A long plate girder is subjected to a maximum bending moment M in which permitted allowable bending stress is f_{b}. The economical depth d_{e} and the thickness of the web t_{w}, is given by

In a grillage column footing maximum bending moment M occurs at the centre of grillage beams. Its value is (where P, L and a are axial load, length of the beam and length of the column base respectively).