UK Steel Fire Protection — EN 1993-1-2 Design Guide

This reference covers fire protection for UK steel design per EN 1993-1-2:2005 and UK Building Regulations Part B. Fire resistance of steel structures in the UK is achieved through a combination of active systems (sprinklers) and passive protection (coatings, boards, encasement).

Design requirements, worked examples, and practical design guidance are provided for common design office applications.

Code Reference: EN 1993-1-2:2005 and UK Building Regulations Part B

Fire Resistance Periods (UK Building Regulations Part B)

Critical Temperature Method (EN 1993-1-2 Clause 4.2)

The critical temperature method determines the steel temperature at which the member fails under the fire limit state load:

• For protected steel: the fire resistance period (30, 60, 90, 120 min) is achieved by limiting steel temperature rise
• For unprotected steel: the critical temperature is reached when the member's load ratio is exceeded

Degree of utilisation (load ratio): [ \mu0 = \frac{E{fi,d}}{R_{fi,d,0}} ]

Where Efi,d is the design effect under fire (typically 1.0 × dead + 0.3-0.7 × imposed) and Rfi,d,0 is the ambient temperature resistance.

Critical temperature (for steel with μ0 determined): [ \theta_{cr} = 39.19 \ln\left(\frac{1}{0.9674 \mu_0^{3.833}} - 1\right) + 482 ]

Critical Temperature vs Load Ratio

Steel Temperature Rise — Protected Sections

[ \Delta\theta*{a,t} = \frac{\lambda_p A_p / V (\theta*{g,t} - \theta*{a,t})}{d_p c_a \rho_a (1 + \phi/3)} \Delta t - (e^{\phi/10} - 1) \Delta\theta*{g,t} ]

Where the key parameter is the section factor (A_p/V) — the ratio of heated perimeter to steel volume.

Section Factors for Common UK Sections

Lower section factor = slower heating (more favourable for fire design).

Fire Protection Materials Comparison

Worked Example — Fire Resistance of a 533×210 UB 92

Given:

• Beam: 533×210 UB 92 in S355 (4-sided exposure)
• Section factor: A_p/V = 160 m⁻¹
• Required fire resistance: 60 minutes
• Load ratio: μ0 = 0.55
• Fire limit state load: 1.0 Gk + 0.5 Qk

Step 1 — Critical temperature: For μ0 = 0.55: θcr = 39.19 × ln(1/(0.9674 × 0.55^3.833) - 1) + 482 = 589°C

Step 2 — Unprotected section check: Time to reach 589°C for unprotected steel (simplified): θa,t at 30 min ≈ 750°C (for A_p/V = 160 m⁻¹) → exceeds 589°C → Unprotected steel fails at ~25-28 minutes

Step 3 — Protection thickness requirement: For 60 minutes fire rating, select intumescent coating: Using manufacturer's data for A_p/V = 160 m⁻¹, 60 min rating, S355: Required dry film thickness: ~1.5-2.0 mm (depending on manufacturer and load ratio)

Step 4 — Verify protection adequacy (simplified): Section factor 160 m⁻¹ with 1.8 mm intumescent coating: Estimated steel temperature at 60 min ≈ 520°C < 589°C → Satisfactory

Design Resources

• UK Beam Design — Beam flexural design
• UK Column Design — Column buckling
• UK Steel Properties — Material data at elevated temperature
• UK Steel Beam Sizes — Section factors
• UK Framing Systems — Fire compartmentation
• All UK References

Frequently Asked Questions

How is fire resistance achieved for UK steel structures?

EN 1993-1-2 provides methods for fire design. UK practice commonly uses intumescent coatings for up to 60 minutes, board protection for 90-120 minutes, or concrete encasement. The critical temperature method (Clause 4.2) compares the steel temperature at a given fire exposure to the critical temperature based on the load ratio. Section factor (A_p/V) governs how rapidly the steel heats up. The fire limit state combination uses reduced partial factors: typically 1.0 Gk + ψ1 Qk where ψ1 = 0.3-0.7 depending on occupancy.

What are the standard fire resistance periods in UK buildings?

UK Building Regulations Part B specifies: 30-60 minutes for single-storey, 60-90 minutes for buildings up to 18 m height, 120 minutes for high-rise (≥ 30 m). Depends on building use and height. Approved Document B provides a comprehensive table of minimum periods by building type, height, and compartment size. For buildings with sprinklers, the fire resistance period may be reduced by 30 minutes.

How does the UK NA modify EN 1993-1-2 material factors?

The UK NA to EN 1993-1-2 specifies: γM,fi = 1.00 for fire design (no additional safety factor beyond ambient design). The reduction factors for yield strength and elastic modulus at elevated temperature follow Table 3.1 of EN 1993-1-2 without modification. At 600°C (critical temperature for many members), steel retains approximately 47% of ambient yield strength. For concrete, the UK NA to EN 1994-1-2 uses γM,fi = 1.00 and γs,fi = 1.00.

What is the section factor and why does it matter?

The section factor A_p/V (or A_m/V) is the ratio of the heated perimeter to the steel volume per unit length. It controls the rate at which the steel heats up. A high section factor (narrow exposed profile, e.g., 406×178 UB 60 at 220 m⁻¹) heats quickly, requiring more fire protection. A low section factor (chunky profile, e.g., 305×305 UC 97 at 120 m⁻¹) heats slowly. For 60-minute fire rating, a beam with A_p/V = 220 m⁻¹ needs approximately 2.5 mm intumescent coating, while A_p/V = 120 m⁻¹ needs only ~1.2 mm.

What protection is needed for connections in fire?

Connections must achieve the same fire resistance as the members they connect. Fire protection of connections is often overlooked in design. Methods: (a) encasing the connection within the beam protection (extending intumescent coating over connection plates and bolts), (b) using thicker coating locally at connections (the increased steel mass of the connection plates offsets the higher A_p/V ratio), or (c) using board protection at critical connections. For end plates, the protected zone must extend at least 500 mm each side of the connection.

Reference only. Verify all values against the current edition of EN 1993-1-2:2005 and UK Building Regulations Part B. This information does not constitute professional engineering advice.