To most people, tents are what you have to tolerate if you want to go camping. They`re temporary, flimsy, too hot or too cold, and blow away in storms, usually when you`re sleeping under them.
To Horst Berger, however, tents deserve a place in the architectural mainstream. From his cramped offices in the shadow of the Chrysler Building, Berger, a structural engineer, talks of skylines punctuated with tents, tents sprouting above convention centers, even housing developments snuggled beneath tents.
Berger is one of a small band of architects and engineers who preach the virtues of fabric roofs built to last 20 years or more. These roofs use a translucent fabric woven of fiber glass strands coated with Teflon or, occasionally, with silicone. In Berger`s tent roofs, the material is stretched between masts or arches at the high points and walls or ground anchorages at the low points. Tension pulls the fabric into smooth, taut curves, often quite complex and beautiful to behold. ”With fabric, form must follow function,”
says Berger. ”The beauty of a fabric structure is that you can see where the stresses are, the way you can with a suspension bridge.”
In some tent roofs, a single swoop covers the better part of an acre, the fabric drawn so tight that a person can walk across it without any noticeable sag. The Taj Mahal of these structures is the Jeddah International Airport in Saudi Arabia and its five and a half million square feet of gleaming, white fabric.
But a corporate client planning new headquarters in mid-Manhattan is bound to have trouble accepting a building that looks like a translucent bat in flight. ”The skepticism is tremendous,” says Berger. ”If these structures captured just one percent of the market, we`d be doing 40 times the business we`re now doing.” Once he submitted a tent-roof design for a terminal building at Kennedy Airport. ”The man in charge said, `This is nice. It`s beautiful. But fabric?”`
It`s not just that it looks different. It looks flimsy.
In fact, it is anything but. Stretched and fixed in position, even delicate fabric exhibits remarkable strength. This is what allows an umbrella to hold its shape in the wind–most of the time, at least.
Shape is also critical. Like a horse`s saddle, the roof`s surface curves up in one direction and down in the other. The more pronounced the curvature, the stiffer the surface. Without curvature, even a tightly stretched fabric will yield when pushed. This principle gives a trampoline its bounce.
Bill Moss found this out the hard way. He is the founder of Moss Tent Works in Cameden, Maine, best known for its lightweight camping tents. Like Horst Berger, Moss thinks tents are suited for long-term structures. ”I spent a lot of time in the Middle East, where fabric is accepted for housing,” he says. ”I put up a little community of tents that the wind blew down immediately.”
Moss started over. He threw out all the flat surfaces and came up with a tent made entirely of saddle shapes. His Optimum 350 resists high winds, and other manufacturers have begun to copy Moss`s ideas.
”Form follows climate,” says Moss, who was trained not as an engineer but as an artist. ”After I designed the Optimum, I had it wind-tested by a group at MIT. We found that the wind will blow down a conventional tent, while it will help hold up my tents.” Curves and undulations are a Moss trademark. The curves of his Optimum tent allow winds to pass relatively smoothly over its surface, avoiding uneven pressures that can knock other tents down.
A fabric structure must stand up to forces other than the wind: sunlight, fire, vandalism, grime, rain, snow, and just plain old age. Early attempts at long-term tent roofs used vinyl-coated polyester, which stretches little by little over time, leading to wrinkles, flutter, and rips.
The sun`s ultraviolet rays weaken the stuff and turn it sticky so that it traps grime, gradually becoming opaque. Worse, vinyl produces a thick, toxic smoke when burned, so it ran afoul of building codes as a permanent structural material.
The coated fiber glass is strong, doesn`t burn, and barely stretches at all–a mixed blessig, because there`s little room for error when the fabric is cut into patterns. The Teflon coating is chemically inert, so corrosion and discoloring do not occur. Rain washes it clean.
But a square foot of Teflon-coated fiber glass costs three to four times as much as one of vinyl-coated polyester. And while it is sturdy enough to be stretched drumhead tight, knife-wielding vandals could slice through without much trouble. ”The answer is to keep these roofs out of people`s reach,”
says Horst Berger.
Besides cost and vulnerability, fabric is a terrible insulator; a single layer is about as effective as no insulation at all. There is, however, a new and expensive kind of fabric roof that is quite energy-efficent.
It consists of a sandwich made of a top layer of Teflon-coated fiber glass, followed by 8 to 16 inches of woolly fiber glass insulation, several inches of dead air, and, at the bottom, a vapor barrier of a plastic called Tedlar to prevent condensation.
