Wind Speeds in Structural Design
Among the least understood aspects of structural design for single-family homes is lateral loading, particularly wind loads. Until roughly 20 or 30 years ago, at least in New England, single-family homes were typically built without involving structural engineering services. This, combined with the fact that the region has not experienced a hurricane since one named Bob in 1991, helps explain why some contractors with 30+ years of experience still scratch their heads when reviewing plans that specify wood shear walls, holdown units, or gulp, steel moment frames.
But it is extremely important to understand, design, and build structures to withstand these lateral loads. Although we have been spared since 1991, the wind events of the past will certainly occur again in the future; one could surmise this even without considering climate change.
So let us explore some definitions and numbers. First, there are different ways to measure and report wind speeds. Weather stations continuously collect weather data (duh), with wind speeds being one parameter of interest, measured in miles per hour (mph). Sustained wind speeds are many individual measurements averaged over a time period of 1-minute or more. Peak gust wind speeds are a similar concept, but the many individual measurements are averaged over a much shorter time period, usually between 1 and 3 seconds. Hurricane Bob struck with sustained wind speeds between 75 and 100 mph, with a peak gust wind speed of 125 mph. Remember this peak gust wind speed because it will help serve as an anchor for current structural design standards.
Structural engineers determine loads on buildings using the American Society of Civil Engineers specification, “Minimum Design Loads and Associated Criteria for Buildings and Other Structures” (ASCE 7). Much progress has been made with each subsequent edition of ASCE 7, as research on wind, seismic, and other loads has advanced considerably. In 1995, the first edition published following Hurricane Bob had a total specification length of 236 pages. The current edition, published in 2022, is a whopping 1,046 pages, with roughly 300 pages devoted solely to wind loads.
To design homes, we use the 3-second gust wind speed, corresponding to a 7% probability of exceedance for a 50-year wind event. In simpler terms, this amounts to designing homes to withstand a 700-year storm, that is, a storm with a 1-in-700 chance of occurring in any given year. This does not mean a 700-year storm would occur exactly once every 700 years; realistically, there could be two 700-year storms within the same year, decade, or whatever time frame you typically default to.
To reference a couple of projects recently completed: in Cambridge, MA, the ASCE 7 wind speed used in design was 120 mph, while in Chappaquiddick, MA, it was 134 mph. Comparing these to Hurricane Bob’s 125 mph, the ASCE 7 derived wind speeds appear reasonable, that is, they are neither too high nor too low (at least in my humble opinion). Yet, with such a seemingly small difference in wind speeds, how could the wind loads for the Chappaquiddick project end up more than twice those for the Cambridge project? For the millions of readers of the Koa Rose blog, we will find out next time.