Design fire

A design fire is a quantified, idealised description of a fire that is used as input for the fire safety design of a building. It is usually expressed as a curve of heat release rate against time, often supplemented by the production of smoke and toxic gases. A typical design fire has a growth phase, a peak or fully developed phase and a decay phase. The growth phase is commonly described as a t-squared fire, in which the heat release rate rises with the square of time.

Design fires are a central input of performance-based design. They turn a fire scenario, a description of how a fire might start and develop in a particular building, into numbers that hand calculations and computer models can use. They are normally meant to be conservative. New Zealand's verification method C/VM2 (second edition, 2025) describes them as representing the worst credible scenarios that will challenge the fire safety design. The choice of design fire can critically affect the outcome of a fire safety engineering analysis.

Purpose and use

In a fire engineering analysis, the design fire defines the heat and smoke that the building and its occupants must cope with. The fire models used in engineering, such as zone models and computational fluid dynamics (CFD) models, usually need a heat release rate curve as input. Introduction to Fire Safety Engineering lists the inputs that a fire model needs: data on the fire itself, on the building and its openings, and on the heat release rate and energy. The design fire provides the last group. It sets how fast the fire grows, how large it becomes and how long it lasts.

The International Fire Engineering Guidelines (IFEG, 2005) place the selection of design fires early in the design process, in the fire engineering brief agreed between the parties involved. At that stage, each fire scenario is formalised as a design fire and drawn as a simple graph called a schematic design fire. The numbers are then quantified during the analysis. In the SFPE Handbook, Notarianni describes design fires as descriptions of fire events, such as a grease fire on a stove or a smouldering cigarette fire on a sofa.

A design fire must suit the problem it is used for. The Australian data sheets for the Fire Safety Verification Method list the factors that matter. These are the geometry and materials of the room, the location and character of the fire, the building use and the ventilation. The fire safety question and the method of analysis matter as well. A design fire for checking escape routes therefore focuses on the early growth of the fire. A design fire for checking the structure focuses on the fully developed fire and its duration.

Parameters of a design fire

Design fires are usually characterised by one or more of the following parameters: the heat release rate over time, the peak heat release rate and the production of smoke and toxic gases. Temperature or heat flux over time, the duration of the fire and its position can also be part of the definition. C/VM2 (second edition, 2025) uses a similar list, which adds the fire load energy density and the heat of combustion.

C/VM2 gives fixed values for many of these parameters. For most buildings, it prescribes a soot yield of 0.07 kg/kg, a carbon monoxide yield of 0.04 kg/kg and a heat of combustion of 20 MJ/kg before flashover. The radiative fraction is 0.35. After flashover, or once the fire becomes limited by ventilation, the yields rise to 0.14 kg/kg for soot and 0.40 kg/kg for carbon monoxide.

The size of a fire is often linked to its area through the heat release rate per unit area. Introduction to Fire Safety Engineering gives about 250 kW/m² for a developed fire in an average building other than storage. The Dutch national annex to Eurocode 1 uses 250 kW/m² for dwellings, offices and many other uses, and 500 kW/m² for theatres, cinemas, libraries and larger shops such as supermarkets. In a CFD model under C/VM2, the fire is modelled as an area sized from 500 to 1000 kW/m² for most buildings. In a zone model, 250 kW/m² is used.

Stages of a design fire

A design fire for a room is commonly divided into four stages: incipient, growth, fully developed and decay. A cooling stage may be added when the behaviour of the structure after the fire matters. The IFEG distinguishes three basic types of schematic design fire. A smouldering fire has a maximum heat release rate typically below 5 kW. The other two types are a flaming fire that does not reach flashover and a flaming fire that does.

For flaming fires, the IFEG generally assumes that the heat release rate grows with the square of time after ignition. It then reaches a steady value limited either by the fuel or by the ventilation, and finally decreases when the fuel runs out. In a fire that reaches flashover, the jump in heat release rate is assumed to be instantaneous and is drawn as a vertical line. C/VM2 assumes that flashover occurs when the average upper layer temperature first reaches 500 °C. The Australian data sheets mention an SFPE engineering standard that uses an upper layer temperature above 600 °C or a heat flux to the floor above 20 kW/m².

The incipient stage before flaming is usually left out. C/VM2 regards it as too unpredictable to include in design. The Australian data sheets use it mainly for smouldering fires in sleeping areas.

The growth phase

Growing fires are commonly described as a t-squared fire, or as a series of such fires that cover the range of credible fires in a room. A single growth coefficient then describes how quickly the fire develops. The heat release rate then increases with the square of the time since the start of flaming: $$\dot{Q}=\alpha t^2$$

Symbol	Description	Unit
$\dot{Q}$	heat release rate of the fire	kW
$\alpha$	fire growth coefficient	kW/s²
$t$	time from the start of the growth phase	s

Four standard growth categories are in common use. They are named after the time the fire needs to reach about 1 MW: slow (600 s), medium (300 s), fast (150 s) and ultrafast (75 s). The matching growth coefficients are 0.00293, 0.0117, 0.047 and 0.188 kW/s². The Australian data sheets define the reference size as 1055 kW, while Introduction to Fire Safety Engineering uses exactly 1 MW. The difference is about 5% and rarely matters in practice.

The growth rate is chosen mainly on the basis of the contents of the space. Introduction to Fire Safety Engineering lists values from the Dutch national annex to Eurocode 1. Dwellings, hotels and classrooms are given a medium growth rate, theatres and cinemas a fast one, and museums a slow one.

