Hybrid Steel and Concrete Nodes with PP-FRC Filled Steel Tube Infill under Transverse Compression

Discrete reticulated shells rely on the transverse compressive performance of joints where multiple linear members converge, and concrete filled steel tube (CFST) nodes have emerged as a competitive solution thanks to the interaction between the confining tube and the concrete core. Although axial CFST behaviour is well documented, evidence on transversely loaded CFST nodes infilled with polypropylene fibre reinforced concrete (PP-FRC) remains scarce and the mechanism by which fibres redistribute stress after split tensile cracking has not been quantified for this geometry. This study tests seven short circular S355J2H tubes of 273 mm outer diameter and 10 mm wall thickness in three configurations covering bare reference tubes, tubes filled with C25/30 normal concrete, and tubes filled with PP-FRC carrying 5 kg/m 3 macro polypropylene fibres, supported by a closed form beam theory model and a three dimensional Menetrey Willam plasticity finite element model in Ansys Mechanical 2022 R2. Concrete infill raised the load bearing capacity by more than fifteen times relative to bare tubes, the supplementary fibre specimen reached 2387 kN at a bearing to compressive strength ratio of 3.83 with controlled post peak softening, the fibre dosage did not alter the splitting load yet converted the failure mode from brittle to ductile through residual tensile transfer at crack widths below 0.5 mm, and the simplified model recovered the pre and post cracking stiffness within 16% and 5% respectively. The findings identify residual tensile strength of macro polypropylene fibres as the governing parameter for ductile design of transversely loaded CFST nodes and support their adoption in long span gridshell connections where progressive collapse robustness is required.