Net Allowable Bearing Pressure
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Learning zone
The ultimate capacity q_u is a collapse load and no one builds on it. Divide by a factor of safety — conventionally 3 for shallow foundations, the highest routine factor anywhere in structural engineering, because soil is variable and never gets a mill certificate. The subtlety is "net": the ground at founding level was already carrying q from the soil that was excavated away, so only the increase over that needs a safety margin. With q_u = 944 kPa, q = 30 kPa and FS = 3, the net allowable pressure is (944 − 30) ÷ 3 = 305 kPa.
The trap is mixing gross and net. If you compute q_all as net, the structural load you compare it against must also be net — the column load minus the weight of the soil displaced — or you will penalise the footing twice. The second trap is thinking that a factor of 3 against collapse means the building is fine. On clays and loose sands, settlement usually governs long before capacity does, and a footing perfectly safe against bearing failure can still settle 50 mm and crack every partition in the building. Check both, and check differential settlement between footings hardest of all.
- = Net allowable bearing pressure
- = Ultimate bearing capacity
- = Surcharge at footing level
- = Factor of safety
- Net allowable bearing pressure — Effective Stress (Terzaghi, σ′ = σ − u), Terzaghi Ultimate Bearing Capacity (Strip Footing)
- Ultimate bearing capacity — Terzaghi Ultimate Bearing Capacity (Strip Footing), Total Vertical Stress (σ = γz)
- Surcharge at footing level — Terzaghi Ultimate Bearing Capacity (Strip Footing), Total Vertical Stress (σ = γz)
- Factor of safety — Factor of Safety, Factor of Safety Against Sliding