The R, E and I criteria
The fire performance of a member is expressed by criteria followed by the duration, in minutes, for which they are met:
- R is the load-bearing capacity criterion: the member keeps carrying its loads.
- E is the integrity criterion: the member does not let flames or hot gases through (walls, floors).
- I is the insulation criterion: the unexposed face does not heat beyond the limit (walls, floors).
Steel columns and beams are load-bearing, so the R criterion is what matters. "R 90" means the member keeps its load-bearing capacity for at least 90 minutes under the standard fire.
The ISO 834 standard curve
R classes refer to the nominal standard temperature-time curve in EN 1991-1-2, cl. 3.2.1:
| Class | Duration | θg at the end [°C] |
|---|---|---|
| R 15 | 15 min | 739 |
| R 30 | 30 min | 842 |
| R 60 | 60 min | 945 |
| R 90 | 90 min | 1006 |
| R 120 | 120 min | 1049 |
For the standard curve, the convective heat transfer coefficient is αc = 25 W/m²K.
Other nominal curves
- The hydrocarbon curve (petroleum fires) uses αc = 50 W/m²K: θg = 20 + 1080 · (1 − 0.325·e−0.167t − 0.675·e−2.5t).
- The external fire curve (members outside the compartment) uses αc = 25 W/m²K: θg = 20 + 660 · (1 − 0.687·e−0.32t − 0.313·e−3.8t).
How a steel member is checked
- The required class is set (for example R 60) by the fire safety strategy, according to the regulations in force.
- The critical temperature of the member is calculated from the actions in the fire situation.
- The steel heating under the fire curve is calculated, depending on the section factor and protection.
- The time for the steel to reach the critical temperature is the fire resistance time. The member passes if it is at least equal to the duration of the required class.
Unprotected sections usually reach R 15-R 30. Higher classes need fire protection (intumescent paint, boards, sprayed coatings).
Informative material summarising the provisions of EN 1991-1-2 and EN 1993-1-2. It does not replace the text of the standards, the fire safety regulations or verification by a qualified engineer.