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RE: SEISMIC: Site-Specific Response Spectrum

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Bill, I'm not sure this is an answer but we had a job where we designed to the code spectrum, then the geotech found a local branch of a seismic fault within feet of our structure and we had to go with the site specific spectrum. 

 

At that point, the geotech made recommendations for the earthquake parameters and they seemed very "voodoo" to us structural engineers.  Why do geotechnical seismic recommendations look so different from structural engineer seismic recommendations, or is that just my experience? 

 

With structural engineers, it's all about seismic base shear.  With geotechnical engineers, it's all about Pleistocene and base formations and by the time you get to their numbers, their pretty well not based on anything apparent.

 

Bob Garner, S.E.

 


From: bill(--nospam--at)polhemus.cc [mailto:bill(--nospam--at)polhemus.cc]
Sent: Thursday, April 30, 2009 7:04 AM
To: seaint(--nospam--at)seaint.org
Subject: SEISMIC: Site-Specific Response Spectrum

 

Although I have no personal experience with it, I observe that ASCE 7-05 11.4.7 allows a site-specific ground motion procedure for "any structure." In particular, 21.2 sets forth requirements for obtaining a site-specific MCE response spectrum.

Aside from the cases of seismic isolation or damping systems, as specified in 11.4.7, are there any other situations where you would want a site-specific MCE response spectrum for design rather than relying upon the code-specified response spectrum?