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Site Specific Seismic Coefficient

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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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