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RE: Shear Key in Basement walls

• To: seaint(--nospam--at)seaint.org
• Subject: RE: Shear Key in Basement walls
• From: "Jake Watson" <jwatson(--nospam--at)utahisp.com>
• Date: Fri, 11 Jul 2003 13:32:23 -0600

```Sometimes I spout off a little too soon.  The first time I read you posting
I thought you were asking about a shear between the footing and the wall.
Many people design walls with a 2x4 or 2x6 keyway between the wall and the
footing for shear transfer.  I thought you were trying to eliminate this

Now trying to answer your real question, let me make a few more assumptions.
EFP of soil: 45 PCF
N.W. concrete: f`c=2500 psi

With a 9' wall and no top support, your bottom reaction is
45*(1/2)(9^2)=1823 plf

stress on the slab between walls assuming they push against each other:
=1823/(12*5) = 30.4 psi = .012 f`c

Different sections of ACI have different limits on when you can ignore axial
load in elements.  For example, in walls if axial stress is below 0.05f`c
you include it for P-delta but can "ignore" the effects of compression on
the wall.  All you really check is bending.  I would argue the stress is low
enough you can ignore its effects and check the slab for bending.  Your slab
is on grade, therefore no bending (and deflection), therefore no issue.
Gravity pushes down and holds the slab against the ground.  It could
potentially still buckle up, but it seems like a long shot to me.

As an alternate, you could call the slab a diaphragm and dowel it to the
side walls.  Use the sidewall mass and the mass of the slab itself to resist
any un-balanced loading.  Never seen this done, but there is a first time
for everything.

This is a *very* liberal interpretation, but one that can be supported by
experience.  I don't recall ever reading a story where the basement slab has
failed in compression.  That said, I believe if you call the slab on grade a
structural element, there are other minimums to consider.  Minimum
reinforcement etc.

In short, I wouldn't lose site of the forest from the trees.  You are
correct to worry about a 100' long wall rotating and possibly sliding if
they backfill without a slab or a floor diaphragm.  If you have resolved
rotation through a footing, the slab should easily handle the sliding.

Sorry for the original confusion,

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

-----Original Message-----
From: G M [mailto:newabhaju(--nospam--at)hotmail.com]
Sent: Friday, July 11, 2003 10:54 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Shear Key in Basement walls

Jake:

I am kinda confused on the shear friction concept.  I agree that one can use
shear friction to transfer the active soil pressure from the stem wall to
the base slab.  Part of that load can be transferred to the foundation
through friction between the base slab and the foundation.  But what about
the base slab itself - what do you design that for ?   I would appreciate a
clarification on process you use - especially the  base slab design.

PS: The retained earth is 9 feet.  This is a residence.   I do not have the
building dimensions off hand - the building is at least 40 feet wide x 100
feet long.

Gautam

>From: "Jake Watson" <jwatson(--nospam--at)utahisp.com>
>To: seaint(--nospam--at)seaint.org
>Subject: RE: Shear Key in Basement walls
>Date: Fri, 11 Jul 2003 08:24:05 -0600
>
>If this a standard house and you have reinforced the walls, shear friction
>should do the trick.  Then you won't have to worry about a fancy analysis
>to
>justify it.
>
>Jake Watson, P.E.
>Salt Lake City, UT
>   -----Original Message-----
>   From: Tripp Howard [mailto:tripphoward(--nospam--at)yahoo.com]
>   Sent: Friday, July 11, 2003 6:45 AM
>   To: seaint(--nospam--at)seaint.org
>   Subject: Re: Shear Key in Basement walls
>
>
>   Just curious how many people use shear keys.  I've moved away from them
>since they can be hard to form and make installing waterstops difficult.  I
>typically just roughen the surface between the wall and the footing and
>check it with the shear friction provisions.  What do others think about
>doing it this way?
>
>   On another note, I've gone away from using the traditional rubber
>waterstops due to the difficulty in installing them properly.  If I need a
>waterstop, I've been using a swellable hydrophilic waterstop such as
>Volclay's Waterstop RX.  You can adhere it to roughened surfaces and to
>join
>ends you just but them together.  No welding or special glues required.
>What are others opinions on these alternate types of waterstops?
>
>   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.
>
>
>     Gautam
>
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>
>
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