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94 UBC and Rigid Plywood Diaphragms??????

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         (__)                  Dennis S. Wish PE                    

I'm embarrassed to admit that I could not answer Ben Yousefi's question:

The 1994 and 1997 UBC both contain the same sections that requires the 
engineer to test the diaphragm for stiffness and to determine whether or not 
it is to be designed as a rigid element. However, building officials (and the 
professional community) did not endorse this method for residential 
construction in the life of the '94 code.
If they did, then I must have slept like Rip Van Winkle during the '94 Code 
cycle since no one that I know considered a plywood diaphgram as rigid.

Therefore, what changed? When and why did it become mandatory in the 1997 
code to follow the methodology that was presented in a review of wood framing 
seminars for the '97 UBC.

I thought it might have had something to do with the lack of an adequate 
method for determining the true deflection of an unblocked diaphragm.

Can someone educate me (and Ben) as to why it was not supported by building 
officials during the reign of the 94 code and what occured that (other than 
Northridge) that made it mandatory in the '97 code.

I am somewhat embarrased because I have been so adamently against the rigid 
diaphragm provisions in the '97 code that I have failed to do my homework and 
discover that it was already existing in the '94 code.

Can someone enlighten us?


Dennis S. Wish PE
Structural Engineering Consultant