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Contents
  1. 1Purpose and principle
  2. 2The standard fire curve
  3. 3How the test is carried out
  4. 4Failure criteria
  5. 5Ratings and their use
  6. 6Lessons from the World Trade Center floor tests
  7. 7Limits and common misconceptions
  8. 8See also
  9. 9Notes
  10. 10References

A fire resistance test is a standardised furnace test, a form of fire testing, for building elements such as walls, floors, columns, beams and doors. The element is exposed to a prescribed temperature-time curve to find out how long it keeps performing its function in a fire.[1][2] The element is judged on whether it keeps carrying its load, whether it stops flames and hot gases from passing, and whether its unexposed side stays cool enough. The time until the first of these criteria fails gives its rating, expressed in minutes or hours.[3][4]

The test is the traditional way to establish the fire resistance of construction. Elements that pass are rated and listed for the duration of the test, for example 1 or 2 hours.[5] Well-known test standards include ASTM E119 in the United States and AS 1530.4 in Australia, and the International Organization for Standardization has adopted a standard fire curve in ISO 834.[1][4][6] The heating follows a standard fire curve, which in the ISO version rises to about 945 °C after one hour (calculated). This curve is an agreed reference that makes tested constructions comparable, not a prediction of a real fire.[2] The results are therefore comparative, and using them in fire safety engineering requires care.[1][7]

Purpose and principle

A fire inside a building passes through a growth period, a fully developed period and a decay period.[8] During growth, temperatures are still relatively low, and their influence on structural members is negligible. The risk of failure of structural members and fire separations begins when the fire is fully developed and reaches temperatures of about 1000 °C or higher.[8] In fire resistance studies, the growth period can therefore be disregarded.[8] In principle, the heating in the test is intended to relate to a severe exposure from a fire in a room.[1]

The amount and arrangement of combustible material in a building change over its life, so the actual fire that an element will face cannot be known when the building is designed. It is possible, however, to define a temperature-time curve that will probably not be exceeded during the life of the building, and to use it as a basis for design.[8] This is the idea behind the standard test: all elements are exposed to the same heating, so that their performance can be compared and graded.[3][2]

The standard test is one of three ways to prove that an element performs acceptably. The other two are a special experiment and an analytical calculation.[5] Standard tests take time to specify, schedule and analyse, and they can be costly. For old types of construction, the original materials may no longer be available to build a test specimen.[5] For these reasons, calculation methods have been developed, many of them based on data from standard tests.[5]

The standard fire curve

In a fire resistance test the temperature of the furnace gases follows a fixed curve. Many countries have their own standard curve, but the SFPE Handbook shows that there are no significant differences between them.[6] The curve adopted by the International Organization for Standardization in ISO 834 rises quickly at first and then more and more slowly.[6] It is a logarithmic function of time: T−T0=345log10(8t+1)

Symbol Description Unit
T furnace (fire) temperature °C
T0 initial temperature °C
t time from the start of the test min

The equation applies over the whole duration of a test, which can run to several hours. The SFPE Handbook tabulates standard curves up to 8 hours.[6] As a worked example, take an initial temperature of 20 °C. After 30 minutes the term inside the logarithm is 8 × 30 + 1 = 241, and log10(241) is 2.38, so the temperature rise is 345 × 2.38 = 822 °C. The furnace is then at about 842 °C.[6] The same calculation gives about 945 °C after 60 minutes, 1049 °C after 2 hours and 1153 °C after 4 hours (calculated).[6]

Introduction to Fire Safety Engineering calls this curve the Standard ISO Cellulosic Curve and notes that building elements are tested with it. Elements of facades that face a fire in the open air are tested with a less severe External Fire Exposure Curve.[2] Some curves are more intense than the standard curve, such as the Rijkswaterstaat curve for tunnels and the hydrocarbon curve, which simulates an oil or gas fire.[2]

How the test is carried out

The test specimen is built into the opening of a furnace, which in North America is heated by gas burners.[1] The burners are controlled so that the furnace temperature follows the standard curve. Floors and roofs are loaded and heated from below.[3] ASTM E119 covers columns, floor and roof assemblies and walls.[1]

Elements are tested either with or without load. Floor and roof assemblies and loadbearing walls are always loaded, at the maximum stress allowed in normal design. Columns may be tested with or without load. Steel beams and girders may be tested without load if the design load cannot be applied in the laboratory.[1] In the World Trade Center floor tests, for example, the load was applied with concrete blocks and containers filled with water.[9]

