Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Tornadic Gust Wind Speeds vs Seismic

[Subject Prev][Subject Next][Thread Prev][Thread Next]
I agree.  It would be impractical to design for 250+ mph wind force.
But it might be logical to consider the effects of small to moderate
tornadoes.  These are the ones that might have mean recurrence intervals
in the 500 year to 1000 year range.  I think that it might be logical to
treat these forces in the same way seismic loads are treated, going to
nonelastic behavior and trying to avoid total collapse while accepting
some damage.  Another consideration is requiring that a resistant
shelter or room be furnished buildings in relatively high risk areas
(some building owners do this on their own around here - I was in a
plant in north Alabama earlier this week that has designated tornado
shelters).  I understand that an average of 42 people are killed each
year in the US by tornadoes.  This could be reduced.

Ed Marshall, PE
Simons Engineering
Atlanta

> -----Original Message-----
> From:	Al Grathwol [SMTP:AGrathwol(--nospam--at)BoyleEngineering.com]
> Sent:	Friday, May 08, 1998 1:39 PM
> To:	'seaoc(--nospam--at)seaoc.org'
> Subject:	RE: Tornadic Gust Wind Speeds vs Seismic
> 
> 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
>