Course · lesson 3 of 7

Strip and combined footings

Walls sit on strip footings, and columns too close together or too close to a boundary share a combined footing. This lesson designs both: a strip per metre with an off-centre wall, and a combined base sized so its pressure is uniform and designed as an upside-down beam, with the STATIX strip and combined footing calculators.

1. Learning outcomes

After this lesson you can:

design a strip footing per metre, including an off-centre wall;
place the main and distribution steel of a strip;
size a combined footing so the resultant falls at its centre;
draw the shear and moment along a combined footing and place the top and bottom steel;
check one-way shear, punching at each column and the transverse bands.

2. Before you start

RC Design 1, lesson 7 (bending, shear and punching of a pad) and lesson 2 (eccentric pressure).
The free strip footing and combined footing calculators; the Learn pages for strip and combined footings as background.
A pocket calculator and about 55 minutes.

3. Strip footings

A strip footing under a wall is designed one metre at a time. Every quantity is per metre run: the wall load in kN/m, the pressure in kPa, the moment in kNm/m and the steel in mm²/m. The STATIX strip footing calculator works this way.

Across the strip. The soil pushes up under the two projections, which bend as cantilevers from the faces of the wall: bending is checked at the face, one-way shear at d from it. This is RC Design 1 lesson 7's pad, per metre and in one direction only.

Wall eccentricity. When the wall cannot sit on the centre line (at a boundary, or to clear a services trench) the pressure becomes a trapezoid and one projection is longer. The footing's own weight and the soil on it are centred, so they reduce e; the calculator takes them into account.

The steel. The main bars run across the strip at the bottom and come from the transverse bending; they are often at the minimum (0.13% of b h for high yield steel in SANS 10100-1). The distribution bars run along the wall: minimum steel that ties the strip together and spreads local loads.

per metre: q = N/B (1 ± 6e/B), N with the footing and the soil on it (SLS) across: cantilever from the wall face on the ULS net pressure; shear at d from the face steel: main bars across (at least 0.13% b h), distribution bars along

4. Combined footings

Why combine. A column at the site boundary cannot have a pad centred under it, and two close columns may need pads that overlap. One base under both, a combined footing, solves both; a strap footing (two pads tied by a stiff beam) is the alternative the calculator also offers.

Sizing. Make the centre of the base fall on the resultant of the column loads (G + Q): then the service pressure is uniform. With column 1 at the edge, that fixes the length L = 2 × the distance from the edge to the resultant, so the base runs on past the heavier column. Then B comes from the allowable pressure. The calculator does both when L and B are left at zero.

The base as an upside-down beam. The soil pushes up along the length and the columns push down: shear changes sign at the columns, and between them the moment is hogging (tension at the top), with sagging under and beyond the columns. The design uses the net pressure from the factored column loads only; the base's own weight and the soil on it bend nothing.

Across the base. Under each column a band (the column width plus d each side, a practice rule the calculator states) carries the column load out to the edges as two cantilevers; elsewhere minimum steel is enough.

Punching at each column. The critical perimeter at 1.5d (SANS 10100-1) is cut short where it meets the edge of the base, as at the boundary column; the net upward pressure inside the perimeter is deducted.

Two pressures. The bearing check uses the service loads plus the base and the soil on it. The concrete design uses the net pressure of the factored column (or wall) loads only, because the weight of the base and the soil sits directly on the soil under it.

size: x_R = Σ(P x) / ΣP (G + Q); L = 2 x_R from the edge; B = ΣN / (L q_all) beam: line load w = ΣP_u / L; V = 0 between the columns gives the largest hogging moment

5. In STATIX

In STATIX: the strip footing calculator takes the wall width, its offset and the loads per metre; the combined footing calculator takes the spacing, the edge distance and each column's G and Q, and sizes L and B when they are left at zero.