Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Fw: UBC 97 Drift

[Subject Prev][Subject Next][Thread Prev][Thread Next]
"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