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RE: More Drift Talk

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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.

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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
>