When two or more footings are interconnected to make them a single unit; it is called a combined footing. It is provided to transfer the load coming from the superstructure to the soil below in a uniform and efficient manner. Selection of shape and size of combined footing should be done carefully for different cases otherwise tilting of building, settlement of the foundation, cracks in walls, etc may occur.
Provision of combined footing also reduces the vertical and horizontal displacement of the building which is caused by seismic waves.
When To Provide Combined Footing
a. When two or more isolated footings are very close causing overlap. Two or more footings are connected either with beam or with the slab or both combined.
b. When the bearing capacity of the soil is not good enough.
c. When the extent of load in columns is highly varied. Example: Let us consider, we have two columns naming A and B. Column A have to carry 20KN/m and B have to carry only 10 KN/m. In this case, the combined footing is provided so that load is transferred uniformly.
Types of Combined Footing
There are 3 types of commonly used combined footing according to their shape. They are:
a. Rectangular Combined Footing.
~ It is rectangular in shape.
~ It is used when one column have to carry more load than others.
b. Trapezoidal Combined Footing.
~ It is trapezoidal in shape.
~ It is provided when the width of the footing is restricted.
c. Tee-Shaped Combined Footing
~ It is tee shaped.
The centre of gravity of the combined footing should be concerned very carefully.
If the load on the foundation too much high and the bearing capacity of the soil is too low then Mat Footing is provided.
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Download Combined Footing Design Excel Sheet
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