The sandwich both insulates and lets light through; earlier insulated fabrics could do one or the other, but not both. The new Lindsay Park Sports Centre in Calgary is covered by a roof of this material, the equivalent of up to four inches of standard insulation. A single layer would have been folly in subarctic Alberta, where winter temperatures often stay below zero for weeks. Fabrics will never replace steel or concrete. Manufacturers will promise only 20-year lifetimes, since the oldest fabric structures are less than 15 years old. Structural engineer Mario Salvadori has other objections: ”A conventional roof has some weight and is steady, while a fabric roof changes shape and moves and vibrates in the wind, rain, and snow. With fabric, you can`t build floor over floor.”
Salvodori, a partner in the engineering firm of Weidlinger Associates in New York City, adds: ”If you don`t want a fabric roof to move, it must be highly prestressed, which leads to problems. The moment you have to prestress, you have to have something to prestress against.”
This means expensive anchorages and supporters. The tent roofs of two Bullock`s department stores in northern California actually cost more to design and erect than conventional flat roofs would have. Understandably, the biggest and fanicest tent roofs are turning up in Saudi Arabia, where the climate is friendly and the money plentiful.
There is a cheaper way to span large spaces with fabric, however. Instead of stretching a roof out like a tent, inflate it like a balloon. David Geiger, a structural engineer and one-time partner of Horst Berger, specializes in these balloon roofs. Of 11 major covered stadiums built in the United States in the past decade, eight are topped by balloon roofs he engineered.
Geiger`s softly curved expanses of fabric are inflated by electric blowers that maintain an inside air pressure only a fraction of a percent above the pressure outside. The air within acts as a huge, invisible column, propping up the fabric and giving it the stiffness it needs to resist wind, rain, or snow. Masts or arches are unnecessary. A concrete ring around the outer edge of the roof–usually the top of the stadium wall–is its only anchorage.
In contrast to the bulbous curves of inflatable tennis-court covers, the almost flat fabric domes of stadiums like the Pontiac Silverdome and the Metrodome in Minneapolis act much like an airplane wing. The difference in air pressure as wind passes over the top tends to lift the roof more or less evenly across its surface. While loath to try it, engineers suspect that on windy days an air-supported roof could remain in place even without blowers.
Balloon roofs bring their own special headaches, however. The most vexing is snow. Last winter, a sudden storm dumped wet snow on the roof of the Pontiac Silverdome in Pontiac, Mich., accumulating to a depth of seven feet in some spots. One overloaded panel drooped several feet and snagged a metal light support tower. The fabric ripped, the roof began to deflate, and the weight of the snow extended the rips. When the storm had passed, the Silverdome needed a new, $8 million roof–this time with more clearance beneath the top.
Some stadium managers have avoided disasters by quickly dispatching workers to trudge across the blimplike rooftops, clearing away snow by hand. Circulating hot air under the fabric shell is fancier and more expensive.
When a sudden snowstorm threatened to collapse the roof of the Indianapolis Hoosier Dome just before last winter`s NBA All-Star game, the maintenance staff pumped 130-degree air between the roof`s two layers–a top layer of Teflon-coated fiber glass and a bottom layer of canvas–to melt the snow on top.
Fabric domes have other special drawbacks. Revolving doors and pressure barriers are needed even at small entrances. Moist air can condense on the inside surface and drip on the crowd. And reverberating noise can be a headache.
But the low cost of a balloon roof is enticing. The Carrier Dome in Syracuse, New York, cost less than half as much per spectator seat as the steel-roofed Houston Astrodome. Balloon roofs are light–typically one-thirtieth the weight of a long-span steel roof the same size–and trimming pounds means saving dollars. Fabric domes also go up more easily and quickly than traditional dome roofs can.
A fabric roof is made at a factory, not built by hand on site. A typical stadium balloon roof is shipped in 40-foot-wide panels that are then clamped together like a huge zipper. Rips are fixed by zipping in replacement panels. The 460-by-260-foot roof of the U.S. pavillion at the Osaka world`s fair in 1970, the first long-span balloon roof and the project that launched David Geiger`s career, arrived at the site on a single truck. Erecting even the largest balloon roofs takes only about six weeks, not several months. Cranes or helicopters crisscross steel cables across the site, and the roof is pulled over them.
Still the image of the tent lingers. Bill Moss, designer of the Optimum 350, has lately been designing what he calls space articulators–graceful fabric shelters for urban plazas and housing developments–and is pushing fabric as a medium for sculpture. One of his eye-catching sculptures, an arch- supported tube of reflective fabric called Stargazer II, is part of the permanent collection of the Museum of Modern Art.
”But my new direction is housing,” he says. Moss sees no reason why not, except for cultural shortsightedness. Geiger has drawn up plans for sheltering a mining town in northern Canada and a college campus in Alaska beneath balloon roofs. ”We know we can do it,” he says. ”We just need a client.”
