Physical and mechanical response of lightweight assembled frame tunnel roof structure before and after fire

The incorporation of cavity structures not only reduces the weight of prefabricated frame tunnel components but also alters their bearing capacity and fire resistance. To investigate the physical and mechanical response laws of lightweight prefabricated frame tunnel roof structures before and after fire exposure, this study examines the influence of cavity configurations on lining temperature fields through fire tests. Combined with 3D numerical simulation, the bearing capacity, deformation, and stress variation of tunnel roof structures with different cavity configurations were analysed, and the influence of cavity configurations on the physical and mechanical response of tunnel roof before and after fire was evaluated. The results show structural lightweighting influences the temperature field distribution of tunnel linings. However, when cavities maintain a sufficient distance from the fire-exposed surface, high-temperature-induced thermal variations exhibit negligible effects on the thermomechanical properties of the overall structure. Consequently, the impact of lightweight on temperature fields can be disregarded. A reasonable lightweight roof design enhances the unit mass bearing capacity but compromises fire resistance, necessitating supplementary fireproofing measures. Under uniform vertical loading, stress in spherical closed cavity structure concentrates predominantly near the top of the cavity, whereas circular opening structure exhibit stress concentration near the bottom of the cavity. This results in superior ultimate bearing capacity for circular opening structures with the same cavity ratio. In the roof structure with a height of 1 m, setting the cavity diameter at 560 mm and raising the cavity position to 584 mm can significantly enhance the mechanical performance of the structural materials. Finally, a mid-span deflection-load calculation method is proposed.