Square FRP–HSC–steel composite columns: Behavior under axial compression

• 40 square FRP-concrete composite column specimens are tested under axial compression.
• Specimens consisted of FRP–concrete–steel DSTCs and CFFTs with or without inner voids.
• Effects of concrete, FRP tube, and inner steel tube parameters are investigated.
• Ultimate conditions and stress–strain behavior of concrete in DSTCs and CFFTs are presented.
• Relative performances of DSTCs are established with respect to companion CFFTs.

This paper presents an experimental study, which investigated the compressive behavior of square fiber reinforced polymer (FRP)–concrete composite columns through the tests of 40 column specimens. 24 FRP–concrete–steel double-skin tubular columns (DSTCs), four concrete-filled FRP tubes (CFFTs), and 12 CFFTs with inner voids (H-CFFTs) were tested under axial compression. The majority of the specimens were manufactured using high-strength concrete (HSC). The key parameters examined included the influence of the strength of the concrete, cross-sectional shape (i.e. circular and square) and dimension of the inner steel tube, and presence (or absence) of a concrete filling inside the steel tube. The results of the DSTCs with circular inner steel tubes indicate that concrete-filling inner tubes results in an increase in the ultimate axial stress but a decrease in the ultimate axial strain of concrete compared to those seen in DSTCs with hollow inner steel tubes. It is observed that concrete in hollow DSTCs manufactured with square inner steel tubes develop significantly lower ultimate axial stresses and strains than concrete in companion hollow DSTCs with circular inner steel tubes. It is found, however, that the performance of these specimens improves dramatically when the square inner steel tube is filled with concrete. Comparisons of the results indicate that concrete in filled DSTCs develops larger ultimate axial stresses and strains than concrete in companion CFFTs. Finally, the results demonstrate that H-CFFTs perform significantly worse than DSTCs and CFFTs, and their performance further degrade as the diameter of the inner void increases.