Wind Speeds

Riku Ylipelkonen, Owner, Standard Building Advisors and FRSA Technical Advisor

Living in Florida, one gets a good handle on the wind speeds associated with hurricanes and other wind events but did you know that “wind speed” is not always the same thing? There are multiple terms used to associate the wind speed of a wind event – not all with a commonsense definition. The many variables included in these thoughts are meant to get down to this point: it’s the resulting pressure from the wind that affects our buildings, not how fast it is measured. The best way to think about wind speed is that it’s not like driving your car at the stated speed, it is more like a bunch of cars driving during your morning commute.

We have all viewed the news during hurricane season and noticed the Saffir-Simpson Scale that outlines the perimeters of a hurricane, starting at 74 mph (see Figure 1).

The Saffir-Simpson Scale was developed at the U.S. National Hurricane Center in response to the need for a scale to convey the severity of an event to the public – much like the Richter Scale for earthquakes. These wind speeds are defined as wind speeds recorded over a period of either one minute or ten minutes at a height of 10 meters (33 feet) above the mean sea level of the location. In the U.S., the standard is one minute of average recorded speeds. In contrast, the world standard of ten minutes usually returns a 14 percent lower speed than the one-minute standard in the U.S. So, even within the definition of the Saffir-Simpson Scale, the reported wind speed is not necessarily the actual wind speed. To bring this back to our earlier analogy, this is like measuring the average speed of all the cars passing a point on I-75 for one minute. There are some cars speeding in the passing lane, some out for a slow cruise in the right lane and most cars traveling at the average speed trying to get to their destination. The important point to take away here is that these wind speeds have nothing to do with the wind speeds used in building material testing or approvals.

The Basic Wind Speed is the term used by the American Society of Civil Engineers (ASCE 7) and associated with the wind speed maps referenced in the Florida Building Code. ASCE 7-22 defines the basic wind speed as “Three-second gust speed at 33 feet (10 meters) above the ground in Exposure C” surface roughness. This is also referred to as the Ultimate Wind Speed or Vult, in calculations and considerations during the design of a building. So, if going from a ten-minute recording window to one minute reduces the reported value by 14 percent, what do you think a three-second window does to the reported value? As with our prior analogy, we are now reporting the average speed of all the cars on the highway for three seconds. That number can be a bunch of cruisers going slowly or a bunch of exotic sports cars on a cannonball rally. One thing every Florida resident knows is that the wind gusts change constantly during a wind event. The snippet of averages can really vary during a regular storm (see Figure 2).

For roofing assemblies, the wind speed utilized is the Allowable Stress Design Wind Speed or Vasd, for use with components and cladding of the building. Roofing assemblies fall under the cladding definition. The Allowable Stress Design Wind Speed is defined as about 77 percent of the Ultimate Wind Speed. This reduction for use with cladding is meant to account for the reduced risk of failure of cladding of the structure versus the structure itself. In our analogy, this is like averaging only three out of four lanes on the highway.

The final takeaway for the Florida homeowner is simpler to convey. The wind speed doesn’t really matter all that much when it comes to a building performing through a wind event. In line with our analogy above, we need to know the average speed, but we also need to know how the average speed was recorded to get an understanding of what is going on. A lot can be done when we have all the information. A good consumer tip on this is that any products or marketing slogans saying that a building component or materials rated for “x” miles per hour, is mostly meant to sell you on a product, not convey actual wind event performance of the product. With all the pertinent information, we can look for quality products with the proper testing to get you and your building through any wind event safely, and, most importantly, dry.

FRM

Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years working for Polyfoam Products. When Polyfoam Products was acquired by 3M and the name changed to ICP Building Solutions Group. Riku worked at ICP as Technical Services Manager until March of 2023, when he left to begin his own company. Riku is an engineer and is working as a consultant with FRSA. He is a member on FRSA’s Codes Committee, Codes Subcommittee, Tile Committee and on the FRSA-TRI Manual Rewrite Committee. Riku is also a member of the American Society of Civil Engineers (ASCE).