Wind Velocity Pressure (qz = 0.613 Kz Kzt Kd V²)

The velocity pressure equation is Bernoulli with the trade's constants folded in. Stagnation pressure is $\tfrac{1}{2}\rho V^2$, and with the standard air density of 1.226 kg/m³ that half-rho becomes 0.613 — so with V in metres per second the answer lands in pascals. The exposure coefficient $K_z$ accounts for terrain roughness and height, $K_{zt}$ for hills and escarpments that speed the wind up, and $K_d$ for the low odds that the worst wind direction coincides with the building's weakest axis. A 45 m/s basic wind (about 100 mph) on flat open ground with $K_z = K_{zt} = 1.0$ and $K_d = 0.85$ gives $0.613 \times 0.85 \times 2025 = 1055$ Pa, roughly 22 psf.

This page carries the general SI form of the ASCE 7 equation so the arithmetic can be checked and rearranged. It is not a substitute for the standard: $K_z$ comes from a table indexed by exposure category and height, $K_{zt}$ from a topographic procedure with three separate multipliers, the basic wind speed $V$ from hazard maps tied to risk category and return period, and the imperial version of this same equation reads $q_z = 0.00256\,K_zK_{zt}K_dV^2$ with V in mph and $q_z$ in psf. Use the adopted edition of the code for all of them.

The square on velocity is the fact worth carrying around: a 20% higher wind is a 44% higher pressure, which is why a jump of one hurricane category is so much more than one step. And $q_z$ is not yet the load on anything. It becomes a design pressure only after multiplying by the gust-effect factor and the external and internal pressure coefficients, and those coefficients are frequently negative — the suction on a leeward roof slope is usually the worst pressure on the entire building. Roofs are far more often lifted off than pushed in.