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

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Your interpretation of Bill Cain's response is not correct.  Please
refer to Tables and of "NEHRP Recommended Provisions
for Seismic Regulations for New Buildings", 1994 Edition, May 1995, FEMA
222A.  The seismic coefficients, Ca and Cb, always increase with
increasing effective peak acceleration, Aa, and effective peak
velocity-related acceleration, Av.

However, in Tables and of the same document, the site
coefficients Fa and Fv decrease as shaking intensity increases for soil
profile type E.  This is due to soil non-linear behavior as described in
Bill Cain's response.

Hope the above helps to clear the confusion in the latest code


Chang Chen, Ph.D., P.E.
Apollo Consulting, Inc.

Stan Johnson wrote:

> Bill Cain, S.E. wrote:
> > 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."
> >
> So then, if I design using the factor given in the book, perhaps my
> building
> would survive The Quake, but not a quake somewhat less in magnitude???
> Stan Johnson, PE, Heee heee heee heee :o)))