Effect of Footing Rigidity on Contact Stress

Contact stress is related to the normal pressure at the contact surface between the foundation and the supporting soil. Contact stress distribution under the footingdepends on the footing rigidity. Rigidity of the foundation-soil system is qualified according to DIN (2005) using the coefficient of rigidity (K). At very low values of K, contact stress is highly concentrated under the footing center, and some unrealistic tension stresses are produced at edges due to the linear elastic soil. This tension disappeared with increasing K, and low compression stresses are produced. With increasing the rigidity, the center contact stress decreased and the edge compression stress increased. When the footing is rigid, the edge stress is highly concentrated and center stresses are low. The increase of concentration of edge stresses leads to increases in the resulting maximum moment. Thus, increasing the footing rigidity is increasing the maximum moment. Footing design using standards always results in a semi-rigid section, which produces concentration of stresses at edges and low stresses at the footing center. Therefore, the calculated maximum bending moment by assuming distributed contact stress, as standards and references, is lower than that calculated considering the effect of soil structure interaction. Most references state that the contact stress in cohesionless soil is concentrated at the footing center and is low at the footing edges. It cannot be done, according to the numerical analysis, except in two cases. Case 1 is the flexible footing, and it is not applicable because the designed footing is considered semi-rigid. Case 2 is the failure condition, which cannot be applicable by calculating footing dimension considering the allowable bearing capacity with an appropriate factor of safety.