------------------------------ | :--------: | :-------: | :------: | :------: | | 1-2 storey, business (sprinklered) | 45 min | 45 min | 45 min | None | | 3 storey, business (sprinklered) | 1 hour | 1 hour | 1 hour | None | | 4-6 storey, business (sprinklered) | 1 hour | 1 hour | 1 hour | 45 min | | > 6 storey, business (sprinklered) | 2 hours | 2 hours | 2 hours | 1 hour |

Unprotected Steel Exceptions

Per NBCC 2020 Article 3.1.5.1, steel may remain unprotected if:

  1. Single-storey building with low occupancy load
  2. Sprinklered throughout and building height ≤ 3 storeys
  3. Roof construction with no occupancy above (in most cases)
  4. Parking structures meeting specific requirements
  5. Exposed structural steel in one-storey buildings with non-combustible construction

Fire Protection Methods

Spray-Applied Fire Resistive Materials (SFRM)

Type Material Density (kg/m^3) Typical Thickness Cost ($/m^2) Application
Cementitious Vermiculite/cement 240-400 12-50 mm Low Interior, concealed
Mineral fibre Rockwool/gypsum 150-300 12-50 mm Low Interior, concealed
Intumescent Epoxy/acrylic 1200-1500 0.5-3.0 mm High Exposed, architectural

SFRM Thickness for 1-Hour Rating

Section Factor A/P (m^-1) Cementitious Thickness (mm) Mineral Fibre Thickness (mm)
≤ 100 12 12
100-150 16 16
150-200 20 20
200-300 25 30
> 300 35 40

Section Factor (A/P or A/V)

The section factor is the heated perimeter (Hp) divided by the cross-sectional area (A), or equivalently A/P per unit length:

For W-shapes: A/P = Area / (2 × d + 4 × bf - 2 × tw) — perimeter exposed to fire

Section A (mm^2) Hp (mm) A/P (m^-1) Fireproofing Need
W310×39 5,060 1,350 267 Moderate
W410×60 7,550 1,650 218 Moderate
W530×82 10,400 1,970 189 Moderate
W610×125 15,900 2,200 138 Low
W360×216 27,500 2,130 77 Low

Higher A/P means faster heating and more fireproofing required.

Intumescent Coatings

Intumescent coatings expand when heated (to 30-50× original thickness), forming a char that insulates the steel:

Rating Dry Film Thickness (mm) Typical Cost Appearance Best For
30-45 min 0.3-0.6 $$ Near-invisible Exposed steel look
1 hour 0.6-1.2 $$$ Slight texture Architectural
2 hour 1.2-2.5 $$$$ Thick, visible Heavy fire resistance

Application Conditions

Condition Requirement
Temperature 10-30°C during application
Humidity ≤ 80% RH
Steel surface preparation SSPC-SP6 (commercial blast)
Priming Required — intumescent-specific primer
Top coat UV-resistant if exposed

Concrete Encasement

Concrete-Filled HSS

Per CSA S16 Clause 18, concrete-filled HSS provides inherent fire resistance:

HSS Size Fill Fire Rating (min) Mechanism
HSS 152×152×8 Unfilled 15 Bare steel
HSS 152×152×8 30 MPa concrete fill 60 Heat sink effect
HSS 254×254×9.5 30 MPa concrete fill 90 Heat sink + barrier
HSS 254×254×12.7 30 MPa concrete fill 120 Heat sink + barrier

Concrete Encased Steel (Fireproofing)

Concrete encasement of W-shapes (typically 50-75 mm cover):

Column Encasement Rating Note
Any 50 mm concrete cover 1 hour Normal weight
Any 75 mm concrete cover 2 hours Normal weight
Any 50 mm concrete cover 1.5 hour Lightweight concrete

Structural Fire Design

Fire Limit State

Per NBCC 2020, the fire limit state uses reduced loads:

Steel Strength Reduction at Elevated Temperature

Per CSA S16 Annex M:

Temperature (°C) Fy Reduction Factor E Reduction Factor Strength Remaining
20 (ambient) 1.00 1.00 100%
200 1.00 0.90 100%
300 1.00 0.85 100%
400 0.90 0.75 90%
500 0.70 0.60 70%
600 0.40 0.40 40%
700 0.15 0.15 15%

The critical steel temperature for structural adequacy is typically 550°C for beams in bending (where the load ratio is ~0.6) and 450-500°C for columns (higher load ratio).

