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Re: Shear Walls extending to Basement Level

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The first floor diaphragm rigidity is very stiff when connected to the perimeter basement walls. You can run the shear wall down to the foundation however your lateral forces often are reversed and increased as a result of doing so.

Consider how a braced frame would act in place of the shear wall and leave out the brace from the 1st floor to the foundation. At the first floor diaphragm all of the lateral load would transfer to perimeter basement walls and just vertical loads would transfer through the end columns.

You can do the same with a shear wall but you still need to have the end columns capable of taking the vertical tension and compression loads.

I came across this interesting load path back in my E.I.T. days when one of my fellow engineers wanted to support the basement wall lateral soil pressure through the ends of bar joists. I began looking at bar joist overturning, bar joist axial loads, etc. Finally came to the conclusion that the floor to wall connection should be concrete to concrete and be able to take the building lateral load into the basement walls at the 1st floor. Tried to run the bracing to the foundation however the forces reversed and greatly increased from the 1st floor to the foundation. The cantilever effect.

I have also seen engineers do a two dimensional analysis of this same problem and take the bracing down to the foundation with no concern for the basement walls. This is not the correct load path on account of the rigidity of the floor being tied to the walls.

James






From: "Jim Kestner" <jkestner(--nospam--at)somervilleinc.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: "'SEAINT'" <seaint(--nospam--at)seaint.org>
Subject: Shear Walls extending to Basement Level
Date: Mon, 11 Nov 2002 09:58:45 -0600

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 cantilevered 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 transferred 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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