EN 1993 Fire Rating — Fire Resistance per Eurocode 3 Part 1-2 Guide
Complete guide to steel fire resistance design per EN 1993-1-2:2005. Critical temperature method, section factor A_m/V, fire protection materials (intumescent coating, board protection, spray-applied), load level in fire eta_fi, tabulated fire resistance ratings R30/R60/R90/R120. Worked example for a protected steel beam achieving R60 fire rating.
Quick access: European Steel Properties | Beam Design | All European References
Fire Resistance Ratings (EN 13501-2)
| Rating | Fire Duration (minutes) | Typical Application |
|---|---|---|
| R15 | 15 | Low-rise, small buildings |
| R30 | 30 | 2-3 storey buildings, sprinklered |
| R60 | 60 | Standard for mid-rise (4-8 storeys) |
| R90 | 90 | High-rise (8-15 storeys) |
| R120 | 120 | High-rise (15+ storeys), critical infrastructure |
| R180 | 180 | Very tall buildings, tunnels |
| R240 | 240 | Extreme hazard (chemical plants, tunnels) |
R = load-bearing capacity (Resistance) in minutes.
Critical Temperature Method (EN 1993-1-2 Cl. 4.2.4)
The design resistance of a steel member in fire is:
N_fi,t,Rd = k_y,theta x N_Rd / gamma_M,fi
Where:
- k_y,theta = reduction factor for yield strength at temperature theta
- gamma_M,fi = 1.00 (partial factor for fire)
Steel Strength Reduction at Elevated Temperature (EN 1993-1-2 Table 3.1)
| Temperature (C) | k_y,theta | k_E,theta |
|---|---|---|
| 20 | 1.000 | 1.000 |
| 100 | 1.000 | 1.000 |
| 200 | 1.000 | 0.900 |
| 300 | 1.000 | 0.800 |
| 400 | 1.000 | 0.700 |
| 500 | 0.780 | 0.600 |
| 550 | 0.630 | 0.540 |
| 600 | 0.470 | 0.490 |
| 650 | 0.330 | 0.430 |
| 700 | 0.230 | 0.380 |
| 800 | 0.110 | 0.270 |
| 900 | 0.060 | 0.170 |
| 1000 | 0.040 | 0.090 |
Critical Temperature for Load Level
| Load Level eta_fi | Critical Temperature (C) |
|---|---|
| 0.20 | 745 |
| 0.30 | 680 |
| 0.40 | 635 |
| 0.50 | 595 |
| 0.55 | 575 |
| 0.60 | 555 |
| 0.70 | 515 |
Section Factor A_m/V (EN 1993-1-2 Cl. 4.2.5)
A_m/V = exposed surface area / steel volume (m^-1)
| Section Type | A_m/V Range (m^-1) | Heating Rate |
|---|---|---|
| Heavy UC (HEB 300+) | 50-80 | Slow |
| Medium UC/UB (HEB 200, IPE 330) | 80-150 | Moderate |
| Light sections (IPE 200) | 150-220 | Fast |
| CHS / RHS (small) | 200-280 | Fast |
| Lattice angles | 250-350 | Very fast |
Worked Example — IPE 330 Beam, R60 Fire Rating
Beam: IPE 330, S355, simply supported, 6.0 m span, eta_fi = 0.55
A_m/V = 2 x 0.330 / 0.006260 = 105 m^-1 Critical temperature: T_cr = 575 C (from Table 3.1)
For A_m/V = 105 m^-1 and R60: intumescent coating at 1.0 mm DFT.
| Protection Type | Thickness for R60 | Cost |
|---|---|---|
| Intumescent (thin film) | 0.8-1.2 mm | 40-60/m2 |
| Board (Promatect) | 15-25 mm | 30-50/m2 |
| Spray (vermiculite) | 12-20 mm | 20-35/m2 |
Frequently Asked Questions
What is the critical temperature method in EN 1993-1-2?
The critical temperature method compares the steel temperature under the standard fire to the temperature at which the member load-bearing capacity equals the applied load. For typical load levels (eta_fi = 0.5-0.6), critical temperatures range from 550 to 600 C. If the unprotected steel temperature rise exceeds this, fire protection is required.
What is the section factor A_m/V and why does it matter?
A_m/V is the ratio of heated surface area to steel volume. A high section factor means a slender section that heats up quickly (CHS 48.3x4: ~280 m^-1) vs a heavy section that heats slowly (HEB 300: ~60 m^-1). Protection thickness requirements are directly related to A_m/V.
Related Pages
Educational reference only. Fire design per EN 1993-1-2:2005. Verify protection thicknesses with manufacturer data. Results are PRELIMINARY - NOT FOR CONSTRUCTION without independent verification.