Behaviour of CFST stub columns with initial concrete imperfection : analysis and calculations

Gap between the steel tube and concrete core can be considered as a kind of initial concrete imperfection in concrete-filled steel tubular (CFST) structures.This paper performs a nonlinear analysis of CFST stub columns with a circumferential gap or spherical-cap gap under axial compression. A nonlinear finite element model is developed, where the nonlinear material behaviour and the effect of gap on the interface behaviour of the concrete and steel tube are included. Close agreement is achieved between the test and calculated results in terms of load−deformation response and ultimate strength. In light of the numerical results, the behaviour of CFST columns with a circumferential gap or spherical-cap gap is analysed. Parametric studies are then carried out to investigate the influence of different parameters on the ultimate strength of CFST stub columns with gaps. Finally, the maximum limit of the gap ratio is proposed for CFST stub columns with circumferential gaps, and a simplified formula is proposed to estimate the effect of spherical-cap gap on the ultimate strength of CFST stub columns as well.