Subject: Re: Seismic Upgrade.... Blue Book Commentary on wood diaphragm
Date: Sun, 23 May 1999 16:55:53 EDT
In a message dated 5/23/99 1:23:03 PM Pacific Daylight Time, Byainc(--nospam--at)aol.com
<< For several weeks after the ensuing of this discussion I have wondered if
there were any documentation or verification that the assumption of
flexibility was proven to be incorrect in the past earthquakes. According to
Mr. warren it was. If this is the case I believe those advocating the
diaphragm rigidity analysis for light framed wood structures owe it to the
rest of us to share this information. I am keeping an open mind on this
subject, but I like to see proof that this supposedly erroneous assumption
led to serious undesirable behavior in these types of structures in the past
Ben Yousefi >>
Thanks Ben, I agree with you.
The majority of damage associated with commercial and residential resulted
from inadequate shear walls designed without deflection calculations and
restricted by the arbitrary 3.5:1 aspect ratio. This was clear on multi-story
condo's with damage from the softstory side of the building (not a true
What would have been the consequences if the diaphragm was considered rigid.
I don't think much considering that in the direction of greatest rigidity
(parallel to the open front) there were few if any interior partiitions used
to resist displacment of the diaphragms. Therefore, the rotation would have
needed to be considered. However, design of an open front using rotation was
found years before to be inaccurate for multi-story sturtures and disallowed.
My point is that the solution was not to design by rotation but to create
stiffer shear walls in the open front. If it was not possible to do this with
wood, alternative means had to be introduced.
My conclusion is that the stiffness of the diaphrams supported by wood walls
is not relevent - the story drift in the open front line of shear is what
matters and the need for more rigid resistance.
Dennis Wish PE