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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.

Again, please take this as coming from a guy with insufficiently wide experience in this area, but I do notice that S. K. Ghosh's recent publication "Seismic Design using Structural Dynamics" (a pretty good book for neophytes like me, but definitely a "Version 1.0" effort if you take my meaning) the authors mention that ASCE 7-05 "probably places too much confidence in the geotechnical input." However, they go on to say that the "confidence is now felt justified in view of the controls placed on the geotechnical input by the provisions in ASCE 7-05 Section 21.2" (emphasis added).

I wonder if this doesn't address the concerns you've specifically stated here. I have only had a geotech give me a Site Classification thus far, and it seemed pretty straightforward. But I realize that's a lot simpler thing than a site-specific geotechnical analysis for deriving an MCE response spectrum.