Behavior of large-scale hybrid FRP-concrete-steel multitube concrete columns under axial compression

Abstract

The combined use of a fiber-reinforced polymer (FRP) tube with steel and concrete to form hybrid structural members has attracted increasing research attention. Hybrid FRP-concrete-steel multitube concrete columns (MTCCs) are a new form of such members, comprising an external FRP tube and a number of internal steel tubes, with all the space inside the tubes filled with concrete. Hybrid MTCCs allow the use of small-scale standard steel tube products to construct large-scale columns, and these possess many advantages including excellent ductility, as demonstrated by recent studies. The existing studies on MTCCs, however, have been limited to the testing of small-scale specimens. For a new column form particularly suitable for large-scale construction, the potential size effect on the behavior of MTCCs needs to be clarified. This paper presents the first-ever experimental study on large-scale MTCCs through the testing of specimens with an outer diameter (for circular specimens) or a side length (for square specimens) of 500 mm and a height of 1,500 mm. The configuration of steel tubes in these specimens, designed to be similar to real columns, is different from those in the small-scale MTCCs reported by the existing studies. The test results show that the large-scale MTCCs all possess excellent structural performance including ample ductility and that the size effect appears to be negligible for MTCCs with sufficient confinement. The test results also show that the configuration of steel tubes may have a significant effect on the behavior of the confined concrete in MTCCs. In addition, an analytical method based on the transformed section approach and an existing model for FRP-confined concrete-filled steel tubes is presented and shown to provide close predictions of the test results in the present study.