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RE: Site Specific Seismic Coefficient

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Mike,

Whether or not you choose to use a dynamic analysis is somewhat up to
you unless you have an irregular building or a specific request from
your client. If you building has no mass or stiffness irregularities,
there is no requirement that I know of to do a dynamic analysis in the
UBC.

If you want to use the values from the soils report, that is okay as
long as they do not produce less conservative results than using the
fault maps. In the last 4 years, I have had about 1/2 a dozen occasions
where the soils report couldn't possibly have the correct values. Since
all/most soils reports are simply change the address on the cover page
and collect money(somewhat kidding), it's quite possible that seismic
criteria was not checked. Usually the error is something like the wrong
fault type (A or B) is used, they will say type C soil (that would be a
miracle)and give you answers for type D soil. You should be able to
derive the numbers in the soils report because they should be listing
all the properties you need like Soil Profile, distance to fault, and
fault type. I always go to mapquest, get the location of my site, and
check the numbers in my spreadsheet I use. Then I compare with the soils
engineer. If there is a difference, I ask them to verify there location
of the site. Sometimes, they round their distances to fault and that is
one source of different values as well.

Now, comparing that to IBC may indeed give you lower base shears. This
is the primary reason (as I heard) that the IBC was not adopted in CA.

HTH,
-gerard
Santa Clara, CA

-----Original Message-----
From: Mike Auchter [mailto:mikea(--nospam--at)tetercon.com] 
Sent: Friday, May 09, 2003 9:01 AM
To: seaint(--nospam--at)seaint.org
Subject: Site Specific Seismic Coefficient

I am hoping to solicit some thoughts on a matter regarding the use of
Site Specific Seismic Coefficients (Ca and Z) values and whether it is
appropriate and/or how to go about using such information. 
If a soils engineer is willing to submit site specific Ca and Z values
based on their elastic response spectra curves, is it valid to continue
on with a Static analysis versus using the elastic response spectra
information for a Dynamic analysis? If so, is one able to bypass Table
16-Q of the 97UBC which defines the Ca based on the soil type and
seismic zone and proceed with the soils engineer defined Ca value? 
One of the bits of information that seemingly reassures our firm that we
aren't proceeding forward with a design that is a "life safety" issue in
using a lower design base shear is a comparison between the the site
specific base shear using the modified Ca value and a base shear
generated by the IBC2000 method which utilizes zip code (and/or latitude
and longitude information versus the broader seismic maps of the 97UBC).
The comparison of those two base shears shows that the IBC2000 is
slightly lower than what was calculated using modified Ca values. This
may or may not be true, but it is my understanding that a difference
between the 97UBC maps and the maps of the IBC2000 is that the IBC2000
is using a much larger data base of sampled and tested soils that they
are able to better refine the seismic zoning/classification of
California versus the much more general/broad zoning/classification that
the 97UBC used. If that is the case, then it would seem that the site
specific testing done would be validated by the correlation to the
IBC2000 base shear. Our approach then was to request approval from the
design jurisdiction to utilize the site specific Ca value to generate
the base shear under the 97UBC (not the IBC2000, that later came into
play as a comparison/check of validity).

Unfortunately, our good faith efforts have hit a bump in the road. Thus,
we were wondering if there are any on this list that have successfully
used a similar method? If so, what rationale was presented to get
approval from the building department? Also, please feel free to throw
in words such as "absolutely not", "unacceptable", "what are you
thinking?!?!" if you feel that it is inappropriate to use such a design
method. Any and all thoughts or opinions will be appreciated. 

Thanks in advance, 

Mike A. P.E.


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