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Re: 1997 Building drift limits

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As a point of clarification, the delta_S and delta_M displacements 
DO NOT correspond to two different levels of earthquake.

The displacements at delta_M are intended to represent the "real" 
displacements for the design earthquake (10%/50).  The displacements 
at delta_S are meaningless; depending on the system being used, 
they are somewhere between 15% and 65% of the maximum displacement 
expected.  The system displacements during the Maximum Considered 
Earthquake (2%/50 or deterministic) would be significantly larger 
than the delta_M displacements.

A two-level design of nonstructural elements (and their connections) 
could make sense.  However, the first (lower) level of design would 
need to be based on service conditions.  The service conditions would 
include wind, snow, thermal, etc. loading but generally not 
code-level (10%/50) seismic.  If damage-free cladding performance is 
desired for some level of ground shaking lower than the code design 
level, such would need to be explicitly considered.  However, the 
code does not define that level and the reduced elastic code forces 
do not represent that level.

******************
> Date:          Wed, 12 May 1999 11:44:07 -0700
> To:            seaint(--nospam--at)seaint.org
> From:          Allen Adams <aadams(--nospam--at)ramint.com>
> Subject:       1997 Building drift limits
> Reply-to:      seaint(--nospam--at)seaint.org
> Organization:  http://www.seaint.org

> 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

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Michael Valley                                   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