Contribution of longitudinal GFRP bars in concrete filled FRP tubular (CFFT) cylinders under monotonic or cyclic axial compression

Abstract

Most of the current design guidelines/codes ignore the compressive contribution of glass fiber reinforced polymer (GFRP) bars in compression members, due to: (1) the compressive strength of GFRP bars is significantly lower than their tensile strength due to fiber micro-buckling or shear failure of GFRP bars under compression; (2) the peak axial load of an unconfined concrete column is achieved at a relatively low axial strain at which the GFRP bars develop small axial stresses due to their low elastic modulus. However, existing studies have demonstrated that, if adequate transverse bars are provided, the contribution of longitudinal FRP bars to the total load resistance of the column is significant partially due to the enhanced axial strain at peak axial load because of the confinement provided by the transverse bars. In concrete filled FRP tubular (CFFT) columns, which is an emerging and attractive form of columns for new construction, the ultimate axial strain of concrete is significantly enhanced by FRP confinement and thus the contribution of internal longitudinal GFRP bars is also expected to be enhanced. However, the compressive behavior of GFRP bars in CFFT columns has never been investigated. This paper therefore presents the results of an experimental program on CFFT cylinders with internal longitudinal GFRP bars. The test results demonstrated that FRP bars contribute significantly to the total load resistance of CFFT cylinders. The FRP bars in CFFT cylinders experienced crushing failure with load resistance close to that derived based on the compression tests on bare FRP bars. A simple analysis method for the axial load at FRP bar crushing for CFFT cylinders was finally proposed.