Key Considerations when sizing and designing footings
One of the most important things to consider when determining the depth of your excavation and the location of your footing is its relationship with other structures or buildings. Essential what this means is that there should be no obstructions or structures within the angle of repose of the footing. The angle of repose is the soil directly below the footing including all adjacent soil lying with a 45 degrees angle down from the underside of the footing. This is applicable to all new and existing buildings. When the soil within the angle of repose will compromised, the retention of a structural engineer will be required.
Footings are required in several locations.
Essentially under all load-bearing elements which include:
Although reinforcing is not necessary by most Building Code, it is highly recommended that reinforcing be installed to improve the structural integrity of the footing. Reinforced footings are much less susceptible to cracking and can bridge short areas in the supporting soil that may have weaker soil bearing capacity. This is also true at transition between various soils such as between rock and granular for example.
Footing size varies greatly depending on the size and height of the building the type of soil and also where the building is being built. Although must Building Code prescribe the minimum width and depth of footings, in my experience I have always found these minimum to be much too small for my own peace of mind. The norm for most buildings is 20” wide by 6” or 8” deep.
The other thing that to keep in mind is the proximity of the footing to the water table. Most codes require that the soil bearing capacity of the soil be divided by half in order to design and place footings which are located within a certain distance from the water table.( this is usually within a distance equal to that of the footing with) Therefore some soils become none suitable when their soil bearing capacity is reduced by half (sand and granular soils usually fall within that category).
Another key component is the thickness of the footing. This needs to be the same as the projection of the footing and each side of the wall that it supports. Essentially this means that thickness of the footing cannot be less than the projection of the footing beyond the edge of the wall it is supporting.
In northern climate, the location of the footing always needs to be below frost as this prevents any kind of movement. When you are working with soils like clay and silt that are water retentive and susceptible to frost because of the water it can hold, special preventive measures should be taken.
These are only the key components that have to be considered when design or sizing footings for a typical residential building. Building Codes, local requirements, soil type and other factors will greatly affect the design process when it comes to sizing your footing. If you have any questions please feel free to contact the writer at the links below.
