Experimentally verified behavior and confinement model for concrete in circular stiffened FRP-concrete-steel double-skin tubular columns

Abstract

A hybrid double-skin tubular column (DSTC) is a modern composite column with concrete sandwiched between outer fibre-reinforced polymer (FRP) and inner steel tube. Recently, a new variation form of DSTC, called stiffened DSTC, has evolved with longitudinal stiffeners provided on the outer surface of the inner steel tube. The stiffened DSTCs have better composite action between the DSTC components and mitigate the negative effects of inward local buckling of the inner steel tube. Scant research or design guidelines from standard specifications are presently available on the compressive behavior of this variation form of DSTC. This paper proposes the first ever design-oriented compressive strength model for confined concrete in circular stiffened DSTCs. The proposed axial stress model is based on the experimental results of the present study and a similar test dataset available from the literature. The experimental test results consisted of both normal FRP and large-rupture-strain FRP outer tubes used in circular DSTCs. In total, twelve circular DSTC specimens were tested in this study, and stiffener characteristics including the quantity, configuration, dimensions (i.e., width and thickness), and shape of the stiffeners, were studied. The test results showed that stiffened DSTCs had an enhanced compressive strength of confined concrete (up to about 29% increase in this study), resulting in higher axial load capacity and more ductile behavior compared to DSTCs without stiffeners. The proposed design model for compressive stress performed reasonably well and depicted close agreement with the experimental results for circular DSTCs, using both normal and large-rupture-strain FRP-based outer tubes.