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RE: Design ground accelerations

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William Sherman posted the following:
"From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com <mailto:ShermanWC(--nospam--at)cdm.com> >
To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org <mailto:seaint(--nospam--at)seaint.org> >
Subject: RE: Design ground accelerations

The geotechnical engineer can certainly report ground accelerations in
excess of those prescribed by the building code for a site-specific seismic
evaluation. But the structural engineer and the building official need to
evaluate this information in relation to specific code requirements, not
just accept the given accelerations as design values. 

It should be noted that Section 1631.2 of the 1997 UBC gives parameters to
be used in determining a site-specific design response spectrum for dynamic
analysis. But Section 1631.5.4 permits a reduction in design values to a
magnitude comparable with code prescribed accelerations. Thus, higher
site-specific ground accelerations should not result in design base shears
greater than the code prescribed values, when strictly applying the UBC
requirements. 


William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com <mailto:shermanwc(--nospam--at)cdm.com> "


I get a chuckle out of the tremendous efforts folks go to in order to reduce
the seismic load on a structure to a gnat's behind.  Remember that the
actual load induced by the earthquake we are trying to model probably can't
be determined to within + or - 100%.  If more effort were spent on ductile
detailing and making sure we have a robust load path instead of trying to
minimize a parameter which we know only to an order of magnitude level of
precision, our structures would perform better. 
Regards,
Bill Cain, SE
Oakland CA

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