23-01-2013, 12:58 PM
THEORY OF STRUCTURES PRESENTATION ON SHALLOW FOUNDATIONS
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TYPES OF FOUNDATIONS
Foundations can be can be categorized into basically two types: Shallow and Deep.
Shallow Foundations:
These types of foundations are so called because they are placed at a shallow
depth (relative to their dimensions) beneath the soil surface. Their depth may
range from the top soil surface to about 3 times their breadth (about 6 meters).
They include footings (spread and combined), and soil retaining structures
(retaining walls, sheet piles, excavations and reinforced earth). There are several
others of course.
Deep Foundations:
The most common of these types of foundations are piles. They are called deep
because the are embedded very deep (relative to their dimensions) into the soil.
Their depths may run over several 10s of meters. They are usually used when
the top soil layer have low bearing capacity.
SHALLOW FOUNDATION
•When the depth of foundation is less then or equal to the width of foundation, then it is termed as shallow foundation. It is also known as open foundation.
•A shallow foundation is placed immediately below the lowest part of the superstructure.
•A footing is a foundation unit constructed in brick work, stone masonry or concrete below the base of the wall or column for the purpose of distributing the structural-load over wide area.
PROPERTIES OF SHALLOW FOUNDATIONS
Foundation structures are customarily divided into shallow or deep on the basis of their depth in relation to their width, the typical divide being the unit value for the ratio (B/H), that is, B/H<2.5 for shallow foundations and B/H >2.5 for deep foundations.
The real difference between shallow and deep foundations is based on the structural response as well as the depth to which the foundation is taken.
Bending (flexure) is the predominant structural action in the case of shallow foundations.
COMBINED FOOTING
•A combined footing is a single footing, which supports two columns.
•A combined footing may be rectangular or trapezoidal in plan. The aim of combined footing is to get uniform pressure distribution under the footing.
•For this, the Centre of gravity (CG) of the footing area should coincide with the C.G. of the combined loads of the two columns.
•If the outer column, near the property line, carries heavier load, provision of Trapezoidal column becomes essential to bring the C.G. of footing in line with the C.G. of two column loads.
RAFT FOUNDATION
•A Raft or mat is a combined footing, which covers the entire area below the whole building or structure and supports all the wall and columns.
•When the allowable soil pressure is low or the building loads are heavy, the use of spread footings would cover more than one-half of the area and it may prove more economical to use mat or raft foundation.
•A raft foundation is also needed to reduce settlement on highly compressible soils, by making the weight of the structure and raft approximately equal to the weight of the soil excavated.