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Based on the position with respect to ground level, Footings are classified into two types; 1. Shallow Foundations 2. Deep Foundations

**Types
of Foundations**

Based on the position with respect to ground level,
Footings are classified into two types;

1. Shallow
Foundations

2. Deep
Foundations

Shallow Foundations are
provided when adequate SBC is available at relatively short depth below ground
level. Here, the ratio of D_{f} / B < 1, where D_{f} is the
depth of footing and B is the width of footing^{.} Deep Foundations are
provided when adequate SBC is available at large depth below ground level. Here
the ratio of D_{f} / B >= 1.

**1 ****Types
of Shallow Foundations**

The different types of shallow foundations are as
follows:

�
Isolated Footing

�
Combined footing

�
Strap Footing

�
Strip Footing

�
Mat/Raft Foundation

�
Wall footing

Some of the popular types of shallo w foundations
are briefly discussed below.

a) __Isolated Column Footing__

These are independent
footings wh ich are provided for each column. This type of footing is chosen
when

�
SBC is generally high

�
Columns are far apart

�
Loads on footings are less

The isolated footings
can have diffe rent shapes in plan. Generally it depends on the shape of column
cross section Some of the popular s hapes of footings are;

�
Square

�
Rectangular

�
Circular

The isolated footings
essentially co nsists of bottom slab. These bottom Slabs can be ei ther flat,
stepped or sloping in nature. The bottom of the slab is reinforced with steel
mesh to r esist the two internal forces namely bending moment and shear force.

The
sketch of a typical isolated foot ing is shown in Fig. 1.

b) __Combined Column Footing__

These are common
footings which support the loads from 2 or more columns. Combined footings are
provided when

� SBC
is generally less

� Columns
are closely spaced

� Footings
are heavily loaded

In
the above situations, the area required to provide isolated footings for the
colu mns generally overlap. Hence, it is advantageous to provide single
combined footing. In some case s the columns are located on or close to
property line. In such cases footings cannot be extende d on one side. Here,
the footings of exterior and i nterior columns are connected by the combined
foo ting.

Combined footings
essentially consist of a common slab for the columns it is supportin g. These
slabs are generally rectangular in plan. Sometimes they can also be trapezoidal
in plan (refer Fig. 2). Combined footings can also have a connecting beam and a
slab arrangement, which is similar to an inverted T - beam slab.

c) __Strap Footing__

An
alternate way of providing com bined footing located close to property line is
the s trap footing. In strap footing, independent slabs below columns are
provided which are then connec ted by a strap beam. The strap beam does not re
main in contact with the soil and does not transfer a ny pressure to the soil.
Generally it is used to com bine the footing of the outer column to the adjace
nt one so that the footing does not extend in the adjoining property. A typical
strap footing is shown in Fig. 3.

d)
__Strip Footing__

Strip
footing is a continuous footing provided under columns or walls. A typical
strip footing for columns is shown in Fig. 4.

e)
__Mat Foundation__

Mat foundation covers the whole plan area of
structure. The detailing is similar to two way reinforced solid floor slabs or
flat slabs. It is a combined footing that covers the entir e area beneath

a structure
and supports all the wa lls and columns. It is normally provided when

� Soil
pressure is low

� Loads
are very heavy

� Spread
footings cover > 50% area

A
typical mat foundation is shown in Fig. 5.

**2 Types of Deep Foundations**

Deep foundations are provided wh en adequate SBC is
available at large depth below GL. There are different types of deep
foundations. Some of the common types of deep foundations are listed below.

� Pile
Foundation

� Pier
Foundation

Well
Foundation

Tags : Civil - Design of Reinforced Concrete Elements - Limit State Design Of Footing

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