Pressure is defined as force per unit area. This force has units of pounds and uses the simplified formula of F = P x A where P is the pressure and A is the surface area. Therefore, the larger the surface area, the larger force it will experience. This is the principle behind why sailing ships use such large sails and why hurricanes easily remove house roofs.
Determine the surface area exposed to the wind. Assume there is a billboard that has the dimensions of 20 feet by 40 feet. The surface area is length multiplied by width or 20 times 40, which is 800 square feet.
Determine the wind speed or wind velocity measured in miles per hour. Assume a hurricane has sustained winds of 100 miles per hour (arbitrary value). Air has a density of 0.075 pounds per cubic foot.
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Determine the force of the wind load on the billboard. This is done using the formula F = 1/2 rho x v^2 x A x C where F is the force of the wind load in pounds, rho is the air density, v is the wind velocity, A is the surface area of the billboard and C is a dimensionless drag coefficient (assumed to be 1.0). The calculation yields 1/2 x 0.075 x 100^2 x 800 x 1.0 or 300,000 pounds of force which is substantial.