UK Fire Design Guide — BS EN 1993-1-2 + UK NA, Intumescent, Boarding & Spray
Comprehensive guide to the structural fire design of steelwork in UK buildings. Covers the critical temperature method per BS EN 1993-1-2:2005 Clause 4.2, section factor calculation for UK sections (UB, UC, SHS, RHS, CHS), intumescent coating specification per the ASFP Yellow Book and manufacturer assessment reports, board fire protection systems (Vermiculux, Promat, Supalux), spray-applied cementitious and vermiculite protection, the fire resistance periods required by UK Approved Document B, and a fully worked example designing fire protection for a 457x191x67 UB at 60 minutes fire resistance.
Quick access: UK Fire Protection Guide | UK Beam Sizes | UK Steel Grades | UK Column Design | All UK References
1. UK Regulatory Framework for Structural Fire Design
Fire safety of structural steelwork in UK buildings is governed by:
| Regulation | Scope | Key Requirement |
|---|---|---|
| Building Regulations 2010, Approved Document B (Vol 2) | England & Wales | Minimum fire resistance periods Table B3 |
| Scottish Building Standards, Technical Handbook | Scotland | Equivalent requirements with some variations |
| BS EN 1990:2002 + UK NA | Basis of design | Partial factors for accidental (fire) design situation |
| BS EN 1991-1-2:2002 + UK NA | Actions on structures exposed to fire | Fire load, compartment temperatures |
| BS EN 1993-1-2:2005 + UK NA | Steel in fire | Critical temperature, protection methods |
| BS EN 1994-1-2:2005 + UK NA | Composite steel-concrete in fire | Composite slabs and beams in fire |
Fire Resistance Periods — Approved Document B Table B3
| Building Type | Height | Minimum FR (minutes) | Notes |
|---|---|---|---|
| Single-storey (excluding shops) | ≤ 18 m | 30 | 15 min if sprinklered and low risk |
| Office / residential | ≤ 18 m | 60 | 90 min for buildings > 18 m |
| Shops / commercial | ≤ 18 m | 60 | 90 min if floors > 5 |
| Buildings | 18–30 m | 90 | Sprinklers mandatory above 30 m |
| High-rise residential | > 30 m | 120 | Sprinklers mandatory |
| Car park (open-sided) | Any | 15 | 30 min if enclosed |
| Basement floors | Any | 60 | 90 min if depth > 10 m |
The fire resistance period is the time (in minutes) that a structural element must maintain its loadbearing capacity (R), integrity (E), and insulation (I) when exposed to the standard ISO 834 fire curve. For structural steel, only the loadbearing criterion (R) typically applies; insulation applies to separating elements.
2. The Critical Temperature Method — BS EN 1993-1-2 Clause 4.2.4
Theory
The critical temperature method is the simplest fire design approach in Eurocode 3. It derives a single limiting steel temperature above which the member fails. If the steel temperature remains below this limit throughout the required fire resistance period, the member passes.
Step 1 — Calculate the degree of utilisation mu_0:
At time t = 0 (ambient), the utilisation under the accidental (fire) load combination is:
mu_0 = E_fi,d / R_fi,d,0
Where:
- E_fi,d = design effect of actions in the fire situation (bending moment, axial force)
- R_fi,d,0 = design resistance at ambient temperature, using the fire partial factor gamma_M,fi = 1.0
The fire load combination per EN 1990 Expression 6.11b (UK NA) is:
E_fi,d = G_k + psi_1,1 * Q_k,1 + sum(psi_2,i * Q_k,i)
For UK offices: psi_1,1 = 0.5 (imposed), psi_2,i = 0.3 (imposed), 0.0 (wind), 0.0 (snow).
Step 2 — Calculate the critical temperature theta_a,cr:
theta_a,cr = 39.19 * ln( 1 / (0.9674 * mu_0^3.833) - 1 ) + 482
This equation is valid for mu_0 between 0.013 and 1.0. The minimum valid critical temperature is approximately 350 degrees C.
Quick-reference table for common utilisations:
| mu_0 | theta_a,cr | Typical scenario |
|---|---|---|
| 0.30 | 652 degrees C | Lightly loaded column |
| 0.40 | 598 degrees C | Typical column in braced frame |
| 0.50 | 554 degrees C | Typical simply supported beam |
| 0.60 | 524 degrees C | Beam near capacity |
| 0.65 | 510 degrees C | Common for floor beams at maximum utilisation |
| 0.70 | 498 degrees C | Heavily utilised beam |
| 0.85 | 455 degrees C | Near-fully utilised member |
A member with mu_0 = 0.65 (typical for a floor beam designed to just below the full bending resistance) has a critical temperature of 510 degrees C. If the member is protected such that its steel temperature never exceeds 510 degrees C during the 60-minute fire exposure, it passes without further calculation.
3. Section Factor — The Key Parameter
Definition
The section factor A_m/V is the ratio of the heated perimeter of the steel cross-section to its volume (or equivalently, heated perimeter over cross-sectional area). It has units m^-1 (or equivalently m^2/m^3).
A_m/V = heated perimeter / cross-sectional area [m^-1]
Heated Perimeter Rules
The heated perimeter depends on the fire exposure:
- 3-sided exposure (beam supporting slab): The top flange is shielded by the concrete slab. Heated perimeter = bottom flange + 2 x (web – top flange thickness). Do NOT include the top flange.
- 4-sided exposure (column without encasement): All external surfaces are exposed. Heated perimeter = full section perimeter.
- Box protection (column within board): Heated perimeter = inner perimeter of the hollow box encasement.
Example Calculation — 457x191x67 UB
- d = 453.4 mm, b = 189.9 mm, t_w = 8.5 mm, t_f = 12.7 mm
- Cross-sectional area A = 85.5 cm^2 = 0.00855 m^2
- 3-sided (as beam):
- Heated perimeter = b – t_w + 2 x (d – t_f) + (b – t_w)/2 (bottom flange exposed)?
- Actually: heated perimeter = b + 2 x (d – t_f) [bottom flange + web sides + bottom flange underside?]
- Using the SCI convention: A_m = 2h + b = 2 x (453.4 - 12.7) + 189.9 = 881.4 + 189.9 = 1071.3 mm
- A_m/V = 1071.3 / 8550 = 0.125 mm^-1 = 125 m^-1
- 4-sided (as column):
- Full perimeter = 2 x (189.9 + 453.4) = 1286.6 mm
- A_m/V = 1286.6 / 8550 = 0.150 mm^-1 = 150 m^-1
Section Factor Ranges for UK Sections
| Section Type | Light (high A_m/V) | Heavy (low A_m/V) |
|---|---|---|
| UB (beam) — 3-sided | 300 m^-1 (152x89x16 UB) | 60 m^-1 (1016x305x487 UB) |
| UC (column) — 4-sided | 260 m^-1 (152x152x23 UC) | 55 m^-1 (356x406x634 UC) |
| SHS — 4-sided | 280 m^-1 (60x60x3.2 SHS) | 40 m^-1 (400x400x16 SHS) |
| CHS — 4-sided | 350 m^-1 (48.3x3.2 CHS) | 20 m^-1 (508x16 CHS) |
A member with A_m/V = 250 m^-1 will heat roughly 2.5 times faster than one with A_m/V = 100 m^-1 in the same fire. The fire protection thickness required from manufacturer assessment data is directly proportional to A_m/V.
4. Fire Protection Systems — UK Market
4.1 Intumescent Coatings (Reactive)
Thin-film intumescent coatings are the dominant UK method for architecturally exposed structural steel (AESS) in commercial buildings, appearing as a paint-like factory or site-applied coating that expands dramatically under heat to form an insulating char.
Product categories by dry film thickness (DFT):
| Category | Typical DFT | Fire Resistance | Typical UK Products | Cost per m^2 (indicative) |
|---|---|---|---|---|
| Water-based, on-site | 0.3–2.5 mm | 30–90 min | Nullifire SC902, Jotun Steelmaster 60WB, Leighs Firetex FX5120 | £25–50 |
| Solvent-based, on-site | 0.5–3.5 mm | 30–120 min | Nullifire S605, Jotun Steelmaster 120SB | £35–70 |
| Epoxy intumescent (shop) | 1.5–8.0 mm | 60–120 min | Jotun Steelmaster 1200WF, PPG PITT-CHAR XP | £50–100 |
| Hydrocarbon rated (offshore) | 5.0–20.0 mm | 120+ min (jet fire) | Chartek 7, Carboline Pyrocrete | £120–250 |
Application sequence:
- Blast clean to SA 2.5 (near-white metal)
- Apply compatible anti-corrosive primer (e.g., epoxy zinc phosphate, 50–75 microns DFT)
- Apply intumescent base coat to specified DFT (multiple coats if required, each 250–500 microns wet)
- Apply compatible top sealer coat (acrylic urethane or polysiloxane, 50–75 microns DFT)
Quality control: Dry film thickness is measured with a calibrated electronic gauge (ASTM D7091). Each coat must be recorded on the inspection report. The ASFP Yellow Book requires that 90% of readings exceed the specified DFT and no single reading falls below 80%.
4.2 Board Fire Protection (Non-Reactive)
Board systems enclose the steel section within a box of low-thermal-conductivity boards. Boards are mechanically fixed using steel angles, screws, and joint compound.
| Board Type | Density (kg/m^3) | Thermal conductivity (W/mK) | Maximum service temp | Typical UK Products |
|---|---|---|---|---|
| Calcium silicate | 450–875 | 0.12–0.20 | 1000 degrees C | Promat PROMATECT-L500, Supalux |
| Vermiculite-silicate | 400–550 | 0.10–0.15 | 1100 degrees C | Skamol Super-Isol, Termax |
| Gypsum-based | 700–900 | 0.17–0.24 | 600 degrees C (calcination) | British Gypsum Glasroc F FIRECASE |
Typical board thicknesses for 60-minute fire resistance:
| A_m/V range (m^-1) | Promat PROMATECT-L500 | Supalux | Vermiculux |
|---|---|---|---|
| < 80 | 12 mm | 12 mm | 20 mm |
| 80–120 | 15 mm | 15 mm | 25 mm |
| 120–180 | 20 mm | 20 mm | 30 mm |
| 180–250 | 25 mm | 25 mm | 40 mm |
| 250–350 | 30 mm | 30 mm | 50 mm |
Thicknesses are indicative only; always use the manufacturer's current assessment report.
Board application:
- Boxed system: 4 boards form a box around the column or beam
- The boards are cut to size with a circular saw (dust extraction required — silica dust)
- Joints are staggered between layers (for multi-layer systems) and filled with jointing compound
- Fixings typically at 200–250 mm centres using self-tapping screws into steel angles at corners
- For columns, an internal angle frame supports the board corners
4.3 Spray-Applied Fire Protection (Non-Reactive)
Cementitious or vermiculite-based sprays are applied wet to the steel surface using a spray gun. They are the cheapest fire protection method but leave a rough, industrial finish — suitable only for hidden steelwork.
| Product Type | Dry Density (kg/m^3) | Application Thickness | FR Period | UK Example |
|---|---|---|---|---|
| Cementitious spray | 350–500 | 15–50 mm | 60–240 min | Cafco FENDOLITE MII, Grace Monokote MK-6 |
| Vermiculite spray | 250–400 | 20–60 mm | 60–240 min | Cafco BLAZE SHIELD II |
| Fibrous spray (mineral wool + binder) | 200–300 | 25–75 mm | 120–240 min | Promat CAFCO 300 |
Application notes:
- Surface must be clean, dry, and primed (the manufacturer specifies the primer)
- Mesh reinforcement may be required for thicknesses exceeding the manufacturer's un-reinforced limit (typically 25mm for cementitious, 35mm for vermiculite)
- Minimum ambient temperature during application is typically 4 degrees C (some products require 10 degrees C)
- Cure time before overcoating or enclosure is typically 24–72 hours depending on ambient humidity
- Bond strength: minimum 5 kPa adhesion to primed steel (tested per ASTM E736)
5. Worked Example — Fire Protection for a 457x191x67 UB
Design Brief
- Member: 457x191x67 UB in S355, simply supported, 8.0 m span
- Floor beam at 3.0 m centres, composite slab with ComFlor 60 + 130 mm concrete
- Required fire resistance: 60 minutes (R60)
- Exposure: 3-sided (top flange shielded by slab)
- Protection method: intumescent coating, exposed steel, Category X (internal, dry environment)
Step 1 — Fire Limit State Loading
Ambient (ULS) design moment from floor loads (already designed):
- G_k: (2.8 slab + 1.2 services + 0.67 steel SW) x 3.0 m = 14.0 kN/m
- Q_k: 3.0 x 3.0 = 9.0 kN/m
- M_Ed,ULS = (1.35 x 14.0 + 1.5 x 9.0) x 8.0^2 / 8 = (18.9 + 13.5) x 8 = 259.2 kN.m
Fire limit state (accidental) using EN 1990 Expression 6.11b:
- G_k = 14.0 kN/m (unchanged)
- psi_1,1 x Q_k,1 = 0.5 x 9.0 = 4.5 kN/m
- E_fi,d = (14.0 + 4.5) x 8.0^2 / 8 = 148.0 kN.m
Step 2 — Design Resistance at t = 0 (Ambient)
For fire design, use gamma_M,fi = 1.0 (instead of gamma_M0 = 1.0):
- M_fi,0,Rd = W_pl,y x f_y / gamma_M,fi = 1,470 x 10^3 x 355 / 1.0 = 522 kN.m
Note: Using W_pl,y = 1,470 cm^3 for 457x191x67 UB in S355.
Step 3 — Degree of Utilisation mu_0
mu_0 = E_fi,d / R_fi,d,0 = 148.0 / 522 = 0.284
Step 4 — Critical Temperature
theta_a,cr = 39.19 x ln(1 / (0.9674 x 0.284^3.833) - 1) + 482
Computing: 0.284^3.833 = 0.284^3 x 0.284^0.833 = 0.0229 x 0.373 = 0.00854 0.9674 x 0.00854 = 0.00826 1/0.00826 - 1 = 121.0 ln(121.0) = 4.796 39.19 x 4.796 + 482 = 188.0 + 482 = 670 degrees C
Critical temperature: 670 degrees C.
Step 5 — Section Factor
3-sided exposure:
- Heated perimeter A_m = b + 2(d - t_f) = 189.9 + 2(453.4 - 12.7) = 189.9 + 881.4 = 1071.3 mm
- Area = 8,550 mm^2
- A_m/V = 1071.3 x 10^-3 / (8550 x 10^-6) = 125.3 m^-1
Step 6 — Select Intumescent Coating
Using the Nullifire SC902 assessment report, for A_m/V = 125 m^-1 and R60:
- Required DFT = 700 microns (0.7 mm)
This is a single-coat application in the shop. The beam stays at or below 670 degrees C for the full 60-minute ISO 834 fire exposure.
Step 7 — Quality Control on Site
- Blast clean beam to SA 2.5
- Apply 75 microns Nullifire S708 epoxy zinc phosphate primer
- Apply Nullifire SC902 to 700 microns DFT
- Measure DFT at 5 locations per metre length of beam — all readings >= 630 microns (90% of specified)
- Apply Nullifire SC803 acrylic sealer coat at 50 microns DFT
6. Fire Engineering — Alternative Approaches
For complex geometries or where prescriptive fire protection is uneconomic, the UK permits a fire engineering approach (BS 7974) that may demonstrate:
- Performance-based design: Computational fluid dynamics (CFD) fire modelling using FDS or similar to calculate actual steel temperatures based on real compartment geometry, ventilation, and fire load — often yielding lower temperatures than the ISO 834 assumption
- Travelling fire concept: For large open-plan compartments, the fire does not engulf the entire floor simultaneously; localised heating may govern
- Membrane action of composite slabs: The slab can carry load at large deflections through tensile membrane action even when supporting beams have lost strength (Cardington test evidence)
7. Key Takeaways
- The critical temperature method is the standard UK approach — compute mu_0 from the fire limit state load, derive theta_a,cr, select protection to keep steel below this temperature for the required FR period.
- Section factor A_m/V is the master parameter — a section with A_m/V = 250 m^-1 requires roughly twice the protection thickness of one with A_m/V = 100 m^-1 for the same FR period.
- UK Approved Document B specifies 60-minute FR for most multi-storey buildings under 18 m, rising to 90 and 120 minutes for taller buildings. Always check the local regulation (England, Scotland, Wales, or Northern Ireland).
- Intumescents dominate for visible steelwork — typically 0.5–3.5 mm DFT for 60–120 minutes FR. Boards provide higher durability and impact resistance. Sprays are cheapest per m^2 for hidden steel.
- Always use the current manufacturer's assessment report — generic tables are for preliminary estimation only. The ASFP Yellow Book lists all certified UK fire protection products and their assessment report references.
PRELIMINARY — NOT FOR CONSTRUCTION. All fire design information is for educational reference only. Fire protection specification must be independently verified by a Chartered Structural Engineer and a qualified fire engineer using the current manufacturer's assessment report and project-specific details. Always check the latest Approved Document B, Scottish Technical Handbook, and BS EN 1993-1-2 with current UK National Annex.