An isolated footing's four checks don't just each pass independently — on a well-proportioned pad, bearing, punching shear and one-way shear can all land within a whisker of each other, a sign the plan size and thickness were chosen together to use the pad efficiently on every front at once.
Every check above runs automatically inside the Structyze Isolated Footing module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.
The Example We'll Use
- Column 400 × 400 mm
- Footing 2.6 × 2.6 m, thickness 450 mm
- Cover 50 mm (cast on blinding)
- Axial: dead 600 kN, live 300 kN
- Allowable bearing 150 kN/m²; concrete M20, steel Fe415
Working Through It
Service bearing q = (600+300)/2.6² ≈ 133.1 kN/m² against 150 kN/m² allowable — utilisation 0.89. Factored pressure qu = 1350/6.76 ≈ 199.7 kN/m².
One-way shear at d from the column face: τv ≈ 0.36 MPa against τc ≈ 0.41 MPa (at pt = 0.34%) — utilisation 0.88. Punching shear on the 4(c+d) perimeter: τv′ ≈ 0.99 MPa against ksτc0 = 1.12 MPa. Utilisation 0.88, essentially identical.
Bending at the column face needs only 897 mm²/m against the 540 mm²/m minimum (required governs), covered by 16 mm bars at 150 mm (1340 mm²/m) — utilisation 0.69, the widest margin of the four.
What Governs, and Why
Bearing, punching and one-way shear all sit within a whisker of each other (utilisation 0.88–0.89) — the plan size and thickness were evidently chosen together to use the pad efficiently on every front at once; bending has the widest margin, as is typical once the thickness is set by shear. Where either shear check runs this close to its limit, thickness — not bar area. Is the fix.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Isolated Footing module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.
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Quick FAQ
Why would three different checks land at nearly the same utilisation?
It's not a coincidence for a well-optimised footing — a designer (or an automated sizing routine) iterating plan size and thickness together will naturally converge on a geometry where the governing checks all run close to their limits simultaneously, since over-satisfying one check while another is tight just wastes material.
What's the practical lesson from punching and one-way shear both sitting at 0.88?
Since both are concrete-alone checks with no stirrup reserve, either one failing would need the same fix — more thickness — so seeing them converge is actually a sign of efficient proportioning, not a warning sign, as long as neither has actually crossed 1.0.
Final Thoughts
A well-proportioned isolated footing with its bearing, punching and one-way shear checks all landing within a percentage point of each other is a clean illustration of what 'efficiently sized' actually looks like on paper.