Introduction to footings
In a typical structure built on ground, that part of the structure which is located above ground is generally referred to as the superstructure, and the part which lies below ground is referred to as the substructure or the ‘foundation structure’ (or simply, foundation). The purpose of the foundation is to effectively support the superstructure by
1 transmitting the applied load effects (reactions in the form of vertical and horizontal forces and moments) to the soil below, without exceeding the ‘safe bearing capacity’ of the soil, and
2 ensuring that the settlement of the structure is within tolerable limits, and as nearly uniform as possible.
Further, the foundation should provide adequate safety against possible instability due to overturning or sliding and/or possible pullout. The choice of the type of foundation depends not only on the type of the superstructure and the magnitudes and types of reactions induced at the base of the superstructure, but also on the nature of the soil strata on top of which the substructure is to be founded.
The Code recommendations (Cl. 34) are confined to the design of footings that support isolated columns or walls and rest directly on soil or on a group of piles
TYPES OF FOOTINGS
Footings’ belong to the category of shallow foundations (as opposed to deep foundations such as piles and caissons) and are used when soil of sufficient strength is available within a relatively short depth below the ground surface. Shallow foundations comprise not only footings (which support columns/walls, and have a limited area/width in plan) but also rafts which support multiple columns on a large plan area). The shallow foundation (footing or raft) has a large plan area in comparison with the cross-sectional area of the column(s) it supports because:
• the loads on the columns (axial thrust, bending moments‡) are resisted by concrete under compression and reinforcing steel under tension and/or compression, whereas these load effects are transmitted by the footing/raft to a relatively weak supporting soil by bearing pressures alone;
• the ‘safe bearing capacity’ of the soil is very low (100 – 400 kPa) in comparison with the permissible compressive stresses in concrete (5–15 MPa) and steel (130–190 MPa) in a column under service loads.
For ordinary structures located on reasonably firm soil, it usually suffices to provide a separate footing for every column. Such a footing is called an isolated footing. It is generally square or rectangular in plan; other shapes are resorted to under special circumstances. The footing basically comprises a thick slab which may be flat (of uniform thickness), stepped or sloped (on the upper surface).
The soil bearing pressures from below tend to make the base slab of the footing bend upwards, somewhat into a saucer-like shape (cantilever action), and hence the footing needs to be suitably reinforced by a mesh provided at the bottom of the slab.
In some cases it may be inconvenient to provide separate isolated footings for columns (or walls) on account of inadequate areas available in plan. This may occur when two or more columns (or walls) are located close to each other and/or if they are relatively heavily loaded and/or rest on soil with low safe bearing capacity, resulting in an overlap of areas if isolated footings are attempted.
In such cases, it is advantageous to provide a single combined footing for the columns. Often, the term ‘combined footing’ is used when two columns are supported by a common footing, the term ‘continuous strip footing’ is used if the columns (three or more in number) are aligned in one direction alone, and the term ‘raft foundation’ (‘mat foundation’) is used when there is a grid of multiple columns. The combining of footings contributes to improved integral behaviour of the structure.
A two-column combined footing, in which there is a ‘property line’ which restricts the extension of the footing on one side. In this case, the non-availability of space near the exterior column is circumvented by combining the footing with that of an interior column. The width of the footing may be kept uniform or tapered, as shown. The trapezoidal shaped footing (with a larger width near the exterior column) is required when the exterior column is more heavily loaded than the interior column. Another option is a combined footing which is T-shaped. It is sometimes economical to provide a central beam interconnecting the column bases; this causes the base slab to bend transversely, while the beam alone bends longitudinally.