How to design a pad footing

A pad (isolated, spread) footing is a square or rectangular block of concrete under a column. It does two jobs: it spreads the column load over enough ground that the soil can carry it, and it acts as an upside-down slab, bending and shearing under the soil pressure pushing up. Those two jobs are checked with two different sets of loads.

#Two sets of loads

• Soil checks use service loads. An allowable bearing pressure q_a from the geotechnical report already contains a safety factor, so the plan size is found with unfactored D + L (and service wind or seismic). Eurocode 7 instead uses design soil resistances with its own partial factors (GEO).
• Concrete checks use factored loads. The thickness and bars are designed like any concrete member, from the strength combinations.

#Worked example (ACI 318)

A 400 × 400 mm column carries service loads D = 600 kN and L = 300 kN. Allowable bearing q_a = 200 kPa, base 1.2 m below ground, f′_c = 28 MPa, f_y = 420 MPa.

#1. Plan size from the soil

The footing and the soil on top of it weigh roughly 20 kN/m³ × 1.2 m = 24 kPa, which leaves a net 200 − 24 = 176 kPa for the column load:

Check: q = (900 + 24 × 5.29) / 5.29 = 194 kPa ≤ 200 kPa ✓ (utilization 0.97).

#2. Factored soil pressure

P_u = 1.2 × 600 + 1.6 × 300 = 1200 kN. The footing's own weight doesn't bend it (it sits directly on the soil), so the net factored pressure is q_u = 1200 / 5.29 = 226.8 kPa.

#3. Thickness from punching shear

Try h = 500 mm, d ≈ 410 mm. The critical perimeter is at d/2 from the column face: b_o = 4 × (400 + 410) = 3240 mm.

#4. One-way shear

The section at d from the column face: the cantilever is (2300 − 400)/2 = 950 mm, so the loaded strip beyond it is 950 − 410 = 540 mm wide.

#5. Bending at the column face

R_n = M_u/(φ b d²) = 0.68 MPa → ρ = 0.16 %, A_s = 1541 mm². The shrinkage minimum 0.0018 b h = 0.0018 × 2300 × 500 = 2070 mm² governs → 11 Ø16 (2211 mm²) each way, at about 210 mm.

Eurocode: EN 1997-1 checks bearing and sliding with design actions and resistances (Design Approach depends on the country), EN 1992-1-1 6.4.4 checks punching at control perimeters between the column face and 2d, reducing the shear by the soil pressure inside the perimeter.

#Moments and uplift

A column moment M makes the pressure uneven: q = P/A ± 6M/(B L²). When the eccentricity e = M/P exceeds L/6, part of the base lifts off and the pressure under the rest rises sharply. Fixed-base frames, wind and seismic all cause this — the footing must also be checked against overturning and sliding (EQU).

#Design it in Struxt3D

1. Open a framed template with column supports, for example the Multi-storey frame.
2. Turn footings on. In Supports & loads › Foundations, switch Pad footings on and set the Allowable bearing (kPa) and the Depth of the base below the column base. They apply to every footing; you can also change them in Design settings.
3. Run the design (F6). Each support's reactions become load sets (dead, live, wind, seismic) and the code's own strength, service, GEO and EQU combinations are built from them. The plan is sized in 50 mm steps for the soil checks, then the thinnest slab and the bars.
4. See the footings. Footing blocks appear under the columns in 3D. Click one to see its size, its checks with ratios, and a Footing report button.
5. Review all footings in the Footings table: L × B, h, bars, q max, the governing check and ratio.

#Common mistakes

• Sizing the plan with factored loads against an allowable pressure — double-counting safety.
• Forgetting the footing's own weight and backfill in the bearing check.
• Ignoring moments from fixed bases or lateral loads.
• Too little cover: concrete cast against soil needs about 75 mm.