Spread footings

To be continued >

Soil pressures and deformation of an isolated spread footing

The spread footing behaves like an inverted cantilever with loads applied in the upward direction. As a rule, a spread footing is a quite rigid element therefore, the applied soil stresses are almost linear and in case of a symmetric (with respect to the pedestal) footing, they are orthogonal. These soil pressures are the loads carried by the footing that behaves like a slab and is deformed by the way shown at the figure.
The real deformation is in the order of a millimeter and although it is not visible to the human eye, it always has that same form. The reinforcement is placed at the lower surface of the footing both along the x and y axis.

Reinforcement of an isolated spread footing

The footing is reinforced with an orthogonal rebar grid that may have bars of different diameter and spacing in either direction. Bending the rebars ends helps in the proper anchorage of the reinforcement.

Rigid and flexible spread footings

The reinforcement is the same both for flexible and rigid spread footings either being centrically or eccentrically constructed.

3D view of spread footing reinforced with both upper and lower rebar grid

In certain occasions, the footing may also have an upper reinforcement rebar grid.


Detail of spread footing reinforced with both upper and lower rebar grid


Controlling punching shear, with the use of stirrup cages

Punching shear stresses are similar to the shear stresses (diagonal flexure) and they appear along the perimeter of the column.

Spread footing punching shear mechanism

Punching is a form of column sliding along the side of a cone that forms a 35° angle with the horizontal axis, as shown at the opposite figure. The first stirrup leg must be placed at a maximum distance of 0.5d away from the face of the pedestal and the last stirrup leg at a maximum distance of 1.25d. In this specific spread footing, punching shear is controlled by stirrup cages (figure a) however, it could be also controlled with bundles of properly bent-up rebars (figure b), or with special industrial elements (figure c).

Spread footing with punching shear reinforcement