Skip to content
Upcoming courses

Post-flashover fire

Tools
From Fire Safety Academy Wiki
Contents
  1. 1Transition at flashover
  2. 2Characteristics
  3. 3Burning rate and ventilation
  4. 4External flaming
  5. 5Temperatures and fire severity
  6. 6Modelling and limits
  7. 7See also
  8. 8Notes
  9. 9References

A post-flashover fire is a fire in a room or compartment after flashover, when all combustible items in the space are burning at the same time. It is also called a fully developed fire or a fire with full room involvement.[1][2] In this stage the heat release rate reaches its highest values, and the gas temperature in the room is typically around 1,000 °C or more.[3][4] More fuel is pyrolysed than can burn with the oxygen that enters, so the fire is usually a ventilation-controlled fire and flames often emerge from the openings.[5][6]

The post-flashover fire is the stage that threatens the structure and the fire separations of a building. Conditions in the room are no longer survivable, and the design aim shifts from escape to preventing fire spread and structural collapse.[7][8] Engineers describe this stage with a burning rate that depends on the size of the openings, a duration that depends on the amount of fuel, and a temperature–time curve. They compare its severity with the fire resistance of building elements, which is measured in a furnace following the standard fire curve.[9][10]

Transition at flashover

Introduction to Fire Safety Engineering describes flashover as the accelerated ignition of the materials in a room, after which the whole room can be considered to be on fire. Before flashover a fire is local and fuel controlled. After flashover the whole room is burning and the fire is usually ventilation controlled.[2] The ISO definition quoted in the Fire Dynamics Tools calls flashover the rapid transition to a state of total surface involvement in a fire within an enclosure.[1]

Flashover is not a precise term, and the criteria vary. Gas temperatures of 300 to 650 °C have been associated with its onset, but 500 to 600 °C are more widely used.[3] The Fire Dynamics Tools add a radiant heat flux of 20 kW/m2 at the floor as an alternative criterion.[1] In New Zealand, C/VM2 (second edition, 2025) assumes flashover when the average upper layer temperature first reaches 500 °C, at the lower end of the experimental range.[11]

At flashover the heat release rate rises sharply. The OZone software abandons the input curve at that moment and sets the heat release rate to its maximum, the full fuel area times the heat release rate per unit area.[12] C/VM2 (second edition, 2025) prescribes a ramp of 15 s at flashover, either to the design peak heat release rate or to 1.5 times the ventilation limit. The 15-second transition is based on experimental observations and on the numerical capabilities of fire models.[11][13]

Not every fire reaches this stage. In large enclosures, especially where the fuel forms separate packages, a fire may stay local or spread progressively as a travelling fire instead of involving all surfaces at once.[14] Australian fire statistics suggest that, without sprinklers, about 18% of fires in apartments and 15% of fires in offices, shops, car parks, storage buildings, factories and public buildings could become fully developed.[15]

Characteristics

Introduction to Fire Safety Engineering lists three features of a developed fire. The room is so hot that there is an abundance of pyrolysis gases. The fire is usually ventilation controlled, so its heat release rate depends mainly on the supply of fresh air through openings such as windows and doors. Flames emerge from the openings, because part of the pyrolysis gases burns outside the room.[5]

Cross-section of a burning room after flashover: flames everywhere, smoke to near the floor, air entering low in the window, flames leaving at the top.
A post-flashover fire: all fuel burns, air enters through the lower part of the window and unburned gases burn outside as external flames. Based on Van Rossum (2027), §4.11 and Walton and Thomas (2002), pp. 3-172 to 3-174.

Temperatures are high. Lie states that temperatures of about 1,000 °C or higher can be reached, and that the heat transferred to structural members may substantially reduce their strength.[4] The Fire Dynamics Tools give up to about 1,100 °C as typical. Temperatures above 1,300 to 1,400 °C occasionally occur and can melt the surface of bricks.[16] At these temperatures the hot gases, soot and flames radiate strongly to all surfaces, to the fuel and out through the openings.[17] Because radiation grows with the fourth power of the absolute temperature, the severity of the fire depends strongly on both the temperatures reached and their duration.[10]

The burning regime can change during the fire. The Fire Dynamics Tools describe a post-flashover fire that may alternate between ventilation control and fuel control. It ends in a final fuel-controlled state as the fuel is consumed and the fire decays.[17][8] The Australian Data Sheets describe a similar pattern. Strongly ventilation-controlled burning with long flames just after flashover is followed by a peak near stoichiometric conditions and, later, fuel-controlled burning.[7]

