Abstract: The primary supports of steel-reinforced concrete tunnels often experience deformation and failure due to adverse geological conditions. To mitigate this problem, studs and stirrups are incorporated into the steel arch frame, creating a composite support structure. Small eccentric compression tests under three conditions are conducted: natural bonding, studs on the web, and studs with stirrups on the web. The tests analyze the deformation and failure characteristics as well as the bearing capacity of the structure, leading to the derivation of a theoretical calculation formula for ultimate bearing capacity. The results show that: (1) Under the natural bonding condition, the concrete spalls and the steel twists and deforms, consistent with that observed under the actual tunnel support failure mode. (2) The addition of studs and stirrups on the steel web enhances the shear strength at the interface between the concrete and steel. The stirrups effectively constrain the core concrete, preventing spalling and improving the overall bearing capacity, ductility, and deformation resistance of the structure. Compared to the natural bonding condition, the structures with studs and with both stirrups and studs show increases in bearing capacity of 18.99% and 31.55%, respectively. In addition, the ductility coefficients increase by 26.79% and 46.41%, respectively, and lateral deflection decreases by 11.21% and 37.72%, respectively. Throughout the tests, the structure with stirrups and studs adhered to the assumption of a plane section, whereas the structure under natural bonding and that with studs failed to meet this assumption at 60% and 80% of the ultimate load, respectively. (3) Based on the plane section assumption, a theoretical formula for steel-reinforced concrete composite structures under small eccentric compression is proposed, with discrepancies between calculations and experimental results remaining within 5%, thus providing a theoretical foundation for optimizing tunnel support design.

Key words: tunnel support, steel-reinforced concrete structure, stirrup constraint, small eccentric compression, plane section assumption, ultimate bearing capacity