Structural fire protection

How steel behaves in fire

Steel does not burn, but it loses strength and stiffness as it heats. Structural steel retains roughly half of its room-temperature yield strength at around 550–600 °C, and the thin cold-formed sections used for purlins and girts heat faster than heavy members. How quickly a member reaches its critical temperature depends on its section factor – the heated perimeter divided by the cross-sectional area (Hp/A or A/V in Europe and Australia, W/D in the US). Thin, open sections with high section factors heat fastest and need more protection.

Many single-storey PEB warehouses and factories need no fire-resistance rating for the frame at all: building codes accept unprotected steel for single-storey buildings of many occupancies when the building area, separation distances and sprinkler provisions satisfy the code. Fire protection becomes necessary for multi-storey or low-rise buildings, mezzanines used for occupancy, buildings close to boundaries, fire walls between compartments and certain high-hazard or assembly uses.

Fire-resistance requirements by region

In the US a typical single-storey steel warehouse is classed as Type IIB (unprotected non-combustible) construction, and IBC Section 507 allows unlimited-area single-storey buildings for several occupancies where the building is sprinklered and surrounded by open yards. Equivalent concessions exist in the other codes. The required rating is always set by the architect or fire engineer for the specific building – the steel supplier then provides members and coatings to match.

Passive protection options

• Thin-film intumescent coatings – paints that swell into an insulating char when heated. Typical dry film thickness runs from a few hundred microns for R30 to several millimetres for R120, depending on section factor and critical temperature. Shop application at the fabricator gives the best quality control; the product must be compatible with the primer of the paint system and protected by a topcoat in humid or exterior conditions.
• Sprayed fire-resistive materials (SFRM) – cementitious or mineral-fibre sprays applied on site; economical for long ratings but not decorative, and vulnerable to impact.
• Board encasement – gypsum, calcium-silicate or mineral boards around columns and beams; clean, dry and predictable.
• Concrete encasement or filling – used for columns in multi-storey frames and for impact-prone locations.
• Fire-rated envelopes – mineral-wool sandwich panels tested as fire walls or external walls, and rated ceiling or partition assemblies.

Performance-based design using AISC 360-22 Appendix 4, AS 4100 Section 12 or EN 1993-1-2 can calculate the actual critical temperature of each member under its fire-load level, which often reduces coating thickness compared with prescriptive tables.

Fire walls and boundary walls in steel buildings

Large buildings are divided into fire compartments, and walls near a site boundary may need a rating to stop fire spreading to neighbours. In a PEB the critical issue is that a fire wall must remain standing when the steel on the fire side collapses. Common solutions are an independent masonry or precast wall with its own stability, or a steel-framed wall where the frame next to the wall is protected and the roof framing is detailed so that collapse on one side does not pull the wall over. Interfaces with masonry and precast walls are discussed in masonry and precast interfaces.

Active systems and the steel structure

Sprinklers (NFPA 13, AS 2118.1, EN 12845), smoke vents and detection systems (NFPA 72, AS 1670.1, EN 54) are designed by others, but they affect the steel. Filled sprinkler mains and ESFR systems in high-bay storage add significant collateral load; smoke and heat vents are roof openings to be framed; and pipe hangers need defined fixing points on purlins. World Steel coordinates these loads during design, can apply shop intumescent coatings during fabrication where specified, and documents coating systems and thicknesses in the quality records.