Hybrid FRP–concrete–steel tubular columns: Concept and behavior

Hybrid FRP–concrete–steel double-skin tubular columns are a new form of hybrid columns recently proposed by the first author. The column consists of an outer tube made of fiber reinforced polymer (FRP) and an inner tube made of steel, with the space between filled with concrete. In this new hybrid column, the three constituent materials are optimally combined to achieve several advantages not available with existing columns. In this paper, the rationale for the new column form together with its expected advantages is first explained. A series of axial compression tests on stub columns are then presented to demonstrate some of the expected advantages. These test results confirm that the concrete in the new column is very effectively confined by the two tubes and the local buckling of the inner steel tube is either delayed or suppressed by the surrounding concrete, leading to a very ductile response. The application of the proposed hybrid section form in beams is also examined by presenting the results of a series of tests on beams with a hybrid section in which the inner steel tube is shifted towards the tension side. The test results also show that such beams have a very ductile response and that the GFRP tube in such beams enhances the structural behavior by providing both confinement to the concrete and additional shear resistance.