Axial compression behaviour of GFRP-confined UHPC encased cross-shaped steel columns

GFRP-confined UHPC encased cross-shaped steel (FCUSC) column is a novel composite column composed of ultra-high performance concrete (UHPC), cross-shaped steel sections, and external confinement using fiber-reinforced polymers (FRP). An experimental study on 22 specimens including 14 FCUSCs, 6 FCUs, and 2 bare cross-shaped steel columns was conducted to investigate the axial compression behaviour of composite columns employing different UHPC types, steel sections and GFRP tube thicknesses. The failure modes, failure mechanisms, composite effects were investigated, while the load capacity enhancement and GFRP tube confinement stiffness contributions were also evaluated in this paper. The results revealed that FCUSCs using fiber-reinforced UHPC exhibited greater axial load capacity and excellent deformation compatibility among each component. However, the steel section showed more positive contribution to the relief of nonuniform UHPC dilation than application of steel fibers in UHPC. The composite effect of FCUSCs is primarily governed by the deformation compatibility, which can be achieved by increasing GFRP tube thickness or using fiber-reinforced UHPC. Adopting thicker GFRP tube was more effective to improve load-bearing capacity than employing longer flange steel section.