FRP-HSC-steel composite columns: behavior under monotonic and cyclic axial compression

This paper presents the results of an experimental study that was undertaken to investigate the effects of key parameters on the compressive behavior of fiber reinforced polymer (FRP)-concrete-steel composite columns. Performance parameters for the columns were investigated experimentally by testing 24 double-skin tubular columns (DSTCs), six concrete-filled FRP tubes (CFFTs), and two CFFTs with inner steel I-beams (I-CFFTs). The parameters examined included the loading patterns; concrete strength; diameter, thickness, and end condition of the inner steel tube in DSTCs, and influence of concrete-filling the inner steel tube. The results of the experimental study indicate that concrete-filling inner steel tubes of DSTCs results in an increase in the compressive strength and a slight decrease in the ultimate axial strain of concrete in DSTCs, compared to the values observed in companion specimens with hollow inner steel tubes. The new DSTC system developed in this study, which comprises dual-grade concretes, has been shown to exhibit superior performance compared to conventional normal-strength concrete (NSC) and high-strength concrete (HSC) DSTCs. The results also indicate that the ultimate axial stress and strain of concrete in filled DSTCs tend to increase with an increase in inner steel tube diameter. It was observed that increasing inner steel tube thickness leads to a slight increase in ultimate axial stress and strain of concrete in DSTCs. It was also observed that cyclically loaded NSC DSTCs developed similar strength and strain enhancement ratios with monotonically loaded NSC DSTCs. On the other hand, a slightly increase strength and strain enhancement ratios was observed on HSC DSTCs in the presence of load cycles. Finally, it was found that the concrete in filled DSTCs exhibit slightly larger ultimate axial stress compared to those of the concretes in companion CFFTs and I-CFFTs.