Converting Wind Speeds to Loads

As we mentioned in the previous blog post, “Wind Speeds in Structural Design,” the ASCE 7 wind speed used for the design of our house in Cambridge, MA was 120 mph, while in Chappaquiddick, MA it was 134 mph. Yet the wind loads in Chappaquiddick were more than twice as large. How could that be? To answer that, we need to discuss how wind speed is converted into wind force (often called “load”).

The terms are closely related and often used interchangeably. In structural engineering, “load” simply refers to the external “forces” acting on a structure, like the roofs, walls, and floors. There are many types of force, such as force = mass x acceleration (thank you Isaac Newton) and in our case, force = pressure x area.

If you weigh 150 pounds and stand in the snow, your weight is a force. The size of your feet, specifically, the area in contact with the snow, determines how much pressure you apply to the snow surface. Tiny feet (small area) create higher pressure and cause more sinking, while larger feet spread the force out and reduce pressure. Fluids behave the same way. Water, for example, exerts pressure that increases with depth, which is why you feel more pressure at the bottom of a pool. Wind, though invisible, is also a fluid. As it moves across surfaces like roofs, walls, and floors, it applies pressure, and the faster the wind moves, the greater that pressure becomes.

In ASCE 7, wind pressure is calculated using the following equation:

Wind Pressure = 0.00256 × Kz × Kzt × Kd × V²

We’ll focus on just two of these terms: the wind speed, V, and the velocity pressure exposure coefficient, Kz. First, notice that wind speed is squared. This means small increases in wind speed can lead to much larger increases in wind pressure. Comparing Chappaquiddick to Cambridge: (134 / 120)² ≈ 1.25

So the higher wind speed alone accounts for about a 25 % increase in wind pressure, not double. Clearly, something else is going on. That something else is terrain exposure, captured by the coefficient Kz. Wind speed increases with height above the ground, but it also depends heavily on the surface that the wind travels over before it reaches the building. Rough terrain, like forests, houses, and trees, slows wind down while smooth, open terrain, like water (aka the Atlantic Ocean) does not. Think of driving on a smooth, open highway versus navigating a rough, obstacle-filled road. You can move much faster on the smooth surface, and wind behaves similarly.

A waterfront building, such as one in Chappaquiddick, is exposed to wind coming off open water with very little resistance while a suburban building, like one in Cambridge, is shielded by trees and surrounding structures. As a result, the waterfront building can experience on the order of 80% higher wind pressures due to terrain exposure alone. When you combine the 25 % increase due to higher wind speed with the roughly 80 % increase from open-terrain exposure, the resulting wind loads are more than twice as large. That is why two buildings in the same state, with design wind speeds only 14 mph apart, can experience dramatically different wind loads, and as a result, the structural framing and foundation requirements needed to resist those loads can also look markedly different.