In practice, you don't have to do this engineering on most houses. 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. It might be a little less or a little more than the code assumes. 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. 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. 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. 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. The allowable bearing capacity of the soil under the footing has to equal the load imposed by the structure. These numbers come from assumptions about the weights of building materials and the live and dead loads on roofs and floors. Under an 8-inch-thick wall, that's the same as saying you have no footing. But the lightest buildings on the strongest soil require footings as narrow as 7 or 8 inches. As you can see, heavy houses on weak soil need footings 2 feet wide or more. You can look up the recommended footing size, based on the size and type of house and the bearing capacity of the soil. Footing depth - varies based on frost line and soil strength (some footings can be shallow, while other must be deep)Ĭoncrete Calculator - Figure out how much concrete you'll need for your footings.Source: Table 403.1 CABO One- and Two- Family Dwelling Code 1995. Here are the minimum widths for concrete or masonry footings (inches): Load-Bearing Value of Soil (psf)Ĥ-Inch Brick Veneer Over Wood Frame or 8-Inch Hollow Concrete Masonry 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.įind nearby slab and foundation contractors to help with your footings. The lower the bearing capacity of the soil, the wider the footing needs to be. So, how does soil bearing capacity relate to the size of footings? The footing transmits the load into the soil.
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