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[SEAOC] Re: [SEAOC] Re: [SEAOC] Re: [ Reroofing (Wa...

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In regards to applying new plywood over space sheathing:

I don't agree with Dennis Wish's statement - "the increase in the demand of
diaphragm can be detrimental to the existing shearwalls and should be a
consideration before recommending a new diaphragm". 
Yes, the diaphragm capacity has increased but why is it stressed to maximum
capacity to cause failure in shear walls?  Where is the load coming from? The
seismic demand on the structure has not increased except perhaps slightly due
to increased weight from the plywood.  Also, the capacity of existing shear
walls have not changed therefore, it will transfer approximately equal amount
of force to the diaphragm whether it is sheathed with plywood or not.  
I can agree that by applying plywood over spaced sheathing, the failure mode
is isolated to shear walls but only if the shear wall capacity was deficient
to begin with. 

Brad Smith