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Re: Shear Key in Basement walls

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I assume you mean a shear key extending below the horizontal base of the
wall to give you more passive resistance to sliding.

First, if designed as a true cantilever wall using active pressure, then by
definition the top of the wall will move inward, and could damage the
building above.  If the wall cannot move (or if movement will result in
damage), then it should be designed for the much higher at-rest pressures
(about 30-40% higher than active pressures).

If this is a fairly small rectangular basement, then the walls can be
designed as a 3-sided slab with moment connections at the wall returns and a
hinged connection at the base.  The footing acts a deep beam spanning
between the footings on the return walls.  Almost all of the sliding force
is resisted by the return walls and the opposite force acting on the other
wall.  Shear keys should not be needed.

If excavation costs are an issue and the slab can be structurally connected
to the footing (not a "floating" slab), then a band around the perimeter of
the slab can be thickened and reinforced.  This band acts as a tension
flange along the face of the wall.  The load is transfered in shear to the
return walls or footings.

Hope this helps,

----
Jason W. Kilgore, PE, SE
Project Engineer
Leigh & O'Kane, L.L.C.
jkilgore(--nospam--at)leok.com
816-444-3144
816-444-9655 (FAX)
----------------------------------------

G M <newabhaju(--nospam--at)hotmail.com> wrote:
To all:

I recently designed a basement wall as a cantilever retaining wall with a
shear key. The slab within the two ends of the retaining walls is a 5 inch
slab on grade. Due to field condition, the contractor does not want to
intstall the shear keys.

One way to reduce the shear demand at the base of the retaining wall is to
account for the fact that the first floor provides some lateral support.
However, because the first floor consists of wood construction, I am
hesitant to do this and prefer to be conservative. ( I do reinforce the wall
stem for this condition,though).

The second alternative is to assume that the active force on the retaining
wallls at the opposite ends of the basement cancells each other out -
therefore, a shear key is not necessary. This condition should hold true no
matter how far the two retaining walls are located - in finite terms, thoguh
- provided they are of the same height and have simillar backfill. The
slab-on-grade will be subjected to compressive forces from the retaining
wall and should act as a plate with compressive forces on all four sides (
four sides of the basement).

My question to you all is - how do you design the slab? I am thinking of
taking a unit width of slab and finding the allowable compressive load based
on Euler's formula with a factor of safety. Since water will be drainded,
moment due to water pressure should not be an issue.

If the wall design were to include seismic forces - (Mononobe-Okabe
equation)- how would one dissipate the shear for the worst case scenario for
seismic forces acting in the same direction for the two opposite walls.

Your comments would be highly appreciated.

Gautam



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