One correction to Scott's reply:
Sds and Sd1 ARE equal to 2/3 of Sms and Sm1, but are NOT "for 475
return period earthquakes [10% probability of exceedence in 50
Although 2/3 of the MCE motion is roughly equal to 10%/50 motion for
some California locations, 2/3 of MCE is not equal to any specific
probability of exceedance (since the hazard curves are nonlinear and
other steps go into the MCE maps). There are locations where 2/3 of
MCE is more than five times the 10%/50 values and other areas where
2/3 of MCE is noticeably less than 10%/50.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E. E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200
1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
Sent: Tuesday, May 01, 2001 10:00 AM
Subject: RE: Seismic Base Shear
Ss and S1 are spectral responses for a site class B [rock] MCE
considered earthquake] with 5% damping. Ss and S1 are derived for a
return period earthquake [2% probability of exceedence in 50 years].
S1 are modified by Fa and Fv for other soil types [Sms=Ss*Fa and
Sds and Sd1 are for 475 return period earthquakes [10% probability of
exceedence in 50 years] and are used for design [Sds=2/3*Sms and
In the 1997 UBC the EPA was Ca and that was multiplied by a 2.5 fault
amplification factor to get a spectral pseudo acceleration. The EPA
built into S1 and Ss IBC spectral values.
Scott M Haan P.E.
Plan Review Engineer
Building Safety Division http://www.muni.org/building
Development Services Department
Municipality of Anchorage
From: Sunho Shim [mailto:sunho.shim(--nospam--at)som.com]
Sent: Tuesday, May 01, 2001 7:58 AM
Subject: Seismic Base Shear
I am comparing seismic base shear for several codes.
UBC and ASCE seem to use Effective Peak Acceleration and Effective
However NEHRP 97 AND IBC200 are using Spectral Acceleration at short
period and 1 second.
How can I convert EPA,EPV to Ss, S1?
Thank you for your answer.
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