How the spread footing calculator works
This free spread footing calculator checks an isolated rectangular footing under one column carrying axial load and moment, using ACI 318-19. Enter the footing length, width and thickness, soil cover, column size and location (interior, edge or corner), service dead and live loads, allowable bearing pressure, f'c, fy, bar size and cover. Results update as you type.
What it checks
- Service soil bearing pressure: q = (P/A)(1 + 6e/L) when e ≤ L/6, and a triangular distribution q = 2P/[3B(L/2 − e)] when e > L/6. Footing self-weight (150 pcf) and soil cover (120 pcf) are included when selected.
- Two-way (punching) shear at d/2 from the column face: vc is the least of 4λ√f'c, (2 + 4/β)λ√f'c and (2 + αsd/bo)λ√f'c, with φ = 0.75 and αs = 40, 30 or 20
- One-way shear at a distance d from the column face: φVc = 0.75·2λ√f'c·B·d
- Bottom flexure at the column face: required steel, minimum steel 0.0018bh, number of bars and spacing
Method
Bearing is checked with service loads. Shear and flexure use the factored load (1.2D + 1.6L or 1.4D) spread uniformly over the footing as qu = Pu/(L·B). The effective depth is d = thickness − cover − one bar diameter, and the required flexural steel uses a lever arm of 0.9d.
Assumptions and limitations
- Factored shear and flexure use a uniform soil pressure, so the effect of moment on the factored pressure is not included.
- The punching perimeter bo always has four sides. Only αs changes for edge and corner columns, so check footings with a shorter perimeter separately.
- One-way shear and flexure are checked in the L direction only. Check the B direction for rectangular footings.
- The ACI 318-19 size-effect factor λs (Tables 22.5.5.1 and 22.6.5.2) is not applied. It reduces shear strength when d exceeds 10 in. and no minimum shear reinforcement is provided, so results can be unconservative for thick footings.
- Settlement, sliding and overturning of the footing are not checked.
Frequently asked questions
What is punching shear in a footing?
Punching (two-way) shear is the tendency of the column to push through the footing along a pyramid-shaped failure surface. ACI 318-19 checks it on a critical perimeter located d/2 from the column face.
Should bearing pressure use service or factored loads?
Service loads. Allowable bearing pressures from a geotechnical report are working-stress values, so the calculator compares them with the bearing pressure from unfactored dead and live loads plus self-weight.
Why does minimum steel often control?
Footings are thick, so the minimum reinforcement of 0.0018 times the gross section area frequently exceeds the steel required for flexure. The flexure card shows MIN STEEL when that happens.
What do the interior, edge and corner options change?
They set αs in the punching shear equation to 40, 30 or 20 and move the column in the plan drawing. The perimeter length itself is not reduced.
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For educational and preliminary design purposes only. Always verify results with a licensed PE.