Load-Bearing Behavior and Load-Transfer Mechanism of CFST Combined Arches Under Uneven Load in Large-Section Tunnels

To investigate the load-bearing mechanism of concrete-filled steel tube (CFST) combined arches under localized additional loads, numerical simulations were conducted comparing shaped steel combined arches and CFST combined arches. The CFST section contains about 25% more steel and has a lower in-plane EI than I20a but a much higher axial stiffness. The results show that the CFST combined arch can withstand a localized additional load of 235.1 kN at the vault, which is 99.6% higher than that of the shaped steel combined arch. The initial vault settlement of the CFST arch is reduced by 15.7% compared to the shaped steel arch. The load-transfer mechanism is clarified: the localized additional load applied to the middle arch is transferred to adjacent arches through longitudinal connections, and the middle arch carries the major share while the adjacent arches provide spatial coupling support. The deformation and failure modes are dominated by local buckling of the steel tube and crushing of the core concrete at the vault. Based on the simulation findings, practical engineering suggestions are provided for the application of CFST arches.