Design Footing Area with Biaxial Bending

A direct method of proportioning a rectangular footing area subjected to biaxial bending is proposed as an alternative to the trial and error method of solution. Formulas for the dimensions of the footing area are derived using the ordinary flexure formula and the limiting conditions that the maximum and minimum pressures are developed at the critical corners which are diagonally opposite each other. In addition, the maximum pressure is equated to the allowable bearing capacity of the soil while the minimum pressure is equated to zero. This condition assures the validity of the application of the flexure formula since the footing area is in full compression. Th analysis yielded the basic relationship of the footing area dimensions as 12 times the eccentricities of the total vertical load about the centroidal axes while the minimum area is controlled by the allowable soil bearing capacity. Once the area of the footing has been determined, adjustments can be made in the magnitude of the total vertical load to satisfy the above relationships.