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RE: Tornadic Gust Wind Speeds vs Seismic

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I would think it very impractical to "design" for tornado forces.  Many
couldn't afford to buy their homes !  How could you design in an economic
manner for a 250+ mph wind force?

> ----------
> From: 	Ed Marshall[SMTP:elmarshall(--nospam--at)HASimons.com]
> Reply To: 	seaoc(--nospam--at)seaoc.org
> Sent: 	Friday, May 08, 1998 9:39 AM
> To: 	'seaoc(--nospam--at)seaoc.org'
> Subject: 	FW: Tornadic Gust Wind Speeds vs Seismic
> Importance: 	High
> 
> > 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]
> > 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]
> > > 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
> > 
> 
> 
>