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Contents
  1. 1Definitions
  2. 2What a scenario describes
  3. 3Identifying potential scenarios
  4. 4Selecting design fire scenarios
  5. 5From scenario to design fire
  6. 6Scenarios in risk analysis
  7. 7Limits and misconceptions
  8. 8See also
  9. 9Notes
  10. 10References

A fire scenario is a description of how one particular fire could start and develop in a building, from ignition through growth, spread and decay to burnout, as modified by the building's fire safety system.[1] It is described by the times at which its key events occur, such as detection, sprinkler operation, flashover or the failure of a barrier.[1] A fire scenario is broader than the fire alone. It includes the ignition source and the first item ignited, the arrangement of fuel and enclosures, the status of fire protection measures, the occupants and, where relevant, external conditions such as wind.[2][3]

Fire scenarios are the starting point of almost every fire safety analysis. Watts and Hall describe fire risk analysis as answering three questions, the first being what could happen. A sequence of events ending in a fire loss is called a scenario.[4] Because a building can burn in countless ways, engineers analyse only a limited set of design fire scenarios.[5] These are chosen to be severe but credible, and each is turned into a design fire for calculation.[5] Several regulations now prescribe a minimum set of design scenarios, such as the ten scenarios of New Zealand's Verification Method C/VM2.[6]

Definitions

The International Fire Engineering Guidelines (IFEG, 2005) define a fire scenario as the ignition, growth, spread, decay and burnout of a fire in a building as modified by its fire safety system.[1] The glossary of NUREG-1805, a report of the U.S. Nuclear Regulatory Commission, gives a similar definition. There a fire scenario is a set of conditions that defines the development of fire and the spread of combustion products through a building or part of a building.[7] Bukowski, writing about fire hazards in passenger rail, describes a fire scenario as the sequence of events that must occur to result in an uncontrolled fire.[2]

Three related terms are easily confused. A fire scenario is any possible course of a fire, whereas a design fire scenario is the subset selected for design purposes.[8] The IFEG define a design fire scenario as a fire scenario that is used as the basis for a design fire.[9] A design fire in turn is a representation of a fire by the variation of its heat output with time.[9] The scenario is therefore the story, and the design fire is the quantified heat release curve used to tell it in a calculation.

The NUREG-1805 glossary stresses the purpose of a design fire scenario. It is a set of conditions that describes the critical factors for determining the outcomes of trial designs.[10] New Zealand's C/VM2 uses the shorter term design scenario. The first edition defined it as a specific scenario on which a deterministic fire safety engineering analysis is conducted.[11]

What a scenario describes

A fire scenario combines the fire with the building and the people in it. The IFEG list the factors that a scenario takes into account. These are the nature, quantity, arrangement and burning behaviour of combustibles, the geometry of the enclosure, and the number of enclosures and the connections between them.[5] The fire protection measures in the building and their effect on the fire complete the list.[5]

A scenario represents one particular combination of outcomes for the types of fire, its development and the external environment.[3] The fire may be smouldering, flaming without flashover, or flashover.[3] Its development is influenced by the size and type of ignition source, the distribution and type of fuel, the fire load density, the location of the fire, ventilation, construction materials and air handling equipment.[3] External conditions include the season, wind speed and direction, and the stack effect.[3]

The IFEG also define a scenario by the outcome of each of the six sub-systems of a fire safety system.[12] Typical alternatives are whether smoke management operates, whether doors are open or closed, and whether detectors and sprinklers succeed. Others are whether occupants are awake or asleep, and whether the fire service rescues and extinguishes successfully.[12] A simple example with sprinklers and barriers gives three scenarios. In the first, sprinklers control the fire; in the second, sprinklers fail but the barriers contain the fire; in the third, both fail and the fire spreads to adjoining enclosures.[12]

Event tree: ignition, sprinklers succeed (scenario I) or fail; if they fail, barriers hold (scenario II) or fail (scenario III).
Event tree for a fire with sprinklers and barriers: each path from ignition to an outcome is one fire scenario. Based on National Research Council of Canada et al. (2005), pp. 2.2-3–2.2-4.

