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Re: Wood Fr-Upgr : What I'll do and two more questions

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Dennis, Bob Bossi's clarification (below) makes sense just fine, but I
caution you to be wary when assigning an upper capacity to the existing roof
diaphragm for this purpose. I think you said it is made of 2x6 T&G deck
planks. There only maybe is an accredited lower bound allowable diaphragm
shear value you may cite in your calculations. But you can hardly surmise
from that what an upper limit of strength might be.

As always, each structural element will behave the way IT wants to, not the
way desired of it. It's like schoolchildren-- there are underachievers and
there are overachievers. We are well aware of the underachievements here and
there in Northridge, but who investigated all the houses that won't calc out
yet failed to fail? The overachieving elements remain unknown as to their

I think you are better positioned if you design for tributary forces in
well-accepted ways, with due consideration for the uncertainties of relative
rigidities among the roof diaphragm and useful walls, and avoid relying on
the roof diaphragm protecting you like a fuse blowing.

Charles O. Greenlaw,  SE     Sacramento CA
>What I was suggesting was to:
>1.  calculate the EQ load the existing structure would deliver to the common
>wall using the current code tributary area methods and compareit to the
>of the existing roof diaphragm.  Use whichever is less (most likely the
>diaphragm capacity) since the wall won't see more load from the original
>structure than the existing diaphragm can deliver.
>2.  calculate the full code load for the addition.
>3.  add 1 and 2 together and design the common shear wall for the combined