Skip to content
Upcoming courses

Steel in fire

Tools
From Fire Safety Academy Wiki
Revision as of 08:07, 2 November 2026 by Sanne (talk | contribs) (Add diagrams fse-steel-in-fire-v1.svg, fse-steel-in-fire-v2.svg, fse-steel-in-fire-v3.svg (level-1 visual plan))
Contents
  1. 1Heating and section factor
  2. 2Strength and critical temperature
  3. 3Thermal expansion and restraint
  4. 4Intumescent and passive protection
  5. 5Structural fire engineering
  6. 6Fire resistance and performance criteria
  7. 7Limits and common misunderstandings
  8. 8Assessment and review
  9. 9See also
  10. 10Notes
  11. 11References

Steel in fire describes how structural steel members heat, expand and lose strength and stiffness during fire exposure. Although steel is noncombustible, its mechanical properties and structural performance can deteriorate substantially. The outcome depends on fire exposure, member shape, protection, loading, connections and end restraint.[1][2]

Steel conducts heat efficiently, so exposed regions can warm quickly and transfer energy to adjacent parts and connections. Protection can delay this heating, while member geometry and loading determine how the rising temperature affects stress, buckling and deformation. Structural response must therefore be evaluated rather than inferred from noncombustibility alone.[1][3]

Assessment follows three linked stages: characterise the fire exposure, calculate heat transfer into the member and evaluate structural response at elevated temperatures. Each stage has its own data and uncertainty.[1][4]

Heating and section factor

Steel temperature results from the balance between incident heat, thermal storage and losses. Exposed surface area receives radiation and convection, while conduction redistributes energy through the section and connected members. The actual fire and boundary conditions must be defined before a meaningful steel temperature can be calculated.[1][5]

Section factor relates the heated perimeter or exposed area of a steel section to its volume or mass. A slender, highly exposed section can respond differently from a compact section with more steel mass. Protection changes the exposed boundary and should be represented in the heat-transfer model.[6][4]

Two steel sections with different exposed perimeters and masses receiving the same nominal heat flux.
Section geometry affects steel heating. Based on Milke (2002), pp. 4-219–4-221.

Insulation reduces heat transfer to the steel and delays the time to a critical condition. Its thickness, conductivity, continuity and attachment determine performance. Joints, penetrations and damaged regions can create local thermal bridges that an average model would miss.[1][7]

The heat-exposure history matters as well as the peak fire temperature. A short severe exposure can produce different internal gradients from a longer moderate one. Conductive storage means steel can continue heating after the fire environment begins to cool.[5][8]

Strength and critical temperature

Strength reduction at elevated temperatures affects yield strength, tensile strength and modulus of elasticity. Fleischmann and Buchanan show different strength-temperature curves for hot-rolled, cold-drawn prestressing and high-strength alloy steels. The steel type and manufacturing history therefore matter when selecting properties.[9]

Reduced stiffness can increase deflection even before strength becomes inadequate. Structural analysis should evaluate both stress and deformation at elevated temperature, using properties appropriate to the expected range. Ambient-temperature values alone can be misleading.[1][2]

Critical steel temperature is a selected temperature at which structural performance becomes unacceptable under the stated loading and criteria. It is not a universal physical constant. Different loads, steel types, end conditions and failure definitions can justify different critical temperatures.[10][1]

North American practice described by Kodur and Harmathy uses 538 °C for structural steel in some ASTM E119-related calculations, typically for columns under full design load. Reinforcing and prestressing steel have different values in their review, including 593 °C and 426 °C respectively. These examples must not replace a project-specific structural assessment.[10]

Schematic strength curves for hot-rolled, prestressing and alloy steel that fall at different temperatures and cross.
Steel products respond differently to elevated temperature. Based on Fleischmann and Buchanan (2002), p. 4-241.

Milke similarly notes that temperature endpoints are often based on ASTM E119, but their inaccuracies depend on temperature-dependent properties and heating descriptions. A calculation can use a selected critical temperature, yet still require structural analysis of the actual member and loading.[1]

Thermal expansion and restraint

Steel and surrounding materials expand when heated. Restrained thermal expansion occurs when connections or adjacent construction prevent free deformation, producing additional forces and changing buckling behaviour. Restraint can therefore alter both the member's stress state and its stability.[2][11]

ASTM E119 distinguishes restrained and unrestrained construction systems according to geometry, connections and framing. Welded, riveted or bolted steel framing is listed as restrained in the method's examples, while other systems are classified differently. These categories belong to the test arrangement rather than to a generic label for every structure.[12]

Thermal expansion is not always detrimental. Fleischmann and Buchanan note that it can improve fire resistance in some concrete and composite arrangements by changing the structural response. Its effect depends on the system and cannot be assessed from expansion alone.[2]