The people form part of the scenario as well. The IFEG describe occupant response scenarios in which people notice the fire directly, respond to an alarm, or respond later to new cues or to warnings from others.[13] Each occupant response scenario is combined separately with each fire scenario.[14] For escape analysis, Introduction to Fire Safety Engineering lists the possible or expected fire scenario among the aspects that determine whether people can escape safely.[15]

Identifying potential scenarios

The first task is to find the potential fire scenarios. The IFEG suggest reviewing the information assembled for the fire engineering brief, especially the hazard analysis.[5] Other sources are the published literature, fire statistics, and the experience and knowledge of the team.[5] The Australian handbook for the Fire Safety Verification Method (FSVM) names several hazard identification techniques. These are checklists, what-if analysis, hazard identification (HAZID), hazard and operability analysis (HAZOP), failure mode and effects analysis, and reviews of the literature and of historic records.[16] The handbook recommends a mix of structured and informal methods, so that free thinking can reveal significant events of low probability and high consequence.[16]

For a systematic search, the IFEG recommend event tree analysis. An event tree starts with an initiating event, such as ignition, and branches at each system or event into success or failure.[17] Each path through the tree, from the initiating event to a final event, defines one fire scenario.[17] Where the probability of an event is not known, fault trees can be used to calculate it from the failure mechanisms of the system.[17]

The number of scenarios grows quickly. Meacham gives the example of a building with three compartmented rooms.[8] If only the room of origin and the order of spread are considered, there are 15 possible paths of fire propagation. Adding simultaneous spread to the second and third rooms gives 18 scenarios.[8] That count is reached before open or closed doors, the characteristics of the people present, or whether they are asleep are even considered.[8]

Plan of three rooms in a row with a list of the five spread paths for a fire starting in room 1; 15 paths in total.
A building of three rooms already has 15 possible paths of fire spread, or 18 with simultaneous spread. Based on Meacham (2002), p. 5-162.

Selecting design fire scenarios

A fire engineering analysis can consider only a limited number of the potential scenarios.[5] The IFEG describe a three-step approach. The team first determines the potential fire scenarios, then selects the design fire scenarios, and finally specifies a schematic design fire for each.[5] Usually, the selected scenarios are severe, have a reasonable probability of occurrence and have a significant potential for loss.[5] Events with a very low probability should not be analysed unnecessarily. If their consequences could be very high, however, they deserve consideration at least in the sensitivity studies.[5]

Funnel: many potential fire scenarios narrow to a few design fire scenarios, each turned into a design fire curve.
From potential fire scenarios to design fires in three steps. Based on National Research Council of Canada et al. (2005), p. 1.2-27.

Deliberately started fires are treated in the same way as accidental ones when design scenarios are chosen.[5] The IFEG recognise, however, that some large arson attacks and terrorist events may be beyond the capacity of the fire safety system and need other measures.[18] The selection is made by the stakeholders in the fire engineering brief.[5] At that stage, the design fires are defined and described as far as possible without calculation, in a largely qualitative review of the design.[19]

Some regulations prescribe a minimum set of design scenarios. In New Zealand, Verification Method C/VM2 (second edition, 2025) contains ten design scenarios that must each be satisfied separately.[6] They range from a fire that blocks an exit and a smouldering fire to horizontal and vertical fire spread, firefighting operations and a challenging fire.[20] The scenarios are intended to give a diverse range of fire events that will challenge the design and its fire safety systems.[6] Many can be satisfied by inspection or by providing features such as fire separations or automatic alarms.[6] Only a few need a comparison of the available safe egress time (ASET) with the required safe egress time (RSET), or other computational modelling.[6]

The challenging fire scenario of C/VM2 represents credible worst cases in normally occupied spaces. Design fires are placed in various locations in the building, using a single fire source for each location.[21] In this scenario, active and passive fire safety systems are assumed to perform as designed.[22] A separate scenario then assumes the failure of each key fire safety system in turn.[23] The commentary to the first edition explains that the framework was modelled on structural design, which also specifies design events such as wind, earthquake and snow.[24]

In Australia, the FSVM specifies a minimum of twelve design scenarios. Each is considered in one or more locations to compare the proposed design with a reference building that fully complies with the deemed-to-satisfy provisions.[25] The handbook explains that prescribing scenarios reduces the risk that critical design scenarios are missed. It notes similar approaches in C/VM2, in NFPA 5000 in the United States and in ISO 16733-1.[26]

Hazard identification may show that some prescribed scenarios do not apply. For example, the smouldering fire scenario near sleeping areas is not needed in an office without sleeping accommodation.[27] Buildings with innovative or unusual features may need additional scenarios.[27]

From scenario to design fire

Once a design fire scenario has been chosen, it must be quantified. The Australian handbook lists what is needed: a design fire, the status of active and passive fire protection, occupant characteristics where required, and acceptance criteria.[16] The IFEG compare this step with structural engineering, where the loads must be specified before safety can be evaluated.[5] The validity of the whole fire engineering evaluation rests on the validity of the design fires.[5]

