calculation of width of continuous footing

Question:

What's the procedure for calculating the width of continuous footing?

Answer:
In order to calculate the footing size for any structure, you need to know the soil pressure and the weight of the building.

1. Soil pressure: the allowable stress that a particular soil can take is determined by a soil engineer or geotechnical engineer. Determining allowable soil pressure for design purposes requires testing of the soil. This process can take weeks because it requires the geotechnical engineer to bring special equipment to the job site and remove soil samples at various locations in order to determine the load carrying capacity of the soil.

Depending on the type of soil and the nature of the structure to be built, removing of soil samples may require to dig down to a few hundred feet deep. Considering the amount of time and effort to obtain the soil samples as well as the special equipment to not only remove the soil samples but to test the soil samples for strength mean that this process can cost a lot of money. For this reason, vast majority of homes are build on soils that were not professionally tested. In most cases a soil pressure of 1500 psf is used for untested soils.

2. Weight of the structure - to calculate the size of the footings, we must know how much the building weigh. This will require to estimate the weight of the roofs, walls, floors, and any equipment that will be potentially on the structure.

The formula for footing design is Soil pressure = P/A, where P is the weight of the building and any live load, A is the area of the footing.

Let's assume we have a building load of 3500 plf and we need to find out the width of the continuous footings.

A=P/soil pressure = 3500/1500 = 2.3 SF

Area = L x w, w = area/L = 2.3/1
For every foot of the continuous footing we need 2'-4" width of footing.