Thickened Edge Slab on Grade - Two pours 3

[CANeng11]

We typically specify a monolithic pour for a slab on grade with a thickened edge, but we have a contractor requesting to construct it in two pours, with the slab resting on the footing. This is for a small garage. I don't see any issues with doing it this way, and it may in fact be better for cracking. Am I missing anything that may be an issue?

 

[dik]

Bad idea... best to have the slab integral with the stiffener... accomodates any moments from eccentrically loaded exterior walls, etc. and usually non-formed surfaces with the exception of the exterior.

Dik

 

[CANeng11]

Is the problem that the wall above would be sitting on the edge of the footing, thus inducing a moment that wouldn't be restrained by the slab?

 

[canwesteng]

Probably better to split the wall from the slab than this proposal.

 

[BAretired]

I don't know why the contractor wants to pour such a wide footing for a small garage. A four inch ledge should be adequate.

Whether the slab is poured monolithic or separate is probably not too serious for a small garage. The two pours can be properly tied together to restrain eccentric moment, making it effectively monolithic.

If this structure is in Alberta, frost heave can be expected. Looks like a layer of insulation between the granular fill and the concrete, right?

I assume that the two vertical lines at the bottom are dimension lines, not concrete piles, right?

BA

 

[Enhineyero]

if the width of the thickening is not that large, such as what you have shown here, I would ask the contractor on why do intend to do this. It doesn't seem to be logical/practical to me.

Should the construction move forward in this direction, I would probably add a tie bar (most likely S-shaped) tying the footing and the slab-on-ground.

 

[RayGon]

The good thing about the monolithic pour is it will create so much redundancies that any movements and/or stresses will be shared by the structure.

When you design the footing and slab separately and build them separately the redundancies is gone and the footing and slab will have to rely that their design is adequate for the loads and expected movement of the structure. Hence, I don't see that it will be a problem provided that the footing and slab is properly designed. But,I would advise to do what Enhinyero suggested above as its best practice to tie them together. Also, here in Australia, they will need to be tied to comply with the requirements for termite proofing the construction.

 

[dauwerda]

if the width of the thickening is not that large, such as what you have shown here, I would ask the contractor on why do intend to do this. It doesn't seem to be logical/practical to me.

I believe the contractor wants to do it in two pours because of the curb wall. Supporting the inside form for the curb wall while trying to simultaneously place and finish the floor slab does not look like anything I would expect a contractor to be excited about.

 

[CANeng11]

That's exactly the reason dauwerda.

So I'm thinking a 4 inch ledge with tie bars between footing and slab.

 

[r13]

IMO, the turndown slab should be made one pour, and leave the curb for the second. You should provide a J (L with 135° bent) hook from the turndown to the curb tying the curb bars.

 

[dik]

Something like:

Dik

 

[r13]

Yeah, like this (only differ in J hook).

 

[r13]

This is a stirrups made from 3 pieces - 2 Js + 1 cap tie.

 

[Enhineyero]

I would recommend Dik's detail, that's how I have always drawn it. Good outcome from thickening and slab being monolithic, amount of formwork is minimised, efficiency in the concreting work.