Subject: Re: For Urgent Review and Comment - Blue Book Commentary
Date: Sun, 23 May 1999 16:33:00 EDT
In a message dated 5/23/99 10:34:44 AM Pacific Daylight Time, RLFOLEY(--nospam--at)aol.com
<< Another idea would be to limit the shear wall stress to 1/2 values
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
structure due to a number of reasons unrelated to our computational
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