Effect of Spread Footing Flexibility on Structural Response

Spread footings are normally used under individual columns of buildings and bridge piers. They are economical to use and are applicable for any soil conditions where the bearing capacity for the applied loads is adequate. Structural design codes and specifications allow a linear soil pressure distribution to be assumed for the design of spread footings. This approach is valid for infinitely rigid footings. Past experience has shown that the assumption of a linear pressure distribution is satisfactory for most footings; however, there are some cases in which a shallow foundation must be analyzed as a flexible structure, particularly if the footing is excessively long/wide and thin. In this study, a relative stiffness factor, $K_r^{\prime }$ , is developed that can determine whether a footing can be considered rigid for the purposes of structural analysis and design. This factor is a modified version of an expression first proposed by Meyerhof in 1953, but takes into account the size of the column supported on the footing. The study is based on modeling square and rectangular spread footings subjected to concentric and eccentric loadings by finite elements. The footings are modeled using thick rectangular plate elements and the soil with elastic springs. The results of the study showed that a footing with $K_r^{\prime }$ factor greater than 1.0 indicates that it can be analyzed as a rigid footing with reasonable accuracy. This includes determination of soil pressures, vertical footing displacements, shear forces, and bending moments. The study also showed that maximum shear forces within a spread footing are less sensitive to changes in the stiffness of a footing than bending moments.