Fire severity evaluation. The potential risk P is decisive for the severity. by the combination of the 3 factors q, i and v. The fire load factor q measures the possible fire duration, and the i and v factors modify this value for the initial fire growth and for the flash-over conditions.   Natural fire models usually show a slow growing beginning, usually a t² - curve, representing the initial development of fire before flash-over, a nearly horizontal curve for a fully developed fire and a declining tail (linear or t²) to represent the extinguishing phase of the fire. The nearly horizontal part of the temperature-time curve covers two current scenarios in real fire conditions: it can be either a post-flash-over ventilation controlled fire or a situation where the heat output of the fire is nearly in balance with the heat absorption potential of the water flow applied by the fire brigade and sprinklers. In both cases a nearly constant RHR (rate-of-heat-release) is assumed, and the duration of the fire is almost linear linked with the available fire load. The tail end of the fire extinguishing is not very interesting in risk assessment, as the key question is to define when and how often the thermal action will be sufficiently strong to cause the undesirable event. Additional parameters define the shape of natural fire curves. Generally, local conditions (ventilation, compartment size, etc.) are taken into account to transform the standard curve into a less or more severe fire model. The "equivalent time"-concept simplifies the fire severity evaluation to a comparison between the peak of a natural fire curve with the standard ISO 834 fire curve. Fire Duration expression.   Most mathematical models express the severity of the thermal action of a fire as a function of the duration of the fire. The standard fire curves, (see Eurocode EN1991-1-2) are logarithmic temperature-time curves. The curves have a fast growing head, representing the start of flashover conditions and a more horizontal body, representative of a severe fire with a more or less constant rate-of-heat-release. In FRAME the duration part of the fire model is also represented by the logarithmic expression used for the fire load factor q. q = 2/3 * log (Qi + Qm) - 0.55 The basic formula for q is logarithmic, which corresponds with the logarithmic distribution of fire damage as found in fire statistics, and with the standard temperature-time fire curves. The 0.55 correction can be seen as that part of the fires’ heat output that is lost in the growing phase, goes into the smoke and is left in the extinguishing phase. Fire growth and rate-of-heat-release.   Introducing a beginning phase in a fire model is more significant, as it gives an indication of the time delay before the severe thermal action starts, and influences greatly the effectiveness of defensive actions such as fire operations and sprinkler actuation. Most fire models are very elementary when dealing with the heat release of fires. Yet, this aspect of fire development could be a key issue, especially for human safety, as the developing phase of the fire defines the time available for the escaping from the fire area. Scientific literature refers to a simple t²-curve with a growth parameter value for slow, medium, fast and ultra-fast fire development. There is almost no research into what parameters influence fire growth. In FRAME, three influence factors have been identified as contributing to the fire growth and hence to the fire severity: the volume/area ratio of the combustibles, the combustibility of the surfaces and the ignition characteristics of the surface materials. These have been identified by three parameters and combined in the fire spread factor i. i = 1 - ( T/1000) - 0.1* log m + ( M / 10)   The combination and balancing of the three parameters is the result of reasoning and experienced guesswork, there is no scientific evidence available to support the combination as such, but there is NEITHER any scientific material that indicates that the combination is WRONG. The combination and balancing of the three parameters is the result of reasoning and experienced guesswork, there is no scientific evidence available to support the combination as such, but there is NEITHER any scientific material that indicates that the combination is WRONG. The value of i will vary in the range of 0.5 to 1.65. The first value is representative for a storage of large blocks of concrete. The last one is typical for a heap of chips of expanded polystyrene. For most houses, the value of i will be about 1.2, assuming e.g. that m= 0.1, T = 200 and M = 3. Considering the logarithmic aspect of the basic formula, and the value for i, a "residential" fire (i=1.2) is then comparable to an ISO 834 standard fire, the i-value of 0.5 means that a fire in a storage of concrete blocks should be comparable to 20 % of an ISO fire, the polystyrene fire (i=1.65) could be 3 times as severe. Any other guess or approach is welcome, but the range seems fairly reasonable.   Most fire models use a pre-defined fire growth curve for the initial stage of the fire development. More information on the link between the fire growth rate and the i-factor of FRAME can be found on the "firegrowth page" . Fire growth to flash-over.   Generally speaking, localised fires are easier to handle: They do not impose a severe action on the building elements and can be approached for extinguishment. The transition from a localised fire to a fully developed fire is described in the scientific literature and expressed as a function of the fire heat release, the (square root) of the height between ceiling and floor, and the area of available ventilation openings. (E.g. Thomas’s flashover correlation, ventilation limit theory by Kawagoe). In FRAME this relationship is found back in the ventilation factor v, which is calculated in a similar way with the log of the mobile fire load, the venting ratio k, and the (square root) of the height: The effect of this factor in the potential risk P reflects an increased severity for high fire loads inside the compartment, and a decrease in severity when favourable ventilation conditions allow for localised fires. Whether the expression is a correct transcription of the scientific theories cannot be proven, but in practice properly engineered smoke venting systems always give a v-value slightly below 1, meaning that the fire severity is reduced, which is exactly what smoke venting systems do.   PRINT  THIS SECTION  (pdf)