Structural Engineering Blog

In this post we’ll be looking at seismic. Specifically, let’s chew on something that’s been a hot topic since the NBCC 2020 dropped: the growing gap in seismic design philosophies between Part 4 (Structural Design) and Part 9 (Housing and Small Buildings). If you’re juggling projects that dance on the edge of Part 9’s limits or work across different seismic zones in Canada, this is a conversation you need to be in on.

We all know the NBCC is a living document, constantly evolving. But sometimes, different parts of it evolve at different speeds. This post will break down what’s new in Part 4’s seismic world, why Part 9 hasn’t quite kept pace, and what that practically means for us on the ground.

Part 4: Embracing the 6th Generation Seismic Hazard Model

Part 4 of the NBCC 2020 really stepped up its game when it comes to earthquake loads. The biggest headliner? The full adoption of Canada’s 6th Generation Seismic Hazard Model (CanadaSHM6).

What does this mean for your Part 4 projects?

New Seismic Hazard Values: We’re looking at updated spectral acceleration values, peak ground acceleration (PGA), and peak ground velocity (PGV). You’ll need to be comfortable using the NBC 2020 Seismic Hazard Tool provided by the Geological Survey of Canada to get these site-specific values.
- Figure from Seismic collapse risk of RC-timber hybrid building with different energy dissipation connections considering NBCC 2020 hazard
Detailed Site Property Considerations: There’s a much stronger emphasis on understanding your site.
- Site Designation (X): This is now critical. It’s derived from parameters like the average shear wave velocity in the top 30 meters (\(V_{s30}\)), average standard penetration resistance (\(\overline{N}_{60}\)), or average undrained shear strength (\(\overline{S}_u\)).
- No more casually picking a site class without some homework! A site-specific geotechnical evaluation might be required, especially for \(X_F\) sites (those requiring site-specific evaluation, like liquefiable soils).
Updated Performance Objectives: Part 4 has sharpened its focus on performance, particularly for:
- Post-Disaster Buildings: These structures need to remain operational after a significant seismic event. Think hospitals, emergency response centers.
- High Importance Category Buildings: Think schools or community centers. These structures have more stringent requirements to ensure life safety.
- Article 4.1.8.23. lays out additional verification requirements for these buildings, often demanding checks for elastic behaviour and stricter drift limits under specific spectral acceleration values.

Pro-Tip: Don’t underestimate the impact of \(V_{s30}\) on your design spectral acceleration \(S(T)\). Getting this right is crucial for an accurate seismic assessment under Part 4. If your geotech report is vague, push for clarity!

Part 4 continues its reliance on Limit States Design (LSD), with detailed load combinations and requirements for ductility-related (\(R_d\)) and overstrength-related (\(R_o\)) force modification factors. The commentaries in the “Structural Commentaries (User’s Guide – NBC 2020: Part 4 of Division B)” – especially Commentary J on Seismic Effects – are indispensable for navigating these complex provisions.

Part 9: The Echo of NBCC 2015

Now, let’s switch gears to Part 9, which covers “Housing and Small Buildings.” When it comes to seismic provisions in the NBCC 2020, Part 9 essentially carries over the requirements from NBCC 2015. This is where the divergence really becomes apparent.

No CanadaSHM6 Integration (Yet): Part 9 does not directly incorporate the new seismic hazard values from CanadaSHM6 in the same way Part 4 does. This means the baseline seismic inputs for a Part 9 building can be quite different from a Part 4 building right next door.
Prescriptive Approach Continues: Part 9 relies heavily on prescriptive solutions – “if you build it like this, it’s deemed-to-comply.” This is great for straightforward projects but can be limiting and, as we’ll see, potentially less robust in higher seismic areas.

This lag has led to what many in the industry are calling a potential “protection gap.” A significant portion of Canada’s building stock, especially residential, falls under Part 9. If these buildings aren’t designed to the latest understanding of seismic risk, are we inadvertently accepting a lower level of safety?

When Part 9 Says “Time to Call in Part 4”

Even within its prescriptive framework, Part 9 has triggers that push a design (or parts of it) into the realm of Part 4 engineering. For seismic design, the big one is often tied to the 5%-damped spectral response acceleration at a 0.2-second period, \(S_a(0.2)\):

Low to Moderate Seismic Forces (\(S_a(0.2) \le 0.70\)): Part 9 offers fairly straightforward prescriptive bracing requirements.
High Seismic Forces (\(0.70 < S_a(0.2) \le 1.8\)): The bracing requirements become more stringent. You’ll see more detailed rules for braced wall panels and bands (Subsection 9.23.13.). While still prescriptive, the intensity is higher.
Extreme Seismic Forces (\(S_a(0.2) > 1.8\)): This is a critical threshold. If your \(S_a(0.2)\) value (derived from NBCC 2015-era data for Part 9 context) exceeds 1.8, Part 9 essentially throws its hands up and says the bracing must be designed in accordance with Part 4 or “good engineering practice” (which, let’s be honest, points back to a Part 4 level of rigour).
- Similarly, anchorage often needs a Part 4 design if \(S_a(0.2) > 1.8\).

Key Takeaway: Just because a building can be shoehorned into Part 9 doesn’s always mean it should be, especially in regions with moderate to high seismicity. That \(S_a(0.2) = 1.8\) trigger is a line in the sand.

Other triggers pushing elements to Part 4 design from Part 9 include:

Certain foundation cripple wall configurations if they don’t meet prescriptive limits.
Exceeding specific height or span limitations (e.g., structural member spans > 12.2 m).

Proposed Changes to Part 9

The good news is that this discrepancy hasn’t gone unnoticed. There’s ongoing work to update Part 9’s seismic provisions. One of the key proposed changes you might have heard about involves the