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RE: Seismic Recurrance Intervals

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The FEMA 303 Commentary to the NEHRP Recommended Provisions for
Seismic Regulations for New Buildings and Other Structures (Sec.
4.1.1) contains a more complete discussion of the MCE ground motion
parameters.  For most of the U.S., the MCE values have a 2%
probability of exceedance in 50 years (2475 yr m.r.i., noted 2%/50).
However, near many known faults the values are lower because they are
based on 150% of the median deterministic ground motion.  At such
locations the mean recurrence interval is less than 2475
years--sometimes much less.

The shape of ground shaking hazard curves vary considerably across the
U.S.  FEMA 273 provides an approximate way to calculate the ground
shaking hazard parameters for probabilities of exceedance other than
those provided on the maps(MCE and 10%/50).  If you are charged with
designing for an earthquake with a recurrence interval of about 1000
years, the 5%/50 motion is probably intended (975 yr m.r.i.).  Rather
than using the FEMA 273 approximate method, I would use the
probabilistic values provided by USGS (2%/50, 5%/50, 10%/50)at

If the design is based on 5%/50 ground motions, it may or may not
satisfy the 2000 IBC (since 5%/50 motions may be less than 2/3 of
2%/50 motions at some locations).

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

-----Original Message-----
From: Ritter, Mike [mailto:mritter(--nospam--at)]
Sent: Wednesday, July 05, 2000 11:36 AM
To: SEAOC List Server (E-mail)
Subject: Seismic Recurrance Intervals


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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