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Re: Rigid vs Flexible diaphragm discussion

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In a message dated 8/10/99 8:56:33 AM Pacific Daylight Time, MWJ(--nospam--at) 

<< Most of the comments about this topic have seemed to me to have been
 missing the real issue of how we are designing diaphragms versus how they
 in fact behave.  >>

I believe your post was well presented with good arguments and, in theory, I 
would tend to agree. However, I think the bigger picture that is being missed 
is to ask just why it matters if there is little to show in the way of life 
threatening issues or major structural damage. As of this date, the 
Seismology committee has revised their assessment in the Blue Book of the '97 
UBC methodology as not having sufficient information that justifies the 
change of methods. Admittedly, homes performed well with the exception of 
those with "tuck-under" parking.
We can try to idealize a methodology so as to predict exactly how the 
diaprhagm will behave, but this may not be practical for those who design 
wood structures for a living. Therefore, we need to address the core issues 
like stiffness of walls and separate them from a method of tedius and costly 
analysis into a broader simplified approach that will solve the problem.
Your examples may be valid on paper, however, there is little documentation 
that failures due to the rigidity difference between two four foot shearwalls 
and an adjacent 12' wall. I'm not suggesting that the physic's is wrong, only 
that it is not practical. At some point we can design as close to earthqake 
proof as possible, but not be able to afford to build it or sell the 
aesthethics. Therefore, we must work within the constraints of the real world 
- those designers, owners and architects that are unwilling to comprimise 
aesthetics for strength. 
Somewhere in the middle lies an agreeable solution but I think that we should 
neither be generic or get lost in the infinite details.