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RE: Shear Wall Rhos
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Shear Wall Rhos
- From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
- Date: Tue, 13 Aug 2002 09:10:39 -0800
Ditto. The code requires the redundancy to apply
to the building not the bracing line or element. The idea is for the
whole system to be stronger to pick up the slack if the component fails, not for
the component to be stronger by itself.
The redundancy factor
is applied to the entire building not each line of resistance. Even along the
height of the structure, if you notice, the UBC, in calculating the "r max"
factor, considers the highest "r" for lower two third of the height of the
building. In the IBC, even this was eliminated and the highest r along the
entire height of the building should be used in calculating Rho for all
levels. The only break is given when you determine the force level in each
direction. In that case, you may have different rhos for each direction of
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.