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RE: Philosophy: Seismic Design Standards

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Bill Sherman stated:"One area I would like to see developed further in the
code is in the area of seismic standards for "damage control" vs. "life
safety", specifically for usability after a significant seismic event.  This
could be done in an optional Appendix to be selected by the Owner or
engineer, as applicable."  This pretty much defines the goals of Vision 2000
which emphasizes a performance based methodology.

I often wondered we, as a professional community, know enough about natural
disaster to stick our necks out define just how well a structure will
perform. Had we started to advertise the idea that our codes were
performance based before the Northridge earthquake occurred, would we have
been surprised to find ourselves embroiled in numerous lawsuits for the
damage that occurred? I think we might have been very embarrassed at the
least when we were forced to admit that this quake was atypical of the
others that we used to define our codes.

Personally, I think most of us in private practice had an easier time
designing to a life safety measure. If we wanted better performance, we knew
how to do it without making implied guarantees in performance. If a
structure was damaged and no lives were lost we satisfied the goals of our
job.

Again, this is based on your perspective. Residential structures should be
treated differently than commercial and industrial. The primary reason is
that residential structures are more complex in design (unless it is a
simple prescriptive home). There is a great degree of redundancy in
multi-residential structures which tends to reduce the complexities, but
when you have a creative architect you must contend with very complicated
designs.

Therefore, should we restrict our goals for performance based design on the
complexity of the structure? I'm not sure how to answer this one.

The bottom line in my book is to resolve the existing conflicts then improve
the design. Once this is done, work on performance.

Regards,
Dennis S. Wish, PE
Structural Engineering Consultant
(208) 361-5447 E-Fax