How does a monolithic foundation distribute weight?
I’m getting ready to build a house on a mono-pour slab foundation, a first for me. One question that has been rattling around my brain is how does the footer portion distribute the weight of the building to the soil below?
With a separate stem wall, the weight of the house sits roughly on the middle of the footer, allowing the weight to be evenly disbursed, but with a monolithic pour, the 2×6 walls will be sitting on the very outer edge. Doesn’t this cause an uneven weight distribution to the soil beneath? Why even have a 12″ wide footer if the weight will be held by the outer 6″? Wouldn’t a 6″ footer be just as effective?
I know the answer to my above questions is “no,” but I don’t understand why, and it’s a question that has been nagging me.
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A monolithic foundation is by design, very stiff. Whether you apply a load to the outside or inside of the "footer" portion of the wall doesn't really matter, because it still pushes down on the whole thing. With the slab attached and the stiffness of the whole shebang, the footer can't rotate under eccentric loading, so the soil still sees the same distributed load. If you load an isolated pier eccentrically, the whole pier can rotate and then it only loads the soil at the corner of its footer. But with a stiff monolithic foundation, the whole thing acts together and spreads the loads evenly.
Thanks for the explanation!
+1 on what Peter said. You will have less soil load and less differential movement (vs a slab poured inside a stem wall/footing).
Thanks!
Not much I can add here, but it may help to visualize this by thinking of the end of a column supported I beam. While you'd normally support such a beam with the column centered under the web, which is similar to a regular foundation wall in the center of the footer, you can also support the beam with a column NOT centered on the web. When off center like this, you need to keep the flange from bending up, or the web from buckling due to the way the forces shift. With a monolithic pour, you generally have sufficient strength in the wall to prevent buckling, so there can be no twisting motion. The footing also has sufficient strength to prevent any significant bending, so you continue to have loss distribution.
You can always keep loads distributed by controlling deformation of the support structure. This is how your “regular walk in the middle of the footing” works too. If it didn’t work this way, only the middle of the footing would support the load of that wall. The load is spread out because the footing itself does not significantly bend and acts to distribute the load of the wall over the entire area of the footing.
This is one of those things that is much easier to explain with a drawing, but hopefully you’ll be able to visualize the examples to get an idea of what’s going on.
Bill
Yes, that's helpful, thanks!