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Re: Wall weight in base shear computation

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Eddie, 

I always include the tributary mass of walls in both directions in the mass of
the diaphragm at each level -- if it is a rigid diaphragm analysis.  

If the shear walls all had the same rigidity and the same mass, the effect of
doing this would be the same as if the load due to the mass of the wall were
added in after the loads distributed from the diaphragm were completed.  The
same would be true if the mass of each wall is in proportion to its rigidity.

If the mass of the wall is not in proportion to its rigidity, it would correct
to include it in the total mass of the diaphragm so that the load due to the
mass of the walls get appropriately distributed to each of the other walls.
If a wall has rigidity that is very low relative to other walls, the stiffer
walls may actually provide more bracing for that wall than the wall itself
does.  

Besides, it makes the bookkeeping a little less complicated.  

I'm sorry to have missed you at the Wood Solutions Fair -- it was fun to meet
the faces behind some of the postings on our list.

Nels Roselund
Structural Engineer