Fire Resistance Levels per the NCC

The National Construction Code (NCC 2022) specifies Fire Resistance Levels (FRLs) as three numbers: structural adequacy / integrity / insulation, each in minutes. For structural steel, the first number (structural adequacy) is the primary focus:

AS 4100 Section 12 allows demonstration of structural adequacy via calculation (Clause 12.2), standard fire test (AS 1530.4), or a combination. The calculation method is the most common for steel because tabulated protection thicknesses are well-established.

Limiting Steel Temperature per AS 4100 Clause 12.2

The limiting steel temperature theta_lim is the temperature at which the member can no longer sustain the design action effects at the fire limit state. Per Clause 12.2.2:

For beams in bending (no axial load):

theta*lim = 905 - 120 * rf where rf = M*_fi / (phi _ Ms) ≤ 1.0

M*_fi is the design bending moment in fire and Ms is the nominal section moment capacity at ambient temperature. At rf = 0.4 (typical dead + reduced live case), theta_lim = 857 degrees C.

For columns in compression:

theta*lim = 905 - 120 * rf where rf = N*_fi / (phi _ Ns) ≤ 1.0

Columns typically have lower utilisation (rf ≈ 0.3), giving theta_lim ≈ 869 degrees C.

For tension members:

theta_lim is typically higher because tensile capacity degrades more slowly with temperature. With rf = 0.5, theta_lim ≈ 845 degrees C.

Critical temperature limits (regardless of utilisation):

• Grade 300 steel (fy = 300 MPa): max 620 degrees C for members relying on plastic capacity
• Grade 350 steel (fy = 350 MPa): max 590 degrees C
• Bolted connections in shear: 450-500 degrees C (bolts lose preload above 400 degrees C)

Section Factor ksm and Heating Rate

The section factor (ksm) = exposed perimeter / cross-sectional area, expressed in m^-1 (or mm^-1). It is the single most important parameter determining how quickly a steel section heats in fire:

A section with higher ksm heats faster and requires thicker fire protection. This is why shallow, light sections in roof framing need proportionally more fire protection than heavy columns.

Passive Fire Protection Systems

Board Encasement (Vermiculite or Calcium Silicate)

Factory-made boards fixed mechanically around the steel section. Typical Australian products: Promat PROMATECT-L500 or BGC Fire-rated plasterboard.

• Advantages: Clean, dry installation; predictable thickness; good for columns; can be decorated
• Disadvantages: Labour-intensive for complex shapes; board joints need sealing; difficult around connections
• Typical thickness: 20-40 mm for 120-minute FRL on moderate sections

Spray-Applied Vermiculite (Sprayed Fibre)

Cementitious or mineral-fibre spray applied directly to the steel surface. Australian products: Cafco FENDOLITE MII, Monokote MK-6.

• Advantages: Conforms to complex geometry; fast application; lower cost than board
• Disadvantages: Messy application; thickness control variable; susceptible to mechanical damage; not suitable for external or high-humidity environments
• Typical thickness: 15-35 mm for 120-minute FRL

Intumescent Coatings

Thin-film epoxy or acrylic coatings that expand and char when exposed to fire, forming an insulating foam layer typically 25-50 times the original dry film thickness.

• Advantages: Architecturally invisible — preserves the steel aesthetic; thin (0.5-3 mm dry film); can be top-coated in any colour; ideal for architecturally exposed structural steel (AESS)
• Disadvantages: Higher material cost than spray or board; requires certified applicators; moisture-sensitive during curing; thickness must be verified with DFT gauge
• Typical DFT: 1.0-2.5 mm for 60-minute, 2.0-4.0 mm for 120-minute FRL (varies significantly with section factor)

Concrete Encasement

Traditional method — steel encased in concrete. Now less common except for composite design or heritage works.

• Provides very high fire resistance (240+ minutes achievable)
• Adds substantial dead load and section depth
• Requires formwork and reinforcement

Simplified Design Method per AS 4100 Clause 12.2.3

For standard fire exposure (ISO 834 time-temperature curve), the protection thickness may be determined from:

1. Calculate the section factor ksm
2. Determine the required limiting temperature theta_lim from utilisation ratio
3. Select protection system and read required thickness from manufacturer's certified test data
4. Verify that the protection material can deliver the required insulation to prevent theta_steel exceeding theta_lim at the design FRL duration

Critical Detailing Requirements

Connection fire protection: Per NCC requirements, connections and bolts must achieve the same FRL as the connected members. Intumescent coatings work well here because they can be applied to bolted connections after assembly.

Thermal expansion joints: Portal frames longer than 60 m should allow for thermal expansion. At 600 degrees C, a 50 m steel frame expands approximately 350 mm — sufficient to cause column distress if ends are rigidly restrained.

Slim floor and partially encased sections: Partially encased beams (concrete between flanges) have significantly reduced ksm, often eliminating the need for additional fire protection for 60-90 minute FRLs. This is an under-utilised design strategy in Australia.

Verification requirements per NCC: Fire protection thickness must be verified by an accredited certifier. Manufacturer's test reports must cover the actual section shape, size, and orientation — extrapolation between test configurations is not permitted without an engineering assessment.