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Re: site specific spectrum vs. published curves

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The available site specific spectrum COULD be appropriate depending 
on the following factors.

1)  It depends on what you mean by "a few years ago."  If knowledge 
of the regional seismicity hasn't changed (no new seismogenic 
sources, no revisions to the characterization of the existing 
sources, no important changes in attenuation relationships), then 
there is nothing wrong with the estimate just because it is a few 
years old.  However...

2) ... was the previous study based on ground motions with the same 
probability of exceedence as the FEMA 302 motions?  Most site 
specific studies "a few years ago" were based on a 10% probability of 
exceedence in 50 years.  The FEMA 302 ground motions (and the entire 
design approach) are now based on a 2% probability of exceedence in 
50 years (2/3 of which is not generally equal to the 10%/50 values).  
In order to be used with FEMA 302, the site-specific study must 
conform to Section 4.1.3.  Also note that FEMA 302 requires that the 
site specific design accelerations be at least 80% (NOT 70%) of the 
values from the general response ("published") values.


**********
> From:          "Khosrownia, Ghassem SPK" <GKhosrownia(--nospam--at)spk.usace.army.mil>
> Subject:       site specific spectrum vs. published curves
> Date:          Thu, 28 Oct 1999 08:00:17 -0700

> Situation:
> Site specific response spectrum is available for a site where an existing
> irregular structure requiring dynamic analysis had to be evaluated a few
> years ago. A new building is to be built in the same vicinity now. Seismic
> criteria is in accordance to FEMA 302 / 303.
> 
> Question:
> If the site specific response spectrum indicate ground accelerations less
> than the map values published by FEMA would you think the available site
> specific response spectrum (assuming it is reliable) would be a good source
> to do seismic design? I think FEMA's acceptability criteria is no less than
> 70% of the published map values regardless of the site specific response
> spectrum.

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201