Standard fire curve

The standard fire curve is an agreed relation between time and gas temperature that a furnace follows when building elements are tested for fire resistance. The best-known versions are the curve of the international standard ISO 834 and the curve of the American standard ASTM E119. Introduction to Fire Safety Engineering calls it the standard ISO cellulosic curve and notes that construction elements are tested for fire resistance against it. The temperature rises quickly in the first minutes, passes about 840 °C after 30 minutes and keeps rising slowly for as long as the test lasts.

The curve was chosen to represent a severe, fully developed room fire, but it is not a model of any real fire. Its main value is that every test follows the same heating, so that results of different products can be compared. Fire resistance ratings in minutes, such as 60 or 120 minutes, therefore refer to exposure to such a standard curve. Engineering methods such as time equivalence translate the severity of a real fire into an equivalent duration of the standard curve.

Purpose and origin

The temperature course of a real fire cannot be known when a building is designed. It depends on parameters such as the amount, surface area and arrangement of the fuel, which change with time and vary from room to room. Surveys show that deviations of 50 % or more from the most probable fire load are common. This alone can shift fire temperatures by hundreds of degrees and fire durations by 50 % or more. It is possible, however, to give a temperature–time curve that will probably not be exceeded during the life of the building. Such curves are a useful basis for fire-resistive design.

Lie divides a fire into a growth period, a fully developed period and a decay period. During growth the temperatures are relatively low, and their influence on the fire resistance of structural members is negligible. The real risk to structural members and fire separations begins in the fully developed stage, the post-flashover fire, when temperatures of about 1,000 °C or higher can be reached. Because ventilation-controlled fires are usually the more severe and quite likely to occur, fire resistance requirements are commonly based on the assumption that fire severity is controlled by ventilation.

Lie also shows that the exact shape of a design curve matters less than its total heating effect. In his example, a theoretical fire curve for a building with a chance of being exceeded once in 50 years is replaced by a simpler curve. That curve assumes a constant burning rate until the fuel is consumed, followed by a linear fall to room temperature. Although the two shapes differ, their heating effect is about the same, so they provide virtually the same fire safety. The simpler curve has the advantage of being easier to define.

In a fire resistance test, gas burners heat the furnace so that its temperature follows the standard curve. Under ASTM E119, the furnace temperature is taken as the average of at least eight thermocouples, read at intervals of no more than 1 minute. Milke notes that the ASTM E119 curve is intended to relate to a severe exposure from a room fire. Its use for structural members outside a building, exposed to fires outside it, is therefore questionable. Versions of the American test method are also published as NFPA 251 and UL 263.

Principle of a furnace test: burners are controlled so that the average furnace temperature follows the standard curve. Based on Milke (2002a), p. 4-210, and Iqbal and Salley (2004), p. 17-2.

The ISO 834 relation

The curve of ISO 834 has a simple analytical form. The rise of the furnace temperature above its starting value grows with the logarithm of time. The factor 8 in the logarithm makes the temperature rise steeply in the first minutes, and the curve then flattens without ever reaching a maximum.

$$T-T_0=345{\log }_{10}(8t+1)$$

Symbol	Description	Unit
$T$	fire (furnace gas) temperature	°C
$T_0$	initial temperature	°C
$t$	time from the start of the test	min

The relation applies to the furnace gas temperature in the standard test. It does not describe the temperature of the tested element, and it does not include a decay phase. With an initial temperature of 20 °C, the curve gives about 576 °C after 5 minutes and 678 °C after 10 minutes. After 30, 60 and 120 minutes the temperature is about 842 °C, 945 °C and 1,049 °C. After 4 hours, the longest period usually required, it reaches about 1,153 °C.

As a worked example, a wall is required to achieve a rating of 60 minutes. With an initial furnace temperature of 20 °C, the term 8t + 1 equals 481 at 60 minutes. Its logarithm is 2.682, so the temperature rise is 345 × 2.682 = 925 K. The furnace must therefore be at about 945 °C at the end of the required period. In the first 3 minutes alone the temperature already climbs to about 500 °C.

