Compressive behavior of aramid FRP-HSC-steel double-skin tubular columns

This paper presents the results of an experimental study on the behavior of fiber reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) under concentric compression. Influence of column parameters was investigated experimentally through the test of 16 normal- and high-strength concrete-filled DSTCs and six concrete-filled FRP tubes (CFFTs). The column parameters examined included the thickness of FRP tube; the concrete strength; the diameter, thickness, and shape of inner steel tube. The results of the experimental study show that concrete in a DSTC system is confined effectively by FRP and steel tubes. Both the normal- and high-strength concrete DSTCs tested in the present study exhibited a highly ductile compressive behavior. The results also show that increasing the inner steel tube diameter leads to an increase in the ultimate axial stress and strain of concrete in DSTCs. It is found that, for a given nominal confinement ratio, an increase in the concrete strength results in a decrease in the ultimate axial strain of DSTCs. It is also observed that DSTCs with square inner steel tube confined concrete ineffectively. In addition, it is found that concrete in DSTCs developed similar ultimate axial stresses but higher ultimate axial strains compared to concrete in companion CFFTs. The results of the present study and those from the previously reported studies are then compared with the only existing stress-strain model proposed for DSTCs. Finally, a new design-oriented model that provides improved predictions of the ultimate conditions of concrete in DSTCs was proposed.