Net Allowable Bearing Pressure
Worked example: 944 kPa ultimate, 30 kPa surcharge, FS 3 → 304.7 kPa — press Try an example to run it live, then adjust anything.
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Net Allowable Bearing Pressure explained
The ultimate capacity 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 = 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 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 formula
- = Net allowable bearing pressure (kPa)
- = Ultimate bearing capacity (kPa)
- = Surcharge at footing level (kPa)
- = Factor of safety
Missing one of these? Work it out first, then come back
- 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), Bearing Stress on a Pin or Bolt (σ = P/dt)
- 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