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Re: For Urgent Review and Comment - Blue Book Commentary

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In a message dated 5/23/99 10:34:44 AM Pacific Daylight Time, RLFOLEY(--nospam--at)aol.com 
writes:

<< Another idea would be to limit the shear wall stress to 1/2 values 
(similar 
 to uninspected masonry) if a rigid diaphram analysis is required but not 
 elected to be  performed. 
 
 These ideas are put forth in order to keep the computational effort required 
 for most single family residences, condominiums, or small limited occupancy 
 structures comensurate with the fees an engineer may realistically expect to 
 receive. The benefit to cost ratio of a rigid diaphram analysis is 
 questionable, and further may not more accurately predict the response of 
the 
 structure due to a number of reasons unrelated to our computational 
abilities. >>

Personally, I don't agree - although it is a good start. We need to look at 
the damage what occured by failure of properly designed and implimented 
shearwalls. I don't feel that there is any need to reduce the capacity of the 
walls when the code has already implimented reductions in shear for plywood 
walls (in the 94 code) and has modified the lateral components for active 
fault proximities. Considering the penalty we take for embedded columns, the 
code reduces capacities by more than 40% without the rigid diaphragm concept.

I'm not sure if this is accurate, but from the data I've seen the shear due 
to torson contributes around 15% into the surrounding shearwalls. As a 
comprimise, I could see a 15% reduction as the very most. We already have a 
restriction as to the walls aspect ratio (2:1) in the 94 code that should 
compensate for too much flexibility.

Here is something to think about. Most engineers that I have spoken with use 
an Rw of 6 for a plywood shearwall system when an Rw of 8 is adequate. If the 
restrictions in the 97 code were in force, many of the same engineers would 
compensate by increasing the Rw for an appropriate plywood system. This may 
seem counterproductive, but then there is no real evidence that properly 
implimented designs are in question.

Dennis S. Wish PE