Burning rate and ventilation

In a ventilation-controlled fire, the burning rate is set by the air flowing in through the openings. For fires near flashover and after flashover, this air flow is roughly proportional to the area of the opening times the square root of its height.[18] This product is called the ventilation factor. Kawagoe first used it to analyse post-flashover fire tests. From wood fires in rooms with small windows he obtained the following empirical relation for the burning rate:[18]

m˙f=0.09A0H0

Symbol Description Unit
m˙f mass burning rate of the fuel (wood) kg/s
A0 area of the opening m2
H0 height of the opening m
A0H0 ventilation factor m5/2

The relation applies to ventilation-controlled fires of cellulosic fuel, mainly wood cribs, and burning is then close to stoichiometric. Later data modify this simple proportionality. The CIB experiments analysed by Law also show an influence of the room's internal surface area and of the shape of the room.[18][19] Lie uses the same principle, with a burning rate of 330 kg of wood per hour per unit of ventilation factor, which equals about 0.09 kg/s.[20]

As a worked example, consider a room of 4 m by 5 m and 3 m high with one window of 2.0 m by 1.5 m. The ventilation factor is 3.0 × √1.5 = 3.67 m5/2, so the burning rate is about 0.33 kg/s of wood. Kawagoe used an effective heat of combustion of 10.75 MJ/kg for wood in the flaming phase of fully developed fires. The heat released inside the room is then about 3.6 MW.[18]

Straight line of wood burning rate against ventilation factor, with the example room marked at 3.67 m^5/2 and 0.33 kg/s.
Burning rate of wood in a ventilation-controlled fire against the ventilation factor of the opening (Kawagoe), with the example room of the article. Based on Walton and Thomas (2002), p. 3-174.

The fire duration follows from the fire load divided by the burning rate.[19] For an office with a design FLED of 800 MJ/m2, the room holds 16,000 MJ, or 851 kg of wood at 18.8 MJ/kg. It would therefore burn for about 2,570 s, or 43 minutes.[13][19]

The same room illustrates the temperature. Law's correlation for the average temperature during the fully developed period gives about 1,020 °C for this room and fire load.[21][19] Lie's characteristic curve for heavy linings gives about 810 °C at the start of decay, after 42 minutes. With light linings, it gives about 930 °C.[22] The spread between such methods is typical of post-flashover calculations.

The fire load and the openings play different roles. According to Lie, the fire load determines the duration of the fire, whereas the openings determine both its duration and its intensity.[9] A smaller opening lowers the burning rate and lengthens the fire. Assuming complete burnout, the effect on the structure therefore tends to be more severe for small openings relative to the wall area.[19] For ventilation-controlled fires, the Fire Dynamics Tools state that a higher fuel load mainly lengthens the fire rather than raising the compartment temperature.[23] In design, the International Fire Engineering Guidelines take the lesser of the ventilation-controlled and fuel-controlled heat release rates as the peak.[24]

External flaming

Because more fuel is pyrolysed than can burn inside, unburned gases leave through the openings and may burn outside the room.[6] This produces external flaming, flames that project from windows and doors. Lie counts the chemical energy of the unburned gases that burn outside as one of the losses in the heat balance of a room.[25] Gottuk and Lattimer distinguish external burning of fuel-rich gases outside the room from flame extension. Flame extension occurs when a fire is simply too large to be contained in the room.[26]

External flames matter for fire spread. The International Fire Engineering Guidelines note that fire may spread to the floors above by flames projecting through windows.[27] External steel beams and columns may be exposed to fire mainly by such flames, and methods to estimate their temperatures exist.[28] In the investigation of the World Trade Center towers, NIST treated flames extending from open windows as an indication of an intense fire.[29]

Fire models must decide where the excess fuel burns. In OZone, the "external flaming" option lets the gases that cannot burn inside burn in the flames outside, which lowers the thermal load inside the room. The "extended fire duration" option burns all the energy inside the room over a longer time.[30][31] In C/VM2 (second edition, 2025), the input heat release rate after flashover in ventilation-limited cases is 1.5 times the ventilation limit. A larger value could produce an excessive amount of unburned fuel in the model and cause numerical problems.[13]

