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I don't have any experience with STAAD, but I am pretty familiar with ETABS.

I use "g" (386.4 for k-in-s) for the scaling factor when doing an RSA, since
the response spectra is normalized.

The thing is, we are required by code (UBC94) to have the dynamic base shear
at LEAST equal to Vb static for irregular structures, and 90% for regular

Since an RSA is used in an ELASTIC analysis, I usually adjust the dyamic
base shear in the design files (CONKER or WALLER), or in the load
combinations defined at the end of the ETABS input file (which using
ETABSOUT you can modify on the fly).  That way, I don't have to rerun the

In other words, the scale factor in the RSA portion of the ETABS input file
doesn't have to mean a whole lot --> the important thing is that the dynamic
base shear be factored, up or down, such that it matches the static Vb (or
portion thereof) before you do any design or look at any results.

Does this answer your question?

T. Eric Gillham PE
GK2 Inc.
PO Box 3207  Agana, Guam  96932
Email - gk2(--nospam--at)
Ph:  (671) 477-9224
Fax: (671) 477-3456
-----Original Message-----
From: Francis.Ang(--nospam--at) <Francis.Ang(--nospam--at)>
To: seaint(--nospam--at) <seaint(--nospam--at)>
Cc: gk2(--nospam--at) <gk2(--nospam--at)>
Date: Thursday, April 01, 1999 4:27 PM
Subject: Response Spectrum Analysis

>I'm having a problem with regards to the seismic analysis of structures
>using response spectrum analysis. We usually use this dynamic analysis for
>multi-storey structures. For 5 storey-high structures and below, we use the
>UBC minimum lateral force procedure which normally gives a conservative
>value of EQ forces.
>With regards to response spectrum analysis, using STAAD or ETABS, there is
>portion there which requires a a value like the SCALE of the forces. I know
>of some who input on this blindly like using a value of 4.445 or something.
>I commonly use the value obtained from the dynamic base shear divided by
>base shear from the minimum Lateral Force Procedure of UBC.
>Am I right on this or have I missed out some things. I tried to read other
>materials but it doesn't mention the scale to be used?
>Any reply, comments and suggestions would be greatly appreciated.