Foundations(Part-2) Shallow Foundation

Shallow foundation: The foundation whichis very near to surface is called shallow foundation.These type of foundation is generally formed by open excavation.The depth is generally less than its width.

Types of Shallow Foundation:

1.Spread Footing:These type of footing is used when there is need for spreading applied load over a large area.In these type of footing, we provide a continuous longitudinally bearing.These is generally used for providing foundation to wall and column.

The above figure is example of strip footing. Fig.a is example of strip footing without offset(Simple footing) and Fig.b is example of strip footing with offset(Stepped footing).

Simple footing is used when load is small.The width of footing is small and equal to width of wall.But in case of Stepped footing,load applied on foundation is large.The steps is formed on concrete base whose width is decreasing as going above.

2.Combined Footing:These type of footing is used to connect two or more columns and wall.if it connects more than two columns is called continous footing.It is provided in following cases:

• when columns is very nearer to each other such that footing is overlap
• when columns is nearer to boundary wall
• when foundation soil has low bearing capacity

Types:The combined footing has two types-

1.             Rectangular footing
2.             Trapezoidal footing

The aim is to get uniform pressure distribution under the footing.For this,the centre of gravity of combined load of two columns should coincide with centre of footing area.

a.Rectangular footing-The combined rectangular  footing is shown in below picture of load W1 and W2 carrying two columns A and B respectively and spaced l centre to centre. Then area of footing is given by

                  A=(W1+W2+W')/qs
Where W' = weight of footing
              qs = safe bearing capacity

The net upward pressure p is given by
                    = (W1+W2)/L.B
The distance of C.G of column load from centre of column A is given by
                 =W2×l/W1+W2
b.Trapezoidal footing-It is provided when

•           Outer columns carrying differnet load
•            Limiting space beyond columns

 The area of footing is given by
                  A=W1+W2+W'/qs
    Net upward pressure is given by
              p= (W1+W2)/0.5 (B1+B2)L
 The centre of gravity from column A is
                      =W2×l/W1+W2
The combined footing may be of reinforced  cement concrete or of grillage type

3.Strap footing:These type of footing is used to join footing of two or more columns or walls when there is large distance between them and column or wall is bear to property line.

  The main function of these footing is to evenly distributed the unequal load carried by two different columns.

4.Raft foundation:  In these type of footing entire area of foundation  which support alls walls and columns is covered.It is also called mat footing.

Raft or mat is defined as flat concrete bed of uniform thickness.

 It is used where

•      Building load is heavy
•       Soil pressure is loose such that differential settlement would be difficlut to control.