Shopping under the big top

From the air it looks like the top of a giant lemon-meringue pie. Eight creamy-looking peaks protrude skyward from the flat landfill bordering San Francisco Bay.

Yet these peaks are not made from egg whites. They are the Teflon-coated material atop the largest fabric-covered department store in the world.

The octagonal-shaped structure, part of the new San Mateo Fashion Island Shopping Center 20 miles south of San Francisco, is the second fabric-roofed store in Bullock's West Coast department store chain. Its tent roof covers over half of the store's 135,000 square-foot area.

The fabric, woven from glass fiber, is supported by eight A-frame cables clamped around the edges to the store's brick walls. This translucent canvas, coated with Teflon to give it added strength and waterproofing, is only 1/32nd of an inch thick. The Teflon also creates a slippery surface that is self-cleaning in the rain. The fabric is so sturdy that construction workers last summer spent their breaks sliding down from the slick, steep peaks. The roof will withstand winds up to 80 miles an hour, and lab tests simulating a 20 -year time period show that the fadeproof fabric actually becomes denser and stronger with age.

The translucence of the roof creates ''an extremely exciting environment in which to shop,'' said Paul Heidrick, chairman of the board at Bullock's when the store opened last fall. ''The effect of the natural light on the merchandise is aesthetically pleasing,'' he continues. ''For the first time, shoppers are able to see merchandise the way it's going to look outside. During the day you can see things through the roof, like the shadow of a passing sea gull. Then when clouds come over, the light changes. It's super when it rains. It sounds like rain on a tin-roof barn.''

The aesthetic values of the new store are not its only advantages. The fabric roof is quite practical in its potential for reducing the store's energy bill. Conventional stores spend some 50 percent of their energy bill on lighting. Since the Bullock's store relies on natural light transmitted through the roof during the day, this cost is virtually eliminated. Electric lights are turned on only during evening hours.

Also eliminated is the heat generated by fluorescent and incandescent lights. This heat buildup typically creates the need for an energy-intensive air conditioning system, says L. Gene Zellmer, designer and architect of the new Bullock's store. By circumventing some of the lighting and the air-conditioning requirements, Mr. Zellmer predicts the store may save between $70,000 and $100, 000 a year in energy costs.

After a year's operation, says Bullock's manager William Thayer, the San Mateo store's energy bill has been approximately one-third lower than the average bill incurred by stores of a comparable size. Larger savings were possible, but the store encountered constant problems with temperature control under the big top. ''Last winter it was so cold in the store that women wouldn't try clothes on,'' laments Fred Neilson, division manager of store planning and construction. ''Then in the summer it was too warm. We had to close the restaurant.''

The temperature problem results from an inadequate system of ducts to distribute air throughout the store. The system is being redesigned this fall and the problem should be corrected before winter weather sets in.

For Mr. Zellmer, the new store represents ''the culmination of a 20-year career objective.'' The architect became enamored with the concept of using fabric as a building material while in Europe in 1961. Four years later he wrote his master's thesis at the Massachusetts Institute of Technology (MIT) on fabric-applied structures. He subsequently experimented with constructing roof sections for office buildings in Fresno, Calif. Yet the concept did not come to full fruition until 1977, when he and Mr. Heidrick happened to sit next to each other on a plane flying between New York and San Francisco. ''We just started talking,'' Mr. Heidrick explains. ''He had written this thesis at MIT, and I was very intrigued.''

Intrigue led quickly to implementation. Mr. Heidrick was just completing plans for a Bullock's store in San Jose, and the design included a small skylight over the escalator. ''We redesigned the original plans,'' says Mr. Heidrick, and a 16,000 square-foot piece of fabric was installed

This test of fabric as a building material was a cash-register smash. Sales were substantially higher in the areas under the fabric, compared with similar departments in conventionally roofed stores. And, says Mr. Heidrick, the San Jose store has been by far the most efficient in its energy use, saving some $21 ,000 annually in energy costs.

The San Mateo store has also been popular with shoppers, despite the temperature irregularities. ''People vote for or against it at the cash register ,'' says Mr. Heidrick, ''and they seem to be voting for it. We've had amazing crowds.''

Would such a structure be feasible in parts of the country with more extreme climates? Sure, say Mr. Zellmer and Darryl T. Roberson, president of Environmental Planning & Research Inc., the San Francisco firm that designed the Bullock's interior. Fabric roofs are already in place at Florida's Sea World; the Pontiac Silverdome in Michigan; several universities; and in a number of small skylight areas in retail stores. An entire fabric-roofed shopping mall in Houston should be completed by 1983. ''The fabric structure has application in 80 percent of the United States,'' Mr. Roberson says.

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