No one associates architectural adventure with McDonald's, under whose golden arches more than 50 billion hamburgers have been consumed since its rounding 36 years ago. Over the years, the chain has commissioned nothing but carbon-copy buildings; nearly all of its 8,000 fast-food restaurants in 33 countries sport the same rustic mansard roof, walls of warm red brick and windows that are cosily colonial. So what happens when the hamburgergiant puts itself in the hands of the most audacious architects alive? The provocative--and tremendously popular--answer is now on view in a Berwyn,Ill., shopping center near Chicago. Challenged by mall owner David Bermant to come up with a more daring design, McDonald's reluctantly agreed to commission three radical New York-based designers named Alison Sky, Michelle Stone and James Wines. Collectively known as SITE, their firm strikes fear in the heart of the gray-flannel architecture profession. It is notorious for radical reversals, for buildings whose facades seem to be crumbling apart or tilting on their sides or that are "invaded" by hungry trees, plants and vines.

Magnet: Radical reversal is precisely what SITE has done to McDonald's standard design. While all the familiar parts are still there, they are dramatically altered. From afar, the new emporium seems poised for flight. The roof floats weightlessly above the rest of the building. Parts of the facade are raised above the ground on unobtrusive steel piers; the resulting gaps are paneled with glass, exposing the feet and legs of customers inside to passers-by. The customers can enjoy a view of the sky through the glass clerestoryjust below the building's roof. Dubbed "The Floating McDonald's," the razzle-dazzle structure is attracting gapers and tourists. For the first time, McDonald's is a magnet for devotees of architecture as well as of hamburgers.

SITE's iconoclastic McDonald's comes at a time when the group seems perched on the edge of respectability for the first time. Wines, 52, its spokesman, has been appointed chairman of environmental design at the respected Parsons School of Design in New York. Two months ago, SITE won a hotly contested competition to design a major pavilion at the 1986 World Exposition in Vancouver, Canada. And early in December, two exhibitions of SITE's work opened. One, at the Chrysler Museum in Norfolk, Va., is devoted solely to the new McDonald's. The second, at New York's Decorative Arts Center, covers several new projects. These include a dramatic showroom in Milwaukee, Wis., for Best Products, the premier mail-order retail chain and SITE's first patron in the early '70s. The showroom's faqade is festooned with ghostly gray replicas of the merchandise inside.

SITE has always proclaimed the joys of what it calls "de-architecture." Certainly the brilliant Berwyn McDonald's defies convention--delighting children, fast-food addicts and architectural fans alike. McDonald's itself is less enthusiastic. But Bermant, a vigorous art collector, insists he'll match up the McDonald's restaurants in his developments with SITE until he gets a string of "radical" stores. He is sorry the company would not go along with SITE's proposal to revamp the burger as well. Dubbed the "Floating Big Mac," their witty design features a top bun that floats above the other ingredients, supported by a giant toothpick. IfBermant and SITE have their way, the floating Mac could be on display-- in selected shopping centers--quite soon.