NBCC 2020 Wind Load Guide — Canadian Wind Provisions

Complete reference guide to wind load determination for structural design per the National Building Code of Canada 2020 (NBCC 2020), Division B, Part 4, Section 4.1.7. Covers the static (simplified) procedure using the reference velocity pressure q50, exposure factor Ce, gust effect factor Cg, and external pressure coefficients Cp. Includes a worked example for a steel industrial building in Toronto.

Related pages: Canadian Wind Load | CSA S16 Steel Design | Seismic Load Calculator | Wind Load Calculator

NBCC 2020 Wind Load Framework

Code Reference: NBCC 2020 Section 4.1.7, Structural Commentaries Part 4

The NBCC 2020 wind load provisions are based on a statistical approach using reference wind pressures derived from hourly-mean wind speed data collected at meteorological stations across Canada. The design approach uses the following general equation for external pressure:

[ p = I_w \times q \times C_e \times C_g \times C_p ]

Where:

• p = specified external pressure acting statically on a surface (Pa or kPa)
• Iw = importance factor for wind (Table 4.1.7.1)
• q = reference velocity pressure (Pa), based on the reference wind speed V (1-in-50 year return period)
• Ce = exposure factor accounting for terrain roughness and height above ground
• Cg = gust effect factor accounting for fluctuating wind
• Cp = external pressure coefficient accounting for building shape and wind direction

For internal pressure, a similar equation applies with Cpi replacing Cp and usually omitting the gust factor (or using a modified Cgi).

Step 1 — Reference Velocity Pressure q50 (NBCC 2020 Table C-1, Appendix C)

The reference velocity pressure q is calculated at a reference height of 10 m in open terrain (Exposure A equivalent) with a return period of 50 years:

[ q = \frac{1}{2} \times \rho \times V^2 ]

Where rho = 1.2929 kg/m^3 (standard air density at 0 degrees C) and V is in m/s.

Selected Canadian Cities — Reference Velocity Pressure q50

Values from NBCC 2020 Appendix C, Table C-1. The q50 value already accounts for the 1-in-50 year return period. For other locations, interpolate from the NBCC wind maps or consult the NBCC Climatic Data tables.

Step 2 — Importance Factor Iw (Table 4.1.7.1)

For a Normal importance building: Iw = 1.0.

The importance factor scales the reference wind pressure up or down based on the consequences of failure. Note that unlike ASCE 7 (which embeds importance in the hazard map), NBCC applies the importance factor as an explicit multiplier on the design pressure.

Step 3 — Exposure Factor Ce (Section 4.1.7.1, Table 4.1.7.3)

The exposure factor Ce reflects the variation of wind speed with height above ground and the roughness of the surrounding terrain.

Terrain Categories (NBCC 2020 Commentary I-14)

The Ce factor is tabulated in NBCC 2020 Table 4.1.7.3 as a function of height above ground:

Exposure Factor Ce Values

For intermediate heights, Ce may be interpolated linearly. Above 300 m, Ce is the same for both terrain categories — at high elevations, the surface roughness effect diminishes and local turbulence controls.

Note: The previous NBCC 2015/2010 combined the gust effect into the Ce factor; NBCC 2020 separates Ce (mean wind profile only) and Cg (gust). However, many provincial building codes still reference NBCC 2015 and use the combined CexCg format. Always verify the applicable edition.

Step 4 — Gust Effect Factor Cg (Section 4.1.7.1, Commentary I-15)

The gust effect factor Cg accounts for the dynamic response of the structure to gusty wind. NBCC 2020 provides simplified values:

Cg for Different Structure Types

For the simplified static procedure (clause 4.1.7.1(5)), Cg is taken as:

• Cg = 2.0 for the main structural system (external pressures on primary framing)
• Cg = 2.5 for cladding and secondary structural members (C&C elements)

Where a more detailed dynamic analysis is warranted (tall buildings, natural frequency < 1 Hz, height-to-width ratio > 4), the full dynamic procedure in Commentary I-24 should be used. This computes Cg as a function of the structure's natural frequency, damping ratio, and the turbulence intensity profile. For most steel-framed buildings under 60 m in height, the simplified Cg values are adequate.

