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RE: Site Specific Seismic Coefficient

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Mike,
[UBC only] Are you confused between using Site Specifiec Response Spectrum
and UBC Spectrum (Fig 16-3 UBC)? The first one is required for specific
cases only (see UBC 1631.2). Both are dynamic analysis procedures.

In any case, you need to calculate base shear using static method (1630.2)
because base shear from dynamic analysis can not be less than 90% of base
shear from the static method for regular structures, 100% for irregular
structures.

Site Specific Seismic Coefficients from soils engineer you mentioned should
be for use in the static method. Soils engineers' values normally come from
UBC Tables 16-R & S. As Gerard mentioned, it is worth verifying soils
engineers' values based on given source type, distance to the source, and
soil type.

UBC does not require dynamic analysis (using either site specific spectrum
or UBC spectrum) for all structures (See 1629.8.4), but you can choose to do
it for any structures. 

HTH.

Suresh Acharya, SE
Richmond, 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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