Usually deemed brittle and temperamental, glass is being used by architects and engineers who are making a career out of using it not only for self-supporting applications but also for structural uses. It only makes sense, says Graham Dodd, an engineer with Arup’s London office: In spite of its lack of ductility, glass matches aluminum alloys in density and stiffness. It resists heat and cold and is nonflammable – claims that even the best new plastics can’t match.
Taking advantage of these properties, several architects have recently invented spectacular new applications of glass – very carefully, of course. “Some designs employ glass-tube columns and laminated glass beams as structural cladding members,” says Charles Clift, president of Curtain Wall Designing & Consulting, Dallas. “However, you can imagine the engineering rigor needed to avoid catastrophic collapse.”
The most common examples of glass structures are horizontal, including staircases, pedestrian bridges and canopies.
One celebrated stair was specified for Apple Stores in San Francisco, Chicago, and New York. Designed by engineer James O’Callaghan and architects Bohlin Cywinski Jackson, the glass-supported staircases employ laminated panels and point-supported connections that rise more than 20 feet over lengths of more than 33 feet. To meet code requirements for fire-resistance and loading up to 100 pounds per square foot, the team used relatively thick, all-annealed glass panels polished after the laminating process.
Horizontal glass plates for structural uses are getting bigger and more transparent. Two new canopies at New York City’s Lincoln Center, almost 90 feet long each, are supported by twin metal columns for an impressive cantilever of half the total length. Glass panels are used between the bent metal columns to stabilize them against lateral movement, according to architects Diller Scofidio + Renfro and engineer Dewhurst McFarlane and Partners.
A Spectacle of Strength
In this case, the laminated glass panels are surprisingly thin – just 2 inches in total depth along the full span. To make it work, the engineers opted against using polyvinyl butyral (PVB), a material synonymous with safety glazing. Instead, the interlayer is an ionoplast resin, which offers high levels of rigidity and strength. The inonoplast allowed the designer to use glass panels that are half the total thickness of a conventional panel using PVB.
Not surprisingly, ionoplast is also being used for seismic design, security glazing and ballistic glass applications. Some ionoplast sealants have displayed five times the tear strength and 100 times the rigidity of conventional PVB interlayers, according to DuPont, which manufactures both materials. Ionoplast also has good acoustic properties, with sound transmission coefficient (STC) values of up to 41 for an insulating glass unit.
Another novel glass technology combines interlayers of PVB and polyethylene terephthalate (PET) to improve a structural glass panel’s strength and structural integrity, says Richard Green, P.E., of The Facade Group, Portland, OR. These interlayers, known as “low-creep laminates,” reduce deflection enough to allow “glass-only structures with ductile failure modes and without the risk of a sudden collapse,” he explains. Ultimately, this means more freedom to use glass and create transparent, self-supporting designs.
In fact, Green and other experts predict a new wave of glass structures as a result – what Arup’s Jan Wurm called “self-supporting skins” in his influential book Glass Structures. These will include glass columns and beams and fin-shaped curtain-wall elements.
The idea sounds far-fetched, but these approaches are being used already, albeit on a limited scale, says Clift. Recent examples include the corrugated glass walls at the Casa da Música in Porto, Portugal, and Antwerp, Belgium’s Museum aan de Stroom. The wavy glass panels aren’t actually supporting the concrete slabs and brick walls on the cultural facilities, but they offer significant improvements over flat glass panels in terms of performance.
For example, the panel spans of 15 feet to 20 feet used in the projects typically require very thick glazing to resist wind loads. Corrugated glass bends much less, so at the Antwerp museum, for example, the panels are a mere 1/2-inch thick. At the Porto music hall, three 13-foot-tall corrugated panels are stacked within the concrete structure, yielding an impressive, three-story glass expanse.
For architects considering the use of structural, load-bearing glass elements, experts offer a few considerations. First, glass performs well against short-term loads rather than sustained tensile stress, says Dodd. The latter leads to tiny cracks in the glass surface, eventually causing sudden, unexpected failures.
Second, careful evaluation of environmental conditions and loads is crucial to a long-lasting glass structure, says Nils Petermann, a project leader in the Efficient Windows Collaborative of the Washington, D.C.-based Alliance to Save Energy. That includes wind and snow, heating and cooling, as well as seismic loads and dead loads. For uniform loads, the standard ASTM E 1300 can be used to determine whether glass will stand up to the challenge – or crack under the pressure.
C.C. Sullivan (firstname.lastname@example.org) is a marketing communications consultant and writer specializing in architecture, design, and construction technology.