From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
Date: Thu, 8 Feb 2001 13:16:06 -0500 (EST)
While I won't claim to be an "expert" on the UBC code (I live in BOCA,
soon to be IBC) country, I do recall some discussions on the list about
this suject in the past.
>From what I recall, UBC 97 "makes" you do drift checks with factored loads
rather than service loads, while as older versions of the code used
service loads. The other item to be aware of is that the drift limits are
expressed in terms of the inelatic drift (at least I believe that is
true). Therefore, one needs to be careful when comparing drifts from a
equivalent lateral force frame model (i.e. typically elastic drifts).
That is typically where the Cd factor comes into play.
Those two items may account for the difference that you are encountering.
On Thu, 8 Feb 2001 croper(--nospam--at)bjgse.com wrote:
> 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