The NBCC Cg = 2.0 is substantially higher than the ASCE 7 gust factor G = 0.85 — this is because the ASCE value is applied to a 3-second gust wind speed while the NBCC applies to an hourly-mean wind speed (~40% lower than the 3-s gust). When converting between codes, the combined wind effect (wind speed x gust factor) should be compared, not the gust factors in isolation.

Step 5 — External Pressure Coefficient Cp (Figure 4.1.7.6)

The NBCC 2020 provides an extensive set of pressure coefficient diagrams for buildings of various shapes. The Cp values are based on wind tunnel studies and are generally similar to those in ASCE 7 and EN 1991-1-4.

Cp for Rectangular Buildings with Flat Roofs (Figure 4.1.7.6-A)

Windward wall:

Where h = building height and d = building depth parallel to wind.

Leeward wall:

Side walls: Cp = -0.70 uniformly (suction).

Roof — flat (<10 degrees slope):

Step 6 — Internal Pressure Coefficient Cpi (Figure 4.1.7.6-G)

Internal pressure depends on the distribution and size of openings. NBCC 2020 commentary provides:

For a typical enclosed industrial building (roll-up doors closed during design wind event, small openings uniformly distributed): Cpi = +/-0.30. Both signs must be checked — the internal pressure adds to suction on the roof and leeward wall (+Cpi) or adds to the windward wall push (-Cpi as internal suction).

Step 7 — Worked Example: Steel Industrial Building in Toronto

Given:

• Location: Toronto, ON (q50 = 0.42 kPa)
• Building: 40 m wide x 25 m deep x 10 m eaves height
• Flat roof, steel portal frames at 8 m centres
• Rough terrain (industrial park, suburban)
• Normal importance (Iw = 1.0)
• Enclosed building, uniformly distributed openings

Design External Wind Pressure (Main Structural System)

Per NBCC 2020 Section 4.1.7.1: [ p = Iw \times q{50} \times C_e \times C_g \times C_p ]

At mean roof height h = 10 m, Rough terrain: Ce = 0.85 (interpolated between 0.8 at 9 m and 0.9 at 12 m).

For main structural system: Cg = 2.0.

Windward wall: h/d = 10/25 = 0.40 < 0.5 → Cp = +0.70 Leeward wall: h/d = 0.40 → Cp = -0.35 (by interpolation: at 0.4, Cp = -0.35)

Windward wall pressure:

Positive external + negative internal (internal suction increases net push): [ p_{w} = 1.0 \times 0.42 \times 0.85 \times 2.0 \times (+0.70 - (-0.30)) = 0.42 \times 0.85 \times 2.0 \times 1.00 = 0.714 \text{ kPa} ]

Or with +Cpi = +0.30 (internal pressure increases net suction on roof/leeward): [ p_{w} = 1.0 \times 0.42 \times 0.85 \times 2.0 \times (+0.70 - 0.30) = 0.42 \times 0.85 \times 2.0 \times 0.40 = 0.286 \text{ kPa} ]

Governing windward: pw = 0.71 kPa (positive, toward building).

Leeward wall pressure:

With +Cpi = +0.30 (internal pressure adds to leeward suction): [ p_{l} = 1.0 \times 0.42 \times 0.85 \times 2.0 \times (-0.35 - 0.30) = 0.42 \times 0.85 \times 2.0 \times (-0.65) = -0.464 \text{ kPa} ]

Governing leeward: pl = -0.46 kPa (suction).

Total horizontal pressure on frame (tributary = 8 m):

Windward: Fw = 0.71 x 10 x 8 = 56.8 kN Leeward: Fl = 0.46 x 10 x 8 = 36.8 kN Total frame shear: V_frame = 56.8 + 36.8 = 93.6 kN

Step 8 — Cladding and Components (C&C)

Per NBCC 2020, C&C uses Cg = 2.5 and higher local Cp values. For a flat-roof corner zone:

Cp (corner roof zone) = -2.5 (suction) Ce (at roof height = 10 m, rough) = 0.85 q50 = 0.42 kPa

With +Cpi = +0.30: [ p_{C&C} = 1.0 \times 0.42 \times 0.85 \times 2.5 \times (-2.5 - 0.30) = 0.42 \times 0.85 \times 2.5 \times (-2.80) = -2.50 \text{ kPa} ]

This is roughly 3.5x the MWFRS pressure, consistent with the localized nature of C&C loading. This controls roof deck fastening, purlin design, and edge flashing detailing.

Comparison: NBCC 2020 vs ASCE 7-22 Wind Load

Important: When working across codes, compare the final design pressure p (in kPa or psf), not the individual coefficients. A fundamentally different reference wind speed (hourly-mean vs 3-s gust) makes coefficient-by-coefficient comparison misleading.

Frequently Asked Questions

What is the difference between q50 and the hourly-mean wind pressure?

The q50 value published in NBCC 2020 Table C-1 already includes the density factor (0.5 x rho) and the square of the 1-in-50-year hourly-mean wind speed. It can be used directly in the pressure equation p = Iw x q50 x Ce x Cg x Cp without further wind-speed-to-pressure conversion. The "50" subscript indicates the 50-year return period — for other return periods consult the NBCC Structural Commentaries.

When does the NBCC require a dynamic wind analysis instead of the simplified static procedure?

The simplified static procedure (Cg = 2.0, Cg = 2.5 for C&C) is applicable to most buildings. However, NBCC 2020 Commentary I-24 recommends a detailed dynamic analysis when: (a) building height > 120 m, (b) building has a fundamental natural frequency < 1 Hz (approximately height > 40 m for steel frames, > 60 m for concrete), (c) height-to-minimum-width ratio > 4, or (d) the building is lightweight and susceptible to dynamic amplification. For a typical 10 m steel portal frame (fundamental frequency ~3–5 Hz), the simplified procedure is fully adequate.

How does terrain roughness change between Open and Rough categories?

For the same height, Ce in Open terrain is approximately 20–30% higher than in Rough terrain. At 10 m height, Ce_open ≈ 1.05 vs Ce_rough ≈ 0.85, so a building in an exposed prairie location (Open terrain) experiences roughly 25% higher wind pressures than the same building in a suburban industrial park (Rough terrain). The terrain category is determined by the upwind fetch — at least 1 km of consistent roughness.

Why does NBCC use a 50-year return period while ASCE 7 uses 3,000-year?

NBCC uses the 50-year return period for wind speed because the gust factor Cg = 2.0 converts the hourly-mean wind (50-year) into an effective peak-gust effect approximating a much longer return period. ASCE 7 uses a 3,000-year wind speed map directly (ultimate wind) and then applies load factors of 1.0 rather than 1.6 to arrive at strength-level demands. Both approaches deliver similar design forces despite the very different return periods — this is a calibration choice, not a safety-level difference. The ASCE 7/CAN-CSA harmonization effort is ongoing but has not yet adopted a uniform return-period basis.

How do I convert between kPa and psf for wind pressure?

1 kPa = 20.885 psf. Common NBCC design pressures are in the range 0.5–2.5 kPa (10–50 psf). Toronto q50 = 0.42 kPa = 8.8 psf reference pressure. After applying Ce x Cg x Cp (typical combined factor 1.5–3.0), the design wind pressure is 0.6–1.3 kPa (12–27 psf). For comparison, an equivalent Exposure C building in the US Midwest (V = 115 mph ultimate) would have qh = 20–25 psf and design pressures of 15–35 psf — similar order of magnitude to the NBCC result.

Reference only. Verify all values against the current edition of the National Building Code of Canada 2020 (NBCC 2020) and applicable provincial amendments. This guide does not constitute professional engineering advice and must be independently verified by a licensed Professional Engineer for the specific project location, terrain conditions, and governing building code edition.