Rectangular double-tube concrete columns with an internal elliptical high-strength steel tube: Concept and behavior

Abstract This paper presents a novel form of rectangular fiber-reinforced polymer (FRP)-concrete-high strength steel (HSS) hybrid columns referred to as rectangular double-tube concrete columns (DTCCs). These columns consist of a rectangular FRP tube filled with concrete and an elliptical HSS tube as internal reinforcement. The inner elliptical steel tube encloses most of the concrete in the rectangular section and therefore most of the concrete in the section is effectively confined. The high susceptibility of the HSS elliptical tube to local buckling is expected to be suppressed by the confined concrete, leading to full utilization of the high strength of HSS. This paper demonstrates the excellent performance of the new columns due to the above beneficial interactions between different components in the column through an experimental study. In the experimental study, twelve rectangular DTCCs covering different FRP tube thicknesses and aspect ratios were tested under axial compression. Finally, an existing stress–strain model for FRP-confined concrete in rectangular columns is modified to be applicable to the concrete in the new column by accounting for the confinement effect from the inner elliptical HSS tube. It is found that the modified model provides more accurate predictions on the stress–strain curves of confined concrete in the rectangular DTCCs than the original model.