Behavior of hybrid FRP-concrete-steel double-skin tubular beams with ultra-high strength concrete and PBL shear connectors under bending

Abstract

Hybrid double-skin tubular beams (DSTBs) with the external fiber-reinforced polymer (FRP) tube-annular concrete-inner steel tube configuration have excellent mechanical performance and corrosion resistance, but are prone to slip problems. In this study, perfobond shear connectors (PBLs) are proposed to reduce the slip between infilled concrete and inner steel tube. Moreover, using ultra-high strength concrete (UHSC) as the annular material could further improve the structural performance and magnify the lightweight merit of hybrid DSTBs, while hybrid DSTBs with PBLs and UHSC have not been investigated yet. To this end, three-point bending experiments were conducted to hybrid DSTBs with PBLs and UHSC to study the flexural behavior. The mid-span load–deflection curves revealed a three-branched behavior: (i) a first linear ascending branch up to the first yielding at the extreme tension fiber of inner steel tube; (ii) a transition branch to the peak load with a significant slope deterioration when infilled concrete in the compression side reached the unconfined failure strain; (iii) a post-peak residual branch characterized by progressive load reductions over a number of load plateaus. The adoption of UHSC could significantly delay the plastic hinge formation in inner steel tube, and meanwhile improve the bond with PBLs. Thus, the slips between infilled concrete and inner steel tube were minimal (less than 0.2 mm) for most of the specimens, except for those with foamy sheet insertions between PBLs (at around 1 mm). In the end, the design loads of the hybrid DSTBs were predicted by a sectional analysis with high level of accuracy (average error at 6%).