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Basic Wind Speeds

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Recent posts discussed use of a 3-second gust wind speed vs. a fastest-mile
wind speed. The current edition of "Structural Columns", a quarterly
publication issued by the Structural Engineering Institute of the ASCE,
includes an article which provides a methodology for converting a 3-second
wind speed to a fastest mile wind speed and vice versa. It utilizes Fig. C6-1
which is located in the Commentary section of ASCE 7-95. 

Per one of the examples included within the article, suppose you wish to
convert a 80 mph non-hurricane(the lower curve is for non-hurricane winds; the
upper curve is for hurricane winds) fastest-mile wind speed to a time averaged
wind speed such as a 3-second gust speed.
1. Determine the averaging time for the fastest-mile wind speed.
t = (3600 seconds/hr)/(80 mph) = 45 seconds/mile.
2. From Fig. C6-1, determine the ratio of fastest mile wind speed to mean
hourly wind speed by locating 45 along the horizontal axis and proceed
vertically up to the lower curve. At where it intersects the lower curve,
proceed left to the vertical axis for a ratio of 1.28.
3. The mean hourly wind speed can be determined by 80/1.28 = 62.5 mph.
4. Determine the ratio of the 3-second gust to the mean hourly wind speed by
locating 3 sec, on the horizontal axis and again proceeding vertically up to
the lower curve. Proceeding left to the vertical axis provides a ratio of
1.53.
5. The 3-second gust speed is therefore determined by 1.53*62.5 = 95.6 mph.


Charlie Canitz, PE
Bel Air, MD