The real deformation is in the order of a millimetre 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. The column rebars can be anchored inside the footing either with a straight anchorage length or with a 90ο hook at their end (resembling a ‘shoe’). The chosen type of anchorage depends upon the footing’s total depth and the level on which the column is considered fixed which is usually the upper part of the adjacent connecting beams. Most of the times, that level is high enough to allow for a straight anchorage length.
Practically, the need for hooks at the base of columns arises mainly in the case of a solid raft foundation with a relatively small slab thickness. The reinforcement is the same both for flexible and rigid spread footings either being centrically or eccentrically constructed.
The footing is reinforced with an orthogonal wire mesh that may have horizontal and vertical bars of different diameter and spacing. Bending the rebars ends helps in the proper anchorage of the reinforcement. According to the rule of practice, the minimum allowable rebar diameter in a footing is Ø12 and the maximum acceptable spacing between the reinforcement bars is 150 mm. In certain occasions, the footing may have a reinforcement wire mesh both at its upper surface (see following figures).
some other cases, the spread footing may have punching shear reinforcement as well. This reinforcement is usually provided in the form of a peripheral stirrup cage.
Punching shear stresses are similar to the shear stresses (diagonal flexure) and they appear along the perimeter of the column. Punching shear is a type 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.
Note:
In this specific spread footing, punching shear is controlled by stirrup cages (figure a) however, it could be also controlled by bundles of properly bent-up rebars (figure b), or by special industrial elements (figure c).
The density and rebar diameters of the reinforcement (punching shear reinforcement included) for the 2500/2500 footing with a 400/400 column are given bellow:
