pair of canopies at New York’s Lincoln Center. “They really just wanted a slab of nothing to keep the rain off people walking from the buildings to the street,” recalls Tim Macfarlane. The nearly 90-foot-long canopies connect to existing building columns at one end and are supported by a pier toward the middle, leaving close to 40 feet of overhead glass to cantilever above.

“The real innovation here is that the glass structure stabilizes the steel,” Macfarlane says. “We’ve gone from steel supporting glass, to glass supporting glass, to glass supporting steel.” The canopies consist of a series of 14-foot-6-inch-by-7-foot-7-inch glass panels, joined by weathering silicone, underneath a pair of bent steel beams, which, according to the engineers, would buckle were it not for the glass below. A small glass panel located between the legs of the pier provides lateral support to the canopy.

The 2-inch-thick laminated glass, fabricated by the German manufacturer Seele, incorporates another recent innovation. An ionoplast seal binds the layers of glass together for full composite action, making the material itself part of the structure and allowing for a very stiff and relatively thin panel. Panels using earlier bonding materials like polyvinyl butyral (PVB) were not as strong, because the laminated sheets would behave independently rather than as a single unit. “If we didn’t use ionoplast, the glass would probably be 3 to 4 inches thick,” Macfarlane explains. “It’s a big savings in terms of weight.” Another project by Dewhurst Macfarlane, for a balustrade at the Victoria & Albert Museum in London, uses ionoplast to bond glass to metal.

Since the canopies’ unveiling earlier this year, Macfarlane and his team have noticed that some visitors — who don’t seem to have as much faith in the glass as the engineers do — prefer to walk around the glass structures rather than beneath them. For an underground entrance at Dilworth Plaza in Philadelphia, “There’s no ‘walking around it’ option,” Macfarlane jokes.

Designed by KieranTimberlake in collaboration with Olin, two 17-foot-wide, all-glass pavilions connected by a single arcing gesture are planned as gateways to a below-grade transit concourse. “A continuous circle is set against a Classical facade,” says Macfarlane. “It’s a simple gesture that deserved a simple solution.”

That solution eliminated any steel structural members or metal connections whatsoever. Instead, the 2-inch-thick laminated-glass panels of the curving roof are joined by structural silicone and supported entirely by vertical glass panels — without fins — of equal thickness. “It’s basically a giant glued structure,” says Dewhurst Macfarlane associate Nicholas Roach. The walls support all vertical and lateral loads, including wind and snow. Wind loads present the greatest challenge, especially at the top of the staircases, where the wall reaches nearly 20 feet in height. “We needed to transfer those loads down the cantilever where it is stiffer,” Macfarlane explains. “The closer you get to the bottom, the more capable the structure is of supporting the load.”

Dewhurst Macfarlane can predict to some extent how the structure will perform, since its parts — including finless vertical glass walls — and scale are similar to the TKTS Booth in New York City’s Times Square they worked on [Record, January 2009, page 47]. But they used NEi Nastran, a finite element analysis program typically used for designing complex machine parts, to understand all the stresses in the structural silicone, or glue. As currently designed, the silicone will only be used for the 1⁄2-inch-wide gaps between the roof panels. Those same gaps between the vertical panels will be left open, keeping the pavilions ventilated and capturing light during the evenings for a dazzling enhancement to a grand urban space.

Structural glass can be used on a much smaller scale to create equally dazzling interior spaces. AMG Design, a high-end steel-and-glass specialty contractor, is using its new facility in Plainview, New York, to showcase its fabrication capabilities. The showstopper is a dramatic glass-and-cable staircase designed by Grimshaw Architects together with Thornton Tomasetti engineers.

The 4-foot-wide staircase is centered in the foyer of a newly built, all-glass box within an existing traditional industrial building. It may be the first staircase to form a true tensegrity structure (Buckminster Fuller — who coined the term — created what is probably the most famous type of tensegrity structure, the geodesic dome). As such, the solid members, or glass, are loaded under pure compression, with the cables providing all the tension.

“What’s unique about the staircase is that we don’t have any stringers in the conventional sense of two bending beams,” says Thornton Tomasetti’s Wilfried Laufs. “We prestressed thin cables like a harp from the floor to the ceiling and clipped the glass treads in between. There are no beams or slabs or walls in this system at all.” Instead, the inclined cables are tensioned against the floor and ceiling, where castellated steel beams receive the cable ends and transfer the loading over to the main building-structure columns.

Underneath the treads, the 1⁄4-inch-diameter cables form a fish-bow truss in order to avoid staircase vibrations and stabilize the treads laterally. Vibration control is most critical compared with static deflection and stress limitation for this type of staircase. EASY Technet, a specialty German software for lightweight surface structures, was used to determine the critical lowest vibration modes both unloaded and loaded with people at different locations.

