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RE: Seismic Recurrence Intervals

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I am not sure, but I believe that you mean FEMA 302, "1997 Edition NEHRP
Recommended Provisions for Seismic Regulations for New Buildings and Other
Structures".  If that is not the FEMA document that you have, you need it.
I would also recommend FEMA 303 (pp 287 - 337), which is the commentary
section that goes through the maps.

I would resist designing to 1000 year mean recurrence interval.  Minimum
life safety is defined at the 2% in 50 (2500 year mean recurrence interval).
You might find that there is a performance expectation based on the 1000
year mean recurrence interval.  I don't mind have enhanced performance for a
lower probability event, but I would oppose using a lower probability as a
basis for life safety.

Harold Sprague

> -----Original Message-----
> From:	Ritter, Mike [SMTP:mritter(--nospam--at)]
> Sent:	Wednesday, July 05, 2000 1:36 PM
> To:	SEAOC List Server (E-mail)
> Subject:	Seismic Recurrance Intervals
> Friends,
> Another seismic question for the group. 
> I'm currently using the 2000 IBC on a project, which utilizes the
> Maximum Considered Earthquake (MCE) maps.  No problem.  I can follow the
> methodology fine.  However, I have project criteria that states I'm
> supposed to be using earthquake demand based on a 1000 year recurrance
> interval earthquake (or, I think, a 5% probability of exceedance in 50
> years).  Most codes have seismic demand based on 10% probability of
> exceedance in 50 years (or about 500 years).  I could not find anything
> listed like that for 2000 IBC, so I pulled out my FEMA 310 which uses
> the same (new) maps.  I found MCE defined as "2% probability of
> exceedance in 50 years...".  That, I think, goes with a 2500 year
> earthquake.  So.  How do I get to the 1000 year earthquake?  
> I understand the 2/3 factor in 2000 IBC (to get Sds and Sd1) does
> something to take into account an approximate 50% reserve capacity in a
> typical code designed structure (i.e., Sds = Sms x 2/3 or Sds = Sms/1.5)
> but I'm not sure if it really reduces the recurrance interval.  If it
> does, what does it go to?  1000 years?  500 years?
> Any help would be appreciated.
> Thanks in advance,  Mike
> Michael D. Ritter, PE
> Senior Structural Engineer
> Lockwood Greene Technologies
> 1201 Oak Ridge Turnpike
> Oak Ridge, TN  37831
> (865) 220-4418
> (865) 220-4310 (fax)

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