The substructure, or foundation, is that part of a structure which is usually placed below the surface of the ground and which transmits the load to the underlying soil or rock. All soils compress noticeably when loaded and cause the supported structure to settle. The two essential requirements in the design of foundations are that the total settlement of the structure shall be limited to a tolerably small amount and that differential settlement of the various parts of the structure shall be eliminated as nearly as possible. With respect to possible structural damage, the elimination of differential settlement, i.e., different amounts of settlement within the same structure, is even more important than limitations on uniform overall settlement.

To limit settlements, it is necessary (1) to transmit the load of the structure to a soil stratum of sufficient strength and (2) to spread the load over a sufficiently large area of that stratum to minimize bearing pressure. If adequate soil is not found immediately below the structure, it becomes necessary to use deep foundations such as piles or caissons to transmit the load to deeper, firmer layers. If satisfactory soil directly underlies the structure, it is merely necessary to spread the load, by footings or other means. Such substructures are known as spread foundations, and it is mainly this type which will be discussed.

Spread footings can be classified as wall and column footings. A wall footing is simply a strip of reinforced concrete, wider than the wall, which distributes its pressure. Single-column footings are usually square, sometimes rectangular, and represent the simplest and most economical type. Their use under exterior columns meets with?difficulties if property rights prevent the use of footings projecting beyond the exterior walls. In this case combined footings or strip footings are used which enable one to design a footing which will not project beyond the wall column.

Such individual or combined column footings are the most frequently used types of spread foundations on soils of reasonable bearing capacity. If the soil is weak and/or column loads are great, the required footing areas become so large as to be uneconomical. In this case, unless a deep foundation is called for by soil conditions, a mat or raft foundation is resorted to. This consists of a solid reinforced concrete slab which extends under the entire building and which consequently distributes the load of the structure over the maximum available area. Such a foundation, in view of its own rigidity, also minimizes differential settlement.

In ordinary constructions the load on a wall or column is transmitted vertically to the footing, which in turn is supported by the upward pressure of the soil on which it rests. If the load is symmetrical with respect to the bearing area, the bearing pressure is assumed to be uniformly distributed. It is known that this is only approximately true. Under footings resting on coarse-grained soils the pressure is larger at the center of the footing and decreases toward the perimeter. This is so because the individual grains in such soils are somewhat mobile, so that the soil located close to the perimeter can shift very slightly outward in the direction of lower soil stresses. In contrast, in clay soils pressures are higher near the edge than at the center of the footing, since in such soils the load produces a shear resistance around the perimeter which adds to the upward pressure. It is customary to disregard these nonuniformities (1) because their numerical amount is uncertain and highly variable, depending on type of soil, and (2) because their influence on the magnitudes of bending?moments and shearing forces in the footing is relatively small.

On compressible soils footings should be loaded concentrically to avoid tilting, which will result if bearing pressures are significantly larger under one side of the footing than under the opposite side. This means that single footings should be placed concentrically under the columns and wall footings concentrically under the walls and that for combined footings the centroid of the footings area should coincide with the resultant of the column loads. Eccentrically loaded footings can be used on highly compacted soils and on rock.