Fire design verification of an immersed tunnel using nonlinear analysis

Fire design is an accidental loading case requiring verification against the ultimate limit state (ULS). Different to other types of loads, fire cannot be represented as a static load; rather, it consists of indirect effects caused by differential or restrained thermal expansion, which typically can only be estimated through advanced methods. This study proposes a procedure to verify the fire resistance and residual capacity of concrete structures using nonlinear finite element analysis (NLFEA). A staggered approach is employed within this procedure to couple transient thermal and mechanical analyses. The method is applied to estimate the fire resistance and residual capacity of a reinforced concrete (RC) as a function of the time exposure. For demonstration purposes a case study has been defined which is inspired by typical rectangular cross-section widely use in the Netherlands for immersed tunnels. Results indicate that the numerical models effectively trace damage and stress development during the fire event, confirming a significant thermal gradient across the thickness of the concrete section. The study concludes that these indirect effects must be addressed in the fire design of tunnels, even if fire protection is used.