The IFEG first sketch each design fire as a schematic heat release rate curve. A smouldering fire that does not become flaming typically has a maximum heat release rate below 5 kW.[19] For flaming fires, the heat release rate is generally assumed to grow with the square of time, reach a steady value limited by fuel or ventilation, and then decay.[28] For a flashover fire, flashover is drawn as an instantaneous jump in heat release rate.[28] The curve is then adjusted for events such as windows breaking, smoke management operating, doors burning through, and suppression by sprinklers, occupants or the fire service.[28]

Three heat release rate curves: a low smouldering curve, a non-flashover curve with growth, plateau and decay, and a flashover curve with a vertical jump.
Schematic design fires for smouldering, non-flashover and flashover fire scenarios. Based on National Research Council of Canada et al. (2005), pp. 1.2-29–1.2-30.

Introduction to Fire Safety Engineering describes the t-squared growth curve as a widely used starting point in a fire scenario. Its four standard growth rates reach 1 MW after 75 s (ultra-fast), 150 s (fast), 300 s (medium) and 600 s (slow).[29] The book stresses that a temperature-time curve is not suitable for representing a fire in a room. The temperature is a result of a fire scenario, not an input to it.[29] A study of fires in indoor car parks makes the same point, representing the car park fire scenario by a heat release rate curve rather than a temperature curve.[30]

Fire models do not choose the scenario themselves. The book points out that a computational fluid dynamics model does not predict how a fire develops. The modeller must prescribe the fire scenario and the growth of the heat release rate.[31] Before any model is used, it must also be established that the scenario lies within the model's field of application.[32]

Scenarios in risk analysis

A deterministic analysis calculates the outcome of each design fire scenario, but it does not indicate the probability of that outcome. A probabilistic analysis also assigns a probability to each scenario and combines the results.[33] The IFEG note that the use of multiple scenarios combined by probabilistic techniques is the key feature of some risk methods.[33] Such methods need the reliability of fire protection systems, because each branch of the event tree depends on whether a system works.[8] Meacham distinguishes availability, meaning a system is ready to perform, from reliability, meaning that an available system actually functions.[8]

The risk-cost assessment model described by Meacham shows how scenarios are built up in practice. It uses six design fires in the room of origin: smouldering, flaming without flashover, and flashover, each with the room door open or closed.[34] The probability of each design fire is based on statistics. In Canada, 18% of apartment fires reach flashover, 63% are flaming fires without flashover, and 19% are smouldering fires.[34] The number of fire scenarios is the product of the number of design fires, the number of apartment units, and whether the occupants are awake or asleep.[34] For an illustrative building of 20 apartments, this already gives 6 × 20 × 2 = 240 scenarios.

Bar chart of Canadian apartment fires: flashover 18%, flaming without flashover 63%, smouldering 19%.
Share of apartment fires in Canada by type, used to give each design fire a probability. Based on Meacham (2002), p. 5-163.

Frequencies must be handled with care. Meacham warns that a single scenario with a frequency of 10−7 per year may suggest that fire is not a concern.[35] If 30 such scenarios are developed, however, their combined frequency is 3 × 10−6 fires per year.[35] In a fire risk assessment, the consequences of each scenario are therefore weighted by its likelihood and summed over all scenarios.[2][36]

In fire probabilistic risk assessment of US nuclear power plants, a fire scenario is defined by the location and burning characteristics of the initiating fire.[37] Scenarios can be defined broadly, for example as a fire anywhere in an area, or in detail with a precise location and initial size.[38] Broad definitions are used where a conservative analysis is enough, and detailed ones where the area is risk-significant.[38]

Limits and misconceptions

A design fire scenario is not a prediction of the fire that will occur. It is a deliberately chosen case for testing a design, and design fires are normally intended to be conservative simplifications.[19] The IFEG warn that fire and its effects are complex and variable, so a fire safety system may not cope with every possible scenario.[39] Owners, occupiers and approving authorities need to understand this limit when they assess a fire engineered solution.[39]

"Worst credible" is not the same as "worst possible". Selection balances severity against probability, and very unlikely events are usually examined only in sensitivity studies.[5] For the same reason, a scenario set chosen for one building cannot be transferred to another without checking. The Australian handbook stresses that practitioners must consider each project case by case and not rely only on tables of scenarios.[40]

Scenarios can also be used without any probability. A Dutch study on indoor car parks with electric vehicles defined three realistic scenarios. These were a long-lasting battery fire in one car, an electric car exposed to radiation from a burning neighbour, and several burning battery packs, for example in a travelling fire.[41] The authors state that no statements can be made about the probability of these scenarios. They served only as a conceptual framework for choosing prevention and firefighting measures.[41] Such qualitative scenarios support decisions in performance-based design, but they do not by themselves quantify risk.

