How To Calculate Wind Loads From Wind Speeds

All outdoor structures need to be able to withstand the force from wind, and so being able to calculate the wind load is crucial if you're going to be designing buildings. However, the calculations get more complex when you consider more factors. This means that for the most accurate wind load calculation it's often better to use an online calculator (see Resources) that accounts for all the relevant factors, given sufficient input data.

If you're just looking for a basic idea of how to calculate a wind load from a wind speed, though, you can perform a quick calculation for a ballpark estimate.

A wind load is a measure of the force exerted on a surface by the wind, which can be expressed as a force on the whole surface or a pressure (which is simply force per unit area). Therefore, the SI wind load unit is Newtons or Pascals. There are actually three types of forces exerted by wind on an average structure: uplift load, shear load and lateral load.

The uplift load is the lifting effect exerted on a roof from the passage of air around it (analogous to the lift on airplane wings). Shear load is the horizontal pressure that could tilt a building. Finally, lateral load is more like a broad "pushing" that could move a structure off its foundation.

For the purposes of this article, the focus will be on the lateral load since the calculations for the others are more complex, and there are many different variables to take into account.

The simplest formula to find the wind load uses the wind speed to determine the magnitude of the force it exerts. The formula you need is:

\(F_w = \frac{1}{2} ρ v^2 A\)

Here, ρ is the density of air (which varies with elevation and temperature but can be taken as 1.2 kg/m³ based on sea level and a temperature of 15 degrees Celsius), v is the wind speed, and A is the area the wind is striking. This equation is therefore a wind speed to force converter, but you can divide through by the area to get wind load as a pressure based on the wind speed.

The challenge of using this equation is finding values for the density of air in your location and an accurate measure of the wind speed you'll need to calculate for (since the highest wind speed determines how much wind load a structure needs to be able to withstand). The area A is easy enough to find for regular shapes. For example, for a flat rectangular surface you simply multiply the width by the height to find the area.

If you need to calculate the force (or pressure) due to the wind on a non-flat surface, incorporating the effect of drag is more important. While for a flat plate you can use a drag coefficient of 1 (so it makes no difference to the formula above), for a cylinder (for example) a coefficient of 0.67 takes account for the reduced impact of the wind on the surface.

You simply add this factor onto the right hand side of the above equation. You can find standard coefficient values for most common shapes and structures in tables.

Unfortunately, there are many other factors that impact the wind load on a given surface, including the variation in wind speed with elevation, the precise qualities of the surface (e.g. smooth glass versus a textured surface) and the impact of surrounding structures on the wind speeds likely to be experienced.

Performing an accurate calculation for your structure will therefore be a more complicated process than this article has made it appear, and you'll need a lot more than just the wind speed and air density to find a reliable answer.