Control joints in long foundations

[canwesteng]

I have a long strip footing (~300'), and having some discussion on the need for control joints. Minimum rebar of .2% will be provided (possibly increasing to 0.5%) to control cracking, but are joints required as well?

 

[gte447f]

I say no, but I am interested to see what others say. I have never given a single thought to providing a control joint in a strip footing, but admittedly I can't recall designing one 300ft long. I would say, who cares if it cracks? It's underground. Presumably if it is reinforced with adequate temp and shrinkage steel, then the cracks should be uniformly distributed and the widths of the cracks thus controlled. Also presumably adequate cover to reinforcing should protect the rebar from corrosion. One caveat might be if there is an expansion joint in the building, then maybe that joint should extend through the foundation also.

 

[phamENG]

I agree that an expansion joint should carry through. You may also want to provide a construction joint detail. 300' long is not a small pour for most contractors. There's a good bit of logistics to make a pour like that work. A lot has to go right to have all those trucks get there on time and without issues in their load.

 

[canwesteng]

This isn't for a building, it's for a structure with no expansion joints at all. The construction joint is detailed with constraints on how they are to be located and constructed, but quantity is up to the contractor. The construction joint is effectively where I anticipate cracking occurring, but the spacing will be much larger than typical control joints.

 

[IceNine]

Another vote for no control joints. I've never seen a control joint in a strip footing, and don't think a crack would have any impact on the performance.

 

[KootK]

Is there some manner of frost wall on top of the strip footing?

 

[KootK]

Were there a building with control joints atop this foundation, I feel that one could make a pretty good argument for not extending the building control joints trough the foundation. My reasoning:

1) At the foundation, the primary source of movement is shrinkage. That will often be:

a) Restrained in a distributed fashion such that a movement joint is pretty useless and;

b) Have taken place substantially prior to thermal cycling of the super structure (I'm assuming steel).

2) Foundations within the earth will usually experience lower thermal extremes than the superstructure.

3) In many cases, one does not want differential settlement across a building expansion joint. Separating the foundations often kicks you into greased dowels and all that jazz.

 

[canwesteng]

No frost wall - the footing extends from grade to below the frost line.

 

[KootK]

Okay, so more like beam proportions.

I don't feel that 0.2% provides meaningful crack control for situations like this. It's fine in your typical strip footing because, even with reinforcement, you're usually not using the longitudinal reinforcement for anything serious anyhow. So we don't care too much if there are large cracks that might compromise the reinforcement from a corrosion perspective. However, if you need the footings reinforcement for structural purposes, I'd be inclined to either:

1) enforce a maximum poor length as is fairly common with frost walls, even if no one follows it or;

2) do the 0.5% thing.

Can we know what kind of superstructure sits on this foundation? Or are you deliberately keeping that under wraps to keep a reign on the kind of responses that you get? Responder scope management after a fashion. Context is usually helpful, or course, particularly whether or not any human is going to care what the top of this footing looks like.

 

[canwesteng]

Essentially half of a large buried culvert sits on the footing, to make a tunnel. In this case the longitudinal reinforcement is still only for crack control, at least nominally. I expect it will actually be stressed in flexure as the strip footing works to distribute bearing pressure evenly over the subgrade, since we except about 2 inches of even settlement. I came up 0.5% using the CPCI design manual estimate of shrinkage strain, equating that to a stress, and figuring out how much bar will be needed to resist that stress. I don't think it's strictly correct, but at least gives some rationale for the increased bar.

For frost walls, what would the maximum pour length be?

I can say that in the past, the contractor hasn't seen others use joints. But that tells us nothing about how these are performing, or how much bar was added to control cracking.

 

[KootK]

For frost walls, what would the maximum pour length be?

I don't recall off of the top of my head and can't spare the time to dig for it. I like the 0.5% solution.