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Re: More Drift Talk

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Chris
I did not quite understand that why you are using a factor of 1.4
when
calculating DeltaM. According to 97UBC1630.91 & 2, DeltaS should be
computed based on the load combinations of section  1612.2.2
considering
PDelta effects wherein the seismic load factor is equal to unity.

croper(--nospam--at)bjgse.com wrote:

> 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
> >
>
>