Skip to content
Upcoming courses

Fire resistance criteria

Tools
From Fire Safety Academy Wiki
Contents
  1. 1The three criteria
  2. 2How the criteria are judged in a test
  3. 3Use in regulation and design
  4. 4Example: the World Trade Center floor tests
  5. 5Variability and limits
  6. 6Common misconceptions
  7. 7See also
  8. 8Notes
  9. 9References

Fire resistance criteria are the conditions that a building element must keep meeting during a standard fire test, or a calculation that replaces it, for its rating to be valid. There are three main criteria. Structural adequacy is the ability to keep carrying the applied load. Integrity is the ability to stop flames and hot gases passing through cracks or holes. Insulation is the ability to limit the heat conducted to the side away from the fire.[1] A load-bearing element needs structural adequacy, a separating element such as a wall or floor needs integrity and insulation, and many elements need all three.[1]

Three sketches of a specimen heated from one side: a sagging loaded element, a crack with flame passing through, and a hot unexposed face with thermocouples.
The three fire resistance criteria: structural adequacy (carry the load), integrity (no flames or hot gases through cracks) and insulation (limited temperature rise on the unexposed face). Based on National Research Council of Canada et al. (2005), p. 2.6-6, and Australian Building Codes Board (2020a), Data Sheet C6, pp. 174–175.

The result of a test is reported separately for each criterion, as the time in minutes for which it was met.[1][2] In New Zealand and Australia the three times are written in a fixed order, so that a rating of 90/60/– means 90 minutes for structural adequacy, 60 minutes for integrity and no requirement for insulation.[2][3] The exact pass and fail limits depend on the test standard. Under the Australian standard AS 1530.4, for example, insulation fails when the average temperature of the unexposed face rises by 140 K or any single point rises by 180 K.[4] Because the criteria are always assessed under one standard heating regime, the rating describes a comparison between products rather than a survival time in a real fire.[5][6]

The three criteria

Fire resistance is the ability of an element to fulfil, for a stated period, the load-bearing function, integrity, thermal insulation or other duty specified in a standard fire resistance test.[7] The International Fire Engineering Guidelines (2005) explain why each of the three criteria exists. Insulation is required for separating elements, to limit the heat conducted through the assembly and so prevent ignition of combustible materials in the adjoining room. Integrity is also required for separating elements, to prevent cracks or holes through which flame or hot gases could pass. This could ignite materials on the other side or make conditions there untenable.[1] Structural adequacy is required for elements that carry loads, to prevent premature collapse and the fire spread that would follow.[1]

The New Zealand verification method C/VM2 (second edition, 2025) defines each criterion as a time. Structural adequacy is the time in minutes for which a specimen continues to carry its applied load within defined deflection limits.[8] Integrity is the time for which a specimen of a fire separation prevents the passage of flame or hot gases. Insulation is the time for which it limits the transmission of heat.[9] C/VM2 adds that the precise meaning of integrity depends on the type of element and on the test standard used.[9]

Which criteria apply depends on the function of the element. The guidelines note that beams normally need only structural adequacy, because unlike slabs they do not separate spaces unless they are built integrally with a slab.[10] A fire door has no structural adequacy requirement, because it does not carry loads. A typical Australian rating for a fire door is –/60/30, which means 60 minutes of integrity and 30 minutes of insulation.[11] A load-bearing fire wall or floor between compartments, by contrast, has to satisfy all three criteria at once.[1]

Grid with element types in rows and the three criteria in columns; ticks show which criteria apply to each element.
Which fire resistance criteria apply to typical building elements. Based on National Research Council of Canada et al. (2005), pp. 2.6-6 and 2.6-8, and Australian Building Codes Board (2020a), Data Sheet C7, p. 209.

