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RE: Drift per 1997 UBC

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Ben,

Thanks for the response. I think there is a problem your that logic in that it pertains to Zone 4 only, I don't see why Zone 3 should be any different, especially when the results from Eq. (30-6) and (30-7) are usually within about 10% of each other in Zone 4.

Say the base shear force is indeed governed by equation (30-7) but the period calculated by method B exceeds the 1.3 limit from method A. What happens is that when you do your vertical distribution for using the actual period and minumum base shear coefficient, you are lumping more force to the roof compared to the method A period vertical distribution - which I think inadvertantly punishes you. Your base shear coefficient is fixed, but the larger the period, the more out of wack your vertical distribution. 

I agree, that ignoring equation (30-7) can greatly reduces the base shear for drift design, but I think it seems a little too arbitrary especially since the Method A period is usually way off from the Method B value for tall structures. 

My question is really on why the difference is based on seismic zone.

>>> Ben.Yousefi(--nospam--at)ci.sj.ca.us 02/02/01 01:25PM >>>
Personally, I am glad the minimum base shear per eqn. 30-7 was kept. Having
no minimum base shear for calculating the drift is a dangerous proposition
in high seismic areas. Many structures are drift controlled and if you
calculate the period by method B you may get a substantially large period
for these buildings, and you may end up designing the building for a drift
based on a small fraction of base shear that you design the elements for. In
my humble opinion, ignoring the 30% limitation on period calculation is
enough of a break

Ben Yousefi, SE
San Jose, CA