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RE: Tornadic Gust Wind Speeds vs Seismic

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Harold,

Thanks for noting the additional reference.  The Wen and Chu paper draws
heavily on the work by Fujita, notes that his scale is based on the
fastest 1/4 mile wind speed, and mentions the need to convert.  I
therefore assume that they did so although they didn't include that
exercise in the paper.

Ed Marshall, PE
Simons Engineering
Atlanta

> -----Original Message-----
> From:	hsprague(--nospam--at)klaalov.com [SMTP:hsprague(--nospam--at)klaalov.com] On Behalf Of
> hsprague(--nospam--at)aspen.klaalov.com
> Sent:	Friday, May 08, 1998 1:08 PM
> To:	'seaoc(--nospam--at)seaoc.org'
> Subject:	RE: Tornadic Gust Wind Speeds vs Seismic
> 
> One of the best papers I have ever seen on tornado probability is by
> Grant Darkow, Professor of Atmospheric Sciences at the University of
> Missouri - Columbia "Tornado Wind Probabilities for Engineers".  I got
> my copy from Professor Darkow so I don't know when or where it was
> published.  I believe it was originally presented in a symposium.
> 
> He also includes some information on micro bursts.  One additional
> thing worth noting is that when you compare wind speeds you should
> convert the tornado wind speeds to 3 second gusts.  The Fujita scale
> is based on  ¼ mile winds.  The SEI just published a good example on
> wind speed conversion.
> 
> Harold Sprague
> Krawinkler, Luth & Assoc.
> 4412 W. Eisenhower Blvd.
> Loveland, CO 80537
> Voice: 970 667-2426
> Fax: 970 667-2493
> Email: hsprague(--nospam--at)klaalov.com
> 
> 
> 
> 		-----Original Message-----
> 		From:	Ed Marshall [SMTP:elmarshall(--nospam--at)HASimons.com]
> 		Sent:	Friday, May 08, 1998 10:40 AM
> 		To:	'seaoc(--nospam--at)seaoc.org'
> 		Subject:	FW: Tornadic Gust Wind Speeds vs Seismic
> 
> 			> Now that I've actually looked back at the
> 1970's paper that I
> 			> mentioned Thursday I'll quote from it
> correctly.  It's "Tornado Risks
> 			> and Design Wind Speed", by Yi-Kwei Wen and
> Shih-Lung Chu, December
> 			> 1973, ASCE Structural Division Journal.  The
> example they give is for
> 			> a Midwestern site.  For a 50 year return
> period (0.02 annual
> 			> probability of excedence) there is no
> significant increase in the
> 			> design wind speed of 80 mph when considering
> Tornado risk.  At 200
> 			> years the increase is from 90 mph for straight
> line wind to 95 mph.
> 			> At 500 years from 100 to 111 mph, and at 1000
> years from about 110 to
> 			> about 165 mph.  Since wind pressure varies
> with the square of the wind
> 			> speed, the wind loading at 165 mph is
> approximately 4.25 times greater
> 			> than at 80 mph.  
> 			> 
> 			> For comparison, the ASCE 7-95 commentary
> states that the mean
> 			> recurrence interval for the values given in
> the seismic maps is 475
> 			> years (10% chance of being exceeded in 50
> years) and I think it may be
> 			> greater than that in the East.  My thought is
> that the risk due to
> 			> Tornadoes in the Midwest and some parts of the
> East is similar to that
> 			> due to the earthquake, yet unlike earthquake
> it is ignored by the
> 			> codes.
> 			> 
> 			> Ed Marshall, PE
> 			> Simons-Engineering
> 			> Atlanta
> 			> 
> 			> -----Original Message-----
> 			> From:	Ed Marshall
> [SMTP:elmarshall(--nospam--at)HASimons.com] <mailto:[SMTP:elmarshall(--nospam--at)HASimons.com]>
> 			> Sent:	Thursday, May 07, 1998 3:56 PM
> 			> To:	'seaoc(--nospam--at)seaoc.org'
> 			> Subject:	RE: Tornadic Gust Wind Speeds vs
> Seismic
> 			> Importance:	High
> 			> 
> 			> Roger,
> 			> 
> 			> 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 I remember, 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
> 			> Atlanta
> 			> 
> 			> > -----Original Message-----
> 			> > From:	Roger Turk
> [SMTP:73527.1356(--nospam--at)compuserve.com]
> <mailto:[SMTP:73527.1356(--nospam--at)compuserve.com]>
> 			> > Sent:	Thursday, May 07, 1998 2:48 PM
> 			> > To:	seaoc(--nospam--at)seaoc.org
> 			> > 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
> 			>