Hybrid FRP-Concrete-Steel Multitube Concrete Columns: Concept and Behavior

This paper presents the conceptual development of a new form of hybrid structural column, termed the hybrid fiber-reinforced polymer (FRP)-concrete-steel multiple-tube concrete column (MTCC). A MTCC consists of an external FRP tube and number of internal steel tubes, with the space inside all the tubes filled with concrete. The new column form allows the use of small circular steel tubes, readily available in the market, to build columns of a much larger scale. In the column, the steel tubes are well-protected from corrosion by the external FRP tube, and the concrete is well-confined by both the internal circular steel tubes and external FRP tube. A series of stub column tests involving 10 MTCC specimens and two concrete-filled FRP tube (CFFT) specimens are presented to demonstrate the advantages of the new column form. The test variables included the type and configuration of internal steel tubes as well as the thickness of external FRP tube. The test results confirmed that the concrete in the test columns was very effectively confined, and that buckling of the steel tubes was effectively prevented, leading to a very ductile column response. The test results also showed that MTCCs generally have a much larger ultimate axial strain than that of CFFTs when the same FRP tube is used. A simple analytical model proposed for the axial load-strain curve of MTCCs is also presented and is shown to provide reasonable and conservative predictions of the test results.