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.
How does the soil's bearing capacity relate to the size of the
footings?
Lets say that you have a soils report 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
The footing of a building transmits load to the soil. The lower
the bearing capacity of the soil, the wider the footing needs to be.
If the soil is firm, the footing can be narrower. Consider some of
the following numbers. Sand has a bearing capacity of about 3000 psf
(pounds per square foot).
Try a
calculation for a typical 2 storey house using common loading
conditions to see what this looks like. To do this, you will need to calculate the live loads and the dead loads.
After this, you will calculate the length of supported floors and
roofs that the footings will support. Then you add the wall loads to get an approximate footing
size for your project.
Live loads on roof 20 psf
Dead Loads on Roof 15 psf
Live Loads on Floor 40 psf
Dead loads from floor 12 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.
When calculating
for footing size, you use the specified loads only.