Australian Steel Fire Rating — AS 4100 & BCA Guide
Quick access: [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
Reference for fire resistance of steel structures per BCA 2022 Vol 1 (Building Code of Australia) and AS 4100:2020 Section 12. Covers FRL requirements, critical steel temperature, fire protection systems.
BCA Fire Resistance Levels (FRL)
| Building Type | Structural Frame | Columns | Beams | Floors |
|---|---|---|---|---|
| Class 2-9 (low rise ≤3 storeys) | 60/60/60 | 60/60/60 | 60/60/60 | 60/60/60 |
| Class 2-9 (4-8 storeys) | 90/90/90 | 90/90/90 | 90/90/90 | 90/90/90 |
| Class 2-9 (>8 storeys) | 120/120/120 | 120/120/120 | 120/120/120 | 120/120/120 |
| Class 5 (office, 3 storeys) | 60/60/60 | 60/60/60 | 60/60/60 | 60/60/60 |
| Open deck car park | — | — | — | 60/60/60 |
| Class 3 (residential, 4 storeys) | 90/90/90 | 90/90/90 | 90/90/90 | 90/90/90 |
Critical Steel Temperature — AS 4100 Section 12
AS 4100 Clause 12.5 defines the critical steel temperature based on load ratio:
| Load Ratio (R) | Critical Temp (°C) |
|---|---|
| 0.30 | 715 |
| 0.40 | 640 |
| 0.50 | 580 |
| 0.55 | 550 |
| 0.60 | 525 |
| 0.70 | 475 |
| 0.80 | 425 |
| 0.90 | 370 |
| 1.00 | 350 |
Steel temperature rise (unprotected): ΔT = (α × hnet × Am/V) × Δt / (ρ × cp)
Where Am/V = section factor (m⁻¹), typically 150-300 m⁻¹ for exposed steel.
Fire Protection Methods
| Method | Typical FRL | Application | Cost |
|---|---|---|---|
| Intumescent coating | 60-120 min | Aesthetically exposed steel | High |
| Spray-applied SFRM | 60-240 min | Concealed steel (ceilings) | Low |
| Board encapsulation | 60-120 min | Columns, beams (fire-rated) | Medium |
| Concrete encasement | 120-240 min | Columns (permanent) | High |
| Water filling (HSS) | 60-120 min | CHS columns | High |
Design Resources
- [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
FAQ
What is the standard FRL for a 5-storey office building? BCA requires 90/90/90 for buildings 4-8 storeys. Structural frame, columns, beams, and floors all require this rating.
What is the critical steel temperature per AS 4100? Typically 550°C at load ratio R=0.55. The critical temperature decreases as the load ratio increases. For R=0.60, Tcrit=525°C.
Which fire protection method is most cost-effective? Spray-applied SFRM is most cost-effective for concealed steel. Intumescent coatings are preferred for exposed architectural steel but cost 2-3x more.
Educational Use Only — This reference is for educational and preliminary design purposes only. All structural designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) in accordance with AS 4100:2020 and all applicable Australian Standards. Results are not for construction.