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RE: Probabilistic MCE

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How can Sds/2.5 be less than peak ground acceleration as determined from ASCE 7 sec 11.4 (for T=0, Sa = Sds*0.4 = Sds/2.5)?
IBC Sec 1802.2.7 does seem to be rather awkwardly worded.  It seems that it could just say to use either: a) the peak ground acceleration equal to Sds/2.5 based on the ASCE 7 seismic maps, since Sds accounts for MCE and soil amplification effects via Fa & Fv; or b) peak ground acceleration from a site-specific study.  Also, note that the discussion of peak ground acceleration in 1802.2.7-2 appears to only relate to evaluation of liquefaction, not the overall seismic analysis of the structure.
Bill Sherman

From: Teo, KP [mailto:KP_Teo(--nospam--at)]
Sent: Wednesday, May 13, 2009 6:36 AM
To: seaint(--nospam--at)
Subject: Probabilistic MCE

Dear List,
I received a call from a friend in regards to this issue and hopefully can find some help on this list.
IBC Section 1802.2.7 has an exception that a site specific study need not be performed, provided that the peak ground acceleration equal to Sds/2.5 is used, where Sds is determined per section 21.2.1 of ASCE 7 (probabilistic MCE, 2% probability of exceedance in 50 yrs). These ground accelerations can be obtained from in which one can obtain the probabilistic peak, 0.2 sec and 1 sec ground accelerations.
What if the Cs value obtained using this probabilistic approach (Sds/2.5) is less than that of the standard procedure outlined in ASCE 7 Sec 11.4?
There are two scenarios that came to mind:
1) Site specific study is Required per section 11.4.7  ASCE 7-05 (ie site class F)
If one has a site class F and is required to perform a site specific study. Can he/she choose to use the exception (Sds/2.5) without performing a site-specific analysis, even though the values turn out to be really low? (say below 0.05W)?  
2) Site specific study NOT Required but used to reduce the forces
If the geotechnical already performed a site specific analysis, but the exception (Sds/2.5) turned out to be much less than either the report and/or the standard procedures. Can he/she make use of the lesser value obtained per the exception and ignore the geotechnical report?
It doesn't seem like a conservative way of doing things to me, or perhaps i have overlooked some important parameters.
Can someone please help on this topic before i mis-guided my friend?