Since their inception, tensile fabric structures have been the subjects of spectacle. From the first major debut of large-scale tensile structures at the Munich Olympics in 1972, to the 2010 collapse of the Metrodome in Minneapolis, these buildings, with their daring simplicity and outstretched configurations, have captivated the general public and captured the media spotlight in a manner that few other building typologies do. But, as so often is the case, the glare of the cameras has done little to illuminate the story behind tensile structures and their curious place in design history and in contemporary practice.
The basic structural behavior of textile-formed structures is reasonably straightforward. “It’s like a dome in reverse,” explains engineer Guy Nordenson, founder and partner of New York–based Guy Nordenson and Associates. Whereas the shell of a masonry or concrete dome is supported primarily in compression, the continuous surface of a tensile fabric structure contains no discrete compressive members. Instead, Nordenson says, “It’s a thing that depends entirely on in-plane forces that are all tensile—a version of the balloon, so that you’re either holding it stretched with air or by giving it some shape” with masts and cables.
In his 2009 study of tensile surface structures, Michael Seidel, a senior scientist at Vienna University of Technology, identifies their most salient feature as “the large clear spans, which can be roofed over very economically without internal support.”
The balloon-like quality of tensile structures belies the durability of the membrane fabric. Long the industry standard for fabric construction, the tough, weather-resistant, and synthetic polytetrafluoroethylene (PTFE) fabric comprises woven fibers of PTFE or, more commonly, woven fiberglass that is emulsion- or extrusion-coated in PTFE. Saint-Gobain Performance Plastics offers PTFE-coated fiberglass products with thicknesses ranging between 20 and 40 mils and strip tensile strengths of 500 to 900 pounds per linear inch. Marcel Dery, global sales manager, architectural, at Saint-Gobain, says that “it would be deceiving” to compare PTFE to other construction products such as steel or wood. Architectural fabric “is a completely different type of building material” because it elongates and achieves full strength when elongation is properly addressed, he says.
PTFE, though, is not the sole product on the market with which to create tensile fabric structures. “ETFE [ethylene tetrafluoroethylene] is a new trend here in the United States, and you’ll see a lot more projects using it,” says Michele Roth, marketing manager for Birdair, a specialty contractor for custom tensile membrane structures based in Amherst, N.Y.
Like PTFE-coated fabrics, ETFE is a polymer that can be used in single or multiple plies. But ETFE is technically not a fabric because it does not comprise individual woven fibers; rather, “it’s a foil,” Dery says. In multilayer ETFE installations, the interstitial voids between plies are filled pneumatically to create a cushion. The air pocket in architectural ETFE acts as an insulator; depending upon the number of layers of plies and cushions, ETFE can achieve R-values between 1.4 and 5—well above the R-value of 1 that a single ply of PTFE fabric achieves, although roofing systems made from multiple PTFE layers that sandwich an intermediate insulating layer, such as aerogel, are available. With constant R&D efforts at manufacturers such as Saint-Gobain, newer and more products will be on the market soon. In 2008, Birdair, Geiger Engineers, and manufacturer Cabot Corp. released a laminated nano-gel fabric that’s both light transmitting and highly insulating, averaging an R-value between 5 and 14 per inch of thickness. Students at Germany’s University of Stuttgart are currently researching the potential of “active” textile membranes that have sensors and mobile parts to adjust the membrane for changing stress factors.