Fire Protection of Structural Steel: Options and Specification
How structural steel is protected from fire: why steel loses strength, fire resistance ratings, intumescent coatings, boards, sprays, concrete encasement and Saudi Civil Defense requirements.
Saudi Steel Work Editorial Team · · 3 min read
Short answer: Structural steel loses strength as it heats, so fire codes require protection to achieve fire resistance ratings such as 60, 90 or 120 minutes. Common methods are intumescent coatings, sprayed cementitious or mineral fibre coatings, fire boards and concrete encasement. The required rating comes from the building code, and protection thickness depends on section size and design temperature.
Steel doesn't burn, but it softens in fire. Fire protection buys time for evacuation and firefighting and prevents premature collapse.
Why protection is needed
As steel temperature rises, its yield strength and stiffness fall. Design temperatures depend on load ratio and code approach, often in the 500–600 °C range. Unprotected light sections can reach these temperatures quickly in a fully developed fire.
Fire resistance requirements
The Saudi Building Code (including SBC 201 and SBC 801) and Civil Defense requirements set required fire resistance by building type, height and occupancy. Ratings are expressed in minutes (e.g. R60, R90, R120).
Section factor
The section factor (Hp/A) measures how quickly a section heats. Lighter sections and hollow sections with thin walls need more protection; heavy sections need less.
Protection methods
| Method | Advantages | Considerations |
|---|---|---|
| Intumescent coating | Thin, decorative, shows steel form; shop or site applied | Higher cost for long ratings; needs suitable topcoat externally |
| Sprayed cementitious / mineral fibre | Economical for long ratings | Bulky, not decorative; used in hidden areas |
| Fire boards | Clean, predictable thickness; dry installation | Labour-intensive; joints and fixings |
| Concrete encasement | Robust, long ratings | Heavy, slow |
| Concrete-filled hollow sections | Inherent fire resistance | Design-dependent |
Specifying intumescent coatings
- Product must be tested and assessed for the section types and ratings required
- Thickness determined from manufacturer's tables by section factor and critical temperature
- Compatible primer and topcoat
- DFT measured and recorded
Interaction with corrosion protection
Fire coatings must be compatible with the anti-corrosion primer. Coordinate both specifications early (paint systems for structural steel).
Composite floors
Composite slabs on steel decking achieve fire resistance through slab depth and reinforcement, often reducing protection needed on beams (composite decking).
See structural steel fabrication for shop-applied coatings.