Fire resistance

Fire resistance is the ability of a building member or assembly to maintain specified functions for a stated time under a defined fire exposure. Depending on the assembly, these functions include carrying loads, preventing the passage of flames and limiting heat transmission. A rating records performance against particular criteria, rather than a general resistance to every possible fire.

Assessment concerns the complete construction and its conditions. Materials, dimensions, protection, loading, connections and restraint can all affect the result. A standard furnace test provides one means of evaluation, while suitable calculations can connect the exposure to temperatures and structural response. Both approaches require a defined endpoint.

The duration in a rating is therefore inseparable from the method used to establish it. It is not automatically the time available for evacuation, the time to collapse in a natural fire or a measure of combustibility. Understanding the exposure and performance criteria is necessary to interpret what the rating demonstrates.

The functions being assessed

Fire resistance criteria identify the functions that an assembly must maintain. Structural adequacy concerns carrying the specified load without unacceptable structural response. Integrity concerns the passage of flames or hot gases through a separating assembly. Insulation concerns heat transmission to the unexposed side. These functions can reach their endpoints at different times.

A separating wall illustrates the difference between integrity and insulation. Heat can conduct through an intact wall and raise the temperature of its unexposed surface. No visible opening is needed for that heat transfer to become excessive under an insulation criterion. Conversely, a crack can permit hot gases or flames to pass even where most of the wall remains comparatively cool.

Structural adequacy is a further, separate question. A load-bearing member can lose strength or stiffness as its temperature rises, and its response also depends on applied loads and the conditions at its supports. A thermal result therefore cannot establish structural endurance unless it is connected to a suitable structural criterion or model.

New Zealand's C/VM2, second edition of 2025, expresses the fire resistance rating through three times in minutes, and the stated order is structural adequacy, integrity and insulation. That notation makes the separate functions explicit. Other test and classification systems require their own interpretation rather than an assumed transfer of symbols or values.

Fire resistance has three distinct performance criteria. Based on Ministry of Business, Innovation and Employment (2025), p. 64.

How a furnace test establishes a rating

A Fire resistance test exposes a defined specimen to a prescribed heating history while monitoring the required endpoints. Milke describes this approach through the United States ASTM E119 framework in the 2002 handbook, and a successful specimen receives a rating for the stated test duration and conditions. The rating belongs to the tested assembly and its permitted applications.

The specimen can include several materials and components. Protection around a steel member, for example, changes the heat entering the steel and its temperature history, and the test observes the interaction between the protection and the member. This is one reason separate material properties do not always reproduce the behaviour of an assembled construction.

Loads and restraint form part of the test arrangement. Restraint describes the extent to which surrounding construction prevents thermal movement. Milke explains that framing geometry and connections affect the distinction between restrained and unrestrained test systems. A result obtained under one arrangement cannot automatically represent another arrangement with different movement conditions.

The monitored endpoints also depend on the assembly. They can include maintaining the applied load, limiting temperatures and preventing the passage of flames or gases capable of igniting a specified target. Some historical ASTM procedures include a hose-stream exposure. These observations have distinct purposes, so a report identifies which endpoints and procedures apply to its specimen.

Fire-resistance testing links exposure to assembly endpoints. Based on Milke (2002), pp. 4-209–4-210.

An unlisted assembly and a failed assembly are not the same evidential category. Milke explains that an assembly without a listing has no established rating unless acceptable performance is demonstrated. That demonstration may involve a standard test, a special experiment or an analytical method. Absence of a rating therefore concerns the available evidence, rather than an observed failure time.

Reading temperature criteria correctly

A temperature rise is a difference from the starting temperature, so it is not the same quantity as an absolute temperature. Confusing them changes the interpretation of an insulation criterion, even when both are printed using degrees Celsius. The initial condition therefore belongs with the stated limit and measurement location.

Milke's historical ASTM E119 table gives wall-insulation examples of a 139 °C average rise and a 181 °C single-point rise. These values apply to measurements on the unexposed side of the stated wall or partition assemblies; they are not limiting furnace temperatures. The table also contains other endpoints for steel members, which require their own interpretation.

