Behaviour of concrete-filled steel tubes with concrete imperfection under axial tension

In concrete-filled steel tubes (CFSTs), a gap between the steel tube and the core concrete can exist due to inappropriate construction, which will result in a change in the load-carrying capacity of the member. To assess the change in tensile strength, the tensile behaviour of circular CFST members with small gaps was investigated. To this end, a finite-element model was established and validated. Two types of gaps were considered in the analysis – a circular-segment gap and a circumferential gap. The full-range mechanical behaviour of CFST tensile members with a gap was evaluated. CFST members with and without a gap were compared in terms of their failure pattern, load–deformation response, composite action and ultimate strength. A parametric study was also performed and simplified formulae for calculating the tensile strength of CFST members with gaps were developed. It was found that the tensile strength of a CFST member with a gap decreases due to reduced composite action. The tensile strength of CFST members with gaps can be assessed reasonably well using the proposed method.