Some elements carry further criteria. AS 1530.4 can add the resistance to the incipient spread of fire, a limit on radiant heat flux, or the period for which a critical service such as an emergency power cable keeps working.[3] For fire doors, the radiant heat flux 365 mm from the non-fire side must not exceed 10 kW/m2.[11] For fire dampers in ducts, the integrity criterion is replaced by a leakage limit of 360 m3/(h·m2), corrected to standard temperature and pressure.[12]

How the criteria are judged in a test

In a fire resistance test, a specimen is heated in a furnace that follows a standard temperature–time curve. It passes for a given period if no end-point criterion is reached before the end of that period.[13] The Australian Data Sheets summarise how AS 1530.4 judges each criterion. Structural adequacy is, in its simple interpretation, the time until the element can no longer support the test load. Deflection limits and critical temperatures are also applied in some cases.[14]

Integrity under AS 1530.4 fails in any of three ways. A cotton pad held over a crack or fissure ignites, by glowing or flaming. A gap opens that is larger than 150 mm by 6 mm, or 25 mm in diameter. Or flaming persists on the unexposed face for 10 seconds or longer.[14] Insulation fails when the average temperature of the unexposed face exceeds its initial value by 140 K, or the temperature at any point exceeds it by more than 180 K.[4]

The American standard ASTM E119 uses similar end points. They cover structural integrity, temperature, passage of flame, ignition of cotton waste and, in some cases, the response to a hose stream.[13] Floor and roof assemblies and load-bearing walls are always tested under load. Specimens tested without load need only stay in place.[13] A hose stream test is required for walls and partitions with a rating of at least 1 hour. The test is failed if openings let water project beyond the unexposed surface.[15]

ASTM E119 also limits temperatures inside some assemblies. For steel beams in floor and roof assemblies the limits are 593 °C on average and 704 °C at a single point. For reinforcing steel the limit is 593 °C and for prestressing steel 426 °C.[16] These limits are based on the largest permissible loss of load-bearing capacity, given the strength of the steel at high temperature and the loads allowed by structural design standards.[13]

For concrete floors, roofs and walls, the insulation criterion often governs the rating rather than the structural one.[17] Their insulation depends mainly on the thickness of the element and on the type of aggregate in the concrete.[10][18] For concrete slabs and walls, integrity is usually assumed to be met when insulation and structural adequacy have been achieved.[10] Gypsum-lined walls derive part of their fire resistance from the water chemically bound in the gypsum, which must evaporate before the temperature can rise further.[19]

Use in regulation and design

Many national fire safety codes express the required performance of fire barriers as a fire resistance time or rating.[3] Introduction to Fire Safety Engineering notes that Dutch regulations aim to preserve the load-bearing structure and the fire compartmentation of a building. The rules set measurable limits for this, such as minutes of resistance to fire spread (WBDBO) and square metres of compartment area.[20] In Australia, the National Construction Code uses the fire resistance level (FRL), which gives the grading periods for structural adequacy, integrity and insulation, in that order. A dash means that there is no requirement for that criterion.[3]

In New Zealand, C/VM2 (second edition, 2025) uses the fire resistance rating (FRR) with the same three numbers.[2] Where a fire in a normally unoccupied room could threaten a crowded space, separating elements can confine the fire. If the room of origin has automatic fire detection, these elements need at least –/60/60, or must be shown to work until the occupied space is evacuated. Without detection they must withstand a full burnout fire.[21] Cavity barriers in concealed spaces without detection need at least –/30/30.[22] The maximum rating required need not exceed 240/240/240 in an unsprinklered firecell or 180/180/180 in a sprinklered one.[23]

C/VM2 also chooses criteria to suit each situation. A fire-rated roof near a relevant boundary must meet at least structural adequacy and integrity.[24] Intermediate floors that together cover more than 40 % of the floor area of a firecell must be rated for integrity, insulation and structural adequacy.[25] An analysis of radiation through windows on escape routes is not required where insulated glazing of at least –/30/30 is used.[26]

Ratings can be obtained by test or by calculation. If a design fire has been modelled, fire resistance can be found by thermal and structural analysis at high temperature, provided a suitable method exists.[1] Such calculations use the same acceptance criteria as the standard test, namely heat transmission and structural integrity.[27] The guidelines note that fire engineers usually predict the thermal effects, while structural engineers analyse the response of the structure. This division of work is part of structural fire engineering.[28]

Example: the World Trade Center floor tests

The investigation of the World Trade Center collapses included four ASTM E119 tests of floor truss assemblies in 2004. The standard test was described as a test of an assembly's ability to contain a fire. It is based on thermal response, namely flame spread and heating of the unexposed surface, and on structural response, namely supporting the applied load.[29] The tests were continued well beyond the first end point to gather as much information as possible.[30]