ASTM E119 wall-insulation endpoint examples (temperature rise). Based on Milke (2002), p. 4-211.

An average and a single-point limit address different aspects of the temperature field: the average represents the group of measured locations, while the single-point value constrains a local high temperature. Keeping both avoids replacing the endpoint definition with one convenient measurement. Neither value alone describes the complete fire exposure or the structural condition of the specimen.

A structural temperature criterion is different again. Kodur and Harmathy explain that a critical temperature corresponds to the material becoming unable to support the applied load under the adopted conditions. It consequently depends on loading and material behaviour. A temperature used in a simplified test-based method is not a universal temperature at which every member collapses.

Standard exposure and a worked example

A Standard fire curve prescribes temperature as a function of time. It provides a repeatable exposure for comparing assemblies. It does not directly describe the heat release rate of a burning object or the temperature in every real room. Van Rossum distinguishes these quantities because they serve different purposes in a fire analysis.

Milke presents the following approximation for the ISO 834 furnace-temperature history, and the logarithm describes the continuing rise in prescribed temperature, while the initial temperature is added separately. In this expression, time is entered in seconds. The coefficient 0.133 carries reciprocal-second units, making the quantity inside the logarithm dimensionless.

$$T_f=T_0+345{\log }_{10}(0.133t+1)$$

Symbol	Description	Unit
$T_f$	Prescribed furnace temperature	°C
$T_0$	Initial temperature	°C
$t$	Elapsed heating time	s

Using an initial temperature of 20 °C gives approximately 841 °C at 1,800 s and 945 °C at 3,600 s. These calculations use the rounded coefficient printed by Milke, and they illustrate the prescribed thermal environment at 30 min and 60 min. They do not predict the temperature of the specimen, which requires a separate heat-transfer calculation.

The equation contains no material thickness, conductivity, loading or support condition, so it cannot calculate whether a particular wall or beam survives either exposure duration. Two assemblies in the same furnace can develop different internal temperatures and structural responses. The standard curve supplies one input to the assessment, not its final result.

Material and assembly behaviour

Materials perform different functions within an assembly. Steel provides load-bearing capacity but changes strength and stiffness when heated. Concrete can carry load while delaying heat transfer to reinforcement. Insulating materials can protect a structural component without carrying significant load themselves. The complete assembly combines these roles under the imposed exposure.

Combustible construction can also retain structural function for a specified period. White describes timber endurance through protective membranes, the extent of charring and the capacity of the remaining uncharred wood. Char itself has very little load-bearing capacity. The existence of combustion therefore does not, by itself, establish the time at which the member loses its required function.

Conversely, noncombustibility does not establish fire resistance: a material can remain unburned while its mechanical properties deteriorate or its unexposed surface becomes too hot. This is the distinction from Reaction to fire, which concerns the material's contribution to fire development. Different tests are needed because the two assessments follow different physical outcomes.

Changes to an assembly can affect several outcomes together. White explains that changing membranes, member spacing or insulation can fall outside the tested timber arrangement. Adding insulation does not produce an automatic improvement in every floor or roof assembly. The relevant evidence is the behaviour of the revised construction, including how its layers remain supported during heating.

Heat transfer through protection can involve physical and chemical changes as well as ordinary conduction. Van Rossum describes moisture evaporation, release of chemically bound water and swelling as processes that change thermal response. These processes can consume energy or alter the heat-conduction path. An assembly calculation using fixed room-temperature properties may therefore miss part of the protective behaviour.

The relevant time is also a property of the complete sequence. The furnace heats the surface, energy moves through the construction, and the specified endpoint is reached at a measurement location or structural state. A material's conductivity describes only one part of this sequence. It cannot by itself supply a duration without geometry, boundary conditions and the selected endpoint.

