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Strip footing design to SANS 10100

A strip footing carries a wall: the load is per metre run, so the footing is designed as a 1 m slice. First the width must keep the soil pressure below the allowable value. Then the two outstands each side of the wall work as cantilevers under the upward soil pressure: they need bottom steel across the footing, and they must not shear off. Bars along the run tie it together. Every number below comes from the free STATIX strip footing calculator with the inputs in step 1.

1. The problem

Input	Value
Design code	SANS 10100-1
Width B	0.9 m
Depth h	0.3 m
Wall width	230 mm
Permanent load N kN per m	60
Imposed load N kN per m	25
Allowable bearing qa, SLS	150 kPa
Soil over the footing	0.5 m
Concrete fcu	30 MPa
Steel fy	450 MPa
Cover bottom bars	50 mm
Exposure	Against soil (blinded)

2. Loads per metre run, and the self-weight

The wall loads come per metre of wall. The footing and the soil on it are added for the bearing check. For the concrete design the factored loads are used.

footing + soil = 12.5 kN/m SLS G + Q: N = 97.5 kN/m (with the footing) ULS 1,2G + 1,6Q (STR): N = 112.0 kN/m ULS 1,35G + Q (STR-P): N = 106.0 kN/m

3. Bearing pressure

At service loads the pressure under a rigid strip varies in a straight line across its width. Its largest value must stay at or below the allowable bearing pressure of the soil. A wall moment or an off-centre wall makes it uneven.

[SANS 10160-5 (allowable pressure, SLS)] SLS G + Q: N = loads + footing and soil 12.5 kN/m; e = 0 mm; q = N/B (1 + 6e/B) = 108.3 kPa 108.3 ≤ 150.0 kPa OK

4. Bending at the wall face

Each outstand is a cantilever from the face of the wall, pushed up by the net soil pressure (the weight of the footing does not bend it). The moment at the wall face gives the bottom steel across the footing.

[SANS 10100-1 4.10.2, 4.3.3] ULS 1,2G + 1,6Q (STR): M = 7 kNm/m on the linear net pressure over the 335 mm cantilever; d = 244 mm; K = 0.004 <= 0.156; z = 232 mm; A_s,req = 77 mm2/m; Y12 @ 275 = 411 mm2/m 77 ≤ 411 mm²/m OK

5. Minimum steel

A footing needs a minimum amount of steel each way, even when the bending is small.

[SANS 10100-1 4.11.4.1, Table 23] 0,13 % b h = 390 mm2/m 390 ≤ 411 mm²/m OK

6. Shear

A strip of the outstand beyond the critical section must not shear off. No links are used, so the shear stress must stay below the concrete strength v_c. At the wall face the stress must also stay below the crushing limit.

[SANS 10100-1 4.10.3.3, 4.3.4.1, eq. (2)] ULS 1,2G + 1,6Q (STR): V = 11.3 kN/m; v = V/(b d) = 0.046 MPa; v_c = 0.356 MPa (100 A_s/bd = 0.168 %) 0.046 ≤ 0.356 MPa OK [SANS 10100-1 4.3.4.1.1] v_0 = 0.171 MPa; min(0,75 sqrt(f_cu), 4,75) = 4.11 MPa 0.17 ≤ 4.11 MPa OK

7. Bars along the run

Distribution bars along the length tie the main bars together, spread point loads and control shrinkage cracks along the wall. They need at least the minimum steel.

[SANS 10100-1 4.11.4.1, Table 23] 0,13 % b h = 390 mm2/m (minimum steel each way); Y10 @ 200 = 393 mm2/m; s <= 732 mm 390 ≤ 393 mm²/m OK

8. Anchorage of the main bars

The main bars must reach far enough past the wall face to develop the force they carry there.

[SANS 10100-1 4.11.6.3, 4.10.3.4] f_s = 73 MPa (A_s,req/A_s,prov); f_bu = 0.5 sqrt(f_cu) = 2.739 MPa (Table 27); l = phi f_s/(4 f_b) = 80 mm; straight length past the face 285 mm: straight bars (shape 20) 80 ≤ 285 mm OK

Wall moments and horizontal loads add the overturning and sliding checks; the calc sheet shows them with their safety factors.

Run this example yourselfOpen the free STATIX strip footing calculator and press Load the example (230 mm wall, G 60 + Q 25 kN/m) under "How it is calculated". Change any value and every step updates, with a calc sheet.
Open the strip 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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