Spread Footing Bearing Pressure
Bearing Pressure Under a Spread Footing
A spread footing must distribute its column load over enough soil that the contact pressure stays within the ground’s safe bearing capacity. This worksheet computes the average (gross) bearing pressure and the factor of safety.
For a concentric vertical load the pressure is q = P/(B·L), the load divided by the footing plan area.
Inputs
A rectangular pad footing carrying a service column load.
P := 600 kN=600000 N
B := 2 m=2 m
L_f := 2.5 m=2.5 m
q_allow := 200 kPa=200000 Pa
Bearing pressure and factor of safety
q := P/(B·L_f)=120000 Pa
SF := q_allow/q=1.67
Results
The bearing pressure must stay below the allowable value, giving a factor of safety greater than 1. If it does not, enlarge the footing plan dimensions. Remember the allowable pressure already embeds a safety factor (often 2.5–3) on the ultimate capacity, and settlement may govern over bearing.
Assumptions & limitations
- Concentric vertical load — no moment/eccentricity.
- Uniform pressure distribution (rigid footing on soil).
- Settlement and ultimate bearing capacity checked separately.