Storage of alcohol or upholstered furniture and chemical plants are given ultrafast growth. These values are meant as a safe estimate, because many real fires grow more slowly. The t-squared curves assume that the fire spreads horizontally in all directions. They are therefore not suitable for fires that also spread vertically, as in high-rack storage.

As a worked example, consider a fire in a dwelling with medium growth, so that α = 0.0117 kW/s². After 300 s, the heat release rate is 0.0117 × 300², or about 1.05 MW. The fire reaches 2.5 MW, the size of a burning sofa, after about 460 s, or just under 8 minutes. After 600 s, it has grown to about 4.2 MW. With a heat release rate per unit area of 250 kW/m², the burning area is then about 17 m².

C/VM2 sets its own growth rates. For most buildings, including storage below 3.0 m, the fire grows as 0.0469 t² up to a peak of 20 MW. This is close to the fast category: the fire reaches 1 MW after about 146 s and 20 MW after about 650 s. Car parks without stacking systems use 0.0117 t², and storage between 3.0 and 5.0 m high uses 0.188 t² up to 50 MW. Higher storage uses a growth with the cube of time.

Peak, fully developed fire and duration

A growing fire cannot grow forever. In C/VM2, the growth phase ends when the fire reaches flashover, its peak heat release rate or the ventilation limit. In sprinklered buildings, it can also end when the sprinklers activate. The ventilation limit is the heat release rate at which the modelled energy release first falls below the design fire because there is not enough oxygen. After flashover or once the ventilation limit is reached, C/VM2 raises the input heat release rate within 15 seconds. It goes either to the peak value or to 1.5 times the ventilation limit, depending on the case.

Ventilation-limited fires are difficult to model. Introduction to Fire Safety Engineering notes that most models handle a ventilation-controlled fire poorly, if at all. The user must therefore tell the model what happens when oxygen runs short. The options are that all gases still burn in the room, that the excess burns outside as flames from the openings, or that combustion stops.

The total energy available sets the duration of the fire. In fire models, the fire load is mainly important for determining how long the fire lasts, because the fire stops when the fuel is used up. For escape analyses this is often less important, since the first phase of the fire matters most. For the thermal load on structures and fire-separating elements, it is essential. The Dutch national annex to Eurocode 1 gives average fire load densities of 780 MJ/m² for dwellings and 420 MJ/m² for offices. At 250 kW/m², each square metre of a dwelling fire could therefore burn for roughly 50 minutes at full intensity.

C/VM2 prescribes design fire load energy densities of 400, 800 or 1200 MJ/m², depending on the activities in the space. For high-hazard storage above 3.0 m, it prescribes 800 MJ/m² per metre of storage height, with a minimum of 2400 MJ/m². For sprinklered firecells, the fire load used to calculate the fire duration may be halved.

Temperature-time curves such as the standard fire curve are not design fires in this sense. They are agreed temperature histories for testing the fire resistance of building elements. Introduction to Fire Safety Engineering stresses that the temperature in a room is a result of the fire scenario, not a starting point for it. Such curves are therefore unsuitable for representing a fire in a room.

Interventions and object fires

A design fire is modified by events during the fire. The IFEG names changes in ventilation, such as breaking windows or the operation of smoke control, and the start of suppression. Suppression can come from automatic systems, from occupants or from the fire service. In C/VM2 life safety analyses of sprinklered buildings, the fire is assumed to be controlled at a constant heat release rate after the sprinklers activate.

Some design fires describe single objects rather than rooms. Introduction to Fire Safety Engineering presents a heat release rate curve for a burning sofa measured by the Netherlands Institute for Public Safety (NIPV). It stresses that this curve applies only to the tested sofa, and that in a building the ignition of nearby objects must be modelled as well. The book also presents the car fire curve of the Dutch standard NEN 6098 for car parks, based on tests by TNO in 1999. In that curve, the fire spreads to the neighbouring cars after 10 to 15 minutes, so that three cars are burning after 15 minutes.

The book warns that such curves can become less conservative as cars contain more plastics. A report on car parks with electric vehicles by the NIPV states that fire scenarios for indoor car parks are represented by a heat release rate curve rather than a temperature curve. It concludes that electric and conventional cars reach similar peak heat release rates, on average about 6 MW. Electric cars with larger battery packs of 100 kWh or more may reach about 8 MW.

Limits and uncertainty

A design fire is a simplification of a process that varies from fire to fire. Notarianni points out that it is impossible to evaluate the performance of a building for every fire that might occur. Designers therefore usually choose worst-case or reasonable worst-case scenarios. It is not always obvious which scenario is the worst, nor how likely it is. The usual selection process does not take the likelihood of the design fire into account.

The outcome of an analysis can depend strongly on the chosen growth rate. In an absolute approach, choosing a faster growth rate can turn an acceptable result into an unacceptable one. The Australian Fire Safety Verification Method therefore compares a design with a reference building that meets the deemed-to-satisfy provisions. The comparison is less sensitive to the assumed growth rate, as long as the growth rate is the same in both buildings. The data sheets recommend considering a range of growth rates to check that the design is not overly sensitive to this choice.

Tabulated values also carry their own uncertainty. The fire load densities from the Dutch national annex are average values, and other sources give different numbers. The national annex also gives 80% fractile values for a more conservative approach. Introduction to Fire Safety Engineering notes that the heat release rate densities of the annex are intended for structural assessment. For escape analyses, additional calculations with higher values may be needed.

See also

• Fire dynamics
• Fire load
• Fuel-controlled fire
• Fire Dynamics Simulator

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• 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.
• Notarianni, K. A. (2002). Uncertainty. 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. 5-40–5-64). 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.