Connections and adjacent members also conduct heat and provide restraint. A member-by-member temperature calculation may therefore be insufficient if the whole frame governs stability. The structural model should reflect the actual load paths and boundary conditions.[1][4]

Intumescent and passive protection

Intumescent material forms an insulating layer during heating, reducing heat transfer to the steel. Its effectiveness depends on expansion, continuity, adhesion and thermal properties. Damage, deformation or incomplete coverage can compromise that protection.[13][1]

Boards, blankets, concrete encasement and other passive systems provide alternative protection. They delay heating and can preserve structural capacity for longer, but their installation and attachment are part of performance. A rating should be tied to the complete protected assembly.[1][7]

Critical-temperature examples 538, 593 and 426 °C for three steel uses.
Critical-temperature examples depend on steel use and method. Based on Kodur and Harmathy (2002), p. 1-163.

Protective systems can also affect structural behaviour beyond temperature. Added weight, changed stiffness and altered connections can influence response, while moisture and ageing can change the protection itself. These effects belong in assessment where relevant.[14][15]

Passive protection does not remove the need to evaluate the unprotected details. Connections, penetrations and partially exposed members can govern heating and stability. A system-level review should include those local exposures.[1][4]

Structural fire engineering

Structural fire engineering links fire dynamics, heat transfer and structural mechanics. The fire model supplies gas temperatures and heat flux, the thermal model supplies member temperatures, and the structural model evaluates stress, deflection and stability. Errors in any stage affect the final conclusion.[4][1]

OZone illustrates this sequence for simple steel elements. It combines compartment-fire models with localised-fire and steel-temperature calculations, then evaluates fire resistance using Eurocode 3 methods. The authors emphasise the value of connecting fire and structural specialisms rather than using one generic critical temperature for every case.[4]

Empirical correlations can be useful within their tested exposure, loading and span. Outside those conditions, heat-transfer and structural analyses are needed. Testing still validates the models and the interactions between steel, insulation and other components.[1][11]

Model uncertainty includes the fire curve, convective and radiative boundary conditions, material properties and end restraints. Sensitivity analysis should include these rather than only varying steel temperature. A precise thermal result does not guarantee an accurate structural prediction.[1][16]

Fire resistance and performance criteria

Steel fire resistance concerns maintaining structural adequacy for the required time. Deflection, strain, stress and stability are compared with established limits, and loading and restraint must match the application. A steel temperature is only an input to that evaluation.[1][11]

ASTM E119 provides one standard exposure and endpoint framework, including loading and restraint variations. Its furnace dimensions and setup can make results comparative rather than exact reproductions of full structures. Any extension to actual construction should be documented.[11]

A noncombustible member can still fail through buckling, excessive deflection or connection failure. Noncombustibility therefore cannot be used as evidence of fire resistance. The complete assembly and its load paths matter.[17][11]

Post-fire assessment may also need the cooling history. Steel and surrounding materials can remain hot after flames decline, and mechanical properties and connections may have changed. Residual condition should not be inferred from appearance alone.[5][2]

Limits and common misunderstandings

Steel does not fail at one universal temperature. Critical temperature depends on the member, steel type, loading, restraint and performance criterion. Values such as 538 °C belong to specific methods and should not be applied indiscriminately.[10][1]

A thicker or stronger member at ambient temperature is not automatically better in every fire. Section geometry, exposed area, protection and load paths affect heating and stability. Ambient and fire conditions must be evaluated together.[6][4]

An intumescent coating does not guarantee protection for every assembly. Adhesion, continuity and exposure determine performance, and local defects can dominate. Full assembly evidence is needed for critical applications.[13][15]

Finally, steel's noncombustibility does not mean fire has no effect. Heat can change strength, stiffness, expansion and stability without any combustion of the steel itself. These are the central issues of steel in fire.[1][2]

Assessment and review

A steel-temperature calculation should identify the heated perimeter, exposed faces and any insulation or shielding. Those features determine how much energy enters the section and how quickly its temperature rises. Using only the section's ambient dimensions would omit the actual thermal boundary.[6][4]

Temperature should also be evaluated through the section rather than only at the surface. Conduction redistributes energy and produces gradients, especially during changing exposure. A surface or average temperature can be useful, but it does not automatically describe every critical point or connection.[5][1]

Property selection should match the steel product. Hot-rolled, cold-drawn prestressing and high-strength alloy steels can have different strength and stiffness curves. Using a generic steel curve for all of them would misrepresent their elevated-temperature response.[9][2]

Critical-temperature criteria should be tied to the actual failure mode. A strength-based limit, a deflection limit and a connection or buckling limit can produce different acceptable temperatures. A single value cannot cover all of these without a structural justification.[10][1]

