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RE: Seismic Base Shear

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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.

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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: Haan, Scott M. [mailto:HaanSM(--nospam--at)]
Sent: Tuesday, May 01, 2001 10:00 AM
To: 'seaint(--nospam--at)'
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].
Ss and
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
Development Services Department
Municipality of Anchorage
phone:907-343-8183  fax:907-249-7399

-----Original Message-----
From: Sunho Shim [mailto:sunho.shim(--nospam--at)]
Sent: Tuesday, May 01, 2001 7:58 AM
To: seaint(--nospam--at)
Subject: Seismic Base Shear

Hello, Engineers

I am comparing seismic base shear for several codes.
UBC and ASCE seem to use Effective Peak Acceleration and Effective
Velocity-Related Acceleration.
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.

Sunho Shim

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