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Re: Report on Wood Diaphragm Issues

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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.