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.
Any advice would be apreciated.





RE: Stem key in Cantilevered Retaining Wall
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
DaveAtkins
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
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
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
RE: Stem key in Cantilevered Retaining Wall
Mike McCann
McCann Engineering
RE: Stem key in Cantilevered Retaining Wall
f-d
¡papá gordo ain’t no madre flaca!
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
Thanks for the feedback.
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
DaveAtkins
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
Thanks for the help.
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
f-d
¡papá gordo ain’t no madre flaca!
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
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
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
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
RE: Stem key in Cantilevered Retaining Wall
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
VOD
RE: Stem key in Cantilevered Retaining Wall
You would just move the joint shear issue up a bit, and complicate the formwork a lot.
RE: Stem key in Cantilevered Retaining Wall
VOD
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
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
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
RE: Stem key in Cantilevered Retaining Wall
and so I never use them.
DaveAtkins
RE: Stem key in Cantilevered Retaining Wall
RE: Stem key in Cantilevered Retaining Wall
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
RE: Stem key in Cantilevered Retaining Wall
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
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
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
RE: Stem key in Cantilevered Retaining Wall
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.