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Seismic Recurrance Intervals

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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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