Jefferson County Blows Less Than Last Year: A Reduction in Design Wind Speed.

by evstudio on November 3, 2010

Everyone who has lived or worked there knows that Jefferson County blows. I am speaking, of course, about the wind. More specifically, I am speaking about the lateral forces imposed on structures due to pressure caused by blowing winds.

July 1, 2007 Jefferson County adopted the 2006 International Residential Code (IRC). As is the right of all local review jurisdictions, when a building code is adopted by a building department, that building department may supplement certain sections of the code so that the needs of that jurisdiction are more closely met. For example, most building departments will typically supplement their own design wind speed, snow loads and frost depths based on their own historical data regarding these weather events. July 1, 2007 Jefferson County’s adopted IRC supplement included a design wind speed of 105 miles per hour for a sustained 3 second gust with an exposure category of “C”.

Excerpt from Jefferson County's 2006 IRC supplement.

January 1, 2010 County adopted the 2009 IRC with a supplement specifying a design wind speed of 100 miles per hour for a sustained 3 second gust with an exposure category of “B”.

Excerpt from Jefferson County's 2009 IRC supplement.

For illustrative purposes, a single story house with a roof plate height of 10’-0” and a mean roof height of 18’-0” (with an 8:12 pitch) would be designed for an edge wall pressure of 17.8 pounds per square foot (psf) and edge roof pressure of 12.2 psf under the current code. Under the 2006 supplement the same house would be designed for an edge wall pressure of 25.3 pounds per square foot (psf) and edge roof pressure of 17.4 psf. These values are derived from the ASCE 7-05 (Minimum Design Loads for Buildings and Other Structures) which is referenced in both the 2006 and 2009 IRC. All correction factors such as topographic effects were assumed to be equal and were neglected in this example.

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