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Re: Questions on UBC-97 and ASCE-95 Seismic Codes

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The 1996 SEAOC Blue Book (6th Edition) addresses the basis for the values
in the Appendix C, Commentary (p. 499) on Section 1662:  "Data from the
Loma Prieta earthquake also indicate an amplification at long-period
spectral amplitudes of three to six times, and the largest amplification
occurred at the natural period of the soil deposit.  However, at stronger
levels of ground shaking, this later data corroborates the results of
previous studies that the motion attenuates in soft soils as the strength
of the soil is exceeded and the soil behaves in a highly nonlinear manner."

At 13:55 11/6/97 -0800, Samuel K Kassegne, Ph.D.
>We have noticed that, in these tables, for soil type SE, the Ca factor
>drops to a value of 0.36Na compared to a Ca value of 0.44Na for soil type
>SD. If one looks closely at the table, it is easy to notice that as the
>soil gets worse, Ca increases in almost all the cases as expected except
>for soil type SE in zone 4. The Ca coefficient directly affects the maximum
>and minimum bound on the base shear these codes subscribe. For,
>particularly, drift calculations and certain products of Rw*Period, these
>bounds will govern.
>We had suspected that there is some discrepancy in this ...

>Clearly, for soil type SE, lower base shears are given. This is very
>suspicious, to say the least. Not only the base shear for soil type SE is
>less than that of SD but also SC. Furthermore, it is also possible that it
>could be lower than base shears corresponding to Soil type SB also.  Even
>though, we didn't run the numbers for ASCE-95 seismic code, similar results
>are expected.
>  ... Any thoughts why the base shears for Soil type SE could be lower
than that of SD, SC and SB?  Is it possible that this is indeed a mistake
and needs correction before people use it widely.