Compression tests on square hybird FRP-concretesteel tubular columns with a rib-stiffened steel inner tube

Hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) consist of an outer tube made of FRP and an inner tube made of steel, with the space between filled with concrete. Extensive research has been conducted on hybrid DSTCs, and has demonstrated their excellent structural performance. In hybrid DSTCs, the outward buckling of the steel inner tube is constrained by the surrounding concrete and the outer FRP tube, but its inward buckling is still possible. Existing research has shown that such inward buckling generally does not have a significant effect on the column behavior, but it can become significant in non-circular hybrid DSTCs, in hybrid DSTCs with a strong FRP tube, and/or in hybrid DSTCs with a thin steel tube. In such cases, the stiffening of the inner steel tube may be necessary to prevent or delay its inward buckling and to minimize the consequent negative effects on the column behavior. Against this background, this paper presents an experimental study in which square hybrid DSTCs with a rib-stiffened steel inner tube were tested under axial compression. Results from a total of six specimens are presented, with the test variable being the number of ribs on the steel tube. The test results confirmed the effectiveness of providing additional rib stiffners on the steel tube and suggested that the use of a larger number of thin ribs is more effective than that of a smaller number of thick ribs with the same amount of rib material.