In Test 1, a restrained assembly with a span of 35 ft and 3/4 in of sprayed fire-resistive material, the criteria were reached at different times. The steel trusses reached the single-point limit of 704 °C after 62 minutes and the average limit of 593 °C after 66 minutes. The single-point limit on the unexposed surface was exceeded after 111 minutes.[30][31] The assembly did not fail to support its load during the test, which was stopped after 116 minutes because collapse was imminent. It received a restrained rating of 1½ hours and an unrestrained rating of 1 hour.[31]

Timeline from 0 to 120 minutes with markers at 62, 66, 111 and 116 minutes and the 90-minute restrained rating.
Times at which ASTM E119 end points were reached in NIST floor test 1 (35 ft span, restrained, 3/4 in fire-resistive material), with its restrained rating of 1½ h. Based on Gross et al. (2005), pp. 94 and 96.

The unrestrained full-scale assembly of Test 2 received a rating of 2 hours. The investigators noted that past experience would have led them to expect a lower rating for the unrestrained assembly than for the restrained one.[32] Testing for ignition of cotton waste on the unexposed surface was not done, because access to the top of the specimens was not permitted for safety reasons.[30] The investigators also observed that tests are normally stopped before the load-carrying capacity is reached, because other criteria are met first or deflections become too large.[33]

Variability and limits

Test results scatter, even for nominally identical constructions. Six tests for the Fire Code Reform Centre used non-loadbearing walls of one layer of 16 mm fire-protective plasterboard on steel studs. Insulation failed after 70 to 79 minutes and integrity after 105 to 112 minutes. The means were 75 and 108 minutes.[34]

Dot plot of six plasterboard wall tests: insulation times between 70 and 79 minutes and integrity times between 105 and 112 minutes.
Insulation and integrity times of six plasterboard wall tests with standard and poor installation. Based on Australian Building Codes Board (2020a), Data Sheet C6, p. 179.

Collapse of the board followed closely after integrity failure, which was caused by flaming on the unexposed side. Deliberate installation faults changed the results only slightly. The standard deviations were 2.9 minutes for insulation and 2.7 minutes for integrity.[34]

In an interlaboratory series on one partition system to ASTM E119, six Japanese laboratories found an average failure time of 67.1 minutes with a standard deviation of 1.1 minutes. Ten North American laboratories found 65 minutes with a standard deviation of 2.8 minutes.[35] In a worked example, the Data Sheets combine a mean insulation time of 75 minutes with a 0.5 % chance that non-fire-grade board is installed by mistake. For a wall required to achieve 60/60/60, the 1-percentile value is then 55.7 minutes. Without a safety margin, the probability of falling below the required rating could be about 50 %.[36]

The test conditions also limit what the criteria can show. Furnace size restricts specimen dimensions, so continuous beams and long columns are not tested at full scale. The ASTM E119 test is therefore only comparative and cannot be used to assess actual performance.[13]

The insulation limits lie below the temperature at which most materials ignite without a pilot flame over a short period. They are measured, however, on a face that is open to convective cooling. If combustible insulating materials touch the surface, the interface temperature can be higher.[4] AS 1530.4 also does not generally measure the spread of smoke and hot gases through an element.[12]

Common misconceptions

A rating in minutes is not the time an element will survive in a real fire. Fire barriers are normally assessed only under the standard heating regime, and their behaviour under other heating conditions is generally not determined directly.[5] Some barriers are sensitive to the rate of heating. Where a barrier is part of the structure, deflections and stresses caused by heating can also start structural failures.[5] Introduction to Fire Safety Engineering describes the furnace test as a comparative model: its thermal load is not based on a real fire but is identical for every test.[6]

The structural adequacy time of a single element is also not the collapse time of a building. Redundancy may let loads redistribute and delay failure. Conversely, thermally induced stresses may cause earlier failure, particularly when fire heats many members.[14] Integrity failure does not always mean immediate fire spread either. Fire can spread at about the time of integrity failure if combustible materials lie close to the crack, but much later if they do not.[4]

