UK Regulatory Framework

Approved Document B (England and Wales):

Fire Protection Materials — UK Market

Intumescent coatings (thin-film):

The dominant fire protection method for UK structural steel in commercial buildings. Applied by airless spray or brush in the fabrication shop or on site.

Board protection:

Calcium silicate or vermiculite boards fixed around steel sections with screws and adhesives:

• Typical board thickness: 12-50 mm for 30-120 minute FR
• UK manufacturers: Promat (Promatect), Knauf (FirePanel), British Gypsum (Glasroc)
• Advantages: factory-controlled thickness, robust site handling, no curing time
• Disadvantages: bulkier than intumescent, slower installation, architectural constraints

Sprayed cementitious/vermiculite:

Low-cost protection for concealed steelwork (plant rooms, car parks):

• Applied by wet spray, typical thickness 10-50 mm
• UK products: Cafco (Promat), Monokote (GCP Applied Technologies)
• Advantages: lowest cost per square metre, good for complex geometries
• Disadvantages: messy application (overspray), requires mesh reinforcement for thicker coats, not suitable for visible steelwork

Section Factor A_p/V — The Key Parameter

The section factor (also denoted A_m/V or H_p/A) controls the rate of steel temperature rise during a fire:

A_p/V = heated perimeter / steel volume (per unit length)

Note: The beam (3-sided) section factor is lower than column (4-sided) because the top flange is shielded by the concrete slab. This reduces the required fire protection thickness for beams compared to columns. Hollow sections have intrinsically lower section factors due to their compact geometry — they inherently achieve 30-minute FR without protection in many cases.

Critical Temperature Method — Worked Example

Problem: Determine the fire protection requirements for a 457 x 191 UB 67 in S355 (office floor beam, 60-minute fire resistance required). The beam is simply supported, fully restrained by a composite slab, and designed for 72 % utilisation at ambient temperature.

Step 1 — Degree of utilisation (load ratio) at fire limit state:

The fire limit state load combination per EN 1990 UK NA:

E_fi,d = Gk + psi_1 x Qk = Gk + 0.5 x Qk (psi_1 = 0.5 for office occupancy)

Assuming dead/imposed ratio of 1.2:1 (typical for UK office floors):

mu_0 = E_fi,d / R_fi,d,0 = (Gk + 0.5Qk) / (1.25Gk + 1.5Qk) (at ambient ULS)

For Gk/Qk = 1.2: mu_0 approximately = (1.2 + 0.5 x 1.0) / (1.25 x 1.2 + 1.5 x 1.0) = 1.70 / 3.00 = 0.567

Using the actual degree of utilisation from the ambient design: 0.72 (given).

mu_0 = 0.72 x 0.567 (fire/ambient ratio) but simplified, mu_0 approximately = 0.65 for this beam.

Step 2 — Critical temperature:

theta_cr = 39.19 x ln(1 / (0.9674 x mu_0^3.833) - 1) + 482

For mu_0 = 0.65: theta_cr = 39.19 x ln(1 / (0.9674 x 0.65^3.833) - 1) + 482 theta_cr approximately = 39.19 x ln(1 / (0.9674 x 0.191) - 1) + 482 theta_cr approximately = 39.19 x ln(5.408 - 1) + 482 theta_cr approximately = 39.19 x 1.483 + 482 = 540 + 482 approximately = 622 degrees C

At 60 minutes of standard fire exposure (ISO 834), unprotected steel reaches 900-950 degrees C for typical UK beam section factors (A_p/V = 150-250 m-1). The critical temperature of 622 degrees C is reached at approximately 13-15 minutes for unprotected steel. Therefore, fire protection is required.

Step 3 — Intumescent coating thickness:

Assuming an intumescent coating with a manufacturer-certified thermal conductivity lambda_p = 0.05 W/mK (typical for thin-film intumescents):

Using the manufacturer's assessment report (carried out to BS EN 13381-8):

For A_p/V = 220 m-1 (3-sided beam exposure), S355, critical temperature 622 degrees C, 60 minutes FR: Required DFT approximately 1,800 microns (1.8 mm) for a typical UK intumescent system.

Actual DFT specified per manufacturer's project-specific assessment — never interpolate from generic data.

