Risk assessment of zero-flow ventilation strategy for fires in bidirectional tunnels with longitudinal ventilation

Two generically different methods are used to assess alternative strategies for responding to the outbreak of fire in a road tunnel. One approach, common in Japan, follows a deterministic process that leads to strong qualitative outcomes, namely either "safe" or "fatal", for any chosen specific scenario. The other approach, developed in Austria, is a probabilistic, system-based process that leads to quantitative outcomes for any specific tunnel and that provides a method of comparing overall safety in different tunnels. The two approaches are used to provide independent assessments of the merits of (i) an actively controlled zero-airflow response to the detection of fire and (ii) a simpler response in which all fans are immediately shut down and the airflows are determined by conditions at the tunnel. Both assessment methods show that the acfive response constitutes a significantly lower risk strategy than the simpler response.