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Re: Z factor definition

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If I understand your situation correctly, what you have (your 0.55g) is probably
an Sd1 value that can be used in the 1997 NEHRP base shear calculations.  This
is analogous to the Cv (or acceleration taken at the 1 second point on the
response spectrum) in the 1997 UBC.  Since the Z is part of the 1994 UBC system,
it is not directly analogous, although it stems from a similar concept.  Using
the 0.55 as Z is mixing codes a bit, so you may not get the result you are
looking for.  You may want to look at the response spectrum approach given in
the 1994 UBC, that may be a better way to apply some of the site specific
information that you have.  Looking at using the 1997 UBC might also be prudent,
since it considers near source effects that may apply in your case.

Paul Crocker

Bob Hanson wrote:

> List,
> I would like clarity in defining terms related to Z in the UBC seismic
> formula.  As I understand it the Z is the estimated peak horizontal ground
> acceleration (.4g for zone 4) on rock with a 10% probability of being
> exceeded in 50 years. I am looking at a site 1.5 km from a type B fault in
> the LA basin on natural alluviam.In the soil report the statement using
> identical terms states the acceleration as .55g. The soil report basis is
> the National Seismic Hazard Mapping Project by the USGS. I recognize that
> site specific soils play into this and my question is should I use a Z value
> of .55? Are the C, N & S  factors for near source going to ratchet the .55
> beyond intent?
> Thanks in advance,
> Bob Hanson