Behavior of hybrid FRP-concrete-steel double-skin tubular columns with a square outer tube and a circular inner tube subjected to axial compression

Hybrid fiber-reinforced polymer (FRP) concrete steel, double skin tubular columns (DSTCs) are a new form of hybrid columns. The most common sectional form of hybrid DSTCs consists of a layer of concrete sandwiched between a circular inner steel tube and a circular outer FRP tube whose fiber directions are close to the hoop detection to provide effective confinement to the concrete. Much recent research has been conducted on circular hybrid DSTCs, which has demonstrated that the combination of the three constituent materials leads to several advantages not available with existing forms of columns. In practical applications, for aesthetic and other reasons, square hybrid DSTCs may be needed. This paper thus extends the existing work on circular hybrid DSTCs to square hybrid DSTCs in which the outer FRP tube is square while the inner steel tube is still circular. Results from a series of axial compression tests are presented and interpreted to examine the compressive behavior of square hybrid DSTCs. In these tests, FRP tubes formed through a wet-layup process were used instead of filament-wound FRP tubes because the latter were not readily available to the authors at the time of the study. The test results show that the concrete in these square hybrid DSTCs is effectively confined by the two tubes, and the behavior of the confined concrete is similar to that of concrete in FRP-confined solid columns. A stress-strain model for concrete in square hybrid DSTCs is also proposed and is shown to provide reasonably accurate predictions of the test results.