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RE: spectral response acceleration definition

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there was a great article in "structural engineer" - february 2005 about the seismic provisions for asce 7-05.  you can download a pdf by clicking here:
this doesn't really answer your question but it does provide supplemental material
David Adie, P.E., LEED AP 2.1
(P) 206.343.0460
(F) 206.343.5691

From: Bruce Holcomb [mailto:bholcomb(--nospam--at)]
Sent: Tuesday, January 03, 2006 8:01 AM
To: seaint(--nospam--at)
Subject: spectral response acceleration definition

I have been having trouble with the term “spectral” in “spectral response acceleration”, so I’ve been doing some studying.  Can someone verify my findings below?  Or set me straight?  I’m supposed to give a presentation at my office and I want to make sure I have it straight before I try to explain to others. 


The “spectral response acceleration” plotted in the “Maximum Considered Earthquake Ground Motion” maps in the IBC is an acceleration with an annual probability of exceedance of 2% in 50 years (return period of 2500 years) and is for a site class “B” soil.  The spectral response acceleration is derived from many possible earthquakes of various magnitudes at various distances from the site.


The spectral response acceleration is plotted for period of 0.2 seconds and 1.0 seconds because these two periods encompass most building periods.  Periods between these values (and to a certain extent beyond these values) can be interpolated (extrapolated).


The reason the “design spectral response acceleration” is 2/3 of the “site adjusted spectral response acceleration” is that the lower bound estimate of the margin against collapse is a factor of 1.5 (approximate… judged by experience).  Therefore, the “design” earthquake ground motion is 1/1.5 (2/3) of the maximum considered earthquake ground motion because we want to prevent collapse, but the construction cost would be too great, if we designed to prevent all damage to a structure.