Specification Requirements — UK Practice

ASFP Yellow Book (Fire Protection for Structural Steel in Buildings):

The definitive UK guidance document published by the Association for Specialist Fire Protection. Key requirements:

1. Fire protection must be tested and assessed to BS EN 13381-8 (intumescent) or BS EN 13381-4 (boards)
2. Manufacturer's assessment report is project-specific and cannot be reused for a different project
3. Dry film thickness (DFT) must be verified on site with electronic gauges — records retained
4. All fire protection installers must be third-party certified (ASFP, FIRAS, IFC, or LPCB)
5. Primer compatibility: the intumescent coating assessment is valid ONLY for the specific primer used in the fire test. Changing the primer invalidates the assessment.
6. Top-coat (decorative): additional thickness beyond the fire-protective DFT. Must be compatible with the intumescent and included in the assessment.
7. Connections: fire protection must extend through connections. Bolts and connection plates require the same FR period as the member.

Cost Comparison — UK Fire Protection Methods

Frequently Asked Questions

Can UK steel beams achieve 30-minute fire resistance without protection?

Yes — many UK steel beams in single-storey buildings and open-sided car parks can achieve 30-minute FR without applied fire protection. The critical temperature method (EN 1993-1-2 Clause 4.2) may demonstrate that the steel temperature at 30 minutes of standard fire exposure does not exceed the critical temperature. This depends on the degree of utilisation (lower utilisation = higher critical temperature) and the section factor (lower A_p/V = slower heating). For UK car park beams (15-minute FR requirement), unprotected steel is standard. For 30-minute FR in single-storey buildings, unprotected steel often works when the degree of utilisation mu_0 <= 0.40 and A_p/V <= 150 m-1.

How do intumescent coatings work?

Intumescent coatings are thin-film paint-like materials applied to steelwork. At ambient temperature, they are inert. When exposed to fire (temperatures approximately 200-250 degrees C), a chemical reaction causes the coating to expand (intumesce) to 50-100 times its original thickness, forming a charred insulating foam layer that protects the steel. The expanded char has very low thermal conductivity, restricting the rate of heat transfer to the steel. After the fire, the charred layer must be removed and the steel recoated. The coating works once — it is not reusable after fire exposure.

What documentation is required for UK Building Control approval of steel fire protection?

Building Control (England and Wales) or Building Standards (Scotland) will require: (1) the structural engineer's fire design strategy specifying the required FR periods per member; (2) the fire protection manufacturer's project-specific assessment report (not generic brochure data); (3) the installer's third-party certification (FIRAS, IFC, LPCB, or ASFP member); (4) on-site DFT measurement records; (5) the primer and top-coat compatibility statement from the intumescent manufacturer. Missing any of these documents is a common cause of Building Control rejection. The designer must specify the fire protection type and required FR period; the specialist sub-contractor provides the detailed specification.

What fire protection do UK steel connections require?

EN 1993-1-2 does not require explicit fire design of connections if the connection has at least the same fire resistance as the connected members and the connection detail is standardised. In UK practice: (a) simple connections (fin plates, end plates, web cleats) — extend the beam fire protection 500 mm each side of the connection; (b) moment connections (extended end plates) — protect the connection zone to the same standard as the members; (c) column splices — protect the splice and extend protection 500 mm above and below. Connections with exposed bolts (e.g., fin plates with bolts outside the protected zone) require additional assessment — the bolts heat faster than the protected member and may lose strength before the member reaches its critical temperature.

Related Pages

• UK Fire Rating Guide — EN 1993-1-2 design overview
• UK Fire Rating Tables — Approved Document B table summary
• UK Steel Properties — Elevated Temperature — fy and E reduction factors
• UK Beam Sizes and Section Factors — A_p/V values for UK sections
• UK Steel Grade Guide — S235, S275, S355, S460
• UK Composite Beam Design — Fire design of composite floors
• European Fire Protection Guide — EN 1993-1-2 EU practice

Educational reference only. All design values are per BS EN 1993-1-2:2005, UK Building Regulations Approved Document B (2019 edition, as amended), and ASFP Yellow Book (5th edition). Fire protection specification must be project-specific and based on test evidence per BS EN 13381-8. Always consult the manufacturer's current assessment report and a FIRAS/IFC/LPCB-certified installer. Designs must be independently verified by a Chartered Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.