Appropriate footing size based on column load

by Deborah
(Richmond, VA)

Question:

If the soil bearing capacity is 2,500 lbs/sf and the column load on a footing is 15,000 lbs., calculate the appropriate size of the footing in square feet that it would take to spread the load out so that the footing wouldn’t exert more than 2,500 lbs/sf on the soil.

Answer:
Hi Debra, thanks for visiting all-concrete-cement.com.

The formula for calculating pressure is applied force (F) divide by area (A) or P=F/A

That is where the unit of pressure lbs/sf or pounds per square foot – sometimes express in square inch as psi (pounds per square inch) or ksi (kilo pounds per square inch).

Since we are looking for the area of the footing, solving the equation P = F/A for area yields A=F/P

Force F is given as 15,000 pounds, and pressure P is also given as 2,500 psf (pounds per square foot) or lbs/sf

A=F/P= (15,000 lbs)/(2,500 lbs/sf)=6 sf

Algebraically, the pounds in the equation above cancelled each other leaving the unit as 6 square feet of footing needed for the load to be transferred to the ground.

At a minimum, the required footing size is 2’-6” squared or 2’-6”x2’-6” which will yield 6.25 square foot area of footing.

The 15,000 pounds given above obviously does not include the weight of the footing itself so we need to calculate the weight of the concrete and add it to the total load of 15,000 lbs.

Normal weight concrete weighs about 150 pcf (pounds per cubic foot). Using 10 inch thick for the 2’-6”x2’-6” footing, the footing volume
V=LxWxH=2.5x2.5x(10/12)=5.2 cubic feet

footing weight=150x5.2=780 pounds

The (10/12) in the formula is to convert the footing thickness from inches to feet.

Adding the 780 pounds to the total load yield 15780 and recalculating the footing area per formula above:
A=F/P= (15,780 lbs)/(2,500 lbs/sf)=6.3 sf

The 2’-6”x2’6” footing picked above provides 6.25 square feet of footing area which is pretty close to 6.3 sf needed per above calculation. I would stick with the 2’-6” squared footing or if you like, you can use 2’-9” squared footing.

Hope this helps
Good Luck