Foundation Load Bearing Capacity Calculator (Terzaghi)
Calculate the ultimate bearing capacity of shallow foundations using Terzaghi's method. Auto-interpolates bearing factors based on friction angle.
About
Determining the bearing capacity of soil is the first step in shallow foundation design. This tool utilizes Karl Terzaghi"s classical bearing capacity theory, which assumes that soil failure occurs along a specific shear surface under the footing. The calculation is highly sensitive to the internal Friction Angle (φ) of the soil, as this value dictates the geometry of the failure wedge.
Manual calculation is often tedious because the dimensionless Bearing Capacity Factors (Nc, Nq, Nγ) are non-linear functions of φ. This calculator automates the derivation of these factors, providing instant estimates for Ultimate Bearing Capacity (qult). It is designed for strip, square, and circular footings, applying the appropriate shape factors automatically.
Formulas
For a Strip Footing, Terzaghi's equation is:
Where:
- c = Soil Cohesion
- q = Effective surcharge (γ × Df)
- B = Footing Width
- N factors depend on φ
Reference Data
| Friction Angle (φ) | Nc (Cohesion) | Nq (Surcharge) | Nγ (Weight) |
|---|---|---|---|
| 0° | 5.70 | 1.00 | 0.00 |
| 5° | 7.34 | 1.64 | 0.50 |
| 10° | 9.61 | 2.69 | 1.20 |
| 15° | 12.86 | 4.45 | 2.50 |
| 20° | 17.69 | 7.44 | 5.00 |
| 25° | 25.13 | 12.72 | 9.70 |
| 30° | 37.16 | 22.46 | 19.70 |
| 35° | 57.75 | 41.44 | 42.40 |
| 40° | 95.66 | 81.27 | 100.40 |
| 45° | 172.29 | 173.29 | 297.50 |