An Introduction to New Elastic Foundation Element Developed in OpenSees: A Simplified Model to Simulate Soil–Structure Interaction

In engineering practice, the dynamic analysis of structures subjected to seismic excitations often assumes a fixed base, neglecting the critical effects of soil–structure interaction (SSI). This simplification is especially problematic for structures on soft soils, where soil flexibility significantly alters the dynamic response. To address this, a novel stiffness formulation for soil–foundation systems was proposed, overcoming the limitations of existing approaches by more accurately capturing soil–foundation behavior. Building on this advancement, the study developed a new soil–foundation element, named ElasticFoundation, in OpenSees, enabling efficient incorporation of SSI effects into structural analysis. The ElasticFoundation element was validated through nonlinear analyses of 25 three-dimensional numerical models of buildings with 1, 3, 5, 10, and 15 stories, from which stiffness matrices were derived. Experimental data further confirmed its accuracy, with both the proposed model and experimental prototype showing maximum roof deflections of 24 mm. The average inter-story drift values for the model and prototype were 0.01 and 0.02, respectively, demonstrating comparable behavior within acceptable tolerances. The element captures critical parameters influencing SSI, including foundation dimensions, foundation vertical safety factor (FSV, the ratio of ultimate vertical load capacity to the applied load), soil properties, superstructure height, and the structure's fundamental period. This dual contribution—an innovative soil–foundation stiffness formulation and its implementation as a computationally efficient finite element in OpenSees alternative to full continuum soil modeling—offers a robust tool for SSI analysis, bridging the gap between accuracy and practicality in structural dynamics.