It is just like a continuous footing. The only difference is that the thickened slab footing is connected monolithically with the floor slab and this may cause problems when the footing settles under load - cracks running parallel to the wall.
I have never been much of a fan of slab thickenings. I would prefer to use a slab capable of distributing the superimposed loads to the soil without reliance on thickenings. This likely means a thicker and more heavily reinforced slab of uniform thickness throughout.
Agree with JAE and BA. Why not just pour the footing separate and build your wall (block?), pour slab up against on either side. If it is a smaller slab, just thicken as BA states. It all depends on loads, floor finishes etc.
I use the PCA guide for designing slabs on grade. It has a table which gives allowable linear loads for various thickened slabs. I think you will be surprised to find out how wide the slab must be--it won't be 8" X 16", but more like 8" X 36". This is to prevent the cracking that JAE mentioned.
Thanks guys,
I thought I was overthinking this, which is still probable, but for all of the reasons above, I have not been able to see the advantages or design procedure that would result in a little thickened slab/footing, other than some depth for anchor bolts. It seems like everyone of these would have to crack at the transition to slab. And yes, the prescriptive PCA gives a really wide footing. Why are there so many of these out there? Or are they based upon a punching shear calculation?
Thickened interior and exterior slabs can worsen restraint cracks in slabs from shrinkage. I have also seen the slab adjacent to a thickened slab edge crack from a perimeter wall load. But yes they are used all the time to allow for a single concrete pour, I try to avoid them at load bearing walls though.
I think most engineers design them incorrectly. But they don't fail, because as JAE mentioned, once cracks form parallel to the wall on each side, the load goes directly into the soil.
Dave, I agree....though I doubt that for the first 2 bezillion of them, no one bothered to calculate anything...just combined an 8x16 typical footing, tied to the floor slab.
You must also consider scour/undermining, so the thickened section should bear at least 12 inches below grade; however, that makes for a 16-inch deep section, tapering to 4 inches at the slab.
I've been refering to the army manual and it gives really wide thickened footings. I guess the idea of these things not cracking under load, the typical 16" thickened slabs, is really optimistic. The older engineer that I am working with avoided the whole issue with the we do it all the time and haven't had any problems response. I sure appreciate everyone giving this some thought.
