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Re: Seismic Upgrade.... Blue Book Commentary on wood diaphragm

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In a message dated 5/23/99 1:23:03 PM Pacific Daylight Time, Byainc(--nospam--at)aol.com 
writes:

<< For several weeks after the ensuing of this discussion I have wondered if 
 there were any documentation or verification that the assumption of 
diaphragm 
 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 
 earthquakes.
 
 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 
softstory). 
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