Cumulative floor loads of a superstructure are supported by foundation structures?in direct contact with the soil. The function of the foundation is to transmit safely the high concentrated column and or wall reactions or lateral loads from earth-retaining walls to the ground without causing unsafe differential settlement of the supported structural system or soil failure.

If the supporting foundations are not adequately proportioned, one part of a structure can settle more than an adjacent part. Various members of such a system become overstressed at the column-beam joints due to uneven settlement of the supports leading to large deformations. The additional bending and torsional moments in excess of the resisting capacity of the members can lead to excessive cracking due to yielding of the reinforcement and ultimately to failure.

Layouts of structural supports vary widely and soil conditions differ from site to site and within a site. As a result, the type of foundation to be selected has to be governed by these factors and by optimal cost considerations. In summary, the structural engineer has to acquire the maximum economically feasible soil data on the site before embarking on a study of the various possible alternatives for site layout.

There are basically six types of foundation structures. The foundation area must be adequate to carry the column loads, the footing weight, and any overburden weight within the permissible soil pressure.

(1) Wall footings. Such footings comprise a continuous slab strip along the length of the wall having a with larger than the wall thickness. The projection of the slab footing is treated as a cantilever loaded up by the distributed soil pressure. The length of the projection is determined by the soil bearing pressure, with the critical section for bending being at the face of the wall. The main reinforcement is placed perpendicular to the wall direction.

(2) Independent isolated column footings. They consist of rectangular or square slabs of either constant thickness or sloping toward the cantilever tip. They are reinforced in both directions and are economical for relatively small loads or for footings on rock.

(3) Combined footings. Such footings support two or more column loads. They are necessary when a wall column has to be placed on a building line and the footing slab cannot project outside the building line. In such a case, an independent footing would be eccentrically loaded, causing apparent tension on the foundation soil.

In order to achieve a relatively uniform stress distribution, the footing for the exterior wall column can be combined with the footing of the adjoining interior column. Additionally, combined footings are also used when the distance between adjoining columns is relatively small, such as in the case of corridor columns, when it becomes more economical to build a combined footing for the closely spaced columns.

(4) Cantilever footings. These are similar to the combined footings except that the footings for the exterior and interior columns are built independently. They are joined by a strap beam to transmit the effect of the bending moment produced by the eccentric wall column load to the interior column footing area.

(5) Pile foundations. This type of foundation is essential when?the supporting ground consists of structurally unsound layers of material to?large depths. The piles may be driven either to solid bearing on rocks or hardpan or deep?enough into the soil to develop the allowable capacity of the pile through skin frictional resistance or a combination of both. The piles could be either precast and hence driven into the soil, or cast in place by drilling a caisson and subsequently filling it with concrete. The precast piles could be reinforced or prestressed concrete. Other types of piles are made of steel or treated wood. In all types, the piles have to be provided with appropriately designed concrete caps reinforced in both directions.

(6) Raft or floating foundations. Such foundation systems are necessary when the allowable bearing capacity of soil is very low to great depths, making pile foundations uneconomical. In this case it becomes necessary to have a deep enough excavation with sufficient depth of soil removed that the net bearing pressure of the soil on the foundation is almost equivalent to the structure load. It becomes necessary to spread the foundation substructure over the entire area of the building such that the superstructure is considered to be theoretically floating on a raft. Continuously consolidating soils require such a substructure, which is basically an inverted floor system.Otherwise, friction piles or piles driven to rock become mandatory.