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

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On Sat, 24 Aug 1996 12:36:08 -0400, you wrote:

>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

I think Dennis' point concerns the philosophy of design.
Theoretically, a stiffer diphragm will increase the base shear from
the standpoint of response spectra analysis. In addition, retrofits
try to achieve a strength based design, not like a house which are
strictly designed for stress levels.

But realistically, the code prescription for the force level will not
change. Generally speaking I am not too concerned about the
modification to the dynamics of a house. Initially, because it is just
a house, not a high occupancy structure. Lastly, little if any
attention was paid to the design in the first place if it has spaced
sheathing. When a high weight roof is added, it should be analyzed for
current code requirements and upgraded accordingly.

Mark E. Deardorff, SE
Deardorff & Deardorff, Inc.
San Diego, CA