Canadian Steel Fire Protection — NBCC 2020 & ULC Standards

Quick Reference: NBCC 2020 Division B Clause 3.1.7 and 3.2.2 govern fire protection of structural steel. Fire resistance ratings (FRR) range from 45 minutes to 2 hours depending on building height and occupancy. Protection methods include spray-applied fire-resistive materials (SFRM), intumescent coatings, gypsum board encasement, and concrete encasement. ULC S101 provides the fire test standard.

Fire protection is a critical design consideration for Canadian steel buildings. Unprotected structural steel loses strength rapidly at elevated temperatures — at 500°C, steel retains only about 60% of its ambient-temperature yield strength and at 700°C, about 20%. The NBCC 2020 fire protection requirements ensure steel structures maintain their stability for sufficient time to allow occupant evacuation and firefighter access.

NBCC 2020 Fire Resistance Requirements

Fire Resistance Ratings (FRR) by Building Type

NBCC 2020 Table 3.2.2 specifies minimum FRR for structural steel members based on building classification:

The exact FRR depends on:

• Building height (number of storeys above grade). Taller buildings require higher FRR.
• Building area per floor. Larger floor areas require higher FRR.
• Sprinkler provision. Fully sprinklered buildings may have reduced FRR (typically 50% reduction).
• Occupancy type. Group B (hospital, prison) has the highest requirements.

Sprinkler Trade-offs

NBCC 2020 permits FRR reductions when the building is fully sprinklered:

Sprinkler reduction is not permitted for:

• Group B (care/occupancy) buildings
• Buildings used for high-hazard industrial occupancies
• Storage buildings with combustible contents exceeding certain thresholds

Fire Test Standards

ULC S101 (CAN/ULC S101-14)

The primary fire test standard for building materials and assemblies in Canada is ULC S101 "Standard Method of Fire Endurance Tests of Buildings and Construction Materials." This is the Canadian equivalent of ASTM E119.

Key requirements of ULC S101:

• Furnace temperature curve: Follows the standard time-temperature curve, reaching 538°C at 5 minutes, 704°C at 10 minutes, 843°C at 30 minutes, 927°C at 60 minutes, and 1,010°C at 120 minutes.
• Acceptance criteria: The assembly must not allow passage of flame or hot gases, must not exceed a 140°C average temperature rise on the unexposed surface (or 180°C at any point), and must sustain the applied structural load (for load-bearing assemblies).
• Restrained vs unrestrained: ULC S101 distinguishes between restrained and unrestrained assemblies. Restrained thermal expansion against adjacent structure can affect fire resistance.

UL/ULC Listings

Fire protection materials (SFRM, intumescent coatings, fire-resistant boards) carry UL/ULC listings that specify:

• Thickness: The minimum dry film thickness in mm.
• Substrate: The specific steel section shape and size (W, HSS, etc.).
• Fire resistance: The achieved FRR in hours (1 hour, 1.5 hours, 2 hours, 3 hours).
• Steel temperature limit: The limiting steel temperature (typically 538°C for columns and 593°C for beams — the "critical temperature").

Steel Fire Protection Methods

Spray-Applied Fire Resistive Material (SFRM)

SFRM (also called sprayed fireproofing) is the most common method for protecting structural steel in Canada:

Pros: Lowest cost per ft², quick application, compatible with complex shapes, widely available. Cons: Messy application (overspray), susceptible to mechanical damage, requires primer on steel, aesthetically unappealing (usually concealed), may need wire mesh reinforcement for overhead applications. Typical thickness: 12-25 mm for 1-hour rating on W310 column sections.

Intumescent Coatings

Intumescent coatings expand when heated to form a thick char layer that insulates the steel:

Pros: Aesthetically acceptable for exposed steel (architectural finish), thin coating, clean application, can be colour-matched. Cons: Higher cost, requires specialist applicator, less impact-resistant than SFRM, may require top-coat for UV resistance (exterior applications), limited to shorter durations (typically up to 2 hours). Applications: Exposed steel in atriums, architectural columns, open-concept office areas where steel is left exposed.

Board Encasement (Gypsum or Mineral Fibre)

Fire-resistant boards mechanically fastened around steel sections:

Pros: Clean application (no overspray), removable for inspection, provides impact protection, predictable fire performance. Cons: Labour-intensive (cutting and fastening), visual impact (boxed appearance), difficult for complex geometries, may not follow tight radius curves.

Concrete Encasement

Full concrete encasement provides the highest fire resistance:

Pros: Excellent fire resistance, very durable, no maintenance. Cons: Significant weight addition (increases column/beam loads), thicker than other methods, labour-intensive, limits future modifications.

