A new steel – UHPFRC composite beam with in-built composite dowels as connectors: concept and design principles

•A new steel –UHPFRC composite beam structure is proposed, aiming at the full and proper utilization of both materials.•The unique use of ① UHPFRC in both tension and compression and ② a half rolled section with continuous in-built steel dowels as shear connectors.•An experimental study is conducted to characterize the flexural and shear responses of the proposed structure.•The design principles for both flexural and shear resistance of the proposed structure are introduced based on the failure mechanism. Aiming at the full and proper exploitation of both steel and Ultra High Performance cementitious Fiber Reinforced Composites (UHPFRC) materials, this paper proposes a new steel-UHPFRC composite beam structure. The unique use of ① UHPFRC in both tension and compression and ② a half rolled section with continuous in-built steel dowels in combination with UHPFRC dowels (forming composite dowels as shear connectors) is highlighted. An experimental study consisting of two composite beams is then conducted to investigate the flexural and shear responses. In particular, the failure mode, cracking pattern, and monitoring of critical crack kinematics are discussed using digital image correlation (DIC) technology. Subsequently, the design principles for both flexural and shear resistance are introduced based on the failure mechanism: ① the determination of flexural resistance is based on the sectional analysis considering the tensile properties of UHPFRC, and ② the determination of shear resistance is based on a specific approach to the lever arm of internal forces and horizontal shear resistance of the composite dowel, considering the tensile contribution of UHPFRC. According to the experimental results, the effective interlocking between UHPFRC and the steel dowel allows efficient interaction between steel and UHPFRC components, benefiting from its higher shear resistance and ductility compared with traditional welded head studs. Both the flexural and shear response of the composite beam can be characterized into five distinguished domains, in which the quasi-elastic domain is introduced especially due to its large contribution to the resistance and high structural stiffness. Finally, the design principles are validated by the tested values.