The American ASTM E119 curve is defined by a table of points rather than one formula. NUREG-1805 lists 704 °C at 10 minutes, 843 °C at 30 minutes, 927 °C at 1 hour, 1,010 °C at 2 hours and 1,093 °C at 4 hours. The curve ends at 1,260 °C after 8 hours. Compared with the ISO formula, the American curve is about 26 K hotter at 10 minutes and nearly equal at 30 minutes. At 1 hour it is about 18 K cooler and at 4 hours about 60 K cooler.

The ISO 834 standard fire curve (T0 = 20 °C) with the ASTM E119 points. Based on Lie (2002), p. 4-206, and Iqbal and Salley (2004), p. 17-2.

Lie concludes that there are no significant differences between the standard curves used in various countries. In his comparison, Australia, Great Britain and New Zealand share one curve, and Belgium, France, the Netherlands, Sweden and several other European countries share another. Canada and the United States share one curve, and the Japanese curve levels off at a lower temperature than the others in long tests. For the North American curve, Lie also gives analytical approximations. One of them, a sum of three exponential terms, deviates from the tabulated values by at most −26 °C at 45 minutes, +48 °C at 3.5 hours and −78 °C at 8 hours.

Other nominal curves

The standard curve is one of several agreed temperature–time curves. Introduction to Fire Safety Engineering names the external fire exposure curve, used to test facade elements against a fire in the open air. It also names two less frequently used curves that are more intense than these two. These are the Rijkswaterstaat curve for tunnels and the hydrocarbon fire curve, which simulates an oil or gas fire. The book stresses that such curves are only agreed temperature courses and in principle do not correspond to the heat development in a real fire.

A temperature–time curve can also be used inside calculation models. The Eurocodes offer a choice between a temperature–time curve and a natural fire, whose temperature course follows from the characteristics of the fire and the room. A parametric fire curve is one form of such a design fire. In a worked example in the Australian Fire Safety Verification Method Data Sheets, a design fire produced a parametric curve that was closely aligned with the hydrocarbon heating regime.

Use in design and regulation

Many national fire safety codes express the required performance of fire barriers as a fire resistance time or rating. In Australia, the fire resistance level is defined as the grading period in minutes obtained by subjecting a specimen to the standard time–temperature curve of AS 1530.4. In the United States, ratings of floors, roofs, beams, walls and columns are based on ASTM E119. Such a rating describes the fire resistance of an element under this one heating regime, and its behaviour under other heating is generally not determined directly.

Required ratings rarely exceed 4 hours. NUREG-1805 notes that no jurisdiction demands fire endurance periods of more than 4 hours. The 2013 commentary to the New Zealand verification method C/VM2 calls ratings above 4 hours in the NZS/ISO 834 test unusual and very expensive. C/VM2 (second edition, 2025) offers several ways to set the rating of an element that must resist a fully developed fire, with a minimum of 20 minutes. One option is an equivalent time of exposure in a standard fire resistance test assuming full burnout.

Time equivalence links real fires to the standard curve. The International Fire Engineering Guidelines (2005) define the equivalent structural fire severity as the time of exposure to the standard test that gives the same thermal impact as a complete burnout of the compartment. The New Zealand formula finds the time on the ISO 834 curve that produces the same maximum temperature in a protected steel member as a post-flashover fire in the space. In the Australian worked example, an element reached 454 °C after 63 minutes in the standard test but after 45 minutes in the design fire. Its failure time in that scenario was therefore taken as 45 minutes.

For separating elements, a different equivalence has been proposed. The International Fire Engineering Guidelines (2005) describe a method by Nyman, which assumes that a barrier fails when it receives the same cumulative radiation dose as at failure in the standard test. The method suits failure by heat conduction through the barrier.