A further source of confusion lies in unit conversions. ASTM E119 limits the rise of the unexposed surface temperature to 250 °F on average and 325 °F at a single point. Milke gives these as rises of 139 °C and 181 °C.[16] Several other sources print 121 °C and 163 °C instead.[17][37][30] These are the Celsius values of 250 °F and 325 °F read as temperatures, not as temperature differences. A rise of 250 °F equals a rise of about 139 K, which is close to the 140 K limit of AS 1530.4.[38]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 National Research Council of Canada et al. (2005), p. 2.6-6.
  2. ↑ 2.0 2.1 2.2 Ministry of Business, Innovation and Employment (2025b), p. 64.
  3. ↑ 3.0 3.1 3.2 3.3 Australian Building Codes Board (2020a), Data Sheet C6, p. 172.
  4. ↑ 4.0 4.1 4.2 4.3 Australian Building Codes Board (2020a), Data Sheet C6, p. 175.
  5. ↑ 5.0 5.1 5.2 Australian Building Codes Board (2020a), Data Sheet C6, p. 173.
  6. ↑ 6.0 6.1 Van Rossum (2027), §8.2.
  7. ↑ Iqbal and Salley (2004), p. F-28.
  8. ↑ Ministry of Business, Innovation and Employment (2025b), p. 69.
  9. ↑ 9.0 9.1 Ministry of Business, Innovation and Employment (2025b), p. 66.
  10. ↑ 10.0 10.1 10.2 National Research Council of Canada et al. (2005), p. 2.6-8.
  11. ↑ 11.0 11.1 Australian Building Codes Board (2020a), Data Sheet C7, p. 209.
  12. ↑ 12.0 12.1 Australian Building Codes Board (2020a), Data Sheet C6, p. 176.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 Milke (2002a), p. 4-210.
  14. ↑ 14.0 14.1 14.2 Australian Building Codes Board (2020a), Data Sheet C6, p. 174.
  15. ↑ Iqbal and Salley (2004), p. 17-2.
  16. ↑ 16.0 16.1 Milke (2002a), p. 4-211.
  17. ↑ 17.0 17.1 Fleischmann and Buchanan (2002), p. 4-242.
  18. ↑ National Research Council of Canada et al. (2005), p. 2.6-9.
  19. ↑ Van Rossum (2027), §5.2.
  20. ↑ Van Rossum (2027), §1.2.
  21. ↑ Ministry of Business, Innovation and Employment (2025b), p. 40.
  22. ↑ Ministry of Business, Innovation and Employment (2025b), p. 41.
  23. ↑ Ministry of Business, Innovation and Employment (2025b), p. 18.
  24. ↑ Ministry of Business, Innovation and Employment (2025b), p. 46.
  25. ↑ Ministry of Business, Innovation and Employment (2025b), p. 55.
  26. ↑ Ministry of Business, Innovation and Employment (2025b), p. 35.
  27. ↑ Fleischmann and Buchanan (2002), p. 4-239.
  28. ↑ National Research Council of Canada et al. (2005), p. 2.6-7.
  29. ↑ Gross et al. (2005), p. xxxi.
  30. ↑ 30.0 30.1 30.2 30.3 Gross et al. (2005), p. 94.
  31. ↑ 31.0 31.1 Gross et al. (2005), p. 96.
  32. ↑ Gross et al. (2005), p. xxxii.
  33. ↑ Gross et al. (2005), p. xxxiii.
  34. ↑ 34.0 34.1 Australian Building Codes Board (2020a), Data Sheet C6, p. 179.
  35. ↑ Manzello et al. (2010), as cited in Australian Building Codes Board (2020a), Data Sheet C6, p. 180.
  36. ↑ Australian Building Codes Board (2020a), Data Sheet C6, p. 180.
  37. ↑ Iqbal and Salley (2004), p. 17-5.
  38. ↑ Calculated from Milke (2002a), p. 4-211, and Australian Building Codes Board (2020a), Data Sheet C6, p. 175.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Fleischmann, C., & Buchanan, A. (2002). Analytical methods for determining fire resistance of concrete 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. 4-239–4-256). National Fire Protection Association; Society of Fire Protection Engineers.
  • 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.
  • 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.
  • 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.).
  • 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.
Retrieved from "https://fire-safety.academy/w/index.php?title=Fire_resistance_criteria&oldid=2569"