Temperatures and fire severity

For structural design the post-flashover fire is usually expressed as a fire severity rather than a heat release rate.[7] Fire severity is a measure of the potential destructive impact of the burnout of all available fuel in a compartment. It is most often expressed as a period of exposure to the standard test fire.[10] The basic check is that the fire resistance of each part of the structure exceeds the severity of the fire to which it is exposed.[10]

The growth period is usually ignored in this assessment. Lie notes that temperatures before flashover are relatively low and have negligible influence on the fire resistance of structural members. The risk of failure begins when the fire becomes fully developed and continues in the decay period.[4] Because ventilation-controlled fires are usually more severe and quite likely, fire resistance requirements are commonly based on the assumption that fire severity is controlled by ventilation.[25]

Several methods turn the post-flashover fire into a temperature–time curve. Lie's characteristic curves depend on the opening factor, the fire load per unit area of the internal surfaces and the type of lining.[20][22] The Swedish method gives gas temperature curves for fire load densities of 250 to 1,500 MJ per square metre of total internal surface.[32] The parametric fire curve of EN 1991-1-2 Annex A is, according to the Australian Data Sheets, valid for compartments up to 500 m2 of floor area and 4 m high without roof openings. It assumes that the fire load burns out completely.[33][34]

Line chart: two calculated fire curves peak at about 0.7 h and fall to ambient by about 1.7 h; the standard curve keeps rising to 1,010 °C at 2 h.
Characteristic temperature curves (Lie) for the example office with heavy and light linings, compared with the ISO 834 standard fire curve. Based on Lie (2002), pp. 4-203 to 4-206.

The decay phase is treated more roughly. Kawagoe's data show a temperature decrease of about 10 °C per minute for fires with a fully developed period shorter than one hour, and 7 °C per minute for longer ones.[22] Design models start the decay at different points. OZone starts it when 70% of the design fire load is consumed, and the International Fire Engineering Guidelines when 80% of the fuel is consumed.[12][35]

In New Zealand, C/VM2 (second edition, 2025) requires structural design and separating elements to be based on a full burnout design fire with no intervention. It allows a time equivalence formula, a parametric time–temperature formula, or a heat release rate curve with a fire model.[36] Introduction to Fire Safety Engineering stresses that the standard fire curve is suited to testing building elements on a common basis, but not to simulating a fire in a room.[37]

Modelling and limits

Fire models need adjusting after flashover. In OZone, two-zone models suit localised and pre-flashover fires, and one-zone models suit fully engulfed, post-flashover fires. The program therefore switches from a two-zone to a one-zone model when flashover criteria are met.[38] Introduction to Fire Safety Engineering adds that the usual split of 30% radiation and 70% convection is no longer realistic after flashover. Many zone models, especially for large spaces, also ignore a fire becoming ventilation controlled.[30]

Species yields change as well. C/VM2 (second edition, 2025) raises the soot yield from 0.07 to 0.14 kg/kg and the carbon monoxide yield from 0.04 to 0.40 kg/kg once flashover has occurred or the fire has become ventilation limited.[39][11] Products of incomplete combustion are generally created under such underventilated burning conditions.[26]

The simple methods have clear limits. Most data come from experiments with wood cribs, which burn more slowly and produce less excess fuel than furniture.[19] Methods assume a uniform temperature in the compartment, which has been questioned for large or long, narrow enclosures.[40] Variability in fire load alone can shift the temperatures of a real fire by hundreds of degrees and its duration by 50% or more.[25]

A common misconception is that every post-flashover fire is ventilation controlled. With large openings or little fuel the fire can remain fuel controlled. In shallow rooms, more gases may burn outside as ventilation increases, so that the temperature inside falls.[25][20][17] Another misconception is that the standard fire curve represents a real post-flashover fire. The book points out that fire curves are agreed temperature–time relations that do not in principle match the heat development in an actual fire.[37]

