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RE: Shear Walls extending to Basement Level

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What I have done in the past that has been permitted in California:

1) Distribute your forces from the roof to the ground floor (elev. = 0) per
code.
2) Model the shear wall down to the basement foundation level (say elev. =
-30'-0" or whatever)
3) Provide pin supports at the ground and foundation levels ignoring any
intermediate basement levels.
4) Design for the shear transfer at the ground floor which will be a large
force requiring thickened slabs and lots of extra steel but feasible.
5) Even though the moment drops off to zero at the foundation, design for a
constant moment and constant shear anyway. 

This approach will give a conservative estimate of the building drift which
is usually the main concern for high-rises. I'm sure that there are lots of
different ways to model it that would be acceptable.


-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Monday, November 11, 2002 7:59 AM
To: 'SEAINT'
Subject: Shear Walls extending to Basement Level


Picture a shear wall extending to the basement level. It would seem that if
the 1st floor diaphragm was tied into the shear wall with rebar (and that
the diaphragm could transfer load to the perimeter foundation walls) that
the shear wall would try to act as a cantlivered beam using the foundation
in the basement and the 1st floor diaphragm as its supports. It would seem
to me that this would potentially create a large cranking force on the 1st
floor diaphragm that would be difficult to design for since the shear would
be magnified.

All my references show examples of shear walls going to grade level and not
to the basement level.

How is this condition normally handled? Is it practical to isolate the shear
wall from the diaphragm at the 1st floor level so that all overturning and
shear forces are transfered to the foundation at the basement level? It
doesn't seem practical to stop the shear wall at the 1st floor level. The
1st floor shear wall would have to be designed as a transfer beam and
columns on either end would have to extend into the basement to transfer the
up and down forces.

How are these designed on high rises where there may be multiple basement
levels to contend with?

Jim K.






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