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]

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.

 

[miecz]

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

 

[JAE]

heh

 

[Assayagr]

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.

 

[msquared48]

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

 

[miecz]

So, is the solution - to thicken the stem?

 

[JAE]

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)

 

[VoyageofDiscovery]

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

VOD

 

[hokie66]

Voyage,

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

 

[VoyageofDiscovery]

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

 

[hokie66]

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.

 

[msquared48]

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

 

[JAE]

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.

 

[cf9]

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.

 

[DaveAtkins]

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

and so I never use them.

DaveAtkins

 

[JKW05]

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.

 

[DarthSoilsGuy]

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]).

 

[BPA827]

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

 

[miecz]

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.

 

[hokie66]

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.