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RE: Tornadic Gust Wind Speeds vs Seismic

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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  1/4 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)

-----Original Message-----
From:	Ed Marshall [SMTP:elmarshall(--nospam--at)]
Sent:	Friday, May 08, 1998 10:40 AM
To:	'seaoc(--nospam--at)'
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)]
> Sent:	Thursday, May 07, 1998 3:56 PM
> To:	'seaoc(--nospam--at)'
> 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)]
> > 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