Worked Example — 1-Hour Fire Rating for Beam

Given: W610×125 beam in a 4-storey office building (sprinklered). Beam spacing = 3.0 m, span = 9.0 m. Required: 1-hour FRR.

Step 1 — Determine Section Factor: W610×125: A = 15,900 mm^2, d = 612 mm, bf = 229 mm, tw = 11.9 mm Hp = 2×612 + 4×229 - 2×11.9 = 1,224 + 916 - 24 = 2,116 mm A/P = 15,900 / 2,116 = 7.51 mm = 133 m^-1 (shielded — protected on top by slab)

Step 2 — Select Fireproofing: For A/P = 133 m^-1, 1-hr rating:

Step 3 — Structural Adequacy (Fire Limit State): Load at fire limit state: D + 0.5L + 0.4S Assume D = 8 kN/m, L = 12 kN/m, S = 3 kN/m W_fire = 8 + 0.5×12 + 0.4×3 = 8 + 6 + 1.2 = 15.2 kN/m M_fire = 15.2 × 9.0^2 / 8 = 154 kN·m Mr (ambient) = 0.90 × 3430 × 350 / 10^6 = 1,080 kN·m Load ratio = 154 / 1080 = 0.14

At load ratio 0.14, the beam can reach > 700°C before failure. 1-hour SFRM of 16 mm will keep the steel below 400°C for 60 minutes. OK.

Result: W610×125 beam with 16 mm cementitious SFRM (or 0.8 mm intumescent) achieves 1-hour fire rating.

Frequently Asked Questions

What fire rating does a steel beam need in a Canadian office building? For a 4-storey office building per NBCC 2020: 1-hour FRR for columns, beams, and floors. For 6+ storeys: 2-hour FRR. For 1-2 storey with sprinklers: 45-minute FRR. The rating depends on building height, occupancy, and whether the building is sprinklered. Buildings with sprinklers throughout may reduce FRR by 30 minutes on some members.

What is the difference between SFRM and intumescent coatings? SFRM (spray-applied fire resistive material) is a low-cost cementitious or mineral fibre coating applied 12-50 mm thick. It is effective but has a rough texture, making it suitable for concealed steel only. Intumescent coatings are thin-film (0.5-2.5 mm) that expand under heat to form an insulating char. They are more expensive but provide an architectural finish suitable for exposed steel.

How does a concrete-filled HSS achieve fire resistance without additional fireproofing? The concrete core acts as a heat sink, absorbing thermal energy from the steel tube. The moisture in the concrete provides evaporative cooling (latent heat of vaporisation). For a 1-hour rating, HSS 152×152×8 with concrete fill typically suffices without additional fireproofing. For 2-hour ratings, HSS 254×254×12.7 or larger is needed, depending on load ratio.

What is the critical steel temperature for structural fire design? The critical temperature depends on the load ratio (applied load at fire limit state / ambient capacity). For load ratio = 0.6 (typical for beams), the critical temperature is approximately 550°C, where steel retains ~40% of ambient strength. For columns with higher load ratio (0.7), the critical temperature is 450-500°C. Fireproofing is designed to keep the steel below the critical temperature for the required duration.

Related Pages

This page is for educational reference. Fire resistance per NBCC 2020 Division B Part 3. Verify fireproofing thicknesses with product-specific listings (ULC/UL). Fire limit state loads per NBCC 2020 Clause 4.1.5. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.