Biaxial capacity of rigid footings: Simple closed-form equations and experimental results

This manuscript first presents an analytical procedure to derive dimensionless charts for the analysis and design of rigid rectangular foundations under axial load and biaxial moment. It then shows that conditions of symmetry in the normalized domain of the problem lead to practical closed-form equations that provide the entire coupled axial-load-biaxial-bending capacity envelope of shallow rectangular footings. The resulting relations may find direct application in the performance-based seismic analysis/design of soil-structure systems for which the foundations are vulnerable (i.e., prone to uplifting and/or soil plastification). It is demonstrated that the equations are a generalized version of the widely used equivalent width concept proposed by Meyerhof. Results from an experimental program involving 19 tests with 200 mm square and 200 × 300 mm rectangular foundations models under biaxial loading are presented to show that the proposed simple equations provide reasonable estimates of the measured capacity. Further comparisons with large-scale and small-scale foundation models available in the literature suggest that, similar to Meyerhod’s equivalent width concept, the proposed formulation is relatively independent of scale effects.