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Re: Question for East Coast Engineers

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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.
. >