Numerical Analysis on Seismic Performance of Concrete-Encased CFST Columns with Two-Stage Initial Stresses

The three-stage construction of concrete-encased concrete-filled steel tubular (CE-CFST) arches introduces two-stage preloads (namely, two-stage initial stresses) within the members that critically affect their seismic behavior. This paper presents a numerical investigation into this phenomenon, employing a validated nonlinear fiber element model. The key finding is a stress-induced redistribution of internal forces that produces a dual effect: it enhances lateral capacity and ductility at low global axial compression ratios (n < 0.5) but impairs them at high ratios. For instance, at n = 0.3, gains of 3.4% in strength and 18.9% in displacement were observed. Based on parametric and theoretical analysis, a degenerate trilinear restoring force model is proposed, establishing a restoring force model for the seismic analysis of pre-stressed CE-CFST members.