summer billboard image by Jorge Casais from Fotolia.com

If you've ever watched video of a hurricane or ridden a bicycle into a headwind, you're aware that moving air has force. Engineers and architects who design buildings and other structures must create a design that has enough internal strength and reinforcing to withstand wind velocities that are expected to occur at the construction site. Wind load, the wind's force on an object, depends on wind speed, the surface area of the structure and the coefficient of drag of the surface.

Assemble the information needed to calculate wind load, which includes the surface area of the sign or structure (A), the expected wind speed (v), the coefficient of drag of the shape (Cd) and the air density (rho). Air density changes with elevation above sea level, and the coefficient of drag changes with the object's shape. Rho, or air density, is 1.2kg/cubic meter at sea level. For a flat, stationary sign, Cd is approximately 1.28. Values of Cd for other shapes and rho for other elevations are available in reference tables.

Substitute the known values into the general formula for wind load; Fd = (rho * v² * A * Cd) / 2.0. For example, calculate the wind load of a billboard with a surface area of 50 square meters subjected to a wind velocity of 27.7 meters per second (about 100 kilometres per hour) at sea level. After substituting in the known values, the formula becomes Fd = (1.2kg/m³ * 27.7m/s * 27.7m/s * 50 m² * 1.28) / 2.0.

Solve the arithmetic to calculate Fd, the wind load. For this example, Fd = 29,463.94 Newtons.

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- To convert from Newtons to pounds, multiply the calculated value by 7.233. For this example, F is approximately 96615 Kilogrammes of pressure for a 100-kph (62-mph) wind.

- All units must be in the same system.

- summer billboard image by Jorge Casais from Fotolia.com