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RE: Shear Walls extending to Basement Level

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Yes this is possible, and yes it is very tough to do.  Last year we finished
a 5 story precast building in Jackson Hole.  The diaphragm on the upper 3
floors was a 3" topping slab.  On the bottom two floors, the length of the
shearwalls increased dramatically.  We had to increase to a full 10"
cast-in-place slab next to the main upper walls.  One problem you will run
into is rebar development at the shear transfer point.  The wall is likely
12" or so thick (although bigger buildings have bigger walls.)  You have a
hard time developing anything larger than a number 5 horizontally inside the
wall.  You must develop the bar inside the wall for shear friction,
otherwise the transfer mechanism breaks down.  The other part of this is:
where does the force go?  Once it is out of your wall you need to design the
diaphragm to carry the force from the upper wall to the perimeter basement
walls.

One other thought, you might try considering cracked sections directly if
you can.  This might reduce some of the reversal effect by reducing the
stiffness of the wall.  Your short upper wall will crack and soften,
meanwhile the longer lower walls will remain stiff.

Best of luck,

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Jim Kestner [mailto:jkestner(--nospam--at)somervilleinc.com]
Sent: Monday, November 11, 2002 8:59 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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