Exteriors are dressed with a nonstructural skin, equivalent to today's reflective glass or aluminum skin. The length is required to simulate the turbulence of boundary layer winds. Although wind strikes most building sites from just one or two prevailing directions, models are placed on a rotating platform that allows testing at 10 degree intervals, for a total of 36 simulated wind directions as the model turns in a circle. It's the selection of the right upstream roughness. If 7 South is at the center of a circle, the Sears Tower is near one edge, and the John Hancock Building is near the other, along with a bit of Lake Michigan.
Engineers also consider the acceleration of a building's movement.
As winds gust, a building's movements accelerate, which places increased stress on the structure. It's important primarily because designers are continually trying to extend beyond the existing envelope. It's apparent from what's been built in recent years, and what's on the drawing boards, that architects and engineers are leaning toward bolder high-rise designs. Wind tests can show designers which designs have pushed the limits beyond what's practical. Intuition, he adds, is not sufficient.