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FW: R-values in Seismic Provisions

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Ron, with all due respect from a practitioner to an expert, wouldn't the
non-structural parts of a typical building (partitions, glazing, etc.) tend
to reduce the fundamental period rather than increase it?  (At least up to
the point where they fail).

> ----------
> From: 	Ron Hamburger[SMTP:roh(--nospam--at)eqe.com]
> Sent: 	Monday, September 14, 1998 6:10 PM
> To: 	seaint(--nospam--at)seaint.org
> Subject: 	Re: R-values in Seismic Provisions 
> 
> On Sept. 14 Bill Sherman wrote:
> 
> > > An R of 1 would imply an elastic response. < 
> >  
> > While I would agree with the above statement, I have heard some speakers
> on 
> > seismic design state that an R-value of approximately 2.0 under the 1997
> UBC 
> > would relate to a "fully elastic response".  This does not make sense to
> me
> >  can anyone explain the basis for this statement? 
> 
> There are several bases for such a statement-
> 
> 1- The "actual" period of a structure is probably somewhat longer 
> (maybe by as much as a factor of 2) than that calculated using the 
> approximate period formulae.  Thus, the simple base shear formula 
> (absent an R value) could somewhat overstate the response 
> acceleration.
> 
> 2- Most structures incorporate substantial overstrength.  Actual 
> yield strengths of steel and concrete are somewhat higher than 
> minimum specified strength.  Capacities of sections calclulated 
> during design incorproate phi factors - that reduce strength below 
> actual expected values.  Designs typically are not optimized as much 
> as possible.  Engineers arbitrarily throw in more beef than the calcs 
> may demand. 
> 
>  The overall effect of this is that most structures will 
> not actually start to experience signficiant yielding until they see 
> somewhat higher accelerations than the Z value in the code would 
> indicate and will not experience full yield until substantially 
> larger accelerations are experienced.  The factor of 2 probably 
> overestimates the start of significant yielding but underestimates 
> full yielding.  It is probably accurate to say that an R of 2 
> represents "essentially elastic" behavior.
> Ronald O. Hamburger, SE
> Regional Manager
> EQE International, Inc.
> San Francisco, California
> 
> 
>