Steel Temperature and Strength Loss

Critical Temperature Concept

The "critical temperature" is the steel temperature at which the strength reduction is equal to the applied stress ratio. For typical Canadian design:

• Columns under service load: Critical temperature ≈ 538°C — corresponds to approximately 60% of ambient yield strength, balanced against the gravity load level at time of fire.
• Beams (restrained): Critical temperature ≈ 620-700°C — restrained beams can tolerate higher temperatures because adjacent structure provides catenary action.
• Beams (unrestrained): Critical temperature ≈ 550-620°C — governed by moment resistance at mid-span.

Steel Strength Reduction at Elevated Temperatures

At the ULC S101 standard fire exposure:

• Unprotected steel (W310x39): reaches 538°C in approximately 12-15 minutes, depending on the section factor (H_p/A).
• Protected steel (same section, 20 mm SFRM): reaches 538°C in approximately 60-90 minutes.

The section factor H_p/A (heated perimeter divided by cross-sectional area, expressed in m⁻¹) governs the rate of heating. Lower H_p/A = slower heating (better fire resistance). Large, stocky sections (W360x262, H_p/A ≈ 25 m⁻¹) heat up more slowly than light sections (W150x13, H_p/A ≈ 150 m⁻¹).

Section Factor H_p/A

The section factor is the single most important parameter in steel fire design:

For boxed protection (SFRM or board on all four sides of a W-shape), the H_p/A uses all four sides. For exposed-on-three-sides (beam supporting a concrete slab on top), only three sides are heated, reducing H_p/A by approximately 25%.

CSA S16 Fire Design Provisions

CSA S16-19 Clause 28 addresses structural integrity under fire conditions:

Cl. 28.1 — General: Steel structures must maintain load-bearing capacity for the NBCC-required fire resistance period. This can be achieved through fire protection (SFRM, intumescent, board, concrete) or through inherent fire resistance (large section factor, protected by building elements).

Cl. 28.2 — Material properties at elevated temperature: The reduction factors from the Commentary (Annex L) are used for strength and stiffness at elevated temperatures. Canadian reduction factors for 350W steel align with Eurocode 3 Part 1.2 values.

Cl. 28.3 — Connection integrity: Connections must maintain structural integrity during the required fire resistance period. End-plate connections with bolts exposed to fire heat up faster than the connected beam or column. In Canadian practice, connections in fire-rated frames are typically 1.25× the design capacity at ambient temperature to account for elevated temperature strength loss.

Cl. 28.4 — Fire protection continuity: Fire protection at beam-to-column connections and floor penetrations must be detailed to maintain the fire resistance rating at the joint. Gaps in SFRM coverage at bolts or stiffeners must be filled with firestop compound rated for the required duration.

Worked Example: Fire Protection Specification

Problem: Specify fire protection for a W310x97 steel column in a 6-storey fully sprinklered office building in Toronto. Required fire resistance rating = 1.0 hour (sprinkler trade-off from 1.5 hours base).

Given:

• Column: W310x97 (US equivalent W12x65)
• H_p/A = 55 m⁻¹ (boxed, four sides exposed)
• Steel grade: 350W
• Service axial load: C_f_service = 2,500 kN (approximately 50% of ambient C_r)

Option 1 — SFRM (most economical for concealed column):

From UL/ULC listing for W310x97 with cementitious SFRM (density 480 kg/m³):

• 1-hour rating → 12 mm thickness
• 1.5-hour rating → 20 mm thickness
• 2-hour rating → 30 mm thickness

Specify: 15 mm cementitious SFRM (provides margin over 1.0 hour minimum).

Application notes:

• Primer required on steel before SFRM application (rust-inhibitive, epoxy-based)
• Wire mesh for thicknesses > 25 mm and all overhead applications
• Minimum ambient temperature during application: 4°C
• Maximum ambient humidity: 85% RH
• Cure time before concealment: 7 days at 20°C, 50% RH

Option 2 — Intumescent coating (exposed architectural column):

From UL/ULC listing for W310x97 with epoxy intumescent:

• 1-hour rating → 1.2 mm dry film thickness (DFT)

Specify: 1.5 mm epoxy intumescent DFT (provides margin). Final colour to match architectural finish.

Application notes:

• Surface preparation: SSPC-SP6 commercial blast cleaning
• Primer: 75-100 microns epoxy primer
• Intumescent: applied in multiple coats (2-3 coats of 0.5-0.75 mm each)
• Top coat: 50-75 microns polyurethane (UV resistant if exposed to sunlight)
• Confirmation of thickness with wet film gauge during application and DFT gauge after curing

Option 3 — Board encasement (high-abuse area):

Specify: 16 mm Type X gypsum board, three layers for total 48 mm thickness, on each side of column.

Steel stud furring at 600 mm o.c., screw-attached to column flanges with 25 mm air gap.

