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Re: UBC97 question

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The static force procedure contained in the UBC is meant to be a crude
estimation of the first (or second) translational mode response to an
earthquake, hence the loading distribution.

If the structure you are analyzing is pretty symmetric and regular, I would
think that the base shear response from the dynamic analysis wouldn't be
THAT far off from the static base shear.

I would suggest checking the following first;

1) Are you using an Rw>1 (e.g. 6 or 12) for computing the static base shear?
If so, are you accounting for this in the scale factor for the response
spectra used in your dynamic analysis?

2) Is the scale factor for the RS used in the dynamic analysis appropriate
for your system of units? (32.2 ft/s/s or 386in/s/s)

3) Is the structure you are analyzing very irregular, such that a static
analysis might not be capturing significant torsional response? (Check your
mass distribution for your mode shapes and see if translational modes occupy
the first and second modes).

Echoing the previous posts, it is difficult to answer your question without
additional data, but if you check the above and it STILL doesn't make sense,
perhaps you could supply some more information?

Hope this helps.

T. Eric Gillham PE
GK2 Inc.
PO Box 3207  Agana, Guam  96932
Email - gk2(--nospam--at)
Ph:  (671) 477-9224
Fax: (671) 477-3456