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

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Please do not mis-understand, I do not completely believe that a rotational
analysis is the answer, but I do believe that a diaphragm supported by 5
lines of walls, creating 4 diaphragm spans and drags, treating this as a
series of 4 simple spans with half of each span loading contributing to the
wall, regardless of the fact that the interior walls are 2 to 3 times
longer than the exterior (with windows).

The analysis of thesee types of situations should consider the shearwall
support relative stiffnesses, the resulting wall and diaphragm drifts.

Dennis,  Yes a custom home with differing roof diaphragms at different
elevations may act as a simple span, hinged (almost) at the supporting
wall, but the shearwalls should be checked for drift for compatible

Williston "Bill" L. Warren, IV - S.E.
Newport Beach, California

From: Byainc(--nospam--at)
Subject: Re: Seismic Upgrade.... Blue Book Commentary on wood diaphragm
To: seaint(--nospam--at)

Williston "Bill" L. Warren, IV - S.E.

"It was clear from the damage of the Northridge earthquake that the simple
span or flexible diaphragm assumption, or half the width tributary area
assumption abeing supported by rigid supports was not correct." 

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
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

Ben Yousefi