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Fw: Fw: UBC 97 Drift

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Moni:
 
I hear what you are saying about trying to design rationally for seismic loading while being in a sea of "engineers" who don't really understand seismic design.
 
But, I have to ask:  what is it about your particular situation that is requiring you to increase the size of your walls such that it is becoming a problem?  Are you concerned about pounding with adjacent structures, or are you concerned with structural stability at the expected deformation level?
 
In the design of structural walls for expected deformations during an actual design earthquake, I would imagine that you might be concerned with the strain levels in the compression zones of said walls, especially if you have rather high axial loads on them.  But beyond that, unless you have a frame which is being restrained by very few walls, I have some difficulty imagining a system wherein deflections are actually becoming a problem.
 
Could you perhaps go into a little more detail regarding the particulars of your situation?
 
Also, again respecting the fact that I am not all that up on UBC97, it would seem that Section 1630.9.2 is requiring only Delta Max=.7*R*Delta Service, which would be .7*5.5 for a bearing wall system, which is then about 3.85.  This seems pretty reasonable to me, in a conservative sense.  If you really feel that this is too high, then perhaps you could do a moment curvature analysis to determine the equivalent elastic/inelastic stiffness of the walls, then do an analysis based on displacement (using a displacement spectra) which would in turn be based on a more reasonable cracked section period instead?
 
T. Eric Gillham PE
GK2 Inc.
PO Box 3207  Agana, Guam  96932
Email - gk2(--nospam--at)kuentos.guam.net
Ph:  (671) 477-9224
Fax: (671) 477-3456
 
 
-----Original Message-----
From: Sleiman Serhal <mony(--nospam--at)destination.com.lb>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Saturday, March 27, 1999 6:14 PM
Subject: Re: Fw: UBC 97 Drift

"T. Eric Gillham PE" wrote:
 I am not all that familiar with UBC97, and besides Eddie G has already provided what seems to be a reasonable response to the actual code reqts. My comment is general in nature: If you think about the basis for seismic design in general, I think it is apparent that cracked sections MUST be used when computing both structural properties for seismic analysis, as well as expected deformation levels. Remember that the expected accelerations during the actual design EQ are much much higher than those assumed for an elastic analsysis.  This being the case, even if your walls don't crack under ELASTIC forces, they almost certainly will during the actual earthquake.  The elastic analysis used by most designers doesn't really have much relation to the actual inelastic performance of the structure being designed during a real earthquake, primarily because one is elastic, the other inelastic.
 True about the actual high inelastic forces compared to the design elastic ones, but since we are multiplying our elastic Ds with the "inelastic/elastic ratio" R to get Dm, isn't that enough to account for the inelastic deformations ?
Basically, taking Ie=0.35Ig and limiting the drift Dm to 0.02H leads to huge shear walls, and this is what prompted my inquiry. Like I'm I doing something wrong, is the Architect right about me overdimensioning my walls or I'm I doing the right thing and the Architect has been used to undersized walls given by Engineers who aren't going by the "book" ??
(you have to keep in mind here that I ain't in the US and that we have a peculiar situation down here, in Lebanon, where the UBC assigns a Zone 3 to the area but where We, Engineers, are not required by law to perform any seismic designs. So basically nobody checks on the calculations, except on big project where special consultants are hired by the client, and the Engineer can basically tell the client/Architect something pretty vague about him having taken into consideration seismic loads and design, i.e. he could have used a Zone 1 with the Architect/Owner not being aware of its inadequacy. So when we try to go by the "book," we actually end up with much bigger vertical load resisting elements than the Architect/Owner have been used to, and end up being called "wimps" and "incompetent" and nice, well deserved, adjectives like that :))
 
 In structures I have designed for seismic performance, I usually tend to be quite conservative with regard to expected deformations, perhaps because I do mainly R/C work.  I feel strongly that a displacement based approach to seismic design is much more rational than the current force based approach, so that is probably part of it as well. 
The likes of Priestley have been advocating this displacement based approach for a while now !


I would like to add that Section 1921.6.6 for shear walls design is pretty tedious and that we should get paid more for all this complexity :)
 

Thanks again for your input,
Moni Serhal