A unified analytical model for the elastic bending behavior of composite beams with double-slip interfaces considering uplift effects

Composite beam bridges are often strengthened by attaching additional layers to the bottom slab to enhance their load-carrying capacity. During service, slip or uplift may occur at both the upper composite interface and the lower strengthening interface, leading to complex mechanical responses. This study develops a unified analytical model that captures the bending behavior of such double-slip interface systems, incorporating the uplift effect at the upper interface. Within the framework of Euler–Bernoulli beam theory, governing equations are derived to calculate deflection, interfacial shear force, slip, and sectional stress of composite beams with two slip interfaces. Subsequently, a simply supported steel-concrete composite beam was selected for numerical analysis as a case study to validate the proposed model and conduct parametric investigation. The results show that the tensile stress at the upper layer of the concrete and steel beam increase by about 4.1% and 17.4% compared with classical Euler–Bernoulli predictions. The contribution of interface uplift stiffness to static flexural deformation is relatively limited, while the shear stiffness at the upper interface shows a significant influence. Specifically, the average longitudinal shear stiffness across the interface width should not fall below 840 N/mm² to maintain reliable flexural behavior. These findings provide valuable engineering insights for the performance assessment of short- and medium-span bridges.