TheBridgeEng

Spread Footing Designer

Concentric spread footing — bearing, shear & flexure

Size a rectangular isolated footing under a concentric column: service bearing, one-way shear at d, two-way punching at d/2, and the flexural steel per direction, all live as you drag.

Dead load D1200 kN

Live load L600 kN

Footing B3.0 m

Footing L3.0 m

Thickness h0.60 m

Column cₓ×cᵣ0.50×0.50 m

Allowable qₖ250 kPa

Soil depth over pad0.5 m

f′c

Bar

Basis: concentric load, net factored pressure qᵤ=(1.2D+1.6L)/BL; one-way φVₛ=0.75·0.17√f′c·b·d; two-way vₛ=min(0.33, 0.17(1+2/β), 0.083(40d/bₐ+2))√f′c; flexure at column face with φ=0.9 and Aₛ,min=0.0018bh; d = h − 75 mm cover − one bar diameter; fᵢ=420 MPa; concrete 24 kN/m³. Verified against an independent Python implementation on 200 randomised cases (≤10⁻⁹) plus hand-checked spot values. Not covered: moment/eccentric loads, development length, sliding/overturning, settlement, seismic. Preliminary sizing only.

Field notes — what to watch for

Punching usually governs thickness and bearing usually governs plan size — if flexure is governing your design, something unusual is going on and worth a second look. The service bearing check includes pad and soil weight; forgetting them is the most common hand-calc error. Real footings also need development length and, on sloped sites, sliding checks — not covered here.

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