What in the Derecho was THAT?

Here in the Midwest, yesterday morning was littered with broken tree limbs and downed power lines after a storm rolled through with straight-line winds exceeding 100mph for an extended period of time. This storm spanned the whole state of Iowa and lasted for over an hour in many places. 

This storm was what is defined as a derecho: a widespread, long-lived wind storm that typically accompanies a band of rapidly moving showers or severe thunderstorms. According to our trusty friend, Wikipedia, derechos can cause “hurricane-force winds, tornadoes, heavy rains, and flash floods.”

I was working from home at the time and went to look out the window. The trees in my backyard were moving in very unnatural ways and large branches were flying vertically towards the house. The rain on my neighbor’s roof was blowing upward, similar to how snow blows over a mountain peak. We are used to severe thunderstorms around here, but this was very different.

Derechoes are fairly uncommon in this area. Typically, storms may pack short bursts of wind gusts around 70mph. In the structural world, short bursts are easier to withstand than sustained high wind speeds. In Iowa, we are required to design buildings for a 115mph wind speed. This is slightly factored up to align with our equations but it likens to about 90mph actual wind speeds. There are other factors that we have to apply, such as exposure, building type, and topography factors, but our starting point is typically around 90mph. So, most buildings designed in recent history have been designed to withstand 90mph wind speeds.

Yesterday’s storm exceeded 100mph so many buildings were likely tested to their design capacity. Luckily, there are safety factors in place and redundancy within structures so that there was at least a little added capacity in most buildings.

Still, this is a great example of the importance of a wind analysis on a building. These are very real forces that the building will experience over its lifetime and it’s very necessary—life or death, some would say—to have detailing in place to resist these loads. 

What happens when the wind blows? Refer to the sketch on the left. First, wind loads act on the building’s vertical wind resisting elements (1). Then, half of the load goes into the foundation and the other half goes up to the roof diaphragm (2). The roof diaphragm distributes the load to the shear wall elements (3) which take the load from the roof diaphragm into the foundation. Just like a tent needs stakes in the corners to keep it held down in the wind, sometimes shear walls need hold downs (4) to keep the building anchored to the foundation. This is a rather simplistic sketch to illustrate a concept. Sometimes wind analysis can result in thousands of lines in spreadsheets, complex 3D modeling, and hundreds of pages of calculations, but this is the basic theory that carries through all of it.

Want to learn more about wind loads and how they affect structures?  Email me! kari@vector-collab.com