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Foundation Wall Keyways

Foundation Wall Keyways

Foundation Wall Keyways

(OP)
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.

RE: Foundation Wall Keyways

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

RE: Foundation Wall Keyways

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

I never use them.

DaveAtkins

RE: Foundation Wall Keyways

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.

RE: Foundation Wall Keyways

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.

RE: Foundation Wall Keyways

(OP)
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.  

 

RE: Foundation Wall Keyways

(OP)
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?

RE: Foundation Wall Keyways

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)

RE: Foundation Wall Keyways

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.

RE: Foundation Wall Keyways

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.  

RE: Foundation Wall Keyways

(OP)
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.

RE: Foundation Wall Keyways

ACI 318: Section 11.7

RE: Foundation Wall Keyways

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.

RE: Foundation Wall Keyways

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

RE: Foundation Wall Keyways

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.




 

RE: Foundation Wall Keyways

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

 

RE: Foundation Wall Keyways

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.

 

RE: Foundation Wall Keyways

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.

RE: Foundation Wall Keyways

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

RE: Foundation Wall Keyways

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.

RE: Foundation Wall Keyways

Michael,
Not sure if I understand.  When I use a full width key, the entire footing or slab outside the inside face of the wall is at the same level.  So if the footing is 12" thick inside, it is 10" thick from the inside face of the wall outward.   

RE: Foundation Wall Keyways

hokie, I meant the footing outside of the wall, using your numbers, 12" thick inside the wall, 10" under the wall, 12" outside the wall. It's the width of the outside part that interests me, it obviously needs to be the same width as the wall thickness to have the same shear area but what intrigues me is that if I rotate the detail 90 degrees, so that the wall is trying to move downward, that outside looks like a corbel off the bottom of a beam and I wonder why it is not subject to similar requirements.

For clarity, I should say that I have a water tank in mind through these musings.

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

RE: Foundation Wall Keyways

pad:

I think you better to provide a sketch, I am curios to know your concerns, but getting lost here.

RE: Foundation Wall Keyways

We could discuss the theoretical musings of keyways ad nauseum.  When it comes down to it testing would be the only basis for a concrete (pun intended) judgement.

From ACI224.3-08 "Joints in Concrete Construction" (which has just been re-reapproved from 1995's version)
"Shear transfer is provided by shear friction between the old and new concrete, or dowel action in the reinforcement through the joint. Shear keys are usually undesirable (Fintel 1974), since keyways are possible locations for spalling of the concrete. The bond between the old and new concrete, and the reinforcement crossing the joint, are adequate to provide the necessary shear transfer if proper concreting procedures are followed."

I checked the Fintel reference.  It's a book, not a test report.  Anyone know if there's any testing data out there or have I just found my PhD thesis?

RE: Foundation Wall Keyways

I am questioning ACI on this blanket statement.
A member build on the other with rotation caused by large bending moment, there is possibility losing partial contact due to tension at hell, though the compression toe will gain extra force to deliver required shear friction, but in a very limit area (C small). I don't know how reliable the shear friction carried under such condition.

Further more, by any chance once debris get into the joint, there may not have any shear friction capability.

I wondering if there is test done on member subjects to both shear, moment, and stressed to near/at yield.

RE: Foundation Wall Keyways

Quote:

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?

I worry about this too.  My solution is somewhat convoluted.  First, I meet the code by checking the shear friction provisions for the tension steel.  This leaves me feeling uneasy, so I provide dowels across the joint at the compression face that are able to take the shear.  (I would have provided dowels there anyway, so this is usually just a check) On the compression face, the shear force is away from the face of the wall, so the concrete won't tend to spall.   

RE: Foundation Wall Keyways

The only objective I can accept is to be construction friendly, to minimize mistake/potential weakness created by varies reasons, and in application there is no/little chance to loss contact area, dowel alone can not prevent slip accross gap.

RE: Foundation Wall Keyways

JAE: Agreed, a full depth key is better. I always seem to encounter this as ramps (often curving in plan) that need to be keyed into walls.  Contractors complain that, using a full depth key, the key is tough to place accurately and you end up with some of the key beyond the depth of the slab.  Maybe that's not the end of the world.  In the past, I've done these checks:

1) I check slab shear using the usual provisions but assuming that "d" is from the top of the slab to where I reasonably expect the tie reinforcement to land.

2) I check the tie reinforcement for it's ability to resist the tie force generated by some kind of reasonable arch behavior.

3) I check bearing on my assumed arch / comp. strut.

4) Shear friction for sport.

For the particular joint that I sketched, the notion of using two smooth, unkeyed surfaces and dowel/SF only gives me the willies in the worst way.

LTwine: You're quite right, the tension in one part of a SF connected section is compensated for by the added compression elsewhere in the section.  It is common practice -- and necessity really -- to take advantage of this.  Shear wall design is a prime example.

MIECZ: I'm not so sure that spalling doesn't occur for that case too (pushing the dowels towards the centroid of the cross section).  Do an FBD on the rebar on one side of the joint.  Any compression against the concrete on one side of the rebar ultimately has to be balanced by compression on the other side.  That balancing compression imposed on the rebar is seen by the cover concrete as spalling inducing tension.  This is a better situation, to be sure, but I still suspect that spalling could be an issue.
 

RE: Foundation Wall Keyways

Kootenay Kid

I don't see what you're getting at.  Maybe a sketch would help.

RE: Foundation Wall Keyways

Paddington,

Just to clarify, the 10" depth in my example goes outside the wall, all the way to the heel.  That way, the step is made with just a lump of timber tied to the inside face reinforcement.

The design of the heel may well be like typical corbel design, depending on the loading.

On the shear friction issue, I wouldn't like to use a flush joint at a suspended slab to wall connection.  A wall to footing connection is different because of the wall load normal to the joint, but I still prefer bearing rather than clamping.   

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