See also

Notes

  1. ↑ 1.0 1.1 1.2 National Research Council of Canada et al. (2005), p. 0.5-3 (Part 0, Australia).
  2. ↑ 2.0 2.1 2.2 Bukowski (2002), p. 5-231.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 National Research Council of Canada et al. (2005), pp. 2.2-2–2.2-3.
  4. ↑ Watts and Hall (2002), p. 5-2.
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 5.14 National Research Council of Canada et al. (2005), p. 1.2-27.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 Ministry of Business, Innovation and Employment (2025b), p. 7.
  7. ↑ Iqbal and Salley (2004), p. F-28.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Meacham (2002), p. 5-162.
  9. ↑ 9.0 9.1 National Research Council of Canada et al. (2005), p. 0.5-2 (Part 0, Australia).
  10. ↑ Iqbal and Salley (2004), p. F-21.
  11. ↑ Ministry of Business, Innovation and Employment (2013), p. 9.
  12. ↑ 12.0 12.1 12.2 National Research Council of Canada et al. (2005), p. 2.2-3.
  13. ↑ National Research Council of Canada et al. (2005), p. 2.2-5.
  14. ↑ National Research Council of Canada et al. (2005), p. 2.2-6.
  15. ↑ Van Rossum (2027), §7.
  16. ↑ 16.0 16.1 16.2 Australian Building Codes Board (2020b), p. 59.
  17. ↑ 17.0 17.1 17.2 National Research Council of Canada et al. (2005), p. 2.2-4.
  18. ↑ National Research Council of Canada et al. (2005), p. 1.2-28.
  19. ↑ 19.0 19.1 19.2 National Research Council of Canada et al. (2005), p. 1.2-29.
  20. ↑ Ministry of Business, Innovation and Employment (2025b), p. 8.
  21. ↑ Ministry of Business, Innovation and Employment (2025b), p. 56.
  22. ↑ Ministry of Business, Innovation and Employment (2025b), p. 57.
  23. ↑ Ministry of Business, Innovation and Employment (2025b), p. 58.
  24. ↑ Ministry of Business, Innovation and Employment (2013), p. 11.
  25. ↑ Australian Building Codes Board (2020b), p. 7.
  26. ↑ Australian Building Codes Board (2020b), p. 8.
  27. ↑ 27.0 27.1 Australian Building Codes Board (2020b), p. 60.
  28. ↑ 28.0 28.1 28.2 National Research Council of Canada et al. (2005), p. 1.2-30.
  29. ↑ 29.0 29.1 Van Rossum (2027), §8.6.
  30. ↑ DGMR (2015), as cited in Van der Graaf et al. (2021/2023), p. 24.
  31. ↑ Van Rossum (2027), §11.4.
  32. ↑ Van Rossum (2027), §8.1.
  33. ↑ 33.0 33.1 National Research Council of Canada et al. (2005), p. 1.2-22.
  34. ↑ 34.0 34.1 34.2 Meacham (2002), p. 5-163.
  35. ↑ 35.0 35.1 Meacham (2002), p. 5-166.
  36. ↑ Meacham (2002), p. 5-161.
  37. ↑ Siu (2002), p. 5-216.
  38. ↑ 38.0 38.1 Siu (2002), p. 5-217.
  39. ↑ 39.0 39.1 National Research Council of Canada et al. (2005), p. 0.1-4 (Part 0, Australia).
  40. ↑ Australian Building Codes Board (2020b), p. 55.
  41. ↑ 41.0 41.1 Van der Graaf et al. (2021/2023), pp. 32–33.

References

  • Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
  • Bukowski, R. W. (2002). Fire hazard assessment for transportation vehicles. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-227–5-233). National Fire Protection Association; Society of Fire Protection Engineers.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Meacham, B. J. (2002). Building fire risk analysis. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-153–5-175). National Fire Protection Association; Society of Fire Protection Engineers.
  • Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Siu, N. (2002). Fire risk assessment for nuclear power plants. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-214–5-226). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • Watts, J. M., Jr., & Hall, J. R., Jr. (2002). Introduction to fire risk analysis. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-1–5-7). National Fire Protection Association; Society of Fire Protection Engineers.
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