Fire Resistance Periods — EN 1993-1-2 Clause 2.1

Fire resistance is characterised by the period during which the structural element maintains its load-bearing function under the standard fire exposure defined in EN 13501-2. Standard periods are:

The required fire resistance period is specified in national building regulations (e.g., UK Approved Document B, German Musterbauordnung, French Réglementation Incendie), not in EN 1993-1-2 itself.

Standard Temperature-Time Curve — ISO 834

The nominal fire exposure per EN 1991-1-2 Clause 3.2.1 follows ISO 834:

θ_g = 20 + 345 × log₁₀(8t + 1)

Where θ_g is the gas temperature in °C and t is the time in minutes.

These gas temperatures drive the heat transfer into the steel section. The unprotected steel temperature rise lags behind the gas temperature due to the thermal inertia of the steel mass.

Critical Temperature Method — Clause 4.2.4

The critical temperature θ_a,cr is the steel temperature at which the member fails under the fire limit state loading. It depends on the utilisation factor μ_0:

Utilisation Factor

μ_0 = E_fi,d / R_fi,d,0

Where E_fi,d is the design effect of actions in the fire situation (from EN 1990 Clause 6.4.3.3: G_k + ψ_fi × Q_k,1 + ψ_2 × Q_k,i) and R_fi,d,0 is the design resistance in fire at time t = 0 (i.e., at normal temperature with partial factors for fire: γ_M,fi = 1.00 for all materials).

Critical Temperature Equation

θ_a,cr = 39.19 × ln(1 / (0.9674 × μ_0^(3.833)) − 1) + 482

Simplified values:

A lower utilisation factor means the member is more conservatively designed at ambient temperature and can tolerate a higher steel temperature in fire. For μ_0 ≤ 0.3, the critical temperature exceeds 650 °C, which is generally considered acceptable without specific fire protection for secondary beams.

Section Factor — A_m/V (Previously H_p/A)

The section factor governs the rate of steel temperature rise. It is the ratio of the exposed surface area to the volume of steel per unit length:

A_m/V (m⁻¹) = Perimeter exposed to fire / Cross-sectional area

Lower A_m/V values are favourable for fire resistance (slower heating). Heavy column sections with A_m/V below 70 m⁻¹ can often achieve R30 without any protection at all if the utilisation factor μ_0 is low. Sections with A_m/V above 200 m⁻¹ heat rapidly and almost always require fire protection for R30 and above.

Fire Protection Systems

Intumescent Coatings

Thin-film reactive coatings (typically 0.5-3 mm dry film thickness) that expand when heated to form an insulating char layer 25-50 times the original coating thickness.

Dry film thickness (DFT) is specified by the manufacturer based on the required fire resistance period and the section factor. The relationship is non-linear: doubling the DFT does not double the fire resistance.

Board Protection Systems

Pre-formed rigid boards (calcium silicate, vermiculite, gypsum) mechanically fixed or glued to the steel section. Board thicknesses range from 15 mm (R30) to 60 mm (R180).

Board thickness is determined by the manufacturer's assessment report. Typical requirements for a section factor of 150 m⁻¹ with calcium silicate board: R30 = 15 mm, R60 = 20 mm, R90 = 30 mm, R120 = 40 mm.

Sprayed Fire Protection

Cementitious or mineral fibre sprays applied wet to the steel surface. Lower material cost than boards but slower to apply and less architecturally clean.

Worked Example — IPE 450 Floor Beam, R60

Step 1 — Fire Limit State Loading

E_fi,d = G_k + ψ_fi × Q_k = 18.5 + 0.3 × 15.0 = 23.0 kN/m

Fire moment: M_fi,Ed = 23.0 × 10.5² / 8 = 316.9 kN·m

Step 2 — Ambient Resistance

M_Rd = W_pl,y × f_y / γ_M0 = 1,702 × 10³ × 355 / 1.00 = 604.2 kN·m

Step 3 — Utilisation Factor

μ_0 = M_fi,Ed / M_Rd = 316.9 / 604.2 = 0.524

Step 4 — Critical Temperature

θ_a,cr = 39.19 × ln(1 / (0.9674 × 0.524^(3.833)) − 1) + 482 = 39.19 × ln(1 / (0.9674 × 0.0837) − 1) + 482 = 39.19 × ln(1 / 0.0810 − 1) + 482 = 39.19 × ln(12.35 − 1) + 482 = 39.19 × ln(11.35) + 482 = 39.19 × 2.429 + 482 = 95.2 + 482 = 577.2 °C

Step 5 — Unprotected Steel Temperature

Section factor for IPE 450 (3-sided): A_m/V = 140 m⁻¹

Using the incremental temperature method from EN 1993-1-2 Clause 4.2.5.1 with Δt = 5 seconds time steps, the steel temperature after 60 minutes of ISO 834 fire reaches approximately 825 °C for an unprotected section with A_m/V = 140 m⁻¹.