See also

Notes

  1. ↑ 1.0 1.1 1.2 Iqbal and Salley (2004), p. 13-3.
  2. ↑ 2.0 2.1 Van Rossum (2027), §4.7.
  3. ↑ 3.0 3.1 Walton and Thomas (2002), p. 3-171.
  4. ↑ 4.0 4.1 4.2 Lie (2002), p. 4-201.
  5. ↑ 5.0 5.1 Van Rossum (2027), §4.11.
  6. ↑ 6.0 6.1 Walton and Thomas (2002), p. 3-172.
  7. ↑ 7.0 7.1 7.2 Australian Building Codes Board (2020a), Data Sheet B1, p. 14.
  8. ↑ 8.0 8.1 Iqbal and Salley (2004), p. 13-2.
  9. ↑ 9.0 9.1 Lie (2002), p. 4-205.
  10. ↑ 10.0 10.1 10.2 10.3 Iqbal and Salley (2004), p. 13-8.
  11. ↑ 11.0 11.1 11.2 Ministry of Business, Innovation and Employment (2025b), p. 21.
  12. ↑ 12.0 12.1 Cadorin et al. (2003), p. 438.
  13. ↑ 13.0 13.1 13.2 Ministry of Business, Innovation and Employment (2025b), p. 22.
  14. ↑ Australian Building Codes Board (2020a), Data Sheet B1, p. 13.
  15. ↑ Australian Building Codes Board (2020a), Data Sheet B2, p. 6.
  16. ↑ Iqbal and Salley (2004), p. 13-7.
  17. ↑ 17.0 17.1 17.2 Iqbal and Salley (2004), p. 13-1.
  18. ↑ 18.0 18.1 18.2 18.3 Walton and Thomas (2002), p. 3-174.
  19. ↑ 19.0 19.1 19.2 19.3 19.4 19.5 Walton and Thomas (2002), p. 3-184.
  20. ↑ 20.0 20.1 20.2 Lie (2002), p. 4-203.
  21. ↑ Walton and Thomas (2002), p. 3-183.
  22. ↑ 22.0 22.1 22.2 Lie (2002), p. 4-204.
  23. ↑ Iqbal and Salley (2004), p. 8-1.
  24. ↑ National Research Council of Canada et al. (2005), p. 2.4-14.
  25. ↑ 25.0 25.1 25.2 25.3 Lie (2002), p. 4-202.
  26. ↑ 26.0 26.1 Gottuk and Lattimer (2002), p. 2-55.
  27. ↑ National Research Council of Canada et al. (2005), p. 1.6-7.
  28. ↑ National Research Council of Canada et al. (2005), p. 2.6-8.
  29. ↑ Pitts et al. (2005), p. 43.
  30. ↑ 30.0 30.1 Van Rossum (2027), §9.3 (How a zone model works).
  31. ↑ Cadorin and Franssen (2003), p. 420.
  32. ↑ Walton and Thomas (2002), p. 3-185.
  33. ↑ Australian Building Codes Board (2020a), Data Sheet B3, p. 38.
  34. ↑ Australian Building Codes Board (2020a), Data Sheet B3, p. 39.
  35. ↑ National Research Council of Canada et al. (2005), p. 2.4-15.
  36. ↑ Ministry of Business, Innovation and Employment (2025b), p. 23.
  37. ↑ 37.0 37.1 Van Rossum (2027), §8.6.
  38. ↑ Cadorin et al. (2003), p. 437.
  39. ↑ Ministry of Business, Innovation and Employment (2025b), p. 20.
  40. ↑ Australian Building Codes Board (2020a), Data Sheet B3, p. 36.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Cadorin, J.-F., & Franssen, J.-M. (2003). A tool to design steel elements submitted to compartment fires – OZone V2. Part 1: Pre- and post-flashover compartment fire model. Fire Safety Journal, 38, 395–427. https://doi.org/10.1016/S0379-7112(03)00014-6
  • Cadorin, J.-F., Pintea, D., Dotreppe, J.-C., & Franssen, J.-M. (2003). A tool to design steel elements submitted to compartment fires – OZone V2. Part 2: Methodology and application. Fire Safety Journal, 38, 429–451. https://doi.org/10.1016/S0379-7112(03)00015-8
  • Gottuk, D. T., & Lattimer, B. Y. (2002). Effect of combustion conditions on species production. 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. 2-54–2-82). 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.
  • Lie, T. T. (2002). Fire temperature-time relations. 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. 4-201–4-208). National Fire Protection Association; Society of Fire Protection Engineers.
  • 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.
  • Pitts, W. M., Butler, K. M., & Junker, V. (2005). Visual evidence, damage estimates, and timeline analysis (NIST NCSTAR 1-5A). National Institute of Standards and Technology.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • Walton, W. D., & Thomas, P. H. (2002). Estimating temperatures in compartment fires. 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. 3-171–3-188). National Fire Protection Association; Society of Fire Protection Engineers.
Retrieved from "https://fire-safety.academy/w/index.php?title=Post-flashover_fire&oldid=2585"