Wednesday, October 17, 2012

Foundations of Buildings and Structures: Introduction.

Every building consists of two basic components : the superstructure and the substructure or foundations. The super-structure is usually that part of the building which is above ground, and which serves purpose of its intended use. The substructure or foundations is the lower portion of the building, usually localed below ground level, which transmits the load of the super-structure to the sub-soil. A foundalion is therefore that part of the structure which is in direct contact with the ground to which the loads are transmirred. The soil which is located immediately below the base of the foundation is called the sub-soil or foundation soil , while rhe lowermost portion of the foundation which is in direct contact with the sub-soil is called the footing.

The basic function of a foundation is to transmit the dead loads, super-imposed loads (or, live loads) and wind loads from a building to the soil on which the building rests, in such a way that (a) settlements are within permissible limits, without causing cracks in the super-structure, and (b) the soil does not fail. When loads are transmilted to the sub-soil, it settles. If this settlement is sligh and uniform throughout, no damage will be caused to the building. But if the settlement is exccessive or unequal, serious damage may result in the form of cracked walls, distorted doors and window openings, cracked linteis, walls thrown out of plumb etc., and somctimes the complete collapse of the building.

The foundation is thus the most important partof a buildíng. Since it remains below the ground level, the signs of failure of foundation are not noticeable tull it has already affected the building. A foundation should be sufficiently strong to preveni excessive settlement as well as unequal settlement. Unequal settlement or differenrial settlemen may be caused by (i) weak sub-soils, such as made up ground (ii) shrinkable and expansive soils (such as clay), (iii) frost action. (iv) movement of ground water, and uplift pressure, (v) excessive vibrations, due to traffic, machinery etc., (vi) slow consolidation of saturated clays, and (vii) slipping of strata on sloping sites. When designing the foundations, therefore, the above factors must be taken into account.

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