COMBINED FOOTINGS MAY BE RECTANGULAR OR TRAPEZOIDAL OR MAY CONSIST OF ISOLATED FOOTINGS CONNECTED WITH A NARROW BEAM

COMBINED FOOTINGS :-- When two columns are 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 accentric .It may be economical to combine this footing with that of the adjacent internal column in such a manner that the centre 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 the 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 footing will deflect 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 footings will develop hogging moment. Thus at any section, shear is sum of the forces and moment is 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 develops 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 may consist of isolated footings connected with a narrow beam. The latter footing is called combined footing with strap beam. The footings must be checked in one way shear action and two way shear action.