THE COMBINED FOOTING MAY BE TREATED AS A BEAM LOADED ON ITS UNDER SIDE AND SUPPORTED.ON COLUMN.

COMBINED FOOTINGS:-- When it is necessary to have two columns close together,and separate footings would overlap ,it is better to provide a combined footing. Further , If one column is close to a property line or sewer pipe,the load on an isolated base will be eccentric .It may be economical to combine this footing with that of the adjacent internal column in such a manner that  the cetre of gravity of the loads from the two columns coincides with the centre of gravity of the combined footing. If this can not be done ,moments of forces are taken about the centre of base and the maximum pressure is determined from the total vertical load and moment at the underside of the base .The pressure is assumed to vary uniformly along the length of the base .The pressure is assumed to vary uniformly along the length of the base .The footing will reflect in the two directions .In the longitudinal direction the cantilever portion and the portion under the columns will develop sagging moments,and the central portion of the footing will develop hogging moment. As such at any section, shear is sum of the forces and moment of the all forces on one side of the section.The footing may be treated as a beam loaded on its underside , and supported on columns. In the transverse direction , the footing development sagging moments. The moments and shears are found in the same manner as for as isolated footing.Combined footings may be rectangular or trapezoidal in plan or consist of isolated footing connected with a narrow beam .The latter footing is called combined footing with strap beam .The footing must be checked in one way shear action and two way shear action .It is suggested that transverse reinforcement should be provided into groups proportionate in sectional area to the column loads .The transverse reinforcement under each column should be provided with in a band  having a width equal to the width of the column plus two times the effective depth of the foundation .