Hybrid FRP-concrete-steel double-skin tubular columns with a square outer tube and a circular inner tube: Stub column tests

Hybrid FRP-concrete-steel Double-Skin Tubular Columns (DSTCs) are a new form of hybrid columns invented by the second author. Such a column consists of an outer tube made of Fibre-Reinforced Polymer (FRP) and an inner tube made of steel, with the space between filled with concrete. In these columns, the three constituent materials are optimally combined to achieve several advantages not available with existing forms of columns. In this paper, results from a series of stub column tests are presented and discussed to examine the axial compressive behavior of hybrid DSTCs with a square outer tube and a circular inner tube. Three types of specimens were tested: square FRP-Confined Solid Columns (FCSCs), square FRP-Confined Hollow Columns (FCHCs) (i.e. with a circular inner void), and square hybrid DSTCs with a circular inner tube. A total of twenty FRP-confined specimens were tested, with the main parameters examined being the section configuration, the void ratio, and the thickness of the FRP tube. The test results show that the presence of an inner void reduces the effect of external FRP confinement, but this loss of confinement effectiveness is almost completely compensated for through the provision of a suitable steel tube. As a result, the concrete in such hybrid DSTCs is very effectively confined by the two tubes and the behavior of concrete in these hybrid DSTCs is similar to that in FCSCs. The test results also indicate that the thickness of the FRP tube has a significant effect on both the load-carrying capacity and the ductility of such hybrid DSTCs.