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RE: Shear Wall Rho

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I think the code intends that the maximum value of rho be applied across the board to all the elements of the primary lateral force resisting system. It's a penalty for not having enough redundancy. I agree that this design philosophy makes more sense for rigid diaphragm systems, because the benefit of redundant elements can be shared more broadly across the structure. But even with flexible diaphragms, the method gives you a bonus for increased redundancy, say by using longer shear walls.
 
I don't think there is anything in the UBC that implies you should or can apply different values of rho to different parts of the same structure. If you are designing elements according to the provisions of 1632.2, then rho drops to 1.0, but that is not what you are talking about here.
-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)att.net]
Sent: Tuesday, August 13, 2002 8:12 AM
To: seaint(--nospam--at)seaint.org
Subject: Shear Wall Rho

I am designing strengthening of an old multistory wood-framed building with several unreinforced masonry chimneys.  My system for seismic bracing of the building is to install new masonry walls aligned with each chimney in each direction so that the bracing system has enough rigidity to protect the chimneys against seismic loads.  The old building has flexible diaphragms so that each wall acts to brace its tributary area of building; no two walls are aligned.  The lengths of the masonry walls are necessarily as short as practical.  I've determined rho for each wall per equation 30-3, UBC Section 1630.1. 
 
In determining E [per equation 30-1] should a single value of rho, the highest, to be used for all of the walls in a direction, or should each wall be assigned its own rho value?  Since each wall acts independently of the others in a flexible-diaphragm system, it seems that each wall should be designed on the basis of its own value of rho, but the Code seems to imply use of a single value: the maximum.
 
Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net