Cost comparison per linear metre:

SFRM is the most economical for concealed steel. Intumescent costs 2-3× more but provides an architectural finish. Board encasement is intermediate and provides impact resistance.

Unprotected Steel — When Is It Permitted?

Unprotected steel is permitted in Canadian construction under specific conditions:

1. Part 9 buildings (small buildings less than 3 storeys and 600 m²) — FRR is typically 45 minutes which unprotected steel with moderate section factor may satisfy.
2. Roofs where the required FRR is 45 minutes or less and the roof is not used for occupant egress.
3. Parking garages where the structure is open on at least two sides (natural ventilation limits heat buildup).
4. Buildings with engineered performance-based fire design (Alternative Solutions per NBCC 1.2.1.1) — requires demonstration through fire dynamics simulation (FDS) or structural fire engineering analysis.
5. Industrial buildings with low fire load and sprinkler protection.
6. Exterior steel not required for egress path stability.

Note: The lack of fire protection on steel does not change its temperature-dependent strength loss. An unprotected column under full service load will fail within 15-30 minutes of fire exposure depending on the section factor. Unprotected steel must always be justified through engineering analysis.

Related Pages

• Canada CSA S16 Steel Design Guide — Full CSA S16 design reference
• Canadian Steel Beam Sizes — W Shapes, HSS — Complete section tables
• Canadian Steel Grades — G40.21 300W to 480W — Material properties
• CSA S16 Column Buckling — Euler & Fcr Curves — Column buckling theory
• CSA S16 Beam Design — Flexure, LTB & Shear — Beam design per CSA S16
• BEAM Capacity Calculator — Free multi-code beam calculator

Frequently Asked Questions

What is the minimum fire resistance rating for steel columns in a Canadian office building?

For a Group D office building, NBCC 2020 Table 3.2.2 requires: 45 minutes for buildings up to 3 storeys, 1 hour for 4-5 storeys, 1.5 hours for 6-7 storeys, and 2 hours for 8+ storeys. Fully sprinklered buildings may qualify for a 50% reduction (e.g., 1.5 hours reduced to 45 minutes). However, sprinkler reduction is not automatic — it requires the building to be equipped with a fire alarm system and the sprinkler system designed per NFPA 13. The column FRR requirement is typically the same as the floor assembly requirement for the highest floor it supports.

How is intumescent coating thickness determined for Canadian steel sections?

Intumescent coating thickness is determined from UL/ULC listing data provided by the coating manufacturer. The listing specifies the minimum DFT for each section factor (H_p/A) range and required FRR. For a W310x97 with H_p/A = 55 m⁻¹: a 1-hour rating requires approximately 1.2 mm DFT of epoxy intumescent. For a lighter W310x39 with H_p/A = 85 m⁻¹: the same 1-hour rating requires 1.6-1.8 mm DFT — the thinner section heats faster and needs thicker coating. Canadian manufacturers (Carboline, PPG, AkzoNobel, Jotun) have CCMC evaluations listing specific thicknesses for Canadian building code acceptance.

What is the section factor H_p/A and why does it matter for fire protection?

The section factor H_p/A is the ratio of the heated perimeter (H_p) to the cross-sectional area (A), expressed in m⁻¹. It determines how quickly the steel section heats up in a fire. A low H_p/A (stocky section like W360x262 with H_p/A ≈ 25 m⁻¹) heats up slowly, while a high H_p/A (light section like W150x13 with H_p/A ≈ 150 m⁻¹) heats up rapidly. The required fire protection thickness is directly related to H_p/A — a beam with H_p/A = 100 m⁻¹ needs approximately 1.4× the protection thickness of a beam with H_p/A = 50 m⁻¹ for the same FRR. HSS sections, with their enclosed shape, combine moderate H_p/A with inherent thermal benefits (internal heat sink from the enclosed air).

Can Canadian steel buildings use performance-based fire design instead of prescriptive FRR?

Yes. NBCC 2020 permits Alternative Solutions (Division A Clause 1.2.1.1) that achieve an equivalent level of fire safety through engineered performance-based design. This typically involves structural fire engineering analysis using: (1) fire dynamics simulation (FDS or CFAST) to model fire growth and tenability; (2) heat transfer analysis (finite element) to compute steel temperatures during the fire; (3) structural analysis at elevated temperatures (including thermal expansion effects) to verify stability. Performance-based design is more common for complex buildings (large atriums, airport terminals, stadiums) where prescriptive ratings are either impractical or unnecessarily conservative. The design must be submitted to the authority having jurisdiction with a peer review by an independent fire engineering specialist.

This page is for educational reference. Fire protection requirements per NBCC 2020 Division B, ULC S101, and CSA S16-19. Verify all fire protection specifications against current UL/ULC listings and CCMC evaluations. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent P.Eng. verification.