Ever wonder how we landed on the structural design provisions we use today in Canada? The National Building Code of Canada (NBC) hasn’t always looked the way it does. It’s a living document, evolving with research, experience, and a continuous drive for safer, more reliable structures.

This post takes a brief look back, primarily through the lens of the NBC Structural Commentaries (Commentary A in NBC2020-Commentary), at some key milestones, particularly the shift to Limit States Design (LSD) and the rationale behind significant changes.

Introducing Limit States Design (LSD)

Before LSD, Allowable Stress Design (ASD) was the prevailing method. ASD generally involves ensuring that calculated stresses under service loads remain below a certain fraction of the material’s yield or ultimate strength.

Limit States Design (LSD), however, takes a more probabilistic approach. It requires us to consider two main types of limit states:

• Ultimate Limit States (ULS): Deal with safety – collapse, overturning, fracture. We design for factored loads and factored resistances.
• Serviceability Limit States (SLS): Deal with performance and occupant comfort – deflection, vibration, cracking.

According to the NBC 2020 Structural Commentaries (Para 55):

• LSD was first introduced into the NBC in 1975, initially for steel structures.
• It was then developed for concrete (which used ultimate strength design before 1975).
• Subsequently, wood, cold-formed steel, and masonry structures adopted LSD.
• Foundations came under LSD in 1995.

Key Takeaway: The move to LSD was a fundamental shift, allowing for a more rational and consistent approach to safety across different materials and load types by explicitly considering uncertainties in loads and material resistances.

Allowable stress design has been gradually phased out for most primary structural materials in the NBC, though it might still linger in some unreferenced standards.

Key Changes and Their “Why”: Insights from NBC 2005 & Beyond

The NBC 2005 was a particularly significant edition, introducing several changes that shaped the code we use today. The commentaries (Paras 56-60 in NBC2020-Commentary) shed light on the thinking:

1. Standardizing Importance Categories and Factors (Pre-2005 vs. NBC 2005):

• Before 2005: Buildings were categorized more simply (e.g., “post-disaster,” “regular,” “schools”). The treatment of importance wasn’t consistent across wind, snow, and earthquake loads. For instance, wind loads had different return periods based on importance, while snow had no specific importance factor.
• NBC 2005: Introduced the Importance Categories (Low, Normal, High, Post-disaster) in Table 4.1.2.1. and standardized Importance Factors (\(I_E, I_W, I_S\)) for earthquake, wind, and snow/rain. This brought consistency.

Why the change? To create a more uniform and logical approach to how building importance influences design loads for different hazards.

2. Separating Snow (S) and Live (L) Loads:

• Before 2005: Snow and live loads were often grouped or treated with less distinction in combinations.
• NBC 2005: Clearly separated load due to snow and rain (S) from live load due to use and occupancy (L).

Why the change? This allows for more refined load factors and combinations because S and L have different variabilities and probabilities of acting simultaneously at their maximums.

3. Modifying Return Periods for Climatic Loads (Snow, Rain, Wind):

• NBC 2005: Changed the return period for specified snow, rain, and wind loads from 1-in-30 years to 1-in-50 years.

Why the change?

• International Alignment: A 50-year return period is common practice globally.
• Consistency: It aimed to harmonize the calculation of climatic loads using a single return period, modified by importance factors, similar to how earthquake loads were treated. This was done while maintaining a similar target probability against failure as the NBC 1995.

4. Adopting the “Companion Action” Format for Load Combinations:

• NBC 2005: Fully adopted the companion action format for ULS load combinations (those tables like 4.1.3.2.-A and -B we know so well).
• This principle, as we discussed in a previous post, involves one load acting at its principal factored value and others at their reduced companion values.

Why the change? This format is used worldwide and provides a clearer, more physically meaningful set of load combinations. It reflects the unlikelihood of all loads peaking simultaneously. While serviceability criteria were in pre-2005 codes, the 2005 NBC initially placed detailed SLS load combination guidance in the Commentaries, before the NBC 2020 brought them more formally into the main body of the code using the same companion action principles.

The Ever-Evolving Nature of Codes

These are just a few highlights. The evolution of the NBC’s structural provisions is a continuous process driven by:

• Research: New understanding of material behavior, load effects (especially wind and seismic), and structural systems.
• Experience: Lessons learned from structural performance (and failures) in Canada and internationally.
• Data: Better climatic data, seismic hazard mapping, etc.
• Harmonization: Efforts to align with international standards where appropriate.

Pro-Tip for All Engineers: Don’t just use the code; understand its history. Knowing why a provision exists can make you a better, more thoughtful designer. The commentaries are an invaluable resource for this.

Looking Ahead

The NBC will continue to evolve. Future changes will likely address new materials, innovative construction methods, increasing focus on resilience (beyond just life safety), and the impacts of climate change. As structural engineers in Canada, staying updated and understanding the “why” behind the code is part of our professional responsibility.

What historical NBC changes do you think have had the most significant impact on your practice? Or what future changes do you anticipate? Share your perspective in the comments!