Foundation Wall Keyways 1

[EngWade]

Few quick questions for you guys.

Why are keyways typically incorporated into the junction of foundation footings and foundation walls? The only reason I can come up with is for simpler construction. But my intuition tells me that when doing this, you significantly weaken the foundation wall/footing against bending/rotation. Meaning - any moments about the foundation wall/footing will only be transferred by the rebar (if it is specifed) that connects the foundation wall from the foundation footing. If what I am saying is correct, then you should never have keyways in concrete, cast-in-place retaining walls, right?

Thanks in advance.

 

[msquared48]

Keyways are used for alignment and when the resteel is insufficient to resist shearing forces through the plane of the key. They really do not negatively affect the moment capacity of the wall joint.

If anything, keyways complicate forming and are generally frowned upon by contractors. However, they are used all the time and with proper detailing and due dilligence are very dooable.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[DaveAtkins]

Keyways are almost totally worthless from a structural standpoint, and contractors don't like them.

I never use them.

DaveAtkins

 

[paddingtongreen]

As Mike said, they have been used to transmit lateral force from wall to footing, however, their use has been discouraged for many years. The use of shear friction is encouraged.

The advantage is that the keyway is easily visible during inspection, sometimes the roughened surface is not as visible, but if you think about it, with a keyway, the shear capacity is limited to the permitted stress multiplied by the width of the key while the properly prepared surface and the correct reinforcement gives you the full width.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[miecz]

Keyways are useful in unreinforced construction, to transfer the shear from a foundation wall to a footing, but I wouldn't use one in a reinforced retaining wall.

 

[EngWade]

Thanks for all the replies.

So why does the keyway provide any greater lateral resistance than a solid, continuous pour? Intuitively, that doesn't make sense to me.

Paddington, what are you referring to when you say shear friction is encouraged.

 

[EngWade]

And to follow up my own comment, why is the strength of the footing/foundation wall joint not significantly decreased with a keyway? Isn't the keyway essentially just a cold joint?

 

[Ltwine]

Key way, if constructed properly, elong the contact length between stem and base, it also provide extra resistance thru bearing. (By design & theory, the stem will deflect and result in very small area under compression. The remaining area is in tension, which is not considered effective in providing shear friction)

 

[paddingtongreen]

EngWade, at one time, just about everyone used keyways, but they are awkward and self-limiting in capacity, if the key shears off the game is over. The ACI pushed for the use of "shear friction" instead of the keyway, the matching concrete surface would be roughened, and it would no longer be considered a "cold joint" but the same as a continuous placement. A certain minimum of reinforcing is required.

The easiest way to roughen was to spray a retarder on the area where the wall was to be, and then, after the main part was set, hose off the unset concrete.

The keyways were often formed with two pieces of lumber with a dumbbell waterstop in the middle; I stopped using those when the foam/bentonite waterstops came on the scene.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

Solid continuous footing-to-stem forming is difficult.
A monlithic placement like this is hard to do.
So you pour the footing, form the stem, then pour the stem.

 

[EngWade]

Thanks a lot for the help everyone.

Paddington, is there a specific section (perhaps in the ACI Manual) that talks about surface roughening as opposed to keyways? I'd like to stay current with the industry standard, and it seems as though keyways are a thing of the past.

One last note on this, are there any other special considerations that need to be taken into account when doing surface roughening as opposed to keyways? Or are there instances when keyways are really the better practice? I would assume that if you are doing a foundation wall/footing, and may have the potential for groundwater, that you would then place a footing drain at the toe of the footing, as opposed to tucked into the footing/wall "joint." Does this sound accurate?

Thanks again for all your help.

 

[theonlynamenottaken]

ACI 318: Section 11.7

 

[theonlynamenottaken]

And to answer your second question...

If you expect groundwater to be enough of a problem then you'd use a waterstop, not a key. A waterstop is a strip of copper, polymer or other material that is half-cast into the base, left hanging up and out to be cast into the stem. Expensive stuff and also not liked by most contractors.

 

[COEngineeer]

EngWade, surface roughening is not a must. You just have a higher coef. if you roughen it. Go to index and find "shear friction". It should take you to 11.6.

Just calculate the shear and see if it works with "Concrete placed againtst harddened concrete on intentionally roughened". You really dont want to tell them to roughened it because that is extra work (just like keyway).

If it is still not enough then use a keyway. Make sure you use a typical lumber size for the keyway.

Never, but never question engineer's judgment

 

[KootK]

I've always been curious about shear keys. With respect to this thread, I've got three questions/observations:

1) So am I correct to assume that most people are not using keyways these days? There's still something about the mechanical keying that makes me feel all warm and fuzzy.

2) I've always been leery of using shear friction for non-roughened surfaces, even with the lower coefficient of friction involved. My reasoning is this: in most elements, the shear friction reinforcing occurs near the faces of the element (walls/beams etc). And, with relatively smooth contact surfaces, the shear friction becomes mostly rebar dowel action. I worry that these bars placed near the surface of the concrete will cause spalling. Anybody else worry about this?

3) I don't think that shear keys entirely deserve their bad rep. It's not just a matter of shearing off the key. I think that the keyways can provide significant shear transfer capability via arch action (see attached sketch).

What I would really like to see is a reasonable way to calculate the capacity of keyways.

 

[Ltwine]

I would definitely provide shear key in cantilever walls. We used to reinforce critical keyways to increase shear/bearing strength.

 

[JAE]

1) I don't use shear keys in footing-to-wall joints...I use 1/4" intentionally roughened surfaces. Not hard to do as the area between the dowel bars is generally difficult to smooth off in the first place. The finisher need only to stamp his hand float to get the desired effect. Really no big deal.

2) I agree - don't use smooth surfaces with shear friction if I can avoid it.

3) A shear key like your sketch is OK, I guess, but I'd rather key in the whole slab at least 1 1/2" to 2" and then still check bearing on the small ledge. I'd still rely on shear friction (with smooth parameter) in the calcs anyway.

 

[hokie66]

I don't use part thickness shear keys, but setting down the entire wall into the footing gives a mechanical "bumper" without having to rely on shear friction, which I still regard as useful only in desperation when nothing else will work. (Preparing to dodge brickbats)

Likewise, I wouldn't use KK's part depth rebate for a slab to wall connection. A full depth rebate, using pullout bars, is standard practice in high rise construction for slip formed cores.

 

[rowingengineer]

I a fan of keying in the whole slab/ wall when possible similar to Hokie, however on Masonry retaining walls I use shear bars.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[paddingtongreen]

The problem with a full width key is that the key form must encompass the dowels, and the need for the foundation/slab stretch for, at least, the thickness of the wall, beyond the edge of the key.

I look at the extension beyond the key and I sometimes wonder why the requirements for corbels doesn't apply to it.

Michael.
Timing has a lot to do with the outcome of a rain dance.