Shear performance test on demountable double-bolt connectors for steel-concrete composite beams

To enhance the mechanical performance of double-bolt connectors in steel-concrete composite structures, optimization was performed on the configuration of existing double-bolt connectors. Push-out specimens of double-bolt connectors with diameters of M16, M20, and M24 were designed and fabricated, including both original specimens and optimized specimens. Push-out tests were then conducted. Based on the measured shear force-relative slip curves and failure modes of connectors, the shear capacity, peak slip, and shear stiffness of the optimized double-bolt connectors were systematically evaluated. Experimental results show that the final failure mode of all push-out specimens is shear fracture of the bolt shanks, and the fracture surfaces exhibit typical brittle fracture features. For the optimized double-bolt connectors, only slight cracking occurs in the concrete at the lower side, and no crushing or spalling is observed. Compared with those of the original double-bolt connectors, the shear capacity of the optimized double-bolt connectors increases by approximately 30%, and the peak slip increases by approximately 70%, while the average shear stiffness decreases by approximately 65%. In the optimized double-bolt connectors, a steel cushion plate was added between the short bolt and the sleeve, so that the shear plane was located in the unthreaded region of the short bolt shank. This increases the effective shear area and therefore improves the shear capacity of the connectors. The introduction of the steel cushion plate causes a large bending deformation of the short bolt shank during shear loading, which markedly enhances the ductility of the connectors but also leads to a reduction in their shear stiffness. The proposed calculation methods for the shear capacity and shear stiffness can provide theoretical reference for the application of double-bolt connectors in steel-concrete composite structures.