General Design Considerations for Footings
GENERAL DESIGN CONSIDERATIONS AND CODE REQUIREMENTS
Factored Soil Pressure at Ultimate Limit State
The area of a footing is fixed on the basis of the allowable bearing pressure qa and the applied loads and moments under service load conditions (with partial load factors applicable for the ‘serviceability limit state). Once the base area of the footing is determined, the subsequent structural design of the footing is done for the factored loads, using the partial load factors applicable for the ‘ultimate limit state’. In order to compute the factored moments, shears, etc., acting at critical sections of the footing, a fictitious factored soil pressure qu, corresponding to the factored loads, should be considered. It may further be noted that the soil pressure which induces moments and shears in the footing base slab are due to the net pressure qnet, i.e., excluding the pressure induced by the weight ΔP of the footing and the backfill (assumed to be uniformly distributed). Using gross pressures instead of net pressures will result in needlessly conservative designs. The ‘factored net soil pressure’ qu to be considered in the design of the footing at the limit state is obtainable from the factored loads on the column (Pu, Mu).
General Design Considerations
The major design considerations in the structural design of a footing relate to flexure, shear (both one-way and two-way action), bearing and bond (development length). In these aspects, the design procedures are similar to those for beams and two-way slabs supported on columns. Additional considerations involve the transfer of force from the column/pedestal to the footing, and in cases where horizontal forces are involved, safety against sliding and overturning.
Deflection control is not a consideration in the design of footings which are buried underground (and hence not visible). However, control of crack-width and protection of reinforcement by adequate cover are important serviceability considerations, particularly in aggressive environments. It is considered sufficient to
limit the crack-width to 0.3 mm in a majority of footings, and for this the general detailing requirements will serve the purpose of crack-width control.
Although the minimum cover prescribed in the Code (Cl. 184.108.40.206) is 50 mm, it is desirable to provide a clear cover of 75 mm to the flexural reinforcement in all footings.
Thickness of Footing Base Slab
The thickness of a footing base slab is generally based on considerations of shear and flexure, which are critical near the column location. Generally, shear considerations predominate, and the thickness is based on shear criteria.
Except in the case of small footings, it is economical to vary the thickness from a minimum at the edge to a maximum near the face of the column, in keeping with the variations in bending moment and shear force.
In any case, the Code (Cl. 34.1.2) restricts the minimum thickness at the edge of the footing to 150 mm for footings in general (and to 300 mm in the case of pile caps). This is done to ensure that the footing has sufficient rigidity to provide the calculated bearing pressures. A ‘levelling course’ of lean concrete (about 100 mm thick) is usually provided below the footing base.
Design for Shear
The thickness (depth) of the footing base slab is most often dictated by the need to check shear stress, and for this reason, the design for shear usually precedes the design for flexure.
Both one-way shear and two-way shear (‘punching shear’) need to be considered in general [refer Cl. 220.127.116.11 of the Code]. However, in wall footings and combined footings provided with a central beam, the base slab is subjected to one-way bending, and for this reason, need to be designed for one-way shear alone. The critical section for one-way shear is taken, as for beams, at a distance d (effective depth) from the face of the column/pedestal or wall/beam . The effective area resisting one-way shear, (d)] may be rectangular or polygonal, depending on whether the footing is flat or sloped.
The behaviour of footings in two-way (punching) shear is identical to that of a two-way flat slab supported on columns. The critical section for two-way shear is taken at a distance d/2 from the periphery of the column. The design procedures for one-way and two-way shear are identical to one way and two way shear. However, shear reinforcement is generally avoided in footing slabs, and the factored shear force Vu is kept below the factored shear resistance of the concrete Vuc by providing the necessary depth. Where, for some reason, there is a restriction on the depth of the footing base slab on account of which Vu > Vuc, appropriate shear reinforcement should be designed and provided, to resist the excess shear Vu – Vuc.
Finally, it may be noted that in the case of a column/pedestal with a circular or octagonal cross-section, the Code (Cl. 34.2.2) recommends that an equivalent square section should be considered, for the purpose of locating the critical sections for shear (and moment). The equivalent squares should be inscribed within the perimeter of the round or octagonal column or pedestal.
Design for Flexure
As mentioned earlier, the footing base slab bends upward into a saucer-like shape on account of the net soil pressure qu from below. Based on extensive tests, it has been determined that the footing base slab may be designed against flexure by considering the bending moment at a critical section defined as a straight section passing through
• the face of a column, pedestal or wall for a footing supporting a concrete column, pedestal or wall;
• halfway between the face and centreline of the wall for a footing supporting masonry wall.
In one-way reinforced footings (such as wall footings), the flexural reinforcement (calculated for the moment at the critical section) is placed perpendicular to the wall at a uniform spacing. In the perpendicular direction (along the length of the wall), nominal distributor reinforcement should be provided — mainly to account for secondary moments due to Poisson effect and possible differential settlement, and also to take care of shrinkage and temperature effects.
In two-way reinforced square footings also, flexural reinforcement may be placed at a uniform spacing in both directions. In two-way reinforced rectangular footings, the reinforcement in the long direction is uniformly spaced across the full width of the footing, but in the short direction, the Code (Cl. 34.3.1c) requires a larger concentration of reinforcement to be provided within a central band width, equal to the width B of the footing: