Fire Resistance per EN 1993-1-2

EN 1993-1-2 provides the design framework for steel structures in fire conditions. The UK National Annex specifies national parameters for fire design.

Section Factor Am/V

The section factor (Am/V) is the ratio of the exposed steel surface area (per metre length) to the steel volume (per metre length). It is the primary parameter determining heating rate:

Am/V = Perimeter_exposed / A (where A = cross-sectional area)

Lower Am/V → slower heating → higher fire resistance.

Section Factor for Protected Steel

For protected steel, the section factor modifies to:

Am/V_protected = Am/V × (λ_prot / d_prot)

Where λ_prot is the thermal conductivity and d_prot is the protection thickness.

Critical Temperature Method

The critical temperature Tcr is the steel temperature at which the load-bearing capacity reduces to the applied load under fire conditions:

Tcr = 39.19 × ln(1 / (0.9674 × μfi^(3.833)) − 1) + 482 (for μfi ≥ 0.013)

Where μfi = ηfi × NEd / Npl,Rd (load level in fire).

Fire Protection Systems

Worked Example — 533UB Fire Design

Beam: 533UB, S355, 60-minute fire rating Load level in fire: μfi = 0.50 (50 % utilisation at room temperature, ηfi = 0.65 for typical UK office building)

Step 1 — Section Factor

Three-sided exposure (composite slab on top): Perimeter = 2h + b = 2 × 529 + 211 = 1,269 mm A = 13,100 mm²

Am/V = 1,269 / 13,100 × 1,000 = 96.9 m⁻¹

Step 2 — Critical Temperature

For μfi = 0.50: Tcr ≈ 599°C (from table above).

Step 3 — Fire Protection Thickness

For intumescent coating on a 533UB with Am/V = 97 m⁻¹ and 60-minute rating:

• Thickness required ≈ 0.8-1.2 mm (depending on coating formulation — always use manufacturer's data)
• Generic values for thin-film intumescent: 0.8 mm for 60 min (Am/V < 100)

Step 4 — Temperature Check

Steel temperature after 60 minutes (unprotected):

Using the EN 1993-1-2 heating formula for unprotected steel:

Δθa,t = (Am/V) × hnet,d × Δt / (ca × ρa)

For Am/V = 97 m⁻¹, after 60 min with standard fire curve ISO 834: θa ≈ 750-800°C (exceeds critical 599°C) → Protection is required.

With 1.0 mm intumescent coating (λ_prot = 0.2 W/mK): θa ≈ 450-500°C after 60 min → Protection adequate.

Design Guidance

• Am/V < 100: Slow heating (UC sections, deep UB with 3-side exposure) — use thinner protection
• Am/V 100-200: Moderate heating (standard UB/CHS sections)
• Am/V > 200: Fast heating (light sections) — thicker protection required
• Critical temperature: 550°C is a commonly used conservative value for UK design
• Fire protection application: Must follow the manufacturer's approved system and quality plan per BS EN 1090-2 EXC requirements

Design Resources

• UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
• UK Steel Mechanical Properties — fy, fu, and elongation tables
• UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
• UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
• UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
• UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
• All UK Steel Design References — complete library

Frequently Asked Questions

What is the section factor Am/V for a UK steel section?

The section factor Am/V is the ratio of the heated steel perimeter (per metre) to the cross-sectional area. For a 533UB with 3-sided exposure (composite slab on top): Am = 2 × 529 + 211 = 1,269 mm/m, V = 13,100 mm²/m, Am/V = 96.9 m⁻¹. Lower values mean slower heating. UC sections have the lowest Am/V (heaviest, slowest heating). CHS sections have moderate Am/V. Light sections like angles and channels have high Am/V (fast heating, more protection needed).

What is the critical temperature for structural steel in fire?

The critical temperature Tcr is the temperature at which the steel reaches its design resistance under the fire load. For a load level μfi = 0.6 (60 % utilisation in fire), Tcr ≈ 550°C. For μfi = 0.5, Tcr ≈ 599°C. For μfi = 0.3, Tcr ≈ 711°C. The critical temperature is calculated as: Tcr = 39.19 × ln(1/(0.9674 × μfi^3.833) − 1) + 482 per EN 1993-1-2. UK practice commonly uses 550°C as a conservative critical temperature.

What fire protection is standard for UK steel buildings?

Intumescent coatings are the standard fire protection for architecturally exposed steel in UK buildings (0.8-3.0 mm thickness for 60-120 minutes). For hidden steel (above ceilings, in service voids), sprayed cementitious or mineral fibre fire protection is standard (15-50 mm). Board systems (calcium silicate) are used for columns in high-rise buildings where impact resistance is required. The choice depends on aesthetics, cost, and the required fire rating duration.

Does the UK NA modify EN 1993-1-2 fire design parameters?

The UK NA to BS EN 1993-1-2 adopts the recommended fire design parameters: partial factors γM,fi = 1.00 for steel, the standard fire curve ISO 834, and the critical temperature method. The UK NA provides additional guidance on the load level ηfi for different building types (offices, residential, retail). For UK offices, ηfi = 0.65 is typical. The UK NA also references BS 9999 for fire protection strategies in UK buildings.

Related Pages

• EN 1993 Steel Design Overview
• European Steel Properties
• EN 1993 Beam Design Guide
• EN 1993 Column Buckling
• EN 1990 Load Combinations
• UK Steel Chemical Composition
• UK Steel Charpy Values

Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.