Strip Footing Bearing Capacity Calculator
Estimate the ultimate and allowable bearing capacity of a shallow strip footing using Terzaghi's classic equation. You supply the soil strength parameters and the bearing-capacity factors read from a table, and the tool returns the ultimate bearing pressure and the allowable pressure after applying a factor of safety.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the soil cohesion c, the surcharge q at founding depth (usually γ × depth of embedment), the soil unit weight γ and the footing width B.
- Read the Terzaghi bearing-capacity factors Nc, Nq and Nγ for your soil friction angle from a standard table and enter them.
- Set the factor of safety (3 is a common default for shallow foundations) and read off the ultimate and allowable bearing capacities.
How it works
For a long strip footing the shape factors are 1, so Terzaghi's ultimate bearing capacity is qu = c·Nc + q·Nq + 0.5·γ·B·Nγ. The first term is the cohesion contribution, the second the surcharge (overburden) contribution and the third the self-weight contribution of the soil beneath the footing. The allowable bearing capacity is simply qu divided by the factor of safety, which guards against both shear failure and uncertainty in the soil parameters.
Worked example
Worked example. With c = 20 kPa, q = 18 kPa, γ = 18 kN/m³, B = 2 m and factors Nc = 17.69, Nq = 7.44, Nγ = 4.97: qu = 20×17.69 + 18×7.44 + 0.5×18×2×4.97 = 353.8 + 133.92 + 89.46 = 577.18 kPa. With a factor of safety of 3 the allowable bearing capacity is 577.18 / 3 = 192.39 kPa.
Common mistakes
- Using factors for the wrong friction angle — Nc, Nq and Nγ change sharply with angle, so read them for the correct φ.
- Entering the surcharge q as a stress from an unrelated load instead of the overburden pressure at founding depth (γ × embedment depth).
- Applying this strip-footing form (shape factors = 1) to a square, rectangular or circular pad footing, which needs shape factors.
Frequently asked questions
Where do Nc, Nq and Nγ come from?
They are dimensionless bearing-capacity factors that depend only on the soil's friction angle φ. Read them from a Terzaghi bearing-capacity factor table (or chart) for your φ and enter them here.
Why is the allowable capacity so much lower than the ultimate?
The factor of safety (commonly 3) divides the ultimate value to cover uncertainty in soil parameters, workmanship and to limit settlement, giving a safe design pressure.
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Tip: Enter any known values to calculate the remaining results.
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