Canadian Wind Load — NBCC 2020 & CSA S16 Reference

Quick Reference: NBCC 2020 Clause 4.1.7 governs wind load calculation for Canadian steel structures. The reference wind pressure p = I_W × q × C_e × C_g × C_p uses hourly mean wind speed (not 3-second gust). CSA S16-19 Clause 20 provides wind design provisions for steel buildings.

Wind loading is often the governing lateral load for steel buildings in low-to-moderate seismic regions. Canadian wind design follows the NBCC 2020 static wind procedure for most rigid buildings, with dynamic procedures required for flexible or wind-sensitive structures.

NBCC 2020 Wind Load Framework

Basic Wind Pressure Equation

The specified wind pressure at height z above ground per NBCC 2020 Cl. 4.1.7.3 is:

p = I_W × q × C_e × C_g × C_p

Where:

Reference Wind Velocity Pressure q

The reference velocity pressure q is based on the 1-in-50-year hourly mean wind speed at 10 m height in open terrain:

q = 0.5 × ρ × V²

Where ρ = 1.292 kg/m³ (air density at 10°C and standard pressure), and V is the hourly mean wind speed in m/s.

For V = 90 km/h (25.0 m/s): q = 0.5 × 1.292 × 25.0² = 404 Pa = 0.404 kPa

The hourly mean wind speed differs from the 3-second gust speed used in ASCE 7. Typical NBCC hourly wind speeds across Canada:

NBCC Appendix C provides the climatic data for all Canadian locations — these are the legal design values for code compliance.

Exposure Factor C_e

The exposure factor accounts for the increase in wind speed with height and the terrain roughness:

C_e = (h/10)^(0.2) × terrain factor adjustment

Exposure Factor Reference Table

The C_e factor varies with height above ground for each terrain category:

For a 10-storey steel building (30 m height) in downtown Toronto (Terrain C): C_e = 0.87 at roof height.

Gust Effect Factor C_g

NBCC 2020 provides two approaches for the gust effect factor:

Rigid buildings (natural frequency > 1 Hz, height/width < 4): C_g = 1.0 for the main structural system. This covers most low-to-mid-rise steel buildings.

Dynamic buildings (natural frequency ≤ 1 Hz or wind-sensitive): C_g is calculated from the dynamic response including:

C_g = 1 + 2 × g × I_v × sqrt(B + (s × F) / beta)

Where:

• g = peak factor (typically 3.5)
• I_v = turbulence intensity at roof height
• B = background response factor (quasi-static gust)
• sF = resonant response factor
• beta = damping ratio (steel buildings: 0.015 to 0.03 typical)

For steel buildings over 60 m tall or with a fundamental period above 1.0 second, a dynamic wind analysis should be considered. Steel's inherent damping is low (0.5-2.0% of critical) compared to concrete (2-5%), making steel buildings more sensitive to dynamic wind excitation.

Pressure Coefficient C_p

External pressure coefficients depend on building shape and wind direction:

For gable roofs:

Canadian vs US Wind Load Comparison

The key difference is the wind speed definition. NBCC uses hourly mean wind speeds that are approximately 55-65% of ASCE 7's 3-second gust speeds. A 150 km/h ASCE 7 gust corresponds roughly to an 85-95 km/h NBCC hourly wind. The resulting factored wind pressures are comparable despite the different factors.

Topographic Effects (C_t)

NBCC 2020 Clause 4.1.7.5 requires a topographic factor C_t when a building is located on a hill, ridge, or escarpment:

C_t = 1 + k × (tan(θ))^(0.5) × (z/L_d)

Where:

• k = 1.0 for escarpments, 1.2 for ridges, 1.4 for hills
• θ = slope angle (degrees)
• z = height above base of slope (m)
• L_d = distance from crest to half-height (m)

C_t applies when the site is within 50 km of the crest and the slope exceeds 1 in 10 (approximately 5.7°). For buildings on the crest of a 2:1 slope, C_t can reach 1.5 to 2.0, substantially increasing wind loads at the building perimeter.

Dynamic Wind Response for Steel Buildings

Steel buildings are more sensitive to dynamic wind excitation than concrete buildings due to lower damping. Key considerations:

For tall steel buildings, the dynamic response factor (s × F in the C_g equation) amplifies the base shear by 1.2 to 1.8 times the static value. The building's natural frequency must be computed from the structural system (moment frame, braced frame, or tube) and the mass distribution.

CSA S16 Wind Design Provisions

CSA S16-19 Clause 20 provides specific steel design provisions for wind loads:

Cl. 20.1 — Wind load effects: Steel members and connections must be designed for the wind load effects specified by NBCC 2020, including both pressures and suctions (uplift). Members subject to load reversal (windward vs leeward) must be checked for both loading directions.

