From: "Mehdi M. Khabbazan" <MKhabbazan(--nospam--at)allott.co.uk>
Date: Fri, 09 Feb 2001 17:36:33 +0000
The story drift according to 97UBC is calculated based on the amplified seismic
lateral load and the amplification factor is 0.7R. This, for an ordinary steel
braced frame will become approximately equal to 4, meaning that seismic lateral
loads will be enhanced by a factor of about 4 when calculating story drifts. The
drift limits on the other hand have been increased by a factor of about 5 in
this code, and therefore the final result is not as bad as you thought.
> Can someone explain why the drifts/building separations have increased so
> much from 94' UBC to 97' UBC? Was research done to substantiate the change
> or is it simply a by-product of the change in methodology from service-level
> to strength-level design forces?
> The new DeltaM drifts are approx. 5x what they were in 94' UBC, but then
> again the new drift limits have changed correspondingly which makes sense to
> me. What doesn't make sense is the fact that now when you take a look at
> the separation between two structures (for example) it is almost 2x what it
> was in 94'. Is there a reason an expansion joint (for example) now needs to
> be twice the size it used to be?
> I know these questions and this thread may get old, but input from the more
> knowledgeable (dare I say older) engineers is invaluable to us younger ones
> trying to make some sense of the seemingly overcomplicated codes we have
> been talking about so much lately.
> Chris Roper