What is Combined Footing ?

Combined footing is a type of footing which is provided when distance between two columns is small and soil bearing capacity is lower and their footings overlap with each other. When two columns are close together and separate isolated footings would overlap, in such case, it is better to provide a combined footing than isolated footing.

A Combined footings supports 2 or more columns in a row.  The combined footing can be rectangular in shape, if both the columns carry equal loads, or can be trapezoidal if there are space limitations and they carry unequal loads. Generally they are constructed of reinforced concrete.

In the design of footings, the footing is assumed to be rigid on homogeneous soil. The location of centre of gravity of column loads and the centroid of the footing should coincide.

The maximum bending moment is taken as the design value for the reinforced concrete footing and should be checked for maximum shear and bound, etc.

Also Read : Column Failure and It’s Types
Also Read : Caisson Foundation and Types of Caisson

Why Combined Footings are Used ?

• The distance between two columns is small and when bearing capacity of soil is lower and their footings overlap with each other, in this case it is more preferable to use combined footing.
• When one column is close to a property line or sewer pipe, the center of gravity of columns will not coincide with footing. In such cases it is necessary to provide combined footing with that of the adjacent internal column.
• When the dimensions of one side of footing are restricted to some lower value so that column footings may be combined. The combined footings may be rectangular, trapezoidal or t-shaped in a plan.
• The ultimate aim is to get uniform pressure distribution under the entire area of the footings. To achieve this, the center of gravity of the footing area should coincide with the center of gravity of the total loads of the 2 or more columns.

Types of Combined Footings

There are  types of combined footings :

• Rectangular Combined Footings
• Trapezoidal Combined Footings
• T – Shaped Combined Footings

A combined rectangular footing is provided where loading condition is such that either of he two columns are equally loaded or the interior column carries greater load.

The trapezoidal footing or T-shaped is provided when the column load of the property line is much more than the other.

As a result, both projections of the footing beyond the faces of the columns will be restricted. They are provided under any condition of loading.