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Stem key in Cantilevered Retaining Wall

Stem key in Cantilevered Retaining Wall

Stem key in Cantilevered Retaining Wall

(OP)
I'm in the process of designing a 21' cantilevered retaining wall and the shear at the base (between the stem wall and the footing) is a bit too high. I'd like to include a shear key as well as the effects of friction to increase my capacity but I'm not sure how to analize the key in this situation.
Any advice would be apreciated.

RE: Stem key in Cantilevered Retaining Wall

I've never liked the idea of a shear key - basically you would have three little pieces of concrete to break off (the key and the two tongues on either side).

We detail an intentionally roughened surface across the whole stem width (1/4" rough per ACI) and then design for shear friction per ACI chapter 11.  If needed, we add diagonal bars from the bottom heel area up to the front stem area to also take the shear.

RE: Stem key in Cantilevered Retaining Wall

I agree with JAE--if you are using shear friction to resist the shear, I can't believe the small key will increase the resistance significantly.

DaveAtkins

RE: Stem key in Cantilevered Retaining Wall

We don't use keys for reinforced construction.  Shear keys are OK for unreinforced walls, such as in residential construction.  But when vertical reinforcement crosses the joint, the key becomes more of a problem than it's worth, as it reduces the cover for the heel reinforcement.  We don't intentionally roughen, as I'm afraid the vertical reinforcement would get in the way.

RE: Stem key in Cantilevered Retaining Wall

Rather than a shear key within the thickness of the wall, I prefer to cast the footing higher on the inboard side.  That is, provide a step in the top of the footing at the inside face of the wall.  So you have the shear at the bottom of the wall resisted by bearing, and the raised section is checked for the shear.

RE: Stem key in Cantilevered Retaining Wall

To answer your direct question, you would use the passive soil pressure acting on the shear key to resist your lateral forces.  Whether this is a "good" structural practice, I don't know. But as a geotechnical engineer that's what you would do. . . .

One item of note: there may be strain incompatibility between fully mobilized passive pressure and the shear-strength along the interface.  In other words, you may have shear failure along the sliding surface in advance of fully mobilizing your passive resistance.  Because of this, it would seem that you end up designing this as a cantiliver retaining wall afterall.

f-d

¡papá gordo ain’t no madre flaca!

RE: Stem key in Cantilevered Retaining Wall

Assayagr:

I see two responses here to two situations.

Were you asking a question regarding (1) a shear key at the interface of the concrete stemwall and the footing, or (2) a concrete vertical projection below the footing, also referred to as a key, that does offer extra sliding resistsance through passive pressure?

Mike McCann
McCann Engineering

RE: Stem key in Cantilevered Retaining Wall

...well...the OP did say, "between the stem wall and the footing" so I interpreted this to mean concrete to concrete shear...not footing to earth shear per fattdad.

RE: Stem key in Cantilevered Retaining Wall

Yup...

Mike McCann
McCann Engineering

RE: Stem key in Cantilevered Retaining Wall

It's hard to know what the op had in mind if the op isn't commenting.  I would agree, the op is confusing on the meaning of stem key.

f-d

¡papá gordo ain’t no madre flaca!

RE: Stem key in Cantilevered Retaining Wall

...crickets chirping....

RE: Stem key in Cantilevered Retaining Wall

(OP)
Sorry, went home for the night. Here's the deal. I told my boss that the shear at the base of the stem wall was to high and he recomended that I use a key at the base of the stem wall in order to engage the footing slab (then he went out of town). I'm just not sure how to analyse this to say yeah or ney. Widening the base of the stem wall works but due to the length of the wall it would not be cost effective.  I do like the idea of a deeper footing on the heel side to a point where the shear checks out.
Thanks for the feedback.

RE: Stem key in Cantilevered Retaining Wall

I think you mean a thicker footing on the toe side, so that the top of footing on the toe side is higher than the top of footing on the heel side.  That way, the stem bears against the upturned toe.  This detail is used by our DOT.  I don't like it, but who am I?  In any case, I don't think you can combine the resistance of the concrete shear with shear friction.  For shear friction to engage, there has to be some slippage.  By that  point, the concrete has cracked and offers no resistance.

RE: Stem key in Cantilevered Retaining Wall

Why can't you make shear friction work?  Reinforcing can take a lot of shear via shear friction.  I think this would be cheaper than using a key or pouring a thicker toe.

