How ASCE 7 design wind pressure is calculated
ASCE 7 Chapters 26 through 30 build wind pressure from velocity pressure: qz = 0.00256 x Kz x Kzt x Kd x Ke x V², with V as the 3-second gust in mph and qz in pounds per square foot. The exposure coefficient is Kz = 2.01 x (z / zg) raised to 2/alpha, using alpha 7.0 with zg 1,200 ft for Exposure B, 9.5 with 900 ft for C and 11.5 with 700 ft for D. Below the minimum height for each category the coefficient holds flat.
Design pressure on a main wind force resisting system surface is p = qz x G x Cp minus q x GCpi. Note that GCpi already includes the gust effect, so it is never multiplied by G again. Internal pressure is signed to make the case worse: a windward wall being pushed in is combined with internal suction, and a roof being lifted is combined with internal pressure. A 115 mph site at 15 ft in Exposure C with Kzt 1.0 and Kd 0.85 works out to about 24.4 psf of velocity pressure, a useful number to sanity check against.
Cladding and components use the higher peak GCp values from Chapter 30, which are considerably larger near corners, eaves and ridges than the MWFRS coefficients listed here. Buildings that are flexible, in hurricane prone regions or on escarpments need the full topographic and gust factor procedures. Have a licensed structural engineer confirm the pressures before finalizing curtain wall or roofing specifications.
Frequently Asked Questions
Open water and flat open ground create less surface friction, so the wind is already near free stream speed close to the deck. That is why Exposure D uses alpha 11.5 with a gradient height of only 700 ft, giving a Kz roughly 35 to 65 percent larger than Exposure B at the same elevation.
Air entering through openings pressurizes the interior and pushes walls outward. ASCE 7 requires the combination that produces the larger magnitude, so inward external pressure pairs with internal suction and outward suction pairs with internal pressure.