Nonlinear Pushover Analysis of 3D Spatial Frames on Pile Foundations: Integrating Lumped Parameter Soil-Structure Interaction

Assessing the seismic vulnerability of modern structural systems traditionally relies on idealized Fixed Base Building assumptions, which inherently neglect the critical flexibility introduced by the foundation and the surrounding geotechnical environment. This assumption can lead to significant miscalculations in structural demand and capacity. This paper presents an advanced computational framework that integrates a lumped parameter Soil-Structure Interaction model with Nonlinear Static Pushover Analysis to evaluate the true seismic capacity of three-dimensional spatial frames. By representing the multi-layered soil-pile system as a condensed network of dynamic springs and dashpots, the study bridges complex substructure dynamics with performance-based capacity evaluation methodologies. A 9-story steel benchmark building, subjected to varying non-uniform soil conditions, is utilized to demonstrate the proposed analytical framework. The numerical investigation reveals that foundation flexibility significantly alters the fundamental dynamic characteristics of the structural system. In the nonlinear regime, Soil-Structure Interaction significantly shifts the target roof displacement, exacerbates P-Delta effects due to rigid-body base rotation, and redistributes the formation sequence of plastic hinges compared to the conventional Fixed Base Building model. Furthermore, analysis indicates up to a 17.3% increase in column base shear demands under specific soft-soil pile group configurations. The findings provide vital recommendations for structural design engineers, emphasizing that the rigid base assumption may non-conservatively estimate story drift, ultimate ductility, and the overall collapse mechanism of earthquake-resistant high-rise structures.