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RE: Proper seismic R factor to be used in concrete design

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Lynn

As the code states, for a structure with a concrete shearwall LFR system
to be designed as a building frame system with R=5.5 it needs a complete
space frame that supports essentially all of the gravity loads. I feel
that gravity frame columns constructed integrally within shearwalls are
totally acceptable and can perform this functrion provided they are
designed and detailed to maintain the integrity of the gravity frame
through several cycles of lateral drift well into the inelastic range.
We satisfy this criterion by following the requirements of section
1921.7

IMHO, equally important is the layout of the shearwalls. In our office,
we attempt to provide a redundant, well distributed, primary LFR system
by proving wall layouts that conform to 1921.2.1.7(1)  This produces a
system with in multiple lines of resistance that minimize excessive
diaphragm deflections under the design basis ground motion. Although
section 1921.2.1.7(1) specifically applies to precast gravity frame
members, we feel the the intended result of these provisions are
desireable for all structures.

Sincerely,

Brent Koch
Livermore, CA


-----Original Message-----
From: Lynn [mailto:lhoward(--nospam--at)silcom.com] 
Sent: Thursday, January 30, 2003 06:34 PM
To: seaint(--nospam--at)seaint.org
Subject: Proper seismic R factor to be used in concrete design


Dear members of the list:

I have a question that seems to have different
engineers giving different opinions of the appropriate
R factor that should be used for a concrete building we
are involved with.

The building is a shear wall system.  The floors and
roof are concrete constructed of slabs and supporting
beams and girders.  The girders frame into the center
of each exterior concrete shear wall.  Inside the
exterior wall, there is a tied column designed to carry
the gravity loads of the girders.  The columns are dead
center along the length of the wall.

The question is:  Is this a "building frame system"
using concrete shear walls (R=5.5), or a "bearing wall
system" with concrete shear walls (R=4.5)?

The argument for the R of 5.5 is that the shear wall
itself is not load bearing, since there is a column
embedded dead center in the wall designed to carry 100%
of the gravity loads as a column.

The argument for the R of 4.5 seems to suggest that if
the shear walls yields under seismic forces, the
capacity of the column may be jeopardized, so the
girders do in fact rely on the structural integrity of
the shear walls to carry the gravity loads of the
floors.

Any comments or recommendations would be appreciated.

Thanks

Lynn

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