Fire-induced structural response of shield tunnel segments under different boundary constraints and load levels

A thermal-mechanical coupling numerical model of shield tunnel segments was established to investigate the fire-induced structural behaviour of the segment under different boundary constraints and load levels. The results showed that the deformation of the segment located at the vault (under positive load) during heating was less than that of the segment located at the waist (under negative load) due to the difference in boundary constraints. Regardless of whether the segment was under positive or negative load, the heat-induced deformation opposed the load-induced deformation. At low load levels, the heat-induced deformation of the segment was greater than the load-induced deformation, while at high load levels, it was less than the load-induced deformation. There was a stress sandwich in the principal stress of the concrete of the midspan section, and the stress sandwich of the segment located at the vault (under positive load) was more apparent than that of the segment located at the waist (under positive load). The heat-induced stresses of the concrete and main reinforcement were greater than those induced by the load, whether the segment located at the vault or waist (under positive load or negative load). The change on the heated side gradually became smaller than that on the unheated side of the segment located at the vault (under positive load) with increasing load levels. The change on the heated side was always greater than that on the unheated side of the segment located at the waist (under positive load). Segment concrete on the heated side and unheated side were both weak zones during heating. These findings provide scientific guidance for the subsequent fire protection design of the shield tunnel segment. • Heat-induced segment deformation opposes load-induced deformation. • The deformation of the segment during heating depends on the load levels. • The segment section experiences low stress in the middle and high stress at both ends during heating. • The mechanical behaviour of the heated and unheated sides of the segment changes significantly during heating.