Two or more column footings connected by a concrete beam is known as combined footing . This type of beam is called a strap beam. It is used to help distribute the weight of either heavily or eccentrically loaded column footings to adjacent footings.
The function of a footing or a foundation is to transmit the load form the structure to the underlying soil. The choice of suitable type of footing depends on the depth at which the bearing strata lies, the soil condition and the type of superstructure.Combined footings are constructed for two or more columns when they are close to each other and their foundations overlap. Design of combined footings with example is discussed. The function of a footing or a foundation is to transmit the load form the structure to the underlying soil.
Whenever two or more columns in a straight line are carried on a single spread footing, it is called a combined footing. Isolated footings for each column are generally the economical.
Combined footings are provided only when it is absolutely necessary, as
When two columns are close together, causing overlap of adjacent isolated footings
Where soil bearing capacity is low, causing overlap of adjacent isolated footings
Proximity of building line or existing building or sewer, adjacent to a building column.
The combined footing may be rectangular, trapezoidal or Tee-shaped in plan.
The geometric proportions and shape are so fixed that the centeroid of the footing area coincides with the resultant of the column loads.