How necessary is it to have the slab rest on the footing? What do you do for stepped footings? How about when you insulate the slabe edge when you have infloor heat?
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Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
The reason for the footing is not to support the slab but rather to distribute the weight of the structure placed over the slab. Just think about concrete driveways and pads...obviously no footing there, yet they are able to carry a tremendous live load and take frosts without many problems.
It really depends on the purpose of the slab. If it is for a one story garage with 2x4 framing then you can usually usually get away with a simple 8" wide trench footing (depth will depend on your frost level). If it is a house, you will want to have a professional designer or builder look at your site conditions.
gk
I guess my question was misunderstood.
This is the poured-wall basement for a house where the footing supports the wall. This is also a situation where the house is built on a sloping lot so that one wall of the basement opens to daylight and in order to do this, the footings and wall are stepped down the slope so that the footing is always below the frost line.
Now, if the footing steps down, obviously the floor slab will lose the footing support after the first step. My question is: What is the best way to support the slab after that? Or is the compacted earth beneath sufficient?
RSB,
You are correct that the ground will support the slab. As I said before, The footing is there only to distribute the weight of the wall and the fact that many slabs rest on the inner portion of the footing is simply coincidence. In a floor slab situation, the compacted sub base acts like a soil stabilizer, which in turn allows the slab to properly distribute its load across its entire area. Admittedly, there are exceptions to this generalization but from what you describe this will most likely be your situation.
gk
Ahhh, NOW i unnerstand! I believe the term is stem walls...could be mistaken...but there are short walls of concrete block, or poured concrete, that go from the footing to the slab.Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
You have reason for concern if you have a dense clay soil in your area. I have a situation here in Ohio where the slab actually sunk due to the clay drying out over the years. The footing stayed put, the interior slab dropped 1/4 - 1/2". The area under the slab was well compacted stone.
The slab you are going to have is at one level? The footings may be stepped, but the slab will be at one constant plane. As done for exterior brick veneer, have them provide a brick ledge on the interior and rest the slab on that.<P><FONT face="Times New Roman,Times,Serif"><FONT color=purple><FONT size=2><STRONG>Remodeling Contractor just outside the Glass City</STRONG>.</FONT></FONT></FONT></P>
<P><BR><A href="http://quittintime.infopop.cc/ubbthreads/categories.php?Cat=">Quittin' Time</A></P>
I guess the interior brick ledge you suggest is the best way to do this. The foundation wall gets pretty thick however, but the price of a little more concrete is cheap insurance.
Even with the brick ledge, if the compacted interior material drops away a little bit for some reason, now we have the floor slab hanging out there supported only at the edges and depending on the rebar to hold the whole works together.
I understand your concern about the hollow under the slab. I was suggesting you think about the possibility of clay drying and shrinking. No matter how well compacted the base, if the clay drops, the slab could go with it. You should be able to construct the slab much the same as a suspended or tensioned slab. If I'm remembering right, JLC had an article about a tensioned slab, with trenched footings. I've even seen some pictures of a non usual (to me) slab construction used in australia where there are cross hatched footings or thickness increases throughout the pour. With rebar laid properly, the slab could I guess become self supporting with proper footings. Good concrete people reside here, hopefully one will help you out. It's a bitch when it's too late.Remodeling Contractor just outside the Glass City.
Quittin' Time