Cl. 20.2 — Lateral bracing: Wind bracing systems (braced frames, moment frames, shear walls) must provide continuous load paths from the roof and floor diaphragms to the foundation. Steel deck diaphragms must comply with CSA S16 Cl. 20.3 for shear transfer.

Cl. 20.3 — Steel deck diaphragms: The steel deck (roof and floor) acts as a diaphragm distributing wind loads to vertical bracing elements. Diaphragm shear capacity must be verified for the factored wind load. Connection of deck to beams (puddle welds, screws) must develop the diaphragm shear.

Cl. 20.4 — End wall columns: End wall columns supporting wind loads from gable ends must be designed for the tributary wind pressure as beam-columns per Cl. 13.8 (combined axial and bending). The axial load from wind uplift reduces the column compression capacity.

Cl. 20.5 — Wind bracing connections: Connections in wind-bracing systems must be designed for the full member capacity or the factored load, whichever is less. Eccentricity in braced frame connections must be considered per Cl. 20.5.2.

Worked Example: Wind Load on a Steel Building Frame

Problem: Calculate the wind load on a 5-storey steel office building in Toronto (NBCC Normal importance). Building height = 18 m, width = 30 m (windward), depth = 20 m (leeward), located in suburban terrain (Terrain B).

Given:

• Hourly wind speed V = 90 km/h = 25.0 m/s (Toronto per NBCC Appendix C)
• I_W = 1.0 (Normal importance)
• Terrain B (suburban)
• Building: 18 m × 30 m × 20 m, rigid steel frame with concentric bracing

Step 1 — Reference velocity pressure:

q = 0.5 × ρ × V² = 0.5 × 1.292 × 25.0² = 404 Pa = 0.404 kPa

Step 2 — Exposure factor at roof height (18 m, Terrain B):

C_e = (18/10)^(0.2) × 0.85 = 1.125 × 0.85 = 0.956

Step 3 — Gust effect factor (rigid building, height/width < 4):

C_g = 1.0 (rigid building static method)

Step 4 — Internal pressure coefficient:

For a building with nominally sealed envelope (typical office): C_pi = ±0.25 (both positive and negative internal pressure — the governing case governs).

Step 5 — External pressure coefficients:

Windward wall: C_p = +0.80 (uniform over windward face for simplicity; NBCC allows stepped values by height zone — typically 0.55 at base, 0.80 at top)

Leeward wall: C_p = -0.50 (leeward face, depth/width = 20/30 = 0.67) Side walls: C_p = -0.65 Roof (flat): C_p = -1.0 (edge strip 3 m), C_p = -0.6 (interior zone)

Step 6 — Wind pressure on main windward frame:

Tributary width for main frame = 6.0 m (bay spacing)

Windward wall at mid-height (9 m): C_e at 9 m = (9/10)^(0.2) × 0.85 × 1.0 = 0.972 × 0.85 = 0.826

p_windward = 1.0 × 0.404 × 0.826 × 1.0 × 0.80 = 0.267 kPa Force per storey = 0.267 × 18 × 6.0 = 28.8 kN per bay (tributary area per floor)

Simplified per floor level (tributary height = 3.6 m):

• Roof level: p × tributary height × bay width = 0.267 × 1.8 × 6.0 = 14.4 kN (half roof)
• Each floor: 0.267 × 3.6 × 6.0 = 28.8 kN
• Ground level: 0.267 × 1.8 × 6.0 = 14.4 kN (half ground)

Step 7 — Suction on leeward face:

p_leeward = 1.0 × 0.404 × 1.0 × 1.0 × (-0.50) = -0.202 kPa Force per floor = 0.202 × 3.6 × 6.0 = 4.4 kN per bay (suction, adds to windward)

Step 8 — Total lateral wind force per frame bay:

Base shear per frame = sum of floor forces:

Step 9 — Overturning moment:

OTM = 18.8 × 18 + 33.2 × (14.4 + 10.8 + 7.2 + 3.6) = 338.4 + 33.2 × 36.0 = 338.4 + 1,195.2 = 1,533.6 kN·m per frame

Step 10 — Uplift check on roof:

p_roof_edge = 1.0 × 0.404 × 0.956 × 1.0 × (-1.0) = -0.386 kPa (uplift) p_roof_interior = 1.0 × 0.404 × 0.956 × 1.0 × (-0.6) = -0.232 kPa

Net uplift force on roof purlin at 3 m spacing, 6 m bay: W_uplift = -0.386 × 3.0 × 6.0 = -6.95 kN per purlin (design connection accordingly)

The roof purlin-to-beam connections must be designed for this net uplift using CSA S16 Cl. 20.1 (load reversal), combined with snow load per NBCC ULS combination 6b (0.9D + 1.4W).

