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More Drift Talk

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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.

TIA
Chris Roper