A foundation is the lowest and supporting layer of
a structure.

Foundations may be classified as:
-Strip foundation
-Pad foundation
-Raft foundation
-Pile foundation

strip foundations consist of a continous, logitudinal atrip of concrete designed to spread the load from uniformly loaded walls of brick, masonry or concrete to a sufficient area of subsoil.
The thickness of the foundation depends on the strenght of the foundation material. Strip foundations with a wide spread are commonly of reinforced concrete.

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The foundation to piers of brick, masonry and reinforced concrete and steel columns is often in the form of a square or rectangular isolated pad of concrete to spread a concentrated load. The area of this type of foundation depends on the load on the foundation and the bearing and shear strength of the subsoil and its thickness on the strength of the foundation material.
The simplest form of pad foundation consists of a pad of mass concrete, illustrating a pier and foundation beam base for a small building. Heavily loaded pad foundations supporting columns of framed buildings are generally of reinforced concrete showing the base of a reinforced concrete column.

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A raft foundation is continous in two directions, usually covering an area equal to or greater than the base of a building. Raft foundations are used for lightly loaded structures on soils with poor bearing capacity or where variations in soil conditions necessitate a considerable spread of the load, for heavier loads in place of isolated foundations, where differential settlements are significant and where mining subsidence is likely.
There are three types of reinforced concrete raft foundations which are; Solid slab raft, Beam and Slab raft and Cellular raft.

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The word pile means columns, usually of reinforced concrete, driven or cast in the groung in order to carry foundation loads to some deep underlying firm stratum or to transmit loads to the subsoil by the friction of their surfaces in contact with the subsoil. The main function of pile is to transmit loads to lower levels of ground by a combination of friction along their sides and end bearing at the pile point or base.
Piles that transfer loads mainly
by friction to clays and silts are called friction piles and those that mainly transfer loads by end bearing to compact gravel, hard cly or rock are called end-bearing piles.
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