On the fire scenario in road tunnels: a comparison between zone and field models

Some catastrophic fires occurred in the last 15 years in both road and railway tunnels used by millions of people are turning the attention of the engineering community towards the assessment of the structural safety of tunnels, both during and after a fire. To this end, a realistic description of fire propagation and a reliable picture of the thermal field are mandatory, in order to evaluate the thermal load acting on the lining and the partitions. With reference to a rather common road-tunnel geometry, three possible thermal inputs representing as many levels of fire severity are considered here, namely the burning of a car, of a bus and of a heavy-goods vehicle (HGV). The fire scenario is firstly modeled by means of a rather simple but handy code based on a two-zone model for the description of the fire in each compartment. Later, a more complex code based on computational fluid dynamics applied to the simulation of fire development (Fire Dynamics Simulator - FDS) is used to allow a comparison between the numerical results obtained in either way, and to check to what extent some available experimental results concerning similar tunnels can be fitted. The fitting shows that - by properly subdividing the tunnel (that has no compartments) into a suitable number of "virtual" compartments - the agreement between the two codes is very good, as well as that between numerical and experimental results.