## Footings

The minimum depth of a footing is usually determined by the anticipated depth that frost will penetrate below the soil. This depth is determined by using the building code that is applicable for your area. In Canada, the Provinces have adopted building codes that provide climactic conditions including the required depth below soil for footings.

The Ontario Building Code uses an assumed value of 1500 psf (75 KPa) for the bearing capacity of soils. A soils test can  be carried out to determine the actual bearing capacity of the soil. The information derived from this test will provide you with the bearing capacity for the soil on your site.

How does the soil's bearing capacity relate to the size of the footings? Lets say that you have a soils report in hand that says the bearing capacity of the soil is 3000 psf. That's the number to start with. From there, you need to work backwards and figure out the loads acting on the footing in pounds per lineal foot (or KiloNewtons per lineal metre KN/m) - plf or KN/m

Live loads on roof              20 psf x 1.5 =     30.00 psf
Dead Loads on Roof        15 psf x 1.25 =   18.75 psf
Live Loads on Floor          40 psf x 1.5 =     50.00 psf
Dead loads from floor       12 psf x 1.25 =   18.00 psf

The loading indicated above will vary for the roof loads depending on where you live and how much snow you get. In Toronto, the value is 20 psf so we will use this for the calculations. The live loads from floors are the same across Canada being 40 psf. The dead loads are derived based upon the anticipated flooring material. If you are using tile or hardwood flooring on subfloor with wood joists, the dead load value will be around 12 to 15 psf. If you are using a concrete underlay, the value will be around 20 psf. We will use the typical 12 psf for these calculations.

Then you wonder, why the 'x1.5' and the 'x1.25'? These are the values that are used to factor the live and dead load values. When calculating the loading requirements for floor joists and beams, you need to use the factored loads. When calculating for footing size, you use the specified loads only.