Comparison of Wind Load Design: ASCE 7-16 vs NBCC 2015/2020

Wind load design is a critical aspect of structural engineering, especially for buildings in urban environments where wind effects can significantly impact both serviceability and safety. This blog post presents a side-by-side comparison between two major wind load design standards: ASCE 7-16 (used in the United States) and NBCC 2015/2020 (used in Canada). The comparison focuses on the Main Wind Force Resisting System (MWFRS) level, summarizing their formulas, key variables, typical values, and how these are applied in a typical urban scenario.

Explanation of Key Variables

ASCE 7-16

·       V – Basic wind speed (3-sec gust), typical urban: 115–120 mph

·       K_z – Exposure coefficient (height-dependent): 0.70 – 1.0

·       K_d – Wind directionality factor: 0.85

·       K_zt – Topographic factor: 1.0

·       G – Gust factor: 0.85 for rigid buildings

·       C_f – Force coefficient: ~0.8 – 1.4

·       C_pi – Internal pressure coefficient: ±0.18 or ±0.55

NBCC 2015/2020

·       V – Basic wind speed (hourly mean): 108–133 km/h

·       q – Reference velocity pressure: 600 – 1000 N/m²

·       I_w – Importance factor: 1.0 or 1.15

·       C_e – Exposure factor: 0.7 – 1.0

·       C_g – Gust effect factor: 2.5

·       C_p – External pressure coefficient: ~0.8 windward, –0.5 leeward

·       C_pi – Internal pressure coefficient: ±0.25

·       C_t – Topographic factor: 1.0

Comparative Study: Wind Load Provisions in ASCE 7-16 vs NBCC

This blog post presents a detailed comparison between the wind load provisions of ASCE 7-16 (United States) and NBCC 2015/2020 (Canada). Both standards aim to ensure buildings are designed to withstand wind forces, but they use different assumptions, variables, and units. Here, we explore the key components of each code, explain their variables, provide typical values for an urban setting, and demonstrate a side-by-side example calculation for a low-rise building in a typical city environment.

Comparison Table: ASCE 7-16 vs NBCC

Aspect	ASCE 7-16 (U.S.)	NBCC 2015/2020 (Canada)
Design Wind Pressure Equation	p = q_z G C_p - q_i (GC_pi) or p = q_z G C_f	p = I_w q C_e C_g C_p C_t
Basic Wind Speed	V: 3-sec gust at 10 m (mph)	V: hourly mean at 10 m (km/h)
Typical Wind Speed (Urban)	115–120 mph	30–37 m/s (108–133 km/h)
Velocity Pressure	q_z = 0.00256 K_z K_zt K_d V^2 (psf)	q = 0.613 V^2 (N/m²)
Exposure Category / Terrain	Category B (Urban)	Category B → C_e ≈ 0.7–1.0
Importance Factor	I = 1.0 or 1.15	I_w = 1.0 or 1.15
Gust Effect Factor	G = 0.85 (rigid)	C_g = 2.5
Topographic Factor	K_zt (typically 1.0)	C_t (typically 1.0)
Pressure Coefficient	C_f, C_p, C_pi from tables	C_p from tables; ~0.8 / -0.5
Internal Pressure Coefficient	C_pi = ±0.18 (enclosed)	C_pi = ±0.25 (enclosed)
Load Combination	1.0D + 0.6W	1.25D + 1.5W
Unit Consistency	Imperial	Metric

Common Assumptions

- Building height: 10 meters- Location: Urban (Exposure Category B / Terrain Category B)- Building Type: Enclosed, rigid- Importance factor: 1.0- No topographic effect

ASCE 7-16 Calculation

Step 1: Basic Wind SpeedV = 115 mph = 51.4 m/s

Step 2: Velocity Pressureq_z = 0.00256 × K_z × K_zt × K_d × V²K_z = 0.72, K_zt = 1.0, K_d = 0.85q_z = 0.00256 × 0.72 × 1.0 × 0.85 × 115² ≈ 20.7 psf

Step 3: Wind PressureG = 0.85, C_f = 1.0p = q_z × G × C_f = 20.7 × 0.85 × 1.0 = 17.6 psfConvert to SI: 17.6 psf × 47.88 = 842 N/m²

NBCC 2015/2020 Calculation

Step 1: Basic Wind SpeedV = 120 km/h = 33.3 m/s

Step 2: Reference Velocity Pressureq = 0.613 × V² = 0.613 × 33.3² ≈ 680 N/m²

Step 3: Wind PressureI_w = 1.0, C_e = 0.9, C_g = 2.5, C_p = 0.55, C_t = 1.0p = 1.0 × 680 × 0.9 × 2.5 × 0.55 ≈ 841.5 N/m²

Conclusion

Despite different wind speed definitions, both codes yield almost identical wind pressures when equivalent conditions are applied:ASCE 7-16: 115 mph → 842 N/m²NBCC: 120 km/h → 841 N/m²This demonstrates that wind load design under both standards can lead to similar safety levels when interpreted correctly.