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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.


Mike Valley wrote:

> Most engineers have long realized that 3Rw/8 x service level
> displacement grossly underestimates the real, expected deflections.
> Using Newmark's "equal displacement rule", the expected inelastic
> displacement is roughly equal to the unreduced elastic displacement.
> (Structures with very short periods displace even more--according to
> the "equal energy rule.")  The 1997 UBC displacement calculations are
> closer to the expected value (0.7 times the unreduced, elastic
> displacement rather than 3/8 of the unreduced, elastic displacement).
>
> Also note that while the "maximum inelastic response displacement",
> delta_M, is larger than what was previously calculated, the separation
> between adjacent buildings may be based on the SRSS of the individual
> building displacements.  The SRSS of the displacements is based on the
> fact that two separate buildings are unlikely to acheive maximum
> displacements toward each other simultaneously.  Using your example
> (and assuming that the two adjacent buildings have equal
> displacement), the change is really only 1.8/1.414 = 1.27 times the
> separation required using the 1994 UBC.
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
> Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
> 1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201
>
> -----Original Message-----
> From: croper(--nospam--at)bjgse.com [mailto:croper(--nospam--at)bjgse.com]
> Sent: Friday, February 09, 2001 11:03 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: More Drift Talk
>
> 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
> >
>
>
>