Thursday, August 14, 2008
The most familiar inflated membrane structures are airships, from nonrigid blimps to giant vessels such as the proposed 1,003-foot-long (307-meter) ATC SkyCat cargo lifter with a payload of 2,200 tons (2,000 tonnes). The German firm Zeppelin built several rigid-frame airships between 1900 and 1936, including the famous Graf Zeppelin. The new technology had consequences in the building industry. The English aeronautical engineer Frederick W. Lanchester first proposed an air-supported structure in 1917. Immediately after World War II Walter Bird designed and built prototypes of pneumatic domes to house large radar antennae for the U.S. Air Force. Known as radomes, they had many civilian commercial applications and paved the way for a new kind of architecture.
Pneumatic—or air-supported—structures have their form sustained by creating, with the aid of fans, an air pressure differential between the interior of the building and outside atmospheric conditions. The increased air pressure—about the difference between the lobby of a high-rise building and the top floor—is so slight as to be virtually undetectable and causes no discomfort. The structural system enables the achievement of large spans without columns and beams, providing totally flexible interior spaces. Made from laminated membranes such as fiberglass, nylon, or polyester, coated with polyvinyl chloride (PVC) for weather protection, the electronically welded components are tailored to define the building shape. The durability and heat- and light-filtering properties of the membrane are determined by the careful choice of surface finishes and inner lining. Because of its lightness, the air-supported structure is among the most efficient structural forms, combining high-tensile strength materials with the shell form. The capital cost of an air-supported roof is typically up to one-third that of a conventional building; considered on a cost-per-seat basis—they are widely used for sporting venues—the advantage becomes even more obvious.
The United States pavilion at Expo ’70 in Osaka, Japan, was one of several air-supported buildings at the fair. At the time, it was the largest structure of its kind ever attempted, a superellipse 265 feet wide and 465 feet long (81 by 142 meters). Its architects were Davis, Brody and Associates, working with designers Chermayeff, Geismar, de Harak and Associates; the engineer was David Geiger. The sloped sides of the pavilion, covered externally with paving tiles, were a 20-foot-high (6-meter) earth berm that supported a concrete ring 1,000 feet long, 4 feet high, and 11.5 feet wide (306 by 1.2 by 3.5 meters). Crisscross steel cables were locked into the ring to retain the roof once it was inflated. The roof of the pavilion was made of a translucent, closely woven fiberglass fabric, coated on both sides with vinyl. The seams were bonded by heat and pressure. Once inflated, the roof behaved almost as predicted.
More ambitious examples were bound to follow. In 1975, the 80,000-seat Silverdome in Pontiac, Michigan, boasted an air-inflated membrane roof measuring 720 by 550 feet (220 by 168 meters). The Hubert H. Humphrey Metrodome in Minneapolis, Minnesota, designed by Ellerbe Becket architects and completed in 1982, provided seating for up to 63,000 spectators under an air-supported roof of Teflon-coated fiberglass more than 10 acres (4 hectares) in area. Its claim to be the biggest air-supported domed stadium in the world was challenged the following year by the B.C. Place Stadium in Vancouver, Canada. In 1988 the Japanese architectural firm Nikken Sekkei and Takenaka, working with engineer David Geiger, designed the Tokyo Dome, with an air-inflated membrane roof of almost 660 feet (201 meters) span. Since about 1990, it seems that there has been a greater demand for sports arenas with openable roofs.