Wind Load on Components and Cladding

For design of individual cladding elements (roof deck, wall panels, glazing), the gust effect factor is different:

p_c = I_W × q × C_e × C_gc × C_pc

Where:

• C_gc = gust factor for components and cladding (typically 1.5 to 2.5)
• C_pc = local pressure coefficient (higher near edges and corners)

For a corner roof panel: C_gc ≈ 2.5, C_pc ≈ -2.0 (peak suction) p_c = 1.0 × 0.404 × 0.956 × 2.5 × (-2.0) = -1.93 kPa — nearly 5 times the main frame pressure.

Component and cladding wind loads govern the design of connections for roof deck, wall panels, curtain wall mullions, and window framing. Steel girts and purlins in edge zones must account for these amplified loads per NBCC 4.1.7.7.

Related Pages

• Canada CSA S16 Steel Design Guide — Full CSA S16 design reference
• CSA S16 Load Combinations — NBCC ULS & SLS — Canadian load combination guide
• Canadian Snow Load — NBCC Ground & Roof Loads — Snow load calculation guide
• Canadian Seismic Design — CSA S16 Clause 27 — Seismic design provisions
• Canadian Steel Beam Sizes — W Shapes, HSS — Complete section tables
• Canadian Steel Grades — G40.21 300W to 480W — Material properties
• CSA S16 Beam Design — Flexure, LTB & Shear — Beam design per CSA S16
• Wind Load Calculator — Free wind load calculator

Frequently Asked Questions

How does NBCC wind load differ from ASCE 7 wind load?

The fundamental difference is the wind speed definition: NBCC uses hourly mean wind speed while ASCE 7 uses 3-second gust speed. The hourly speed is approximately 55-65% of the gust speed. For example, a Toronto design wind speed of 90 km/h (hourly) corresponds to roughly 150 km/h gust equivalent. The calculation formats differ: NBCC uses p = I_W × q × C_e × C_g × C_p while ASCE 7 uses p = q × G × C_p - q × GC_pi. Despite different factors and format, the resulting factored wind pressures for typical buildings are comparable — approximately 1.2 to 1.5 kPa for the windward wall of a mid-rise office building.

When is a dynamic wind analysis required for a Canadian steel building?

NBCC 2020 requires dynamic analysis for buildings that are wind-sensitive: those with a fundamental natural frequency below 1.0 Hz (period > 1.0 second) or with unusual geometry (large cantilevers, twisted forms). This typically corresponds to steel buildings over 50-60 m tall. Damping in steel buildings (0.5-2.0% of critical) is lower than concrete (2-5%), so the resonant response component is proportionally larger. For buildings over 100 m, wind tunnel testing is recommended. The dynamic gust factor C_g for a typical 80 m steel building may be 1.4 to 1.7 compared with 1.0 for a rigid building, significantly increasing the base shear.

What topographic factors apply to wind loads on Canadian hillside buildings?

NBCC 2020 Clause 4.1.7.5 requires a topographic factor C_t when a building is located on a hill, ridge, or escarpment with a slope exceeding 1 in 10 (5.7°). The factor ranges from 1.0 (level ground) up to 2.0 or more for buildings at the crest of steep slopes. Three terrain features are considered: hills (C_t multiplier = 1.4 base factor), ridges (1.2), and escarpments (1.0). The effect diminishes with distance from the crest and height above the base. For Canadian cities built on escarpments (Hamilton, St. Catharines, parts of Quebec City), this factor can add 20-40% to wind loads on the windward face.

What are the CSA S16 connection design requirements for wind bracing?

CSA S16-19 Clause 20.5 requires wind bracing connections to be designed for either the full member tensile capacity or the factored design load, whichever controls. Eccentric connections (gusset plate with single bolt line) must be designed for the resulting moment per Cl. 20.5.2. All bolts in wind bracing connections must be fully pretensioned when subjected to load reversal. The connection must have sufficient ductility to undergo at least two cycles of the design wind load without failure — this typically means limiting net section fracture and ensuring block shear does not govern. Welded wind bracing connections must comply with CSA W59 with Charpy-tested weld metal for exterior applications.

This page is for educational reference. Wind load provisions per NBCC 2020 Division B Clause 4.1.7. Verify wind speeds against current NBCC Appendix C climatic data. Canadian provinces may have amendments to the NBCC wind provisions. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent P.Eng. verification.