Stress-strain model for concrete in FRP-confined steel tubular columns

Concrete-filled steel tubes (CFTs) are widely used as columns in many structural systems. In CFTs, degradation in steel confinement, strength and ductility can result from inelastic outward local buckling. To overcome this deficiency of CFTs, external confinement of CFTs with an FRP jacket has been explored in recent studies. This paper presents a theoretical model in an incremental-iterative form for circular FRP-confined CFTs (CCFTs) under monotonic axial compression, with the focus being on the stress-strain behavior of the confined concrete. The proposed stress-strain model for concrete in CCFTs is based on the same approach as that commonly adopted by existing models for FRP-confined concrete and includes three components: (a) an active-confinement model; (b) a lateral strain equation; and (c) equations for determining the total confining pressure from the steel tube and the FRP jacket. It is shown that the lateral dilation behavior of concrete in CCFTs differs significantly from that of FRP-confined concrete in the initial stage because the former experiences more severe micro-cracking than the latter in the initial stage of loading; this difference is reflected in the proposed model. In general, the predictions of the proposed model are in close agreement with existing test results. The proposed model provides a useful tool for a parametric study on the stress-strain behavior of confined concrete in CCFTs to produce results for the formulation of simple stress-strain model in closed-form expressions for design use.