Instability mechanism and failure mode identification of axially loaded CFST columns under lateral impact: A modified 2-DOF approach

Abstract

Concrete-filled steel tube (CFST) columns are exposed to accidental lateral impact hazards. The instability-driven fracture mechanism under high axial load or intense impact remains unclear, and the quantitative method for identifying instability failure is lacking. To fill this gap, this research adopted numerical and theoretical research to identify CFST failure modes. A validated explicit numerical model was adopted to characterize the gradient evolution of failure modes and clarify the bidirectional competing mechanism of axial load on impact resistance. The results revealed that axial load influences impact response through the concurrent enhancement of dynamic flexural capacity and geometric second-order effect: axial load improves flexural capacity and suppresses tensile cracking, whereas excessive axial load amplifies the second-order effect and triggers global instability. Based on plane section assumption, a dynamic resistance-deflection constitutive model covering elasto-plastic, plastic and cracking to fracture phases was derived. For statically indeterminate members, an energy conservation-based equivalent lateral force model was proposed to incorporate second-order effect. By combining the resistance with equivalent lateral force, this investigation quantified the critical deflection threshold for instability initiation, and systematically revealed the failure process and mechanisms under zero, low and high axial load levels. A modified two-degree-of-freedom analytical model and a stepwise identification procedure were further proposed to distinguish four typical failure modes, enabling recognition of instability-induced catastrophic failure. The proposed framework shows favorable agreement with numerical and experimental results, and provides an effective theoretical tool for identifying failure modes and avoiding instability failure of CFST columns.