What is Eccentric Footing? its 5 Important Points, & Design

Eccentric Footing

The eccentric footing is a spread or wall footing that must resist a moment in addition to the axial column load and it has the shape of the shoe.

The eccentric footing is designed in such a way that the C.G (center of gravity) of the superimposed load coincides with the C.G of the base area so, resulting in uniform bearing pressure this footing is subjected to concentric loading.

Eccentric Footing

Eccentric footing consists of two isolated footings connected with a structural strap and as a one unit strap connects the footing. The strap behaves like a connecting beam and when the allowable soil pressure is relatively high and distance between the columns is large then an eccentric footing is more economical than a combined footing.

This footing has to be done in cases where neighbor’s construction exists and it is not possible to adopt spread footings. When the allowable soil pressure is relatively high and the distance between the columns is large then an eccentric footing is more economical than a combined footing.

How to Design Eccentric Footing?

The eccentric footing is designed in such a way that the C.G (center of gravity) of the superimposed load coincides with the C.G of the base area so, resulting in uniform bearing pressure this footing is subjected to concentric loading.

It may not be possible to do so in some cases i.e it will not be possible to spread the footing to both the sides of the wall or column if the wall or column under construction is near some other property as shown in fig;

Eccentric Footing problem

W1 is a super-imposed load including the weight of the wall per unit length.

W2 is the weight of foundation &

W is the resultant load on base.

With respect to the center of base width B, this resultant load have an eccentricity e and this eccentric weight is equivalent to;

  1. A centrally placed load W and
  2. A Bending moment which is M = W . e

A trapezoidal soil pressure diagram having pressure intensities q1 and q2 will result due to these two;

example 2

For the soil the magnitude of q1 should not exceed the safe bearing pressure and also q2 should be positive in order that the footing may remain in contact with soil.

In the extreme case q2 = 0, when e = B/6 and this gives the maximum value of eccentricity.

The tension will be developed if e is greater than B/6 and the end B of the footing will have loose contact with the soil in that case.

So,

final equation

Important Points to Remember:

There are the following important points to remember for eccentric footing as given below;

  1. To handle the moment and for the footing to act as a double cantilever footing, eccentric footing requires a strap beam and if the strap beam is not provided than due to the eccentricity of the column the footing cannot handle the moment.
  2. Eccentric footing requires more depth and heavy steel and this type of footing are also known as shoe footing. The eccentric footing is mostly used when space is very less.
  3. To the property line, the eccentric footing is mostly used, and to avoid tilting of the footing in a multi-story building it needs to connect with another footing such as strap beam or combine footing.
  4. It is better to plant a column at the edge of the footing for a multi-story building.
  5. Always provide a strap beam in footing equal to the length of footing.

The Problem of Providing Eccentric Footing:

The major problem is that in some cases the eccentricities may be so large that there may be part of the foundation where there is no bearing pressure and for transient conditions, this state may be satisfactory.

 

 

 

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