Axial load biaxial moment interaction (PMM) diagrams for shallow foundations: Design aids, experimental verification, and examples

Abstract

This paper presents the development of design aids and simplified closed-form equations to estimate the coupled vertical load and biaxial moment capacity (PMM) of solid and hollow rectangular and circular shallow foundations as limited by bearing strength of the supporting soil. It is demonstrated that the resulting simplified equations are generalized versions of the widely accepted equivalent width concept proposed by Meyerhof nearly seven decades ago. The formulation can be used in soil-structure analyses where foundation strength influences the seismic response of bridges or buildings. Whether the design philosophy is to capacity protect the foundations or allow rocking and energy dissipation through soil yielding, the proposed formulation and charts allow sizing shallow footings in an expedite manner and using a pocket calculator. Results from four experimental programs comprising over fifty scaled foundation models under biaxial loading are included to show that the proposed simplified equations are useful to reasonably estimate the measured capacities. In addition, two examples are presented to illustrate the versatility of the formulation and ease of implementation.