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Re: question about ETABS

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Amir,

The most likely explanation for the discrepancy is the contribution of the raft mass to the base shear.  In the raft model, the base shear will be reported at the base of the raft, meaning that the mass of the raft will be included in the seismic force calculations.  In the fixed model, this mass is not present, and therefore the lowest level story shear and the base shear are the same.

Regards,
Daniel Popp

----- Original Message ----
From: Amir Gomma <ec_cai_amir(--nospam--at)yahoo.com>
To: seaint(--nospam--at)seaint.org
Sent: Monday, May 8, 2006 7:36:03 PM
Subject: qustion about ETABS

i am modeling a heigh rise building using ETABS ,  i defined the response spectrum seismic loads by UBC1997, i have a raft 3.0 m thick. the proplem that i am getting response spectrum base reaction differnt from the story shear due to the response spectrum case of loading at the bottom of the basement ( the story shear is less than the response spectrum base reaction ) . i removed the raft and made the supports as fixed . so the problem solved . why the raft make the response spectrum base reaction differnt from story shear at the bottom of first story after raft .


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