The way an element is held at its ends also matters. A restrained specimen is one whose thermal expansion at the supports is resisted by external forces; an unrestrained specimen is free to expand and rotate.[3] ASTM E119 has distinguished restrained and unrestrained ratings since 1971, and a table in the standard relates common forms of construction to the two conditions.[3][1] In the United States, floor assemblies are customarily tested in the restrained condition.[3] According to that table, steel beams welded, riveted or bolted to the framing members count as restrained, while all types of wood construction count as unrestrained.[10]

The size of a specimen is limited by the furnace. ASTM E119 requires a floor test to expose at least 180 ft² (about 16.7 m²), with no furnace dimension under 12 ft (about 3.7 m).[9] Most furnaces are smaller than real floors, so relatively small assemblies have traditionally been tested and the results scaled to practical spans.[9] The SFPE Handbook therefore describes the test as large-scale but not full-scale.[1]

Failure criteria

The performance of an element is judged with three main fire resistance criteria. In the Australian standard AS 1530.4 they are called structural adequacy, integrity and insulation.[4] Structural adequacy is the ability of a loadbearing element to support its load during the test, although deflection limits and critical temperatures are also used in some cases.[11] Integrity is the ability to stop flames and hot gases from passing to the other side. Under AS 1530.4 it is lost when a cotton pad held over a crack ignites, or when a gap larger than 150 mm by 6 mm or 25 mm in diameter opens. It is also lost when flames burn on the unexposed face for 10 seconds or longer.[11]

Insulation is the ability to keep the unexposed face cool. Under AS 1530.4 it is lost when the average temperature of that face rises more than 140 K above its initial value, or when any single point rises more than 180 K.[12] These limits are below the temperature at which most materials ignite without a pilot flame over a short period. However, a combustible material in contact with the face could reach a higher temperature than the bare surface does in the test.[12]

Depending on the element, AS 1530.4 and related methods can apply further criteria. Examples are the resistance to the incipient spread of fire, the radiant heat flux, and the period for which a critical service such as an emergency power cable keeps working.[4] ASTM E119 uses similar main criteria, with temperature limits that depend on the element.[1][10]

Element Measured where Average (°C) Single point (°C)
Walls and partitions rise on the unexposed side 139 181
Steel columns steel 538 649
Floors and roofs rise on the unexposed side 139 181
Floors and roofs steel beam 593 704
Unloaded steel beams and girders steel 538 649

In principle, the temperature end points are based on the largest acceptable loss of loadbearing capacity of the member.[1] ASTM E119 also checks the passage of flame, the ignition of cotton waste on the unexposed side, and in some cases the response to a hose stream.[1] In the Australian rules, a fire door has an additional radiation criterion. The radiant heat flux 365 mm from the unexposed side must not exceed 10 kW/m².[13]

Ratings and their use

The test results are expressed as ratings. In Australia, the National Construction Code uses a fire resistance level (FRL): three periods in minutes for structural adequacy, integrity and insulation, in that order.[4] A dash means that there is no requirement. An FRL of 90/60/– thus requires 90 minutes of structural adequacy, 60 minutes of integrity and no insulation.[4] A typical fire door has an FRL of –/60/30, because a door carries no load.[13] FRLs allow fire barriers to be specified consistently and give manufacturers a clear route to evidence through the standard test.[4]

In New Zealand, Verification Method C/VM2 (second edition, 2025) states how the required fire resistance rating of elements that must resist a fully developed fire may be determined. Options include three times the period for which the element must perform, or the equivalent time of exposure in a standard test that has the same effect as the design fire.[14] The rating must be at least 20 minutes, and need not exceed 240/240/240 in an unsprinklered firecell or 180/180/180 in a sprinklered one.[14] The test specifications themselves are now given in separate Building Product Specifications.[15]

Introduction to Fire Safety Engineering explains that standard curves are mainly used in comparative tests and less often in a fire safety engineering approach. There, the thermal energy of a temperature-time curve can be compared with that of a natural fire.[2] The Eurocodes offer the choice between a temperature-time curve and a natural fire model.[2]

Lessons from the World Trade Center floor tests

The National Institute of Standards and Technology (NIST) investigated the collapse of the World Trade Center towers in 2001. It found no evidence that the floor system of the towers had ever been fire tested.[16] NIST therefore had Underwriters Laboratories carry out four ASTM E119 tests of the composite floor system, with steel trusses protected by sprayed fire-resistive material. The tests varied the thickness of the protection, the restraint condition and the scale.[16][9] Two full-scale specimens spanned 35 ft (about 10.7 m) and two reduced-scale specimens spanned 17 ft (about 5.2 m).[17]

