Footing Sizing — Method
An isolated footing must keep the soil bearing pressure under the allowable value. The plan area is therefore the total load divided by the allowable bearing capacity: area = load / q_allow, and for a square pad the side is the square root of the area. Add an allowance (typically 10%) for the footing self-weight, backfill and any surcharge. Sizing the area is only the first step - the footing thickness and reinforcement are then designed for punching shear around the column and for one-way bending, to ACI 318 or Eurocode 2.
Frequently Asked Questions
Divide the column load by the allowable soil bearing capacity to get the required plan area, then take the square root for a square footing side. 800 kN on 150 kPa soil needs 5.33 m2, about 2.4 x 2.4 m.
The safe pressure the soil can carry, after a factor of safety on its ultimate capacity. It depends on soil type and is taken from the geotechnical report - loose sand or soft clay may be 75-150 kPa, dense gravel or rock much higher.
Yes. Add an allowance (around 10%) for the footing concrete, backfill over it and any surcharge, since these also press on the soil.
No - it sizes the bearing area only. The footing depth and steel are designed separately for punching shear at the column and for bending, to ACI 318 or Eurocode 2.
Use the service (unfactored) load with the allowable bearing pressure to size the area. Use factored loads with the net soil pressure to design the concrete and reinforcement.
Footing and Foundation Sizing
A foundation transfers the building load into the ground without overstressing the soil or settling excessively. For an isolated column, a square or rectangular pad footing spreads the load over enough area to bring the bearing pressure within the allowable.
Sizing the area
area = load / allowable bearing capacity. Add self-weight and surcharge, then choose a practical square or rectangle whose actual bearing pressure stays within the limit.
Beyond the area
The thickness and reinforcement are governed by punching shear around the column and by bending across the footing, designed to ACI 318 or EC2. Combine with the concrete and rebar calculators for materials.
Related: Concrete Mix, Rebar, Retaining Wall.