The standard curve also underlies some design values that are not fire resistance ratings. For fire spread to neighbouring buildings, C/VM2 (second edition, 2025) assumes that unprotected areas of an unsprinklered building emit 83, 103 or 144 kW/m², depending on the fire load energy density. The 2013 commentary explains that this exposure corresponds to radiation from fire gases at the temperatures reached in a standard fire resistance test after 30, 45 or 90 minutes. The emissivity was raised from 0.95 to 1.0 while keeping the same incident radiation.

Emitted heat fluxes of C/VM2 for unsprinklered firecells and the points on the standard curve they correspond to. Based on Ministry of Business, Innovation and Employment (2013), p. 133, and (2025b), p. 44.

The ISO curve indeed gives 842 °C, 902 °C and 1,006 °C at these times. Black-body radiation at these temperatures, multiplied by 0.95, gives 83, 103 and 144 kW/m². The commentary adds that actual gas temperatures in a fire may differ from those in the standard test.

Comparison with real fires

Real fires differ from the curve in several ways. A typical post-flashover room fire reaches a peak temperature of 900 to 1,000 °C, with a maximum of around 1,200 °C. The ASTM E119 curve, by contrast, keeps rising and reaches 1,260 °C after 8 hours. A real fire also decays when the fuel is consumed, which the standard curve does not. Introduction to Fire Safety Engineering explains that the temperature of a room fire depends on the size of the room, its openings, its insulation and the heat release rate. The temperature is a result of a fire scenario and not a starting point for one.

The standard curve keeps rising, while a real fire grows, burns fully developed and then decays. Based on Lie (2002), p. 4-201, and Iqbal and Salley (2004), p. 3-14.

For this reason the International Fire Engineering Guidelines (2005) warn that ratings from ISO 834, ASTM E119 or AS 1530.4 may not represent the actual fire conditions in a building. They should be used in fire engineering only together with a fire severity analysis. The authors of the OZone design tool observed that structural specialists long represented the fire by a single nominal curve, either ISO 834 or ASTM E119. Fire modellers, in turn, thought of the structure only in terms of a critical temperature.

The investigation of the World Trade Center collapses drew similar conclusions. In four standard tests of the floor system, it took about 90 minutes of furnace heating before the steel trusses exceeded 600 °C. The investigators noted that the standard test does not capture important thermally induced interactions between structural subsystems, elements and connections, because these are tested separately. They concluded that a performance objective of burnout without collapse will require standard fire exposures that differ from those currently used.

Common misconceptions

A rating of 60 minutes does not mean that an element will survive 60 minutes in a real fire. The test is comparative, and furnace size limits the dimensions of specimens, so it cannot be used to assess actual performance. Introduction to Fire Safety Engineering adds that the curve suits the testing of a construction from a standardised starting point but is not suitable for simulating a fire in a room.

Another misconception is that the standard curve is the worst case. More intense nominal curves exist for tunnels and for hydrocarbon fires. A short, hot design fire can also bring an element to its critical temperature sooner than the standard test does, as in the Australian example of 45 instead of 63 minutes.

Readers of the third edition of the SFPE Handbook should also note a labelling problem. The table captioned as the ISO 834 relation lists 538 °C at 5 minutes, 704 °C at 10 minutes and 927 °C at 1 hour. These are the ASTM E119 values given in NUREG-1805. The table captioned as the North American relation lists temperature rises that match the ISO formula, for example 925 K at 60 minutes. The two captions therefore appear to be interchanged.

See also

• Structural fire engineering
• Charring rate
• RWS fire curve
• Eurocodes

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• 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
• 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.
• Milke, J. A. (2002a). Analytical methods for determining fire resistance of steel members. 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. 4-209–4-238). 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 Institute of Standards and Technology. (2005). Final report on the collapse of the World Trade Center towers (NIST NCSTAR 1).
• National Institute of Standards and Technology. (2008). Final report on the collapse of World Trade Center Building 7 (NIST NCSTAR 1A).
• 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.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.