Steel Fire Resistance Check
Unprotected steel heats quickly because it has little mass for its surface. EN 1993-1-2 gives the critical temperature from the utilisation and the heating rate from the section factor; the intersection is the time to failure in a standard fire.
| Minutes | Steel (°C) | State |
|---|---|---|
| 15 | 42 | Below |
| 30 | 83 | Below |
| 45 | 133 | Below |
| 60 | 190 | Below |
| 90 | — | Below |
| 120 | — | Below |
A coated member needs the insulation thickness, not this curve.
θcr = 39.19·ln(1/(0.9674·μ₀^3.833) − 1) + 482 · EN 1993-1-2 §4.2.4.
Δθ = (Am/V)/(ca·ρa)·h·Δt · Unprotected steel, §4.2.5.2.
θg = 20 + 345·log₁₀(8t + 1) · ISO 834 standard fire, t in minutes.
A clear path from inputs to results.
Unprotected steel heats quickly because it has little mass for its surface. EN 1993-1-2 gives the critical temperature from the utilisation and the heating rate from the section factor; the intersection is the time to failure in a standard fire.
How to use
- Set section factor am/v (m⁻¹).
- Set utilisation μ₀ at fire limit state.
- Set required fire resistance (min).
- Set resultant emissivity ε.
- Read the headline metrics, then check the table for the detail.
The formula
Reference: EN 1993-1-2 §4.2.4 (critical temperature) and §4.2.5.2 (unprotected heating).
Worked example
At the default inputs
- Section factor Am/V (m⁻¹): 180; Utilisation μ₀ at fire limit state: 0.5; Required fire resistance (min): 60; Resultant emissivity ε: 0.7
- 1
Critical temperature → 585 °C Heat the section → — min
Critical temperature: 585 °C · Time to critical: > duration · Steel at duration: 190 °C
Interpreting the result
At 50% utilisation the critical temperature is 585 °C; an unprotected 180 m⁻¹ section reaches it in more than the fire duration, so the 60-minute requirement is met.
Assumptions & limitations
- Unprotected steel; no intumescent coating or boarding.
- ISO 834 standard fire.
- Uniform temperature through the section.
Frequently asked questions
Why does a higher Am/V fail sooner?
More surface per unit mass means faster heating.
Does coating change the answer?
Yes; the insulation thickness is a separate calculation.
Methodology last reviewed 2026-10-23. Results are informational and not financial or engineering advice — see our disclaimer.