Pad footings and punching

How a pad footing is designed on the Eurocode 2 route of the STATIX pad footing calculator: bearing pressure and the EN 1997-1 design approaches, the net pressure, bending and shear at the column face, and punching on a column base to EN 1992-1-1 6.4.4(2).

1. Learning outcomes

After this lesson you can:

size a pad base for the bearing pressure, with and without a moment;
say what the EN 1997-1 design approaches are, and what the calculator checks instead;
design the bars for bending at the column face, and check one-way shear;
check punching on a column base with perimeters inside 2d, the soil deduction, the 2d/a enhancement and β;
check the concrete at the column face.

2. Before you start

Lesson 3 (v_Rd,c) and lesson 6 (slab steel).
The pad footing calculator, set to Eurocode 2 / EC7 (EN 1992-1-1).
A pocket calculator and about 55 minutes.
The same lesson on the SA code: RC Design 1, lesson 7 (SANS 10100-1).

What the pad page’s Eurocode route uses: EN 1990 6.10 (1.35G + 1.5Q) for the concrete and the characteristic combination G + Q for the bearing; the bearing check against an allowable pressure at the service load (no EN 1997-1 design approach); an optional ULS ground check by EN 1997-1 Annex D when soil strength is given, with the partial factors of SANS 10160-5; the bending of lesson 1 (K′ 0.168, z with 1.134, α_cc 0.85); punching on a column base by 6.4.4(2) with perimeters scanned from the face to 2d, eq 6.48 to 6.51, v_min, and v_Rd,max = 0.4νf_cd with α_cc 1.0.

3. The ground

A pad footing spreads a column load over enough ground. Two things are checked: the ground must carry the pressure (EN 1997-1), and the concrete pad must carry the upward pressure as a slab in bending, shear and punching (EN 1992-1-1).

Bearing pressure. Under an axial load and a moment the pressure varies across the base. The calculator compares the largest pressure under the service (characteristic) load with the allowable pressure you give, from the geotechnical report, and checks that no part of the base lifts.

The Eurocode ground check. EN 1997-1 offers three design approaches with different sets of partial factors; the national annex chooses. The calculator does not run any of the three: it checks an allowable bearing pressure at the service load. Given soil strength, it adds a ULS bearing check by EN 1997-1 Annex D with the South African factors of SANS 10160-5.

The pressure for the concrete design. The weight of the base and of the soil on it press straight down on the ground and bend nothing, so the pad is designed for the net upward pressure from the column’s design load alone (with its eccentricity).

4. The concrete pad

Bending. Each side of the base is a cantilever from the column face. The moment at the face, per metre width, is the net pressure times the projection squared over two; the bars across the base are designed for it like a slab (lesson 6), with A_s,min of 9.2.1.1.

Beam shear. Across the whole width, the upward pressure beyond a section d from the column face must be carried by the concrete alone: v ≤ v_Rd,c of 6.2.2, with no enhancement.

Punching on a column base (6.4.4(2)). For a footing the Eurocode asks for perimeters within 2d of the column, not only at 2d: close to the column the perimeter is shorter and less soil reaction is deducted. The calculator scans the perimeters from the face out to 2d and reports the worst.

The punching load. The soil pressure inside the perimeter goes straight into the column without crossing the shear surface, so it is deducted. The area inside must be the same rounded shape as the perimeter.

Resistance near the column. The closer the perimeter is to the column, the steeper the shear crack and the stronger the concrete: v_Rd,c (with v_min) is raised by 2d/a. That is why the worst perimeter is usually somewhere inside 2d, about d out.

A moment on the column makes the shear around the perimeter uneven; β raises the average stress to the worst. The calculator works β on the same rounded perimeter for each one it scans.

The column face. However deep the base, the concrete next to the column must not crush: the full column load over the column perimeter is checked against v_Rd,max.