Structural steel loses strength as it heats, so in most buildings it must be protected to keep its load capacity for a set time in a fire. UK building regulations set that time as a fire-resistance period, and it's achieved with intumescent coating, boarding or spray protection.
Why steel needs protecting
Steel doesn't burn, but its strength falls as temperature rises. Left unprotected in a serious fire it can soften and lose load capacity. Fire protection keeps the steel below the temperature at which that happens for long enough for people to escape and the fire to be tackled.
How long: the fire-resistance period
UK building regulations (Approved Document B in England) set the required fire-resistance period for the building's height and use, commonly expressed in minutes such as 30, 60, 90 or 120. The structural fire design itself follows BS EN 1993-1-2. The required period is what the protection has to deliver.
The main protection methods
- Intumescent coating: a paint-like film that swells in heat to form an insulating char. Neat finish, popular where steel is exposed
- Boarding: fire-rated boards encasing the section. Robust and predictable, often used where appearance is less critical
- Spray protection: a sprayed cementitious or fibre coating, cost-effective for concealed steel
The right method depends on the required period, whether the steel is exposed or hidden, the section size and the budget.
Section factor
How quickly a section heats depends on its section factor, roughly, how much exposed surface it has relative to its mass. Chunkier sections heat more slowly and need less protection; slender ones need more. This is why fire protection is specified per member, not as a blanket coat.
Designing it in
Fire protection is a design decision that affects appearance, cost and the fabrication finish. Where you want exposed steel, intumescent is usually the answer; where it's hidden, board or spray may be cheaper. Tell us the required fire rating with your enquiry and we'll factor it into the quote.
Common questions
- Does all structural steel need fire protection?
- Not always, it depends on the building's required fire-resistance period under the building regulations. Many commercial and residential buildings need protection to achieve 60, 90 or 120 minutes; some single-storey structures may need little or none. The fire strategy sets the requirement.
- What's the best fire protection for exposed steel?
- Intumescent coating is the usual choice for exposed steel because it gives a neat, paint-like finish while providing the required fire resistance. Boarding and spray are typically used where the steel is concealed.
- What standard governs the fire design of steel?
- The structural fire design of steel follows BS EN 1993-1-2 (Eurocode 3), while the required fire-resistance periods come from the building regulations, Approved Document B in England.