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RE: Tornadic Gust Wind Speeds vs Seismic

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A good example of why I shouldn't rely on my memory.  But even so, my
point is that small and moderate tornadoes occur much more frequently
than a mean recurrence interval of 100,000 years in some areas.  I  have
a copy at home of a paper written in the 1970's that includes a plot
showing the increase in design wind speed vs the recurrence period when
tornadoes are considered in addition to straight line wind.  This was
done for a specific location in "tornado county."  I don't have it in
front of me but, hopefully correctly, that even for a 50 year period (2%
annual probability of being exceeded) there was about a 5 mph increase.
I've read elsewhere that the recurrence period for small tornadoes (or
near misses by larger ones) may be as little as 500 years in some parts
of the county.

Ed Marshall, PE
Simons Engineering

> -----Original Message-----
> From:	Roger Turk [SMTP:73527.1356(--nospam--at)]
> Sent:	Thursday, May 07, 1998 2:48 PM
> To:	seaoc(--nospam--at)
> Subject:	RE: Tornadic Gust Wind Speeds vs Seismic
> Ed Marshall wrote:
> . > Keep in mind that the ASCE 7-95 map of Tornadic Gust Wind Speeds
> . > is for wind speeds exceeding 200 mph.  The wind speeds of most
> tornadoes
> . > are much less but still exceed the code required design wind
> speeds.
> Ed,
> The map that I am looking at (Figure C6-1A, page 157, ASCE 7-95) shows
> tornadic gust wind speeds of 100, 150 and 200 mph and the caption
> states 
> that it is for a mean recurrence interval of 100,000 years.
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona