Axial compression behaviour of hybrid composite FRP–concrete–steel double-skin tubular columns with various fibre orientations

The hybrid double-skin tubular column (DSTC) is a unique composite column composed of an outer tube made of a fibre-reinforced polymer (FRP) and an inner tube made of steel, with concrete filling the space between the outer and inner tubes. The external and internal tubes provide excellent confinement to the concrete. The column is characterised by the outer FRP tube’s excellent corrosion resistance, improved ductility due to the composite effect, and low weight due to its hollow cross-section. Previous studies were limited to the wet-layup of glass fibre reinforced polymer oriented in the circumferential direction of a double-skin tubular column, both experimentally and analytically. A total of 36 specimens were tested for axial compression loading. The behaviour of the column was inferred from the following parameters: orientation of fibre angle (0°,0°/90°and ± 45°) with respect to the circumferential direction, thickness of the FRP tube (2.1 mm and 4.2 mm), concrete strength (NSC and HSC), and void ratio (0.5, 0.59 and 0.67) of the steel tube. The experimental results showed that the fibres oriented in the hoop direction provided higher ultimate load-carrying capacity in comparison with fibres oriented at 0°/90°and ± 45°. The fibres oriented at ± 45° showed an increase in the ultimate strain than the other orientations. It was also found that the normal-strength concrete (NSC) specimens display better strength and strain enhancement ratio than the high-strength concrete (HSC) specimens. An improvement in resisting compressive stress and strain was observed when an increase was made in the FRP tube thickness and steel tube diameter. The results obtained from the experiments were compared to the theoretical prediction model results. The predicted model was found to be accurate in estimating the ultimate stress and strain of concrete in hollow DSTCs.