Concrete Footing Dimensions
So, how does soil bearing capacity relate to the size of footings? The footing transmits the load into the soil. The lower the bearing capacity of the soil, the wider the footing needs to be. If the soil is very strong, the footing isn't even strictly necessary just the soil under the wall would be enough to hold the building up.
Source: Table 403.1; CABO One- and Two- Family Dwelling Code; 1995.
You can look up the recommended footing size, based on the size and type of house and the bearing capacity of the soil. As you can see, heavy houses on weak soil need footings 2 feet wide or more. But the lightest buildings on the strongest soil require footings as narrow as 7 or 8 inches. Under an 8-inch-thick wall, that's the same as saying you have no footing.
These numbers come from assumptions about the weights of building materials and the live and dead loads on roofs and floors. The allowable bearing capacity of the soil under the footing has to equal the load imposed by the structure. Reading down the table, you see that the code calls for a 12-inch-wide footing under a two-story wood-frame house in 2,500-psf-bearing soil. A 12-inch footing is 1 square foot of area per lineal foot, so the code is saying that the portion of a two-story wood house that bears on the outside walls weighs about 2,500 pounds maybe a little conservative, but reasonable. The same size footing is called for under a one-story house if it has brick veneer the brick is assumed to weigh as much as a whole second story.
If you had an engineer design the footing based on soil testing numbers and your prints, he'd add up the actual weights of the concrete, wood, and brick you'd be using in your building, factor in the required live loads, and come up with an estimate of the weight your actual house puts on the footing. It might be a little less or a little more than the code assumes. Then he would take the known bearing strength of the soil what a square foot of the soil can be trusted to support and design the footing so that the area under the footing multiplied by the bearing strength of the soil would equal or exceed the actual load.
In practice, you don't have to do this engineering on most houses. The amount you'd differ from a standard code-compliant footing isn't worth worrying about. Unless you have retaining walls or some other special situation, an engineers fee probably isn't justified.
In any case, I wouldn't recommend that builders cut back on their standard footing size even if they know they're building on strong soil. Regardless of bearing requirements, masons and poured-wall contractors want footings for their block or their forms to sit on. But the lesson to take is that when soils are very strong, (4,000-psf capacity or better), the footings may not be strictly necessary from the standpoint of bearing. This means it is less important, for example, whether the wall is correctly placed in the center of the footing.
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