Behavior of Short-Span Concrete-Filled FRP Tubes with ± 55° Fiber Orientation under Bending and Shear

In this study, the flexural and shear behavior of steel-reinforced concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) with ± 55° fiber orientation is examined extensively through experimental and numerical approaches. A total of 18 CFFT beams, each 1,219.2 mm (4 ft) long and made of ± 55° glass FRP (GFRP) tubes with an inner diameter of 203.2 mm (8 in.), were tested under three-point bending. The test variables included GFRP tube thicknesses of 2.7, 4.7, and 6.7 mm, steel reinforcement ratios 1.85% and 3.70%, and shear span-to-depth ratios 0.5, 1.0, and 2.25. All specimens exhibited a pronounced nonlinear load–deflection response and failed in flexure due to GFRP rupture at the bottom tension zone. Despite the low shear span-to-depth ratio of 0.5, which typically promotes shear failure, no shear-governed failures were observed. Experimental results showed that the combination of ± 55° fiber orientation with internal steel reinforcement, along with reduced shear span-to-depth ratios, contributed to notable improvements in strength, stiffness, and load-carrying capacity. No significant interface slips were observed between GFRP tube-concrete core, and concrete core-steel, establishing strong composite action. Nonlinear finite-element models developed in LS-DYNA (version R10.0) accurately captured the flexural response and failure modes, showing good agreement with experimental results. Overall, the findings underscore the importance of accounting for the nonlinear behavior of ± 55° GFRP tubes in both experimental and numerical modeling of CFFT systems.