In South Florida, where hurricane “season” occupies a full six months of the calendar, AEC industry professionals are especially attuned to the challenge of designing for high winds.
Vincent J. DeSimone, chairman of DeSimone Consulting Engineers, has been there—and knows just where to look for answers. “The most useful tool that structural engineers have to determine the forces on the building skin is wind tunnel testing and the ensuing results,” said DeSimone, who will deliver a talk on “Innovative Facade and Building Design Through Modern Wind Tunnel Engineering” at September’s Facades+ Miami conference.
During a wind tunnel test, explained DeSimone, engineers place a scale model of a building inside a tunnel, then vary the wind speed and direction to determine the pressures on the structure. Sensors detect these pressures, which are then translated into forces acting on the facade. Forces on the facade vary from low to high, he noted, and some “hot spots” on the building envelope can achieve local forces in excess of 200 pounds per square foot. The load for the structure as a whole are generally determined by average these minimum and maximum forces. Per the applicable building codes, Miami-area structural engineers base the wind forces used in wind tunnel strength tests on maximum wind forces for a 50 year wind cycle.
Building movement is another matter, said DeSimone. For architects and builders in Miami, the allowable lateral displacement—the height of the floor divided by 360—is determined using a 25 year wind cycle. In a building with a story height of 12 feet, in other words, the allowable movement is 0.4 inches.
“In seismic zones where movement is expected to be much higher during a seismic event, facades are allowed to be much more flexible,” observed DeSimone. “Knowing that facades do not determine the allowable movement of a structure, doesn’t it stand to reason that here in Miami we are designing buildings much stiffer than they need to be?”
Recalculating allowable movement according to a 10 year wind cycle, for instance, could reduce the building’s shear walls by 22 percent. “This reduction—which, by the way, is used all over the country—results in a true sustainable reduction in material,” he said. “Remember, the most sustainable building is the building you don’t build and, conversely, the building which uses the least material.”
Learn more from DeSimone and other experts in high performance envelope design and fabrication at Facades+ Miami September 10–11.