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

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In a message dated 5/24/99 12:46:38 PM Pacific Daylight Time, 
billw4(--nospam--at)sesol.com writes:

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

No argument - I do as normal practice design my shearwalls for deflection. I 
will go so far as to admit that any damage associated with wall stiffness in 
the Northridge earthquake is, in my opinion, due to the acceptance of a 
prescriptive aspect ratio (ie, 3.5:1). Engineers rarely designed wood 
shearwalls to deflection standards and had they done so would have found that 
highly loaded walls approaching 3.5 to 1 will generally deflect in excess of 
the code allowable story drift. 
Neglect to check the wall deflection was, in my opinion,  the problem 
associated with open front damage - not the action of the diaphram in itself. 
Therefore, why complicate the design method when one possible answer is to 
mandate deflection analysis to prove as close to reality that the expected 
deflection does not exceed code allowable story drift.

Dennis