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RE: Question for East Coast Engineers

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I disagree- the reason you gave for using a greater interval for earthquakes
(large area affected) are good reasons for using a smaller interval !  Many
more structures are affected by earthquakes than tornados.

> ----------
> From: 	Roger Turk[SMTP:73527.1356(--nospam--at)compuserve.com]
> Reply To: 	seaoc(--nospam--at)seaoc.org
> Sent: 	Wednesday, May 06, 1998 3:51 PM
> To: 	seaoc(--nospam--at)seaoc.org
> Subject: 	Re: Question for East Coast Engineers
> 
> 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
> 
> 
> Mike Brown wrote:
> 
> . > After reading this trail about lateral loads on the East Coast, I have
> to
> . > put my two cents in.  I am currently living and working in Boise,
> Idaho.
> . > Seismic zone 2B and 70 mph wind speeds.
> . > 
> . > Mr.. Mike Zaitz brings up an interesting topic: "To my knowledge on a
> . > typical house the framer gets
> . > >to frame it how he or she wants and then the building inspector
> either
> . > >signs off on it or requires an engineer to look at a particular
> problem."
> . > 
> . > This comment is pretty much true here in Boise.  I must say this is
> very
> . > frustrating for a lot of the local engineers.  The builders are able
> to 
> . > draw permits to build houses that do not meet code per structural 
> . > issues.  The comment they get is to provide engineering for:  the
> walls 
> . > at the garage doors, bay windows, etc.. Basically, anywhere there is
> not 
> . > enough shear wall, provide engineering.
> . > 
> . > Sounds O.K., right.  Wrong.  The builders still get the permit and
> build 
> . > the house without engineering.  Here's the catch:  The building
> inspector
> . > reminds them that they need engineering and will not sign off on the 
> . > project until they get the engineering done. Our problem is that the 
> . > house is built and the builders want us to come up with the miracle 
> . > solution to solve their problem, besides "they've built this same
> house 
> . > many times before and never had a problem."
> . > 
> . > The point is this:  The building officials should require engineering
> for
> . > the houses that do not meet current conventional construction before 
> . > issuing a permit. With the new code, how am I going to justify the use
> of 
> . > a 2 foot shear wall with a 9 foot plate height (code requires a 2 to 1
> 
> . > aspect ratio)?
> . > 
> . > Sorry for rambling on.  I believe that many of the engineers
> understand
> . > lateral load paths, but the real battle is with the builders and
> general
> . > public.  In a relatively low seismic zone and wind speeds, the
> builders 
> . > and general public assumes we are over designing since there has not
> been 
> . > any local collapses of structures.  In turn, this means a lot of 
> . > structures may get built without an engineer's input and therefore no 
> . > lateral support.
> . > 
> 
> 
>