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Concrete Footing Designer

Spread footings under a column, post or wall, checked to ACI 318-19, with wind or earthquake uplift, moment and shear.

New to footing design? The concrete footing tutorial works through an example with ACI 318.

Footing

Square column, centered.

The way the moment and shear push.

Loads

Service (unfactored) loads, at the top of the footing. Factored as 1.4D and 1.2D + 1.6L, and with wind or earthquake as 1.2D + L + W and 0.9D + W.

Including snow.

Wind or earthquake

Strength-level loads (W or E from ASCE 7). Leave them at 0 for gravity loads only.

Soil and slab over the footing

From the top of the footing to grade or the slab.

Bottom bars

Each way.

For bars with anchorage this check doesn't cover.

Soil and materials

1,500 without a soils report (IBC 1806.2).

Under the footing, for sliding; 0.25 for sand, 0.35 for gravel.

On the side of the footing below 1 ft; 0 to leave out.

318-14 (IBC 2018) uses 2√f′c one way and no size effect. The other checks stay 318-19.

ACI 318-19 13.2.6.2 allows this for footings, one way and punching.

Results

Design the anchor bolts · Footing size chart

Calculations

Load combinations

Soil pressure

One-way shear

Two-way (punching) shear

Bending and bar area

Bar development

Bar spacing and cover

Bearing under the column

Save to a job

A job keeps the wind loads, rafters, headers, studs, joists, beams and footings for one project together. You'll find it on your dashboard.

How the Footing Designer Checks a Footing

The footing designer checks a concrete spread footing under a column, post or wall, square or rectangular, for the soil and the concrete. Enter its size and loads, choose the bottom bars or let it pick them, and it shows every check worked out step by step. When a footing doesn't work, it suggests the smallest one that does.

The soil is checked with allowable stress loads, and the concrete with factored loads, the way the building code does it (ASCE 7 2.3 and 2.4). The footing, the soil and slab over it count as dead load, and the floor load on the slab as live load.

  • Soil pressure: the most pressure under D + L and, with wind or an earthquake, D + 0.6W, D + 0.75L + 0.75(0.6W) and 0.6D + 0.6W (0.7E for an earthquake), against the allowable. With a moment the pressure is higher on one side; once the load is more than a sixth of the footing off center, part of the footing lifts off the soil and the pressure is a triangle under the rest.
  • Uplift, overturning and sliding: under 0.6D + 0.6W (or 0.7E), the dead load must hold the footing down against the uplift and keep it from tipping, and friction under it plus the soil's passive pressure on its side must stop it sliding, leaving out the top 1 ft.
  • Shear: one-way shear d from the face of the column or wall, and two-way (punching) shear d/2 around the column, with the share of the column's moment that the shear carries (ACI 8.4.4.2). Both take the ACI 318-19 size effect for footings without stirrups (ACI 22.5.5.1 and 22.6.5.2).
  • Bending: the soil pushing up past the face of the column or wall, as a cantilever, each way with its own depth, against the bottom bars. On a rectangular footing, more of the short bars go in a band under the column, 2 / (β + 1) of them (ACI 13.3.3.3).
  • Bar development: straight or hooked, in the length past the face, scaled by the steel needed over the steel provided (ACI 25.4).
  • Bar spacing and cover: at least 0.0018 of the section, spacing no more than 3h or 18 in, tension-controlled, spaced to keep cracks narrow, and the clear spacing (2 bar diameters) and cover (1 bar diameter) that the development length assumes.
  • Plain concrete: when the designer picks the bars and the footing sticks out too little past the column or wall for them to develop, it's checked as plain concrete, with 2 in of its thickness left out (ACI 14.5). Most house footings are plain this way.
  • Uplift on the concrete: when the loads pull the column up, the footing hangs from it by its own weight and the soil on it, bending the top. Plain concrete is checked first, then top bars.
  • Bearing under the column: 0.65 × 0.85 f′c A1, up to twice that for the footing around it (ACI 22.8.3.2), with the dowels a concrete column needs (ACI 16.3.4.1).

Not checked: settlement, frost depth, footings on fill or expansive clay, footings next to a slope, and the post base, anchor or hold-down that ties the column to the footing. For sizes by load, see the footing size chart.

Footing Questions

How big should a footing be?

Big enough that the load, plus the footing and the soil on it, spread over its area, is no more than the soil's allowable pressure. 1,500 psf is the code's value without a soils report (IBC Table 1806.2), so a 10,000 lb post load needs about 7 sq ft, or a footing about 2 ft 8 in square.

How thick should a footing be?

Thick enough for one-way and punching shear, which ACI 318-19 reduces for deep footings without stirrups (the size effect), usually 10 to 12 in for houses and decks and 2 to 4 ft under heavy building columns. A plain concrete footing should be at least as thick as it sticks out past the column or wall.

Does a footing need rebar?

Not always. A narrow footing that sticks out only a few inches past the wall can be plain concrete, and the International Residential Code (IRC) allows plain footings for most houses. Wider footings, footings under heavy columns and footings pulled up by a hold-down need bars.

Why does a footing with a moment get longer?

A moment pushes the soil pressure to one side. A longer footing in that direction keeps the load near the middle, so the pressure stays below the allowable and the footing doesn't tip. That's why a footing under a shear wall's hold-down or a flagpole is often rectangular, long the way the wind pushes.

Do I still need a geotechnical report?

In most cases, yes. The footing depends on the allowable soil pressure, friction and passive pressure, which a geotechnical engineer should set for the site.

Is the footing designer a substitute for a licensed structural engineer?

No. It's an aid for design professionals and students. Final footing designs should be reviewed and approved by a licensed structural engineer, with the project's geotechnical recommendations and local code requirements.