The treads are made up of laminated safety glass. Ionoplast is used to bond the four glass layers to each other, and the stainless-steel connectors to the glazing. As standard practice, the top layer, referred to as the “sacrificial layer,” is assumed to be broken for calculations. “Imagine you would drop a sharp metal suitcase and that top layer would break while you’re standing on it,” Laufs explains. “We already don’t take that into account.”

The designers will use annealed glass, or basic float glass, so that the edges can be polished. (The same polish is not possible with heat-strengthened, or tempered, glass.) A continuous weaving handrail will be assembled along the cables, which will be lit from the floor.

Another project by Thornton Tomasetti, this time in collaboration with Pelli Clarke Pelli Architects, also aims to maximize transparency while minimizing structure. A mixed-use development currently under construction in Washington, D.C., features two structural-glass entry walls that enclose an atrium and provide a daylight-filled walkway through the complex.

ach of the three-story-high glass-and-cable facades incorporates a glass-canopied entrance. “What we tried to do is basically use these pieces that we need anyway and make sense of them structurally in a bit of an innovative way,” explains Laufs. For example, the back wall of the vestibule, containing a second set of entry doors, acts like a column to hold up the vestibule roof. “We figured that if we have to have a second door situation for thermal reasons, let’s use it structurally as well,” says Laufs. Perhaps more interesting, the structural silicone-sealed vestibule roof glazing over the entrance doors uses the glass as shear bracing, acting like a horizontal intermediate truss to further reduce the effective span of the facade.

Only the horizontal glass of the vestibule roof is laminated for redundancy purposes because of its overhead position. According to Laufs, “If the glass would really fail badly, it would still stay in place.”

The engineers used SJ Mepla, another finite element analysis program, for the calculations of the glass structure, but relied on the same tensegrity software used for the staircase design to determine the prestressing of the cables on the facade.

Just past the entrance doors of another building, in this case a new office building in London designed by Michael Aukett Architects, Malishev Wilson Engineers animated an otherwise ordinary lobby by suspending a glass walkway over the reception desk. “The architect thought that the entrance lobby needed some kind of special feature,” recalls engineer Gennady Vasilchenko-Malishev. “The challenge was convincing a conservative client who was very mindful of the budget, without sacrificing the quality we were aiming for.” (As a cost-cutting measure, the designers opted out of using low-iron, or completely transparent, glass — hence the greenish tint to the walkway.)

The walkway’s floor panels are composed of three layers of 1⁄2-inch-thick glass. The laminated panels rest on a series of glass beams. Primary beams, running along the width of the walkway, are spaced 5 feet apart, with every other one hung from the ceiling above by high-strength stainless-steel rods. (The structure is not completely suspended; one end of the walkway is connected to a wall.) Two secondary beams run along the length of the walkway.

“The splice detail between the primary and secondary beams looks fairly simple, but because glass is obviously a very tight-tolerance material, fitting all this together was really quite complex,” says Malishev. The glass works in both compression and tension. The stainless-steel forks that are fixed to the ends of the beams to hang the walkway create high-tensile stresses around the holes in the glass. While the fork slots over the beam, a pin passes through the fork so that the whole weight is transferred through the hole in the glass. A high-strength, epoxy-resin-based mortar completely fills the hole to ensure a good load transfer between the different plies of the glass.

“The real improvement over the last decade is in the detailing,” Malishev notes. “When we carried out tests on earlier projects, we found that the load share was uneven. A full-scale mock-up test of this project revealed that the load transfer between the laminates was very even. That means we’ve achieved a detail with greater reliability.”

The mock-up was built not necessarily to test the glass, but to prove to the client beyond a doubt that structural glass is safe. “We re-created an extreme situation where we broke some of the glass and loaded it to full capacity to show that the walkway would still work in a fail-safe environment,” Malishev explains. The mock-up also revealed to the engineers that they needed to increase some of their tolerances. “When we were assembling the mock-up, we realized that some of our tolerances were too tight, and the builder couldn’t successfully put it together without too much trouble.”

The walkway was a joint venture with U.K.-based F.A. Firman, a pioneering glass manufacturer that worked on such early structural-glass projects as Rafael Viñoly’s Tokyo International Forum, completed in 1996.

A combination of forward-thinking designers and clients who weren’t afraid to push boundaries was needed to get those early projects built. “When we did our first glass staircase for a shop in London in 1985, no one had ever done one before,” Macfarlane recalls. “But once it’s there, and you can walk on it and see that it’s safe, you get confidence to do the next thing. We’re working in an architectural domain where there is much more to think about than limited levels of performance. Each job requires a custom solution. We keep starting from a blank page.”