# Re: Seismic Recurrance Intervals

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Seismic Recurrance Intervals
• From: "fturner" <fturner(--nospam--at)quiknet.com>
• Date: Wed, 5 Jul 2000 12:01:33 -0700
```Mike - Short of obtaining a site-specific study to determine 1000 year
values, try FEMA 356 or FEMA 273. In FEMA 356 on pages 1-29 and 30, there is
a method of interpolating between 2% and 10% in 50 year spectral
acceleration values for 5 regions of the U.S. If your site is outside the
contiguous U.S. you'll need to find some other source for the exponents used
in the interpolation. This method uses the slope of the log probability
curve.

Fred
----- Original Message -----
From: "Ritter, Mike" <mritter(--nospam--at)lgt.lg.com>
To: "SEAOC List Server (E-mail)" <seaint(--nospam--at)seaint.org>
Sent: Wednesday, July 05, 2000 11:35 AM
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.
>
>
> 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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