The results showed the limits of the method. The test assemblies resisted the standard fire for between 3/4 hour and 2 hours.[18] The restrained full-scale floor obtained a rating of 1.5 hours, while the unrestrained floor achieved 2 hours, the opposite of what past experience would lead investigators to expect. With the same protection thickness of 3/4 in (about 19 mm), the reduced-scale test gave 2 hours and the full-scale test 1.5 hours.[18] With the specified thickness of 1/2 in, the reduced-scale test gave only 3/4 hour.[18] NIST concluded that the scale of the test, the restraint conditions and the repeatability of results need further study.[18]

Limits and common misconceptions

A common misconception is that a rating of 60 minutes means that an element will survive 60 minutes of a real fire. The SFPE Handbook stresses that the test is only comparative and cannot be used to assess actual performance, because continuous beams, real floor assemblies and long columns cannot be tested at full size.[1] Introduction to Fire Safety Engineering adds that a temperature-time curve is suitable for testing a construction on a standard basis, but not for reproducing a fire in a room. The temperature of a real fire is the result of a fire scenario, not its starting point.[2]

A second limitation is that elements are tested under one heating regime only. Some barriers, and fire doors in particular, can be very sensitive to the rate and type of heating. Their performance under a different design fire is not directly known from the test.[7][19] When a rating is converted into an equivalent time for a design scenario, the Australian data sheets warn that the conversion adds uncertainty.[7] Test results also depend on how the specimen is supported, as the World Trade Center tests showed.[18]

Fire resistance also depends on processes inside the element. Plasterboard walls, for example, owe part of their fire resistance to the evaporation of water that is chemically bound in the gypsum.[20] Finally, the thermal exposure of a member can be measured more directly than through the gas temperature alone. In the adiabatic surface temperature experiments at SP in Sweden in 2008, plate thermometer measurements near a steel beam were used to supply the boundary conditions for a heat conduction calculation of the beam.[21]

See also

Notes

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 Milke (2002a), p. 4-210.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Van Rossum (2027), §8.6.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Gross et al. (2005), p. xxx.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Australian Building Codes Board (2020a), Data Sheet C6, p. 172.
  5. ↑ 5.0 5.1 5.2 5.3 Milke (2002a), p. 4-209.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 Lie (2002), p. 4-206.
  7. ↑ 7.0 7.1 7.2 Australian Building Codes Board (2020a), Data Sheet C6, p. 173.
  8. ↑ 8.0 8.1 8.2 8.3 Lie (2002), p. 4-201.
  9. ↑ 9.0 9.1 9.2 9.3 Gross et al. (2005), p. xxxi.
  10. ↑ 10.0 10.1 Milke (2002a), p. 4-211.
  11. ↑ 11.0 11.1 Australian Building Codes Board (2020a), Data Sheet C6, p. 174.
  12. ↑ 12.0 12.1 Australian Building Codes Board (2020a), Data Sheet C6, p. 175.
  13. ↑ 13.0 13.1 Australian Building Codes Board (2020a), Data Sheet C6, p. 209.
  14. ↑ 14.0 14.1 Ministry of Business, Innovation and Employment (2025b), p. 18.
  15. ↑ Ministry of Business, Innovation and Employment (2025b), p. 3.
  16. ↑ 16.0 16.1 Gross et al. (2005), p. xxix.
  17. ↑ Gross et al. (2005), p. iii.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 Gross et al. (2005), p. xxxii.
  19. ↑ Australian Building Codes Board (2020a), Data Sheet C6, p. 210.
  20. ↑ Van Rossum (2027), §5.2.
  21. ↑ Stroup et al. (2016), p. 3-17.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Gross, J., Hervey, F., Izydorek, M., Mammoser, J., & Treadway, J. (2005). Fire resistance tests of floor truss systems (NIST NCSTAR 1-6B). National Institute of Standards and Technology.
  • 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. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • Stroup, D., Lindeman, A., McGrattan, K., Peacock, R., Overholt, K., Joglar, F., LeStrange, S., & Montanez, S. (2016). Verification and validation of selected fire models for nuclear power plant applications: Supplement 1 (NUREG-1824, Supplement 1; EPRI 3002002182). U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research; Electric Power Research Institute.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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