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RE: More Drift Talk

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Mehdi,

I understand what you are saying.  Take a braced frame system you spoke of
as an example and I'll try to illustrate what I don't understand using
Building Separations requirements (94' UBC 1631.2.11 / 97' UBC 1633.2.11)
94' UBC:  Separation = 3Rw/8 x service level displacement = 3 x service
level displacement (for brace frames with Rw=8).
97' UBC:  Separation = DeltaM displacement = .7R x DeltaS displacement =
3.92 x strength level displacement (for brace frames with R=5.6) = 3.92 x
1.4 x service level displacement = 5.49 x service level displacement.
Compare the two and the actual separation required by 97' UBC is 1.8 times
what it was in 94'.
Am I wrong in my reasoning or understanding of the provisions?  If not, why
does an expansion joint now needs to be 1.8x what it used to be.

-----Original Message-----
From: Mehdi M. Khabbazan [mailto:MKhabbazan(--nospam--at)allott.co.uk]
Sent: Friday, February 09, 2001 9:37 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: More Drift Talk


Chris
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.
Mehdi Khabbazan
PhD, CEng

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.
>
> TIA
> Chris Roper
>