Experimental investigation of ultimate lateral resistance of superstructure–steel pipe pile foundation–soil systems considering local buckling–induced strength deterioration of pile members

Steel pipe piles supporting buildings may lose lateral resistance during strong earthquakes because local buckling at pile heads degrades member strength under high axial force. Although the lateral resistance of single piles and pile groups has been widely studied, a rational framework for evaluating the ultimate lateral resistance of superstructure–pile foundation–soil systems while accounting for local buckling–induced strength deterioration is still lacking. This study proposes such a framework by integrating experimentally identified collapse mechanisms, finite element evaluation of degraded pile bending resistance, and virtual-work-based system analysis. Centrifuge lateral loading tests on two-pile foundations embedded in sand revealed that the ultimate state is governed by a two-hinge mechanism in each pile and that system collapse is triggered by hinge formation in the embedded portion of the back pile after progressive axial force redistribution and pile-head local buckling. Based on these observations, finite element analyses were used to quantify the degraded bending moment after local buckling through a plasticity-ratio-based procedure, and the system ultimate lateral resistance was then evaluated by accounting for the internal work of both piles and soil, including group pile effects. The study demonstrates that local buckling–induced deterioration is a decisive factor in the maximum lateral resistance of steel pipe pile foundation systems and provides a mechanics-based basis for performance-based design of building pile foundations.