Fire Resistance Ratings Under NBCC 2020 / ULC

The National Building Code of Canada (NBCC 2020) references ULC (Underwriters Laboratories of Canada) standards for fire-resistance ratings. Ratings are expressed in hours, not minutes — a "2-hour rating" corresponds to structural adequacy for 120 minutes under the CAN/ULC-S101 standard fire exposure (equivalent to ISO 834):

ULC listings for fire-protection products are the definitive source for required thickness. A ULC-listed design number specifies the exact assembly — section size, protection material, thickness, and attachment method. Substitution of components is not permitted without a ULC engineering judgement.

Limiting Steel Temperature per CSA S16 Clause 14.2

The limiting steel temperature theta_s is determined from the load ratio at the fire limit state:

ks = (G + psi _ Q) / (phi _ R_n)

where G and Q are the dead and live loads (unfactored), psi is the live load combination factor (0.5 for most occupancies per NBCC), and phi*R_n is the factored resistance at ambient temperature.

For W-shapes in 350W steel with ks ≤ 0.5:

These temperatures are evaluated against the time-temperature profile from CAN/ULC-S101. For a 2-hour rated column with SFRM, the steel temperature must remain below 550 degrees C at 120 minutes.

Section Factor and Its Effect on Heating

In Canadian terminology, the section factor is expressed as W/D (mass per unit heated perimeter) in kg/m per mm, or its inverse A/P (heated perimeter over cross-sectional area). A low W/D (high A/P) heats faster:

This is the Canadian equivalent of the ksm (exposed perimeter / area) used in AS 4100 and EN 1993. Both express the same physics — thin, light sections absorb heat faster.

Spray-Applied Fire-Resistive Materials (SFRM)

SFRM is the dominant fire protection method for structural steel in North America. Canadian practice uses cementitious (Portland cement based) or mineral-fibre formulations, spray-applied directly to primed steel:

Common Canadian SFRM products:

• Cafco FENDOLITE MII (cementitious, ULC listed)
• Monokote MK-6 / Z-106 (cementitious, ULC listed)
• Isolatek Type 300 (mineral-fibre, ULC listed)

Application requirements per ULC:

• Steel surface must be clean and primed — bare or lightly rusted steel is acceptable; oil and grease must be removed
• Minimum ambient temperature during application: 4 degrees C (cementitious) or -7 degrees C (mineral fibre with cold-weather admixture)
• Thickness verified by depth gauge — minimum 80% of specified thickness at any point
• Density verified by core sample — minimum 240 kg/m3 (cementitious) or 180 kg/m3 (mineral fibre)

Typical SFRM thicknesses for W-shapes:

Thickness values are illustrative — always consult the ULC listing for the specific product and section.

Intumescent Coatings

Thin-film intumescent coatings are increasingly popular in Canada for architecturally exposed structural steel (AESS) — particularly in airport terminals, university buildings, and commercial atria where the steel is a visual feature.

Canadian intumescent products must be ULC listed. Key considerations for Canadian climate:

• Cure time: Epoxy intumescents require minimum 16 degrees C for 24 hours — winter construction requires tenting and heating
• Top coat: UV-stable top coat required for any steel exposed to sunlight through glazing — intumescent alone degrades under UV in months
• Thickness verification: DFT (dry film thickness) gauge readings on every member, with minimum reading frequency per CSA S16 or the ULC listing

Board Fire Protection

Calcium silicate or mineral-fibre boards mechanically fixed around the steel section. Common in Canadian mechanical rooms, parking garages, and service risers.

• ULC-listed board assemblies are tested for specific section sizes — field modification of board thickness or attachment spacing is not permitted without re-testing
• Board systems are preferred for exposed exterior columns in cold climates where SFRM would be damaged by freeze-thaw cycling
• Board enclosure systems can incorporate insulation for thermal break, addressing condensation risk on steel passing through the building envelope

Calculation Method per CSA S16 Clause 14.3

For standard fire exposure (CAN/ULC-S101), the simplified calculation method:

1. Determine the load ratio ks at the fire limit state
2. Establish the limiting steel temperature from ks
3. Calculate the section factor W/D or A/P
4. For the selected ULC-listed protection system, verify that the steel temperature at the design rating does not exceed the limiting temperature

CSA S16 provides temperature rise equations for unprotected steel (Clause 14.4) and protected steel (Clause 14.5). For protected steel:

deltaTs = (k_p / d_p) * (W/D)^-1 _ (T_f - T_s) _ deltat / (c_s * rho_s)

where k_p is the thermal conductivity of the protection, d_p is the protection thickness, T_f is the fire gas temperature, T_s is the steel temperature, and c_s*rho_s is the volumetric heat capacity of steel.

Critical Canadian-Specific Considerations

Seismic-fire interaction: In high-seismic zones (Victoria, Vancouver, Montreal), connections detailed for ductile seismic response may lose capacity if fire protection is damaged during an earthquake. CSA S16 does not explicitly require post-earthquake fire resistance, but many Canadian structural engineers consider this in critical facilities.

Cold-region firefighting: In remote northern communities, fire department response times exceed 30 minutes, and water supply may be limited. Some jurisdictions require enhanced fire ratings (2 hours minimum) for essential buildings where active firefighting cannot be relied upon.

AESS and intumescent inspection: For exposed steel in public buildings, the intumescent coating thickness must be verified by a third-party coating inspector. The Canadian Institute of Steel Construction (CISC) publishes AESS category guidance (AESS 1 through AESS 4 plus Custom) that defines the acceptable surface finish for architecturally exposed steel — this governs the acceptable appearance of intumescent coatings.