Restraint should be evaluated at the level of the whole framing system. Connections and adjacent members can restrict expansion and introduce new forces, but they also provide alternative load paths. A member-only model can miss both the additional restraint and the redistribution that follows local weakening.[2][4]

Protection review should include continuity and local details. Joints, penetrations, fasteners and damaged coatings can create thermal bridges even when most of the member is protected. Those details can govern the time to a critical condition and should be represented in the model.[1][15]

Model comparison should use matching exposure and endpoint definitions. A furnace curve, a natural compartment fire and a localised exposure can produce different steel temperatures, and a critical-temperature criterion does not capture every structural failure. Validation therefore requires the complete thermal and structural chain.[8][4]

A final sensitivity review should vary fire exposure, section factor, protection, material properties and restraint. This reveals whether the result is controlled by one uncertain assumption and whether additional full-scale evidence is needed. It also prevents a precise steel temperature from being mistaken for a complete fire-resistance conclusion.[16][1]

The assessment should also distinguish thermal and mechanical limits. A section can reach a selected critical temperature without immediate collapse, or fail earlier through buckling, connection damage or restraint forces. The acceptable condition must be defined structurally rather than inferred from temperature alone.[10][1]

For protected members, the protection's own condition should be evaluated. Intumescent layers and membranes can crack, detach or change conductivity, altering the thermal path during exposure. Those changes can create earlier local heating than an intact-layer model predicts.[13][15]

For structural systems, load redistribution can preserve some capacity after local weakening, but it can also increase demand on connections and adjacent members. Progressive and tensile mechanisms therefore require system-level analysis rather than a simple critical-temperature check for one section.[2][4]

Post-fire assessment should use the cooling and damage history as well as the original member data. Connections, coatings and material properties may have changed, and residual heat can persist. Appearance and noncombustibility alone cannot establish residual structural capacity.[5][2]

A complete report therefore links the fire exposure, thermal boundary, temperature-dependent properties and structural criteria. It identifies where those inputs are uncertain and whether additional assembly evidence is needed. This keeps steel in fire as a structural assessment rather than reducing it to a material-temperature rule.[1][16]

The exposure should also include the regions above, below and beside the member. A hot upper layer can radiate downwards, while a localised flame can produce a very different boundary condition on one face. Treating every side as uniformly exposed would miss those directional effects.[18][4]

A final check is whether the selected model represents the protection and restraint as they actually exist. Idealised insulation, simple supports and a generic fire curve can make the calculation easier, but they can also change the controlling failure mode. Those simplifications should be tested before the steel temperature is used for a decision.[1][11]

The report should also state whether the steel temperature was measured, calculated or adopted as a criterion. Those are different kinds of evidence, and each needs its own justification. A criterion can be useful without claiming that the member actually reached it in a test or in service.[1][10]

For final acceptance, the complete load and thermal history should be compared with the member's required function. Steel's noncombustibility does not answer that comparison, and a critical-temperature value alone does not complete it. The result must be expressed in terms of structural performance and exposure time.[1][11]

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 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 Milke (2002), p. 4-209.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Fleischmann and Buchanan (2002), p. 4-242.
  3. ↑ Rockett and Milke (2002), p. 1-28.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 Cadorin et al. (2003), p. 430.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Rockett and Milke (2002), p. 1-29.
  6. ↑ 6.0 6.1 6.2 Milke (2002), pp. 4-219–4-221.
  7. ↑ 7.0 7.1 White (2002), pp. 4-259–4-260.
  8. ↑ 8.0 8.1 Lie (2002), pp. 4-201–4-203.
  9. ↑ 9.0 9.1 Fleischmann and Buchanan (2002), p. 4-241.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 Kodur and Harmathy (2002), p. 1-163.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 Milke (2002), pp. 4-209–4-210.
  12. ↑ Milke (2002), p. 4-211.
  13. ↑ 13.0 13.1 13.2 Beyler and Hirschler (2002), p. 1-110.
  14. ↑ Kodur and Harmathy (2002), p. 1-156.
  15. ↑ 15.0 15.1 15.2 15.3 White (2002), p. 4-260.
  16. ↑ 16.0 16.1 16.2 Van Rossum (2027), §1.7.
  17. ↑ Kodur and Harmathy (2002), p. 1-155.
  18. ↑ Van Rossum (2027), §5.4.

References

  • Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • 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.
  • 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.). 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.). National Fire Protection Association.
  • Lie, T. T. (2002). Fire temperature-time relations. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). 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.). National Fire Protection Association.
  • Rockett, J. A., & Milke, J. A. (2002). Conduction of heat in solids. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • 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.). National Fire Protection Association.
Retrieved from "https://fire-safety.academy/w/index.php?title=Steel_in_fire&oldid=943"