DaveAtkins

RE: Stem key in Cantilevered Retaining Wall

The key in the wall base does nothing for shear strength.  This is a left over detail from some unknowledgable designer.  Imagine that concrete cracks and that reentrant corners are stress concentrators for crack purposes, thus the keyway either in the footing or in the wall will tend to crack and negate any shear valve addition.  Thickening the base of the wall is my preferred method of increasing shear strength, (increasing f'c or adding horizontal hoops is also possible).

RE: Stem key in Cantilevered Retaining Wall

(OP)
I came to the same resolution. Increasing the width of the stem at the base by 2" solved it but I was trying to keep the yardage down.
Thanks for the help.

RE: Stem key in Cantilevered Retaining Wall

O.K., I missed something here.  I thought we were trying to resist the shear at the joint between the stem and the footing?  How does thickening the stem help with this?

RE: Stem key in Cantilevered Retaining Wall

I'm confused also.  Was the intent of the original post to ask a geotechnical question?  From the replies, I'm guessing this is more of a structural question. . . .

f-d

¡papá gordo ain’t no madre flaca!

RE: Stem key in Cantilevered Retaining Wall

Assayagr - what miecz is suggesting is that shear friction doesn't totally (or usually) depend on the area of concrete at the joint, but rather the area of reinforcing bars crossing the joint (see ACI 318 Chapter 11).

RE: Stem key in Cantilevered Retaining Wall

May I'm missing something here, but, as I see this, it is not all shear friction with the shear key.  

For taller walls, I usually do spec a 2X4 or 6 key in the top of the footing.  The intent is to provide a lateral bearing surface to assist in resisting the lateral load transfer of the stem wall loads to the footing.  In order to generate this, the construction joint plane extending horizontally thru the 2X key will have to resist from being breached or sheared.  In that scenario lies the shear resistance.  An alternate to this would be to provide a shear key in the footing the width of the stem wall and about 1.5 to 2" deep, depending on the force to be resisted.

And, yes fattdad, this is not a geotechnical problem.  It is structural.

Mike McCann
McCann Engineering

RE: Stem key in Cantilevered Retaining Wall

The problem with a shear key is that there really isn't any governing design standard to use in figuring the shear across the key.....2 x sqrt(f'c)? perhaps...but this is a classic situation where ACI Section 11.7, shear friction, applies.

RE: Stem key in Cantilevered Retaining Wall

Sure would be nice to know if Assayagr considers this to be a geotechnical problem...

RE: Stem key in Cantilevered Retaining Wall

heh

RE: Stem key in Cantilevered Retaining Wall

(OP)
No, I do not consider this a geotech question but i am in a foundations forum. my mistake. Also, my boss missunderstood my original question and I did not recognize that. This then led me in the wrong direction of trying to figure out how a key will help increase the stem base shear capacity. The answer: It does not.

RE: Stem key in Cantilevered Retaining Wall

JAE:

If you want to rely on shear friction, fine, but I still feel very comfortable using root f'c across the width of the shear key to be conservative.  I cannot see 2root f'c as the viability of the reinforcing for shear is nebulous at best, even nominally.

Mike McCann
McCann Engineering

RE: Stem key in Cantilevered Retaining Wall

So, is the solution - to thicken the stem?

RE: Stem key in Cantilevered Retaining Wall

Mike,
I guess it's just a difference of opinion, but ACI didn't offer sqrt(f'c) as some magic shear stress capacity (that I'm aware of) for cases such as this.  They only offer shear friction.  

My point was that I have nothing to point an attorney to if there is a shear failure across the stem base.  There would be plenty of attorneys out there who, with the help of another engineer, would ask me to point in the code where it says I can use sqrt(f'c).  

(sorry..I know I'm being a little melodramatic here...please forgive; as honestly I have used sqrt(f'c) in various applications..just never very often)

RE: Stem key in Cantilevered Retaining Wall

Why not pour a portion of this wall monolithically with the footing and this becomes a moot point?

VOD

RE: Stem key in Cantilevered Retaining Wall

Voyage,

You would just move the joint shear issue up a bit, and complicate the formwork a lot.

RE: Stem key in Cantilevered Retaining Wall

I have poured a lot of these type of walls and this has never been an issue, design the wall for shear, roughen the joint and be done with it.  Shear works at approx 45 degrees when a good concrete bond exists.

VOD

RE: Stem key in Cantilevered Retaining Wall

I think the issue here is the direct shear, or sliding resistance, on the joint, which as you say involves good bond.  The 45 degree shear you refer to is diagonal tension, the same as beam shear.

RE: Stem key in Cantilevered Retaining Wall

JAE:

I understand your concerns fully regarding the "L" word.  Obviously we have different opinions, and for lawyers, I guess that is good.

Nevertheless, I know that root f'c is the limit set by ACI as to when nominal stirrups are required in a beam, and 2root f'c as to when more stirrups than the nominal amount are required.  To me, the implication is that use of root of f'c is allowed in a situation when the shear force in the concrete does not exceed that amount.  

All I'm trying to do is develop enough bearing in the shear key to allow the root f'c value to develop across the key.  Nothing more.

Mike McCann
McCann Engineering

RE: Stem key in Cantilevered Retaining Wall

Understand - appreciate the comments.

But shear behavior in a flexural member is a lot different than direct shear across a section at a joint.  

For the lower portion of the stem, in flexure, and adjacent to the joint...no problem with the sqrt(f'c) use.  

But for the joint, it is a completely different behavior in my view.

RE: Stem key in Cantilevered Retaining Wall

Good discussion . . . For me I like the roughened joint/shear friction model w/o diagonal bars.  Tension in the vertical bars should develop enough friction w/o adding diagonals.  However, it is worth noting that diagonals are shown in the CRSI details.  Also, for failure to occur, you have to shear through the vertical bars in the front face of the wall.

RE: Stem key in Cantilevered Retaining Wall

I have never understood the need for diagonal bars a la CRSI--

and so I never use them.

DaveAtkins

RE: Stem key in Cantilevered Retaining Wall

I'm on the fence about using shear keys vs. roughened surface.  When I have incorporate a shear key, I've designed the key as a plain concrete section (ACI 22.5.4). But I think I'm starting to lean toward the shear friction method.

RE: Stem key in Cantilevered Retaining Wall

i avoid "designing" shear keys out of apathy. i've seen them omitted by the contractor too many times. here's what i imagine goes down in contractor QC

1. i look at the plans during formwork placement
2. i look at the plans during rebar placement
3. i look at the plans and double check everything
4. i order the concrete and watch it go in.  Man, i'm good.
or..
4. these anchor bolts have to be right, and i won't let myself get distracted.
or..
4. wow, i barely got those vertical hooks wet-set in there before it got hard.  man, those last ones were tough.
if not the first three options, then...
4. it's just too much of a pain to put a key in with all these verticals in the way. i'll just trowel in a dent.

i've had better luck popping in and finding the additional rebar than i have finding shear keys in hard footings.  in my experience contractors are more hesitant to defend missing rebar over a missing key, even if you make them theoretically equivalent.

i say "designing" because i still detail it... I just don't count it and don't notice it when it's missing (oh, no he didn't [snap-snap-snap]).

RE: Stem key in Cantilevered Retaining Wall

Increasing the stem thickness gives you more "d" and thus more shear capacity. If your trying to economize on yardage, you can batter the soil side of the stem wall or bench the stem wall

RE: Stem key in Cantilevered Retaining Wall

BPA827-
The op had to do with "the base (between the stem wall and the footing)".  While increasing stem thickness helps with shear resistance in the body of the stem, I don't believe it helps much at the joint between the stem and the footing.  In fact, it may hurt, as the depth to steel increases, the compressive force to the bending moment decreases, and so the shear friction decreases.

RE: Stem key in Cantilevered Retaining Wall

miecz,

I am no expert on shear friction, and try to avoid having to rely on it, but if increasing the stem thickness decreases the shear resistance, I will turn in my slide rule.

RE: Stem key in Cantilevered Retaining Wall

Miecz:

The shear capacity I'm talking about is not shear friction, but from the concrete 2 sqrt f'c bd

RE: Stem key in Cantilevered Retaining Wall

Shear from flexure and shear from soil pressure are the same.  The shear found "d" away from the base is the design shear.  The load diagram showing soil pressure is integrated to produce the shear diagram, thus it is the same shear that the makes the moment diagram or flexure.

RE: Stem key in Cantilevered Retaining Wall

BPA827,

The OP was about developing the shear at the stem to footing interface.  Thus all the talk about shear friction, keys, etc.

civilperson,

No one on this post is really arguing how to calculate the shear, just how to develop it across a construction joint.  It is much the same situation which would occur if you made a construction joint in a slab right on the side of a supporting wall.  That is avoided by casting the slab over the wall or keying it in.  A cantilever wall is just a vertical slab, but for some reason it is not respected as such.

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