Slab on Grade Connection to Foundation

[ScratchyFilm]

I have a seen a few different orientations for a slab on grade and its connection into a foundation wall. Is there a more correct way to do it? Specifically for something like a generator or small building station, my thought would be to use the 1st orientation with L-bars to tie everything together, but I have seen some details of the other two as well. Thoughts?

 

[phamENG]

Why are you tying it to the foundation? Most slabs on grade aren't "structural" - they're just used to spread the load out a little to the properly prepared base and sub-base underneath. No load goes to the perimeter foundation walls. Now, if you have a pile supported slab, that's something else entirely.

A turndown onto the top of the wall is okay - and sometimes important if the wall is also retaining the fill under the slab. But then you have to design your slab on grade as a structural diaphragm.

Notching the top of the wall - like a header block in a CMU wall - doesn't really do anything for you unless keeping the slab in precisely the right position along the wall (potentially at the cost of cracking a few feet into the room) is important.

Your L-bar detail is almost not build-able. How do they compact the soil next to the wall? They'd have to have the bars straight, fill and compact, and then field bend the bars while they're embedded. I've had to do this before on a loading dock - the contractor complained the whole time and we almost had to rip out a portion of the wall and redo it. You can really only bend the bars once or you start getting into serious strain hardening issues.

 

[ScratchyFilm]

phamEng, my understanding of the details that have been used in the past is that the load is intended to be transferred to footings below the frost depth - not sit on grade. And as a result, the slab is almost treated as an elevated slab in terms of design - not just an on-grade slab. I believe the Contractors typically only dig out a width necessary to install the foundation walls and then use sand to fill in any remaining gaps. I've been specifically reviewing details for generator pads, FWIW, so application is slightly different than, say, a building.

 

[hetgen]

the slab is almost treated as an elevated slab in terms of design

You could detail it as an approach slab if it is an elevated slab.

 

[ScratchyFilm]

That seems like a decent way to approach this, hetgen.
I guess my question would be is there any issue in installing L-bars from footing to slab if there haven't been constructability issues in the past? Always a little uncomfortable to tell someone to move away from a standard that has been used for a while.

 

[dik]

The bars are placed in the 'keyway form', bent as a Z shape and straightened into the slab after the wall is backfilled. I don't do that detail for SOG... nearly all are totally free of the foundation wall, but on occasions have done that at doorways or door opeings (generally without reinforcing).

Forgot to add... that way, you don't have to drill your forms for the bars.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[canwesteng]

What stops frost from pushing the slab on grade up and lifting the footing? Are you putting a void below the slab?

For generators (small to medium), I would put them on a raft and let them move with the seasons. Otherwise they should sit directly on footings.

 

[MotorCity]

Whats the reason for not allowing to act as a slab on grade (other than "thats the way we've always done it")? If its being treated as an elevated slab, is it being designed as such for flexure and shear? What supports the other end of the slab that is not shown? Supporting the slab as you show can certainly be made to work if designed and detailed correctly, but why add all the extra bars, wall notches etc? Typically, I design it as a slab on grade that is separated by 1/2" expansion material to keep it structurally independent of the grade wall.

 

[dik]

Usually the insulation on the foundation wall... I hate ice forming on the inside part of a SOG. As I noted, I don't connect the two and the dowel rfg is only used for interior structured slabs.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

That's what I do... if the equipment, electrical or mechanical, can move, it goes on a SOG, else, it's structured. Nearly allways spec 1/2" Flexcel joints.

If interior equipment the slab may be 4" or so thicker for a housekeeping pad, but again is isolated with Flexcel. If if doesn't need to be elevated and can be supported by the SOG, no special provisions are made.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[ScratchyFilm]

MotorCity - I don't think there is a specific reason for not allowing it to act as a slab on grade other than the idea of getting everything below the frost line to avoid movement (might not even be a big deal).
Correct, the slab is designed for flexure and shear - same as if it were an elevated slab.

If treated as a slab on grade, I don't even see any reason for footings at all. It would simply be loads sitting on the concrete slab.

 

[canwesteng]

Without a void form below it will just behave as slab on grade anyway, at least for frost heave.