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Combined footing design to SANS 10100

When two columns are too close for separate pads, or one sits on a boundary, one base carries both. The base is sized so the resultant of the two column loads falls near its centre, so the soil pressure is even. Along its length the base then works like an upside-down beam on two supports (the columns): sagging under each column, hogging between them. Across, each column spreads its load in a band. A strap footing joins two pads with a stiff beam instead. Every number below comes from the free STATIX combined and strap footing calculator with the inputs in step 1.

1. The problem

Input	Value
Design code	SANS 10100-1
Type	Combined (one base)
Column spacing s	4 m
Edge to column 1 a1, centre	0.3 m
Depth h	1 m
Column 1 G / Q	600 kN
Column 1 imposed load kN	250 kN
Column 2 G / Q	900 kN
Column 2 imposed load kN	350 kN
Column 1 cx × cy	400 mm
Column 1 size across mm	400 mm
Column 2 cx × cy	400 mm
Column 2 size across mm	400 mm
Allowable bearing qa, SLS	200 kPa
Soil over the base	0.5 m
Concrete fcu	30 MPa
Steel fy	450 MPa
Cover	50 mm
Length L × width B m, 0 = sized for you	m
Base width m	m
Exposure	Moderate

2. Size the base: put the resultant at the centre

The resultant of the two column loads is found by taking moments. The base length is chosen so its centre sits on the resultant (as far as the site allows); the width then follows from the allowable bearing pressure.

columns at x = 0.30 and 4.30 m resultant at x = 2.681 m base L × B × h = 5.40 × 2.40 × 1.00 m (sized by the engine) base + soil = 427.7 kN

3. Bearing pressure

At service loads the base is taken as rigid, so the pressure varies in a straight line. Any small offset of the resultant from the centre tilts it. The largest pressure must stay within the allowable bearing pressure.

[SANS 10160-5 (allowable pressure, SLS)] SLS G + Q: N = columns + base and soil 427.7 = 2527.7 kN; e = -16 mm from the centre; q_max = 198.5, q_min = 191.6 kPa; L sized so that the resultant of G + Q sits at the centre: L = 2 x 2.681 m; B sized from q_a 198.5 ≤ 200.0 kPa OK

4. Bending along the base: sagging steel (bottom)

Along its length the base is a beam loaded upward by the net soil pressure and held down at the columns. Beyond each column (and under it) it sags: bottom bars along the base.

[SANS 10100-1 4.3.3.4; SANS 10100-1 Table 23] ULS 1,2G + 1,6Q (STR): M = 230.4 kNm at x = 4.37 m over B = 2.4 m; d = 942 mm; K = 0.004; A_s,req 658, min 3120 mm2 -> Y16 @ 150 1,300 ≤ 1,340 mm²/m OK

5. Bending along the base: hogging steel (top)

Between the columns the soil pushes the base up like a beam span loaded from below, so it hogs: the top is in tension and needs top bars along the base. This is usually the larger moment.

[SANS 10100-1 4.3.3.4; SANS 10100-1 Table 23] ULS 1,2G + 1,6Q (STR): M = 868.4 kNm at x = 2.16 m; d = 942 mm; K = 0.014; A_s,req 2479, min 3120 mm2 -> Y16 @ 150 1,300 ≤ 1,340 mm²/m OK

6. One-way shear

The base must not shear across its full width at d from a column face. No links are used, so the shear stress must stay within v_c.

[SANS 10100-1 4.3.4.1, eq.(2)] x = 3.16 m: V = 511.9 kN over B; v = V/(B d) = 0.226 MPa; d = 942 mm, rho = 0.142 % 0.230 ≤ 0.240 MPa OK

7. Punching at each column

Each column can punch through the base. Its own load, less the soil pressure inside the perimeter, is checked on the column face and on a perimeter away from it, as for a pad footing.

Punching at column 1: [SANS 10100-1 4.6.2, 4.6.3, 4.3.4.1; beta 1,0] V_Ed = 1120 kN; face u_0 = 1200 mm: v_0 = 0.999 <= v_max 4.108 MPa; perimeter at 1,5d (clipped by the base edges): u = 2241 mm, V_Ed,red = V - 217.8 kPa x area inside = 137.3 kN, v = 0.066 <= 0.240 MPa; d = 934 mm, rho = 0.142 % 0.070 ≤ 0.240 MPa OK Punching at column 2: [SANS 10100-1 4.6.2, 4.6.3, 4.3.4.1; beta 1,0] V_Ed = 1640 kN; face u_0 = 1200 mm: v_0 = 1.463 <= v_max 4.108 MPa; perimeter at 1,5d (clipped by the base edges): u = 2241 mm, V_Ed,red = V - 209.7 kPa x area inside = 299.8 kN, v = 0.143 <= 0.240 MPa; d = 934 mm, rho = 0.142 % 0.140 ≤ 0.240 MPa OK

8. Across the base: bands under the columns

Across the width, each column load spreads out sideways like two short cantilevers. That needs bottom steel across the base in a band under each column, a column width plus about d each side.

Across under column 1: band c_x + 2d = 2252 mm: [SANS 10100-1 4.3.3.4; practice: the column load as two cantilevers across] ULS 1,2G + 1,6Q (STR): N = 1120 kN over B: M = N/B x ((B - c_y)/2)^2 / 2 = 233.3 kNm in the band; d = 926 mm (second layer); A_s,req 678, min 2928 mm2 -> Y16 @ 150 1,300 ≤ 1,340 mm²/m OK Across under column 2: band c_x + 2d = 2252 mm: [SANS 10100-1 4.3.3.4; practice: the column load as two cantilevers across] ULS 1,2G + 1,6Q (STR): N = 1640 kN over B: M = N/B x ((B - c_y)/2)^2 / 2 = 341.7 kNm in the band; d = 926 mm (second layer); A_s,req 992, min 2928 mm2 -> Y16 @ 150 1,300 ≤ 1,340 mm²/m OK

9. Other checks

The strap beam (for a strap footing), the durability cover and any other rows the design adds.

Durability: nominal cover: [SANS 10100-1 4.11.2.2, 4.11.2.4, 4.11.2.10; SANS 2001-CC1:2007 Table 3] moderate exposure, class 30 (f_cu 30): Table 3 40 mm; given 50 mm 40.00 ≤ 50.00 mm OK

The calc sheet repeats each step with every load combination.

Run this example yourselfOpen the free STATIX combined and strap footing calculator and press Load the example (two columns 4 m apart) under "How it is calculated". Change any value and every step updates, with a calc sheet.
Open the combined and strap footing calculator

A worked example for learning, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.

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