Fire resistance within compartmentation

Fire compartmentation uses separating construction to restrict the spread of fire between spaces, and the wall or floor is only part of that separation. Existing openings, closures and penetrations can provide other routes for spread. The 2005 International Fire Engineering Guidelines therefore consider spread through openings separately from failure of the barrier material.

The guidelines distinguish conduction, loss of integrity and loss of stability as different barrier-failure mechanisms. Conduction can heat nearby combustible material without opening the barrier. Loss of integrity includes cracks and failure of firestopping, while loss of stability involves collapse. A fire-resistance rating must be interpreted alongside the boundary details relevant to those mechanisms.

Door use also changes the separation during evacuation, and the same guidelines treat the time of opening a door as information supplied by the occupant-movement analysis. A closed-door test does not describe the period when the door is open. Compartmentation performance consequently depends on the states of its openings as well as the endurance of closed components.

Passive fire protection includes the barriers and protective materials that support these functions. Their performance is expressed through particular constructions and conditions, rather than a general promise that fire remains contained. This connection explains why rated construction and its installation details belong to the same design. A rating on one component cannot close an unprotected route around it.

Calculation methods and validation

Structural fire engineering connects fire exposure, heat transfer and structural response. The first stage defines the thermal environment. The second calculates temperatures within the member, using material properties and boundary conditions. The third evaluates the consequences for stresses, deformation and load-carrying capacity. A calculation of one stage does not independently establish the others.

Empirical correlations provide a simpler route where sufficient test data exist. Milke explains that such correlations are limited by the exposure, loading and span represented in their underlying tests. Their numerical simplicity does not remove those limits. Applying a correlation outside its supporting range introduces a different problem from interpolating within the tested conditions.

Thermal models can represent either standard exposure or a specified nonstandard fire, and their output may be the time to a chosen temperature or a temperature distribution for structural analysis. Temperature-dependent properties and the description of heating both affect accuracy. A model with detailed geometry can still produce a misleading result if those physical inputs are unsuitable.

Testing remains relevant even where calculation is available, and it supplies material behaviour, checks assembly interactions and provides evidence for judging model predictions. New Zealand's 2025 C/VM2 definition explicitly connects its calculation route to verification against standard fire-resistance tests. That connection identifies the evidence supporting a rating rather than treating calculation as evidence by itself.

Limits when applying a rating

A furnace test is a controlled representation with practical size and support constraints. Milke notes that these constraints make some results comparative rather than exact reproductions of entire structures. An extended continuous frame can behave differently from an isolated specimen. The tested configuration and the proposed application therefore remain part of the interpretation.

A real compartment fire can grow, reach a maximum and decay. Its temperature history depends on the fuel, ventilation and enclosure, while the standard curve serves a repeatable testing purpose. A stated rating is therefore not a direct forecast of survival time in every natural fire. Connecting the two requires a method that represents the relevant exposure and structural conditions.

The same rating also does not establish the time available for occupants to escape. The International Fire Engineering Guidelines treat barrier performance, fire spread and other subsystem results as interacting parts of an analysis. Smoke and opening conditions can affect adjacent spaces before a closed assembly reaches its furnace-test endpoint. Escape and containment consequently require related but distinct assessments.

See also

• Concrete in fire
• Timber in fire
• Critical steel temperature
• Time equivalence

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines. Australian Building Codes Board.
• Fleischmann, C., & Buchanan, A. (2002). Analytical methods for determining fire resistance of concrete members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-239–4-256). National Fire Protection Association.
• Janssens, M. (2002). Calorimetry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-38–3-62). National Fire Protection Association.
• Kodur, V. K. R., & Harmathy, T. Z. (2002). Properties of building materials. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-155–1-181). National Fire Protection Association.
• Milke, J. A. (2002). Analytical methods for determining fire resistance of steel members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-209–4-238). National Fire Protection Association.
• Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification method C/VM2 (2nd ed.).
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
• White, R. H. (2002). Analytical methods for determining fire resistance of timber members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-257–4-273). National Fire Protection Association.