Footings is a essential structural element of a building which needs to be designed very carefully and accurately, footings are the one who transfers the whole load of the structure safely to the ground, therefore designer should not go wrong while designing a footing. The choice or selection of type of footings depends upon the type of soil, soil strata and the type of superstructure.
Combined Footing: A Combined footing is a type of footing which supports the load of two or more adjacent columns.
Such footings are provided under following circumstances.
- When the columns are very close to each other such that their individual footings overlap.
- When the Bearing Capacity of Soil is less, due to this condition more area is required under individual footing.
- When the end column is near to a property line such that the footing cannot be extended beyond property line.
These are the 3 major circumstances under which a structural engineer has to design a combined footing.
The Combined footing may be rectangular or trapezoidal in plan, the ultimate aim is to get uniform pressure distribution under the footing to have a safe combined footing, for this CG of footing area should coincide with the CG of combined loads of the two columns. There are usually four types of combined footings generally used.
- Combined Rectangular Footing : Generally used when two columns carry an equal load
- Combined Trapezoidal Footing: If one of the column carries heavier load, generally towards property line, provision of trapezoidal footing becomes essential.
- Strap Footing: A strap footing consists of spread footings of two columns connected by a strap beam.
- Raft Footing: A raft or mat is a type of combined footing which covers the entire area beneath a structure to support all the columns and walls.
Prof. Abhijeet Baikerikar
Founder, All About Civil Engineering.