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62 Glossary AFAC Australasian Fire Authorities Council AFFF Aqueous film forming foam AHJ Authority Having Jurisdiction ASHRAE American Society of Heating, Refrigerating and Air- Conditioning Engineers Aspect ratio The proportional relation between the width and height of the cross section of the tunnel which affects the required critical velocity Backlayering The reversal of smoke and hot gas movement counter to the direction of the ventilation airflow BLEVE Boiling liquid expanding vapor explosion CCTV Closed circuit television CFD Computational fluid dynamics Chimney effect In tunnels that have nonzero grades, particularly those with portals at different elevations, a temperature dependent effect usually referred to as âthe chimney effectâ is often considered. Air will flow through a chimney (in a direc- tion from the lower portal to the higher portal) because of the difference between the weight of the column of air inside, the chimney (tunnel) and an equivalent column of air outside Confinement velocity Air velocity needed to prevent back-layering at a certain loca- tion, that is, to prevent spreading smoke further upstream. It is defined as the longitudinal velocity as induced by the extraction ventilation system which is necessary to prevent smoke layer development after the last exhaust vent of a single point extraction system has been activated. Consequence Outcome of an event in terms of damage to the health of people, to property or environment Critical velocity The minimum steady-state velocity of the ventilation air- flow moving towards the fire within a tunnel or passage- way that is required to prevent backlayering at the fire site EU European Union Event Occurrence of a particular set of circumstances, which may cause harm FEDRO/ASTRA Switzerlandâs Federal Roads Office FFFS Fixed firefighting systems FFSS Fixed fire suppression system FHRR Fire heat release rates Fire decay rate The decrease in the heat release rate of the fire as a function of time Fire growth rate The increase in the heat release rate of the fire as a function of time Fire heat release rate (FHRR) The rate at which heat energy is generated by the fire typi- cally expressed in megawatts (MW) or MBtu/hr Fire size Also equivalent to the maximum heat release rate, the maximum rate at which heat energy is generated by the fire, typically expressed in megawatts (MW) or MBtu/hr FIT European Thematic Network Fire in Tunnels FN curve Graph with the ordinate representing the cumulative fre- quency distribution of N or more units of consequence
63 (e.g., fatalities) et abscissa representing the correspond- ing consequence. FN curves are used for representing societal risk Frequency The number of times a specified event occurs within a spec- ified interval (e.g., accidents per year) Hazard Potential source of harm HRR Heat release rates MTFVTP Memorial Tunnel Fire Ventilation Test Program NFPA National Fire Protection Association PIARC World Road Association PRV Pressure reducing valves Risk Combination of the probability of occurrence of harm and the severity of the harm [2] Risk analysis A detailed examination performed to understand the nature of unwanted, negative consequences to human life, health, property, or the environment Risk assessment Overall process comprising a risk analysis and a risk eval uation [2] Risk evaluation Procedure based on the risk analysis to determine whether the tolerable risk has been achieved Smoke stratification The layering of smoke at the ceiling of the tunnel due to the buoyancy force of the smoke and hot gases leaving a largely smoke free environment below, near the roadway surface Tenable environment An environment that supports human life for a specific period of time TRB Transportation Research Board UN United Nations
