About Fire Resistance Rating Calculator
The fire resistance rating calculator assesses how long concrete, steel, and timber structural members survive a standard fire and reports an REI classification per EN 13501-2. It is used by fire and structural engineers to verify load-bearing (R), integrity (E), and insulation (I) performance against required fire periods.
Choose a member type and material, enter its geometry, exposure, and utilization, and pick a fire curve and target period. The tool computes the gas and member temperatures, the relevant material reduction factors, the residual section, and the achieved REI rating, all updating in real time.
How It Works
- Generate the gas temperature from the selected fire curve (ISO 834, hydrocarbon, external, or ASTM E119) over the target period.
- For concrete, find the 500 C isotherm depth and reduction factors kc and ks; for steel, find the critical temperature and time to reach it; for timber, find the charring depth and residual cross-section.
- Reduce the section and material strengths at the elevated temperature and check the retained capacity against the utilization factor.
- Assign the R, E, and I criteria and report the overall REI rating in minutes against the target period.
Worked Example
A simply-supported concrete slab 200 mm thick with a 40 mm axis distance under the ISO 834 fire. EN 1992-1-2 Table 5.8 gives a tabulated rating of 120 minutes for this thickness and axis distance, so the slab achieves REI 120. The ISO 834 gas temperature at 120 min is 345*log10(8*120 + 1) + 20 = 345*log10(961) + 20 = 1049.5 C.
Formulas
- ISO 834 standard fire curve
T = 345 * log10(8 * t + 1) + 20- ASTM E119 fire curve
T = 750 * (1 - exp(-3.79553 * sqrt(t/60))) + 170.41 * sqrt(t/60) + 20- Steel critical temperature (EN 1993-1-2)
theta_cr = 39.19 * log(1 / (0.9674 * mu^3.833) - 1) + 482- Timber charring depth (EN 1995-1-2)
d_char = beta_n * t- Concrete 500 C isotherm depth
d_500 = 0.5 * sqrt(t) * 25
Standards & References
- EN 13501-2
- EN 1992-1-2
- EN 1993-1-2
- EN 1995-1-2
- ISO 834
- ASTM E119
Frequently Asked Questions
What does the REI classification mean?
REI is the EN 13501-2 fire classification where R is load-bearing capacity, E is integrity (no passage of flames or hot gases), and I is insulation (limiting the temperature rise on the unexposed face). The number is the period in minutes for which each criterion is satisfied.
Which fire curves does the calculator support?
It supports the ISO 834 standard curve, the hydrocarbon curve, the external fire curve, and the ASTM E119 curve. Each defines the gas temperature as a function of time and drives the member heating calculation.
How is steel fire resistance assessed?
The tool computes the critical temperature from the utilization factor using EN 1993-1-2, then time-steps the section temperature (using the section factor Am/V and any protection) to find the time to reach that critical temperature, which gives the R rating.
How is timber charring handled?
Timber uses the reduced cross-section method of EN 1995-1-2: charring depth is the notional charring rate times time, and a 7 mm zero-strength layer is added before checking the residual section capacity.