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1997 Building drift limits

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It very well could be interpreted by some to mean that the cladding needs
to be designed for the Maximum Inelastic Response Displacement, delta M. I
don't have an authoritative answer on this one, but the following approach
is rational, and one used by some Engineers: There are two levels of design
of the cladding and their connections. First, based on the elastic
displacements, delta s, the cladding should be designed such that there is
no "structural" or cosmetic damage. Second, based on the Maximum Inelastic
Response Displacements, delta M, the cladding and their connections should
be designed such that there is no "structural" failure or collapse
(referring to the structural components of the cladding) - this may mean
glass is breaking and popping out at this level of earthquake, but the
cladding would not be falling off of the building. This dual approach seems
reasonable, and is probably better than the old approach which just looked
at a single level of design drifts. Arguably a similar approach
should/could have been done under the 94 UBC with the elastic drifts and
with 3Rw/8 drifts, although I don't think most engineers took it that far.
One important note, though, is that since the elastic drifts are now based
on ultimate level earthquake forces rather than working stress level
forces, the cladding must now be designed for roughly 1.4 times the amount
of drift than was required previously, even when looking at just the
elastic displacements, delta s.

This approach seems reasonable, since it is similar to how we expect, and
design, the structure itself to perform: no structural damage under the
Maximum Probable Earthquake, and no structural collapse under the Maximum
Credible Earthquake (although some structural damage is acceptable).
Allen Adams, S.E.
RAM International


>From: JSadler819(--nospam--at)aol.com
>Subject: Re: 1997 Building drift limits
>To: seaint(--nospam--at)seaint.org
>
>Thanks Allen for your response to my building drift question..  The elastic 
>drift (delta s) is as you say the Inelastic Response Displacement (delta M) 
>divided by )  0.7R.  The problem I see is that section 1630.10.1 Story Drift 
>Limitation says that "story drifts shall be computed using the Maximium 
>Inelastic Response Displacement (delta M)".  I am afraid that this will be 
>interpreted as drift that the building cladding system will need to be 
>designed  for.
>
>Any other thoughts?
>
>Thanks again
>
>Jim Sadler
>