Slab-on-ground edge support? 2

[SteveFehr]

When pouring a 4" slab-on-ground for light duty residence, should the ends be supported by an L-shaped shoe block or similar end support?

The contractors in the area all insist on doing this to prevent the slab from settling too far, but it seems to me that this would essentially turn the slab into a 17' beam and exert stresses a 4" slab can't handle, allowing the slab to crack and settle. I've seen conflicting things online and in different books, too. Is my reasoning sound here, should I insist on an unsupported floating slab, or are the contractors right?

The bearing edge of the block will be insulated per code requirements (R-5 here, 1 or 1.25" foam); if the fill in the center of the slab settles, the entire bearing weight of the slab would be on the edges- I know 3" of foam isn't going to be able to handle the loads of a 17' span of concrete and I would expect it to crush as the fill settles and the stress upon it increases. Which would lead not only to the exact same settling as if it were an unsupported floating slab, but cracking it in the process. By my reasoning, a floating slab would make more sense, as it would settle the same amount but the stresses would be generally uniform and it wouldn't crack.

 

[Ron]

If you want a monolithic slab, the edges should be thickened for two reasons: wall support and to prevent scour undermining.

Typical thickened edges are 12 inches thick with reinforcing. Anything you put under the slab or the integral "footing" should be uniform.

 

[SteveFehr]

I wasn't intending to make a monolithic slab; between the frost line and flood elevation requirements, a monolithich slab would have to be 3' thick at the edges, and that's a lot of extra concrete. So, I was looking to instead put 3 courses of blocks on an 8x22" footer, and pour a 4" slab in the middle. Would I be better off with simple 6" block sill walls and have the slab rest entirely on fill, or to have the edge of the slab resting L-shaped shoe blocks?

If it makes a difference, I had intended to use 3500psi concrete, 6" WWM and fibermesh in both slabs. The 6" garage slab is going to be floating, no question of that, but I was concerned about settling and cracking in the house slab. Thanks!

 

[UcfSE]

I believe you are talking about "chair" block at the top of your stem wall.

Any questions of this sort should be brought to your design professional who did your engineering and drawings.

Using a chair ties the slab to the stem wall and vice versa. This is common with residential construction. It is not necessarily good for commercial practice. If you have a very heavy wall load, you have the potential to develop cracks as the footing tries to settle more than the slab. Either way, your slab will settle how it settles. It will never try to act like a beam spanning 17 feet. As long as the fill is compacted properly you shouldn't have too much cracking though you definitely will have some. Concrete cracks, that's just it. That footer size sounds a little small to me. We usually have a thickness of half the width for typical strip footings. Definitely use 8" cmu.

This is all my opinion. You should have your professional engineer or architect help you with your questions before you actually take any action or make any decisions about how to proceed.

 

[SteveFehr]

They weren't talking about chair block, but man, that looks like the perfect thing to use, exactly what I want.

This is an addition for my house, I'm doing all the design myself. The existing house is slab-on-grade with a 4x16" footer and 2-block stem wall. I'm not a PE yet (nor would I stamp something this far out of my area of expertise if I was!) so I was just trying to stick to the codebook and the experience of the contractor, but it seemed like the contractor's plan required engineering. Guess not I did run everything through the city engineer as I was a little unsure of the foundation and the shear walls, and he approved both and recommended some changes in my gable-ends. All else is just simple construction, right from the code book- I'm looking forward to the project!

IRC requires footers be at least 15" wide, but limits the width on either side to the thickness of the footer (without engineering, at least). With an 8" thick footer and 8" block on top, the footer could be a maximum of 24" wide. With 6" block, an 8" thick foundation can be max 22" wide. I want to use 6" block because my walls are 2x6 and that eliminates stepping and caps and makes a lot of things a whole lot easier; and 2x6 block is plenty strong enough for 2-story residence in my wind zone. With 22" wide footers, my ground pressures will vary from 259psf to 589psf at different points in the house so I'm not expecting too many issues there. I did a ground pressure test (completely unscientific, just for my own curiosity) and it held about 4000psf for 8 hours with no discernable movement, and was approaching 6000psf before the pile of blocks got too tall and toppled. So I'm content to assume my soil is at least 1500psf. The finger test points to around 4000psf too; I'll check the footer troughs before we pour.

 

[SteveFehr]

Is differential thermal expansion a problem with heated monolithic slabs? Would that be reason enough to just go with a simple floating slab? It doesn't sound like my local block suppliers stock 6" chair block and this is too small for a special order.

If my figures are right, in the worst-case scenario, with a warm heated slab on a very cold day, the 17x28' slab will want to expand appx 1/8" longer than the footers, bond beams and block wall. Will that cause failure or just cosmetic cracks?