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RE: Tornadic Gust Wind Speeds vs Seismic

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	Roger Turk wrote:

	"Seismic Zone 2 is a misunderstood seismic zone.  This zone can,
has had, and 
> will have seismic events of magnitudes equal to those experienced in
> Seismic 
> Zone 3.  The only difference is that the events in Seismic Zone 2 are 
> infrequent but can occur today, tomorrow, next week or next century.
> I was just thumbing thru ASCE 7-95 and noticed on page 157 there is a
> map of 
> Tornadic Gust Wind Speeds.  What caught my eye was that it is based on
> an 
> annual probability of recurrence of 10^-5, i.e., a mean recurrence
> interval 
> of 100,000 years!
> I don't want to minimize the effects of tornados; we have seen the
> terrible 
> aftermath on television, in newspapers, and in the technical journals,
> but, 
> comparing the recurrence interval to that for seismic events (50 years
> or so) 
> is astounding!  Each tornado and each earthquake wreaks tremendous 
> devastation, with the tornado's devastation restricted to a narrow
> band and 
> the earthquake's devastation over 100's of square miles, sometimes
> 1,000's of 
> square miles.  It makes me wonder if seismic forces should not be
> based on 
> recurrence intervals considerably greater than what is presently used.
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona" 
	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.
The recurrence interval for these is much less than 100,000 years .  I
seem to recall that the recurrence interval for "design" earthquakes in
the East is greater than in the West.  It seems that the risk from
moderate tornadoes may be at least as significant.  I live in Atlanta
and just about everyone here knows someone who has been effected by a
tornado but nobody has been effected in living memory by an earthquake
occurring here.

	Ed Marshall, PE
	Simons Engineering