What Is Combined Footing and Its Types?

Introduction

The footing is the most important structural component of building, whose main function is to transfer the whole load of structure to the soil safely. There are various types of footing in the construction which we were discussed in types of foundation.  In this article we are discussing the combined footing.

What Is Combined Footing?

·   When more than one column which are supported in row, placed on the common footing is called combined footing.

·    In combined footing mostly two columns are placed on a footing, exceptionally the columns are more than two.

·   Combined footing is used when the load bearing capacity of soil is low, then individual footing area should be increased.

·   The combined footing is useful when there is need of uniformly distribution of superstructure load to substructure. The selection of ideal footing depends on the depth at which hard strata lies, the soil conditions, and the type of superstructure.

 

What Are The Conditions For Combined Footing Preferred?

·   The distance between two columns is small, and their footings overlap with each other.

·   Soil bearing capacity of soil is low.

·  One column is closed to property line or sewer pipe.

·   The center of gravity of footing and column is not equal.

Types of Combined Footing

The main aim of footing is to distribute load uniformly under the whole area of footing. To get it, center of gravity of footing area should coincide with the center of gravity of total loads of the two or more columns.

To achieve this, the combine footing have following shaped in plane,

1.      Rectangular Combined Footing

2.      Trapezoidal Combined Footing

3.      Strap Combined Footing

4.      Raft Combined Footing

 

1.      Rectangular Combined Footing

·   The Rectangular combined footing is the type of combined footing, which is rectangular.

·    The footing is supporting two or more columns, which distributed equal loads to the soil.

 

2.      Trapezoidal Combined Footing

·   The trapezoidal combined footing is the type of combined footing which is trapezoidal.

·   The footing is used where columns on the trapezoidal footing transferred unequal load of superstructure.

·  When column closes to property line, therefore to avoid footing spreading front of property line trapezoidal footing is constructed which helps to construct column.

 

3.      Strap Combined Footing

·   The strap combined footing is constructed when column is very near to property line.

·   By using strap beam and strap footing, which is combination of spread footing two columns are connected.

·   The connecting beam between two spread footings does not transfer any load, only strap footing is transferring load from the heavily loaded column to inner column.

                                          

4.      Raft Combined Footing

·   When the soil has poor bearing capacity, which cannot carry proper load, in such condition raft footing is preferred.

·   Raft footing prevents the structure from overturning, and structure can construct with no. of floors.

·  Columns are much closed to each other, raft is required.

·  Raft footing is constructed where strip footing or spread footing is not constructed.