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Re: 1997 UBC Seismic

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on 5/8 George Barclay wrote:

> Sasha, Ron and Sadre;
>  
> There seem to be significant changes to the '97 UBC seismic compared to
> the '94.  The greatest of these changes, at least to us "east coast
> engineers" in zones 1 and 2, seems to be this omega (i.e. 3R/8) factor
> by which the seismic force is multiplied by for design of certain
> elements.  I still have a minor point of confusion with part of this
> issue.  In section 1633.2.6, collector elements, states that "Em need
> not exceed the maximum force that can be  transferred to the collector
> by the diaphram and other elements of the lateral force resisting
> system."  
> 
> In section 1630.1, Em is defined as omega*Eh.  To me, this is saying
> that if your diaphram is not capable of transferring Em, then the
> collectors, in this case, need only be designed for the force that the
> diaphram can transferr (less than Em, but not less than Eh).  So, this
> Em is not a strict demand than must be placed on the collectors (and
> other elements specified for this particular combination)?  I guess, as
> Sasha said, this is some type of "weak link" concept.
> 
The Omega - Eh quantity is supposed to be an estimate of the "real" 
force demand placed on the element being designed.  Essentially, 
Omega is an average estimate of the overstrength inherent in typical 
structures of different system types.  For some structures Omega will 
under-estimate the real overstrength.  For other's it will 
over-estimate it.  The code permits an engineer to perform a plastic 
analysis (sometimes called a limit state, mechanism analysis, 
collapse analysis, or nonlinear anlaysis) to determine the real 
maximum force.  Limiting collector force to the diaphragm strength is 
one way to do this, however, be aware that the real strength of the 
diaphragm is larger than the determined by the code formulas, because 
it too has overstrength! 
Ronald O. Hamburger, SE
Regional Manager
EQE International, Inc.
San Francisco, California