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Re: seismic forces

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Gordon and Thomas,
The discussion in the 2003 NEHRP is indeed very informative and concise. 
However astronomical the return period is, it has very practical consequence: 
the code-compliant design for identical structures in Hayward and Sikestone, will have the forces for the latter location 1.67 time higher than those for the former.   
As for being "hosed" - imagine the implications for retrofit etc. I also wonder if the plancheck process in Minnesota and South Carolina is as rigorous as in California. 
V. Steve Gordin, SE
Irvine CA
----- Original Message -----
Sent: Monday, November 24, 2008 14:24
Subject: RE: seismic forces

10% has been abandoned; now it?s 2/3 of 2%, 50 yrs.  That?s a 2500 yr return!  So in California, where the frequency of a design event is more like a few hundred years, known faults that have enough historical data use 1 ½ x the characteristic event.  There?s a good, brief  discussion of the rationale behind this change in the commentary to the Ground Motion chapter of the 2003 NEHRP.



Gordon Goodell


From: David Topete [mailto:d.topete73(--nospam--at)]
Sent: Monday, November 24, 2008 2:39 PM
To: seaint(--nospam--at)
Subject: Re: seismic forces


Aren't the accelerations based on the 10% at 50 year event?  So, that means New Madrid and Summerville have the potential for very large accelerations for the 475-year event.  The fact that the Hayward fault (which I live on) is more active is because you have the Pacific plate rubbing constantly on the North American plate, or vice versa.  If I'm off, I'm sure I'll be corrected.  That's my just my view from basement...

On Mon, Nov 24, 2008 at 12:19 PM, SGE Structural <sgordin(--nospam--at)> wrote:



Based on the 2006 IBC (ASCE07-05), the seismic accelerations for Hayward, CA (right on the Hayward fault) appear to be about 60% lower than those for Sikestone, MO, and about the same as for Summerville, SC. 

Historic (1812 and 1886) considerations aside, any comments?


V. Steve Gordin, SE
Irvine CA

David Topete, SE