Thermal Exposure in Concrete Tunnels Subjected to Fire

Abstract

To date, many fire accidents have occurred in tunnels all over the world. In the worst cases, there were catastrophic consequences with numerous human casualties and significant economic impacts. In the occurrence of such tunnel fires, besides the primary concern of saving human life, ensuring a certain level of structural integrity is also desired. Looking from the latter perspective, this thesis is focused on the fire-induced thermal exposure in concrete tunnels, aiming at coupling numerical simulations of idealized fire scenarios in computational fluid dynamics (CFD) software with the use of FEA for the analysis of structures, moving towards a performance-based approach in replacement of the currently adopted prescriptive approach.
In this sense, fire imposes one of the most hazardous conditions for concrete structures, characterizing severe thermal degradation of the mechanical properties of the material. During fire exposure, steep thermal gradients are formed within the concrete structure. Those will always lead to the development of thermal stresses, the state of which being determined by several conditions (i.e. elastic modulus, thickness and supports). Besides the structural consequences of a fire, this thesis also deeply investigates the interface between a fire and the structure, which involves the interaction of three different modes of heat transfer, namely conduction, convection (related to the gas temperatures generated by a fire), and radiation (related to the radiation temperatures generated by a fire). While conduction within the solid phase regulates the amount of heat absorbed by the structure during a fire, radiation overcomes convection and becomes the primary mode of heat supply to the structure. The adiabatic surface temperature (AST) combines both the radiation and convection contributions to the total heat transfer to the structure into a single term, fully characterizing the fire phase in the structural analysis. Based on this concept, the differences between the two methodologies (i.e. prescriptive-based and performance-based) are exposed. From the prescriptive-based perspective, the development of the RWS curve and its indistinct use among protected and unprotected concrete tunnels is deeply discussed, together with the current fire resistance furnace tests. For the performance-based approach, the thermal exposure generated by two pool fire scenarios (200 MW and 30 MW) within the Piet Hein tunnel is addressed and the effects of longitudinal ventilation are highlighted.