Stem key in Cantilevered Retaining Wall

[Assayagr]

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.

 

[JAE]

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.

 

[DaveAtkins]

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

 

[miecz]

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.

 

[hokie66]

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.

 

[fattdad]

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!

 

[msquared48]

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

 

[JAE]

...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.

 

[msquared48]

Yup...


Mike McCann
McCann Engineering

 

[fattdad]

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!

 

[JAE]

...crickets chirping....

 

[Assayagr]

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.

 

[miecz]

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.

 

[DaveAtkins]

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

 

[civilperson]

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).

 

[Assayagr]

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.

 

[miecz]

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?

 

[fattdad]

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!

 

[JAE]

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).

 

[msquared48]

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