Fire Resistance Test and Numerical Simulation on the Tube Structure of Steel–Concrete–Steel Immersed Tube Tunnel

To provide references for the fire prevention design of steel–concrete–steel immersed tube tunnels, four types of test conditions—no fire protection, fireproof coating insulation, single-layer seam fireproof boards, and double-layer seam fireproof boards—were carried out using partial full-size structural test members. Additionally, the thermal insulation effects of various fireproofing technology solutions were contrasted and analyzed. Combined with the numerical simulation analysis, the temperature distribution law inside the tube structure under various fireproofing measures and the temperature rise law of measuring points at different depths were studied, and the protective effect of the fireproof layers on the tube structure under high fire temperature was demonstrated. The results of the numerical simulation and the experimental data agree well. The results show that adding fireproof layers can significantly lower both the steel shell’s surface temperature and the depth of fire impact. Without fire protection, the surface temperature of the bottom steel shell exceeds 300 °C at 69 s, and the member bursts. The fireproof coatings are cracked and flaking and cannot meet the fire resistance limits. Both single-seam and double-seam schemes of calcium silicate boards can meet the fire resistance limit requirements and the latter has a better heat insulation effect.