About Foundation Design Calculator
The foundation design calculator sizes and checks shallow spread footings using the Terzaghi, Meyerhof, and Hansen bearing capacity methods, with shape, depth, and groundwater corrections. It is used by geotechnical and structural engineers to verify allowable bearing pressure, predict settlement, and design the footing reinforcement per ACI 318-19, Eurocode 7, and AS 2159.
Enter the column loads and moments, the footing geometry and materials, and the soil parameters. The tool returns the ultimate and allowable bearing capacity, the factor of safety and bearing utilisation, immediate plus consolidation settlement, ACI 318-19 punching and one-way shear utilisation, and the required flexural steel, all updating in real time.
How It Works
- Compute the bearing capacity factors Nc, Nq, and Ngamma from the friction angle for the chosen method (Terzaghi, Meyerhof, or Hansen).
- Apply shape, depth, and groundwater corrections, and reduce the footing to its effective area B prime x L prime under eccentric loading.
- Assemble the ultimate bearing capacity qu, divide by the factor of safety for the allowable qa, and compare with the actual contact pressure.
- Estimate immediate (elastic) and consolidation settlement, then check ACI 318-19 punching shear, one-way shear, and flexural reinforcement.
Worked Example
Immediate elastic settlement of a 2 m square footing on sand: net bearing pressure q = 150 kPa, B = 2 m, soil modulus Es = 25 MPa = 25000 kPa, Poisson ratio nu = 0.3, and shape factor Iw = 0.56 (square). Si = q*B*(1 - nu^2)/Es * Iw = 150*2*(1 - 0.09)/25000 * 0.56 = 0.006115 m = 6.12 mm, comfortably within the 25 mm sand limit.
Formulas
- Ultimate bearing capacity (general)
qu = c*Nc*sc*dc + q*Nq*sq*dq + 0.5*gamma*B*Ngamma*sgamma*dgamma- Bearing capacity factors
Nq = e^(pi*tan(phi)) * tan^2(45 + phi/2) ; Nc = (Nq - 1) / tan(phi)- Effective area (eccentric load)
B' = B - 2*ex ; L' = L - 2*ey ; ex = |Mx|/P- Immediate (elastic) settlement
Si = q * B * (1 - nu^2) / Es * Iw- Punching shear capacity (ACI 318-19)
Vc = 0.33 * sqrt(fc) * b0 * d
Standards & References
- ACI 318-19
- Eurocode 7 (EN 1997)
- AS 2159
- ASCE 7-22
- EN 1990
- AS/NZS 1170.0
Frequently Asked Questions
Which bearing capacity methods does the calculator use?
It implements the Terzaghi, Meyerhof, and Hansen general bearing capacity equations. They share the same Nc and Nq factors but differ in Ngamma: Terzaghi uses 2*(Nq+1)*tan(phi), Meyerhof uses (Nq-1)*tan(1.4*phi), and Hansen uses 1.5*(Nq-1)*tan(phi), plus shape and depth corrections.
How is settlement estimated?
Immediate elastic settlement uses the Boussinesq form Si = q*B*(1 - nu^2)/Es * Iw, and consolidation settlement uses the one-dimensional log formula with the compression index Cc, void ratio e0, and the stress increase. The total is compared with a 25 mm sand or 50 mm clay limit.
How does eccentric loading affect the footing?
Applied moments create eccentricities ex = |Mx|/P and ey = |My|/P. The tool reduces the footing to its effective dimensions B prime = B - 2*ex and L prime = L - 2*ey (Meyerhof effective area) and checks whether the resultant stays within the middle third for full contact.
What structural checks are performed?
Per ACI 318-19 the tool checks two-way (punching) shear at d/2 from the column face, one-way (beam) shear at d from the face, and the required flexural reinforcement, governed by the minimum steel ratio of 0.0018.