Fire severity

Time equivalence: equivalent fire severity of a compartment

The time of exposure to the standard fire test that has the same effect as a complete burnout of the compartment, from its fire load, ventilation and linings (EN 1991-1-2 Annex F, as used in the International Fire Engineering Guidelines). Compare it with the proposed fire resistance level.

Inputs

Fire load

Occupancy (IFEG 2005 Table 3.4.1b)

Percentile

Fire load density e_f

MJ/m²

Compartment

Floor area A_f

m²

Height H

Vertical openings (windows, doors) A_v

m²

Roof openings A_h

m²

Linings and member

k_b values from

Thermal inertia of the linings, b = √(ρcλ)

Conversion factor k_b

min m²/MJ

Material correction k_c

Proposed FRL (optional)

min

The values shown are an example. Where the linings are not assessed, EN 1991-1-2 allows k_b = 0.07. k_c = 1.0 for protected steel, reinforced concrete and timber.

Result

Equivalent fire severity t_e

–min

Ventilation factor w_f

–

Opening ratio α_v

–

For your report

A_f (m²)	H (m)	A_v (m²)	e_f (MJ/m²)	k_b	w_f	t_e (min)	FRL (min)	Result

Working

Step	Symbol	Expression	Value	Unit

Method

t_e = e_f k_b w_f k_c, with the ventilation factor w_f = (6/H)^0.3 [0.62 + 90 (0.4 − α_v)⁴ / (1 + b_v α_h)] ≥ 0.5. Here α_v = A_v/A_f, α_h = A_h/A_f and b_v = 12.5 (1 + 10α_v − α_v²) ≥ 10 (EN 1991-1-2 Annex F; IFEG 2005 section 2.6.1.3).

The formula applies for α_v from 0.025 to 0.25 (EN 1991-1-2), and IFEG 2005 gives 0.05 to 0.25. IFEG also limits α_h to 0.20. The calculator warns outside these ranges. The value for the report is rounded to the nearest minute. The comparison passes when the FRL exceeds t_e, both as calculated and as rounded for the report.

IFEG 2005 Table 3.4.1b: fire load density by occupancy (MJ/m²)

Occupancy	Mean	80th	90th	95th
Dwelling	780	870	920	970
Hospital	230	350	440	520
Hospital storage	2000	3000	3700	4400
Hotel bedroom	310	400	460	510
Offices	420	570	670	760
Shops	600	900	1100	1300
Manufacturing	300	470	590	720
Manufacturing and storage (< 150 kg/m²)	1180	1800	2240	2690
Libraries	1500	2250	2550	–
Schools	285	360	410	450

Variable fire loads (contents) only, from CIB W14 (1983). IFEG notes that at least the 95th percentile should be used for design.