Structure of NBC 2020
The National Building Code of Canada 2020 (NBC 2020), published by the National Research Council of Canada (NRC), is the model code adopted by most Canadian provinces and territories, with some jurisdictions adopting amendments specific to local conditions. Part 4 of NBC 2020 covers Structural Design — the section relevant to our structural engineering practice. NBC 2020 Part 4 covers structural loads (Clause 4.1), foundations (Clause 4.2), and structural integrity (Clause 4.1.2).
NBC 2020 Seismic Provisions
NBC 2020 seismic design provisions (Clause 4.1.8) are based on probabilistic seismic hazard data — specifically 2% probability of exceedance in 50 years (2%/50yr) ground motions, equivalent to approximately a 2475-year return period. This is more conservative than the ASCE 7-22 value used in the United States (also 2%/50yr for most Risk Category II structures), but the Canadian seismic hazard maps reflect Canada's specific tectonic setting, including the Cascadia subduction zone risk for British Columbia and the St. Lawrence Rift zone risk for eastern Canada (including Ontario and Quebec).
Site class determination in NBC 2020 follows Clause 4.1.8.4, using measured or estimated average shear wave velocity in the top 30 metres (Vs,30) to classify the site as Class A through F. Each site class has associated spectral acceleration modification factors that adjust the reference ground motions for local soil amplification effects. These factors are critical for eastern Ontario sites, where glaciolacustrine deposits can significantly amplify ground motions relative to the rock reference condition.
NBC 2020 Snow Loads
Canada's climate imposes significant snow loads on structures, particularly in Ontario, Quebec, and the western provinces. NBC 2020 Clause 4.1.6 specifies snow load calculation using the specified ground snow load (Ss) and associated rain-on-snow load (Sr) from the NBC 2020 climatic data tables for the project location. Roof shape factors, accumulation factors for valleys and areas adjacent to higher roofs, and wind exposure factors for exposed roof areas all modify the ground snow load to produce the design roof snow load.
For the Ontario steel replacement project, unbalanced snow load drift at the roof edge was a key design consideration. The drift height and weight calculated per NBC 2020 Clause 4.1.6 governs the design of eave beams and columns at the low-roof side of stepped roof configurations — a commonly underestimated design condition that caused failures in older structures designed to less conservative drift provisions.
NBC 2020 vs ASCE 7
NBC 2020 and ASCE 7 are developed independently but share common philosophical foundations — both are reliability-based limit state design codes. The key practical differences for engineers working across both jurisdictions are: different wind speed return periods (NBC 2020 uses 1/50-year for most structures, ASCE 7 uses ultimate design wind speeds with risk category adjustments), different snow load methodology (NBC treats rain-on-snow explicitly, ASCE 7 includes it implicitly in the Cs factor), and different seismic hazard maps reflecting the different tectonic settings. Understanding these differences is essential for engineers who work in both Canadian and US markets — as we do.