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Footing Size Calculator

Q: What is an isolated footing?

An isolated footing is a shallow foundation that supports a single column. It spreads the column load over a sufficient area of soil so the bearing pressure stays within safe limits.

Q: How is footing size determined?

Footing size is determined by dividing the column load by the allowable bearing capacity of the soil. Additional area may be needed if moments are present.

Q: What factor of safety should I use?

For bearing capacity, use FS = 2.5 to 3.0 depending on the reliability of soil parameters and importance of structure.

Q: What is punching shear?

Punching shear is a failure mechanism where the column punches through the footing slab. It is checked along a critical perimeter at d/2 from the column face.

Q: What concrete grade is typical for footings?

M25 (25 MPa) is common for footings. M20 may be used for lightly loaded footings, while M30+ is used for heavily loaded ones.

Calculate isolated footing size, depth, and reinforcement per IS 456, ACI 318, or BS 8110 using allowable bearing capacity.

Foundation Engineering Structural engineers, geotechnical engineers, civil engineering students Commercial Intent: HIGH

Engineering Formulas

Required Footing Area

Areq = P / qall
For concentric axial load

A_req: Required footing area (m²)

P: Axial load (kN)

q_all: Allowable bearing pressure (kN/m²)

Bearing Pressure with Moment

qmax = P/A + Mx/Zx + My/Zy
qmin = P/A - Mx/Zx - My/Zy
Z = BL²/6 (about X), Z = LB²/6 (about Y)

q_max: Maximum bearing pressure

Mx, My: Moments about X and Y axes

Zx, Zy: Section moduli about X and Y axes

Eccentricity Check

ex = My / P, ey = Mx / P
Full contact if e < B/6 or L/6
Partial uplift if e > B/6

e: Eccentricity (m)

B, L: Footing dimensions (m)

Depth for Bending

Mu = qnet × lcant² / 2
d = √(Mu / (0.138 × fck × B))
Per IS 456:2000

M_u: Ultimate bending moment (kNm)

d: Effective depth (mm)

f_ck: Characteristic concrete strength (MPa)

Punching Shear Check

Vu = P - qmax × (a₀ + d)(b₀ + d)
τv = Vu / (b₀ × d)
τc = 0.25 × √fck

V_u: Punching shear force (kN)

b₀: Critical perimeter (m)

τ_v: Punching shear stress (MPa)

τ_c: Shear capacity (MPa)

Worked Example

Square Footing for 1000 kN Column Load

axialLoad: 1000momentMx: 50momentMy: 30sbc: 200factorOfSafety: 2.5columnWidth: 300columnDepth: 300shape: squareconcreteGrade: M25steelGrade: Fe500

Allowable Pressure

q_all = 200 / 2.5 = 80 kN/m²

Required Area

A_req = 1000 / 80 = 12.5 m²

Footing Width

B = √12.5 = 3.536 m → adopt 3.6 m

Eccentricity X

ex = 30 / 1000 = 0.03 m < B/6 = 0.6 m ✓

Bearing Pressure

q_max = 1000/12.96 + 50/7.78 + 30/7.78 = 87.4 kN/m²

Depth Required

d = √(Mu / (0.138 × 25 × 3600)) = 380 mm

Result: 3.6 m × 3.6 m footing, 380 mm effective depth, Ast = 1850 mm²

Engineering Notes

Always include footing self-weight (typically 5-10% of column load)

For eccentric footings, check that full base is in compression

Minimum reinforcement: 0.12% of gross area for Fe415, 0.15% for Fe250

Clear cover: 50 mm for footings on earth, 75 mm if placed on blinding

Development length must be checked at all critical sections

Assumptions

• Rigid footing assumption

• Linear soil pressure distribution

• No soil heave or swelling

• Column is centrally located

• Footing is not subjected to lateral loads

Common Mistakes

✕ Not including self-weight of footing in total load

✕ Using ultimate bearing capacity without factor of safety

✕ Forgetting to check both bending and punching shear

✕ Ignoring moment effects from column eccentricity

✕ Using net allowable pressure instead of gross

Frequently Asked Questions

What is an isolated footing?

An isolated footing is a shallow foundation that supports a single column. It spreads the column load over a sufficient area of soil so the bearing pressure stays within safe limits.

How is footing size determined?

Footing size is determined by dividing the column load by the allowable bearing capacity of the soil. Additional area may be needed if moments are present.

What factor of safety should I use?

For bearing capacity, use FS = 2.5 to 3.0 depending on the reliability of soil parameters and importance of structure.

What is punching shear?

Punching shear is a failure mechanism where the column punches through the footing slab. It is checked along a critical perimeter at d/2 from the column face.

What concrete grade is typical for footings?

M25 (25 MPa) is common for footings. M20 may be used for lightly loaded footings, while M30+ is used for heavily loaded ones.

References & Standards

IS 456:2000ACI 318-19BS 8110

IS 456:2000

Plain and Reinforced Concrete — Code of Practice

ACI 318-19

Building Code Requirements for Structural Concrete

BS 8110

Structural Use of Concrete

Bowles (1996)

Foundation Analysis and Design

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