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Re: Report on Wood Diaphragm Issues[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Report on Wood Diaphragm Issues
- From: SDGSE(--nospam--at)aol.com
- Date: Sun, 12 Dec 1999 12:02:14 EST
Thank you Ron for the prompt action on the issue of rigid vs. flexible diaphragm. In a message dated 12/11/1999 12:29:28 PM Pacific Standard Time, ROH(--nospam--at)eqe.com writes: << 5- Flexible diaphragm assumptions are probably not appropriate for larger residential type construction such as multi-story apartment buildings and hotel/motel type structures, particularly when concrete topping slabs are present on the floors. The poor performance of many such structures in the Sherman Oaks area during the Northridge earthquake, that included a number of partial collapses, confirms the importance of using more rigorous design practices for such buildings. >> This, in my opinion, is a little hasty and too broad of a category of buildings. Most apartment type buildings are boxy and regular in shape. The majority of severe damage or failures during the Northridge earthquake occurred in buildings with little or no plywood shearwalls, designed or constructed based on, believe it or not, on the infamous Type "V" sheet of the City of Los Angeles, because they were of "Conventional Framing." Many buildings did have plywood shearwalls, but had a lot more reliance on drywall and stucco, which are now "outlawed." Drywall shearwall failures increased the demand on the plywood shearwalls considerably and much beyond their capacity leading to their failure. Of course, there is also the quality of construction element that had a major share in the failure of those buildings. Open front type buildings with steel columns failed not because the steel columns were designed as flag poles, rather because they were not. The steel columns were simply gravity-load carrying elements and the diaphragm shear was thought to be resisted by the rear wall by "Rotation," which is a rigid diaphragm concept. Today, however, almost all of the known failure causing inadequacies as mentioned above (at least as we know them) are not allowed by the building code. Add to that the stricter provisions of plywood shearwalls (thickness, 3x, nailing, structural observation, reduced HD values in LA, drift limitations, etc.) and properly designed flag poles, which are in fact inverted moment frames. All this should be more reason not to categorize apartment type buildings as rigid-diaphragm buildings, and leave that decision to the individual engineer to make based on the particular building and its configuration. Perhaps, well thought provisions based on facts, yet flexible to apply to individual cases is more appropriate, but not across the board categorization that may backfire, as we have seen before. Regards, Oshin Tosounian, S.E.
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