Wind Pressure and Surface Force Calculator

How is wind pressure calculated?

The pressure produced when moving air is brought to rest on a surface is the velocity pressure, q = ρ × v² / 2. ρ is the air density (about 1.25 kg/m³) and v the wind speed in m/s. The result is in N/m², which is pascals. Pressure grows with the square of speed: doubling the speed quadruples the pressure.

For a 30 m/s (108 km/h) wind, q = 1.25 × 30² / 2 = 562.5 N/m², about 0.56 kN/m².

From velocity pressure to force on a surface

The pressure on a real surface is not equal to the velocity pressure. Three adjustments apply:

• Exposure factor (c_e): the basic wind velocity in the standards is the mean speed at 10 m height in open country. Once gusts and height are included, the peak velocity pressure can be several times higher.
• Pressure or force coefficient (c_p): depends on the shape of the surface and the wind direction. It is positive (pressure) on the windward face and usually negative (suction) on the leeward face and the roof.
• Structural factor (cs·cd): accounts for the size and dynamic behaviour of the structure.

The force is F = c_s·c_d × c_p × q_p × A, where A is the reference area normal to the wind.

Example

A basic velocity of 28 m/s gives q = 490 N/m². With an exposure factor of 2.1 the peak velocity pressure is 1029 N/m². A panel of 12 m² with a force coefficient of 1.3 then carries 1.029 × 1.3 × 12 = 16.05 kN.

Limits

This tool does the arithmetic of the formula. It does not choose the coefficients. Exposure, pressure and structural factors are given in a standard such as EN 1991-1-4 by terrain category, height and building shape, and choosing them correctly changes the result several times over. Leaving the coefficients at 1 is unsafe in most cases. Use the result for a first assessment.