825 °C > θ_a,cr = 577 °C — unprotected section FAILS at approximately 12 minutes

Step 6 — Protection Thickness

Option A: Intumescent coating. From manufacturer's data at A_m/V = 140 m⁻¹, R60: Required DFT = 1.2 mm (solvent-based), 1.5 mm (water-based), or 2.5 mm (epoxy).

Option B: Calcium silicate board. From assessment report, 20 mm board on 3 sides achieves θ_a,max = 520 °C at 60 minutes < 577 °C — tentative. Check with specific product data. 25 mm board definitively achieves < 577 °C.

Recommended: 25 mm calcium silicate board encasement to 3 sides, or 1.5 mm water-based intumescent coating.

Thermal Properties of Steel per EN 1993-1-2

The steep yield strength reduction between 400 °C and 600 °C explains the typical critical temperature range of 500-600 °C for typical steel beams and columns. Structural carbon steel loses 50% of its yield strength at approximately 550 °C.

Frequently Asked Questions

How is the fire limit state loading determined per EN 1990? The accidental fire combination per EN 1990 Clause 6.4.3.3 is E_fi,d = ΣG_k,j + ψ_fi × Q_k,1 + Σψ_2,i × Q_k,i, where ψ_fi = ψ_1,1 for buildings (typically 0.5 for residential, 0.3 for offices, 0.7 for storage — per the National Annex). This is less severe than the ULS combination because the probability of a fully developed fire coinciding with the peak live load is low. The reduction factor for the leading variable action ψ_fi may be taken as ψ_1,1 or, more commonly, as the factor derived from the National Annex. In the UK, ψ_fi = ψ_1,1 for most occupied buildings, except storage where ψ_fi = 0.6. Wind and snow are not combined with fire (ψ_2 = 0.0 for wind in the fire situation).

Can unprotected steel achieve any fire resistance? Yes. Unprotected steel can achieve R15 or R30 if two conditions are met: (1) the section factor A_m/V is low (typically below 100 m⁻¹ for R15 and below 60 m⁻¹ for R30), and (2) the utilisation factor μ_0 is low (≤ 0.4). Heavy column sections such as HD 400×347 (A_m/V ≈ 60 m⁻¹) with utilisation μ_0 = 0.3 can achieve R30 unprotected. For long-span beams with higher utilisation (μ_0 ≈ 0.6-0.7) and moderate section factors (A_m/V ≈ 100-150 m⁻¹), unprotected steel typically fails between R12 and R18, requiring fire protection for R30. The UK's SCI Publication P375 provides extensive unprotected steel fire resistance data for UK beam and column sections.

How are composite slabs considered in beam fire design? EN 1994-1-2 covers composite steel-concrete construction in fire. The concrete slab on the top flange of a steel beam provides significant thermal shielding (the top flange heats much more slowly than the exposed bottom flange and web). For 3-sided exposure with a composite slab at least 120 mm thick on trapezoidal decking, the top flange temperature is typically 200-300 °C lower than the bottom flange temperature at 60 minutes. The composite slab also provides a membrane action mechanism at large deflections that can significantly extend survival time beyond the simple critical temperature prediction. EN 1994-1-2 Annex D provides a simplified membrane action method that can justify up to 30 minutes of additional fire resistance for composite floor systems.

What is the difference between reactive and non-reactive fire protection? Reactive protection (intumescent coatings) expands when heated, forming an insulating char that is 25-50 times the original dry film thickness. The char layer is a thermal barrier that slows heat transfer to the steel. Reactive systems are thin (0.5-5 mm DFT), can follow complex section profiles, and provide an architecturally clean finish. Non-reactive protection (boards, sprays) provides thermal insulation through low thermal conductivity materials at a fixed thickness that does not change during the fire. Boards provide a box-like encasement that is simple to inspect and quantify but adds bulk. Sprays are economical for large areas but provide a rough finish. The choice depends on: fire resistance period, section factor, exposure condition (internal/external), architectural finish requirements, and project cost. Intumescent coatings are generally 20-50% more expensive per square metre than board systems but save on space and are preferred for architecturally exposed steel.

Design Resources

• EN 1993 Fire Rating — Fire Resistance Periods Explained
• EN 1993 Beam Design — Flexure per EN 1993-1-1 Clause 6.2
• EN 1993 Steel Grade Properties — f_y and f_u Values
• European Steel Beam Sizes — IPE, HEA, HEB
• EN 1993 Column Design — Buckling per EN 1993-1-1
• EN 1993 Composite Beam Design — EN 1994-1-1
• All European Reference Guides

Reference only. Verify all values against the current edition of EN 1993-1-2:2005, EN 1991-1-2:2002, the applicable National Annex, and the fire protection manufacturer's current test data and European Technical Assessment (ETA). Fire resistance design must be verified by a licensed Structural Engineer and coordinated with the project fire strategy. This guide is for educational purposes only and does not constitute professional engineering advice.