A wind load refers to the intensity of the force that wind applies to a structure. Although you can use a simple formula to calculate wind loads from wind speed, building designers, engineers and constructors must incorporate many additional calculations to ensure their structures won't blow over in a high wind.
You can get a general idea of the pressure on a 1-foot–by–1-foot section of a structure by using the following formula: wind pressure per square foot = 0.00256 x the square of the wind speed. For example, a 40-mile-per-hour (mph) wind speed creates a pressure of (0.00256 x (40)^2) = 4.096 pounds per square foot (psf). According to this formula, a structure meant to withstand 100-mph winds must be built to resist a wind pressure of 25.6 psf. Several websites offer multifactor online calculators to determine wind pressures on standard structures.
Translating wind pressure to wind load must take into account the shape of the structure, which determines its drag coefficient (Cd), a measure of wind resistance. Engineers have worked out standard Cd values for different shapes. For example, a flat surface has a Cd of 2.0, whereas the Cd of a long cylinder is 1.2. Cd is a pure number with no units. Complex shapes require careful analysis and testing to determine their Cd values. For example, car manufactures use wind tunnels to find the Cd of a vehicle.
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Armed with pressure and drag data, you can find the wind load using the following formula: force = area x pressure x Cd. Using the example of a flat section of a structure, the area – or length x width – can be set to 1 square foot, resulting in a wind load of 1 x 25.6 x 2 = 51.2 psf for a 100-mph wind. A 10-foot–by–12-foot wall claims an area of 120 square feet, meaning that it would have to withstand a 100-mph wind load of 120 x 51.2 = 6,144 psf. In the real world, engineers use formulas that are more sophisticated and contain additional variables.
Engineers must account for the fact that wind speed can vary with height above the ground, atmospheric pressure, terrain, temperature, ice formation, the effect of gusts and other variables. Different authorities publish conflicting Cd values, which can yield different results depending on the authority chosen. Engineers normally “overbuild” structures so they can withstand wind loads in excess of the maximum wind speed anticipated at the structure's location. Different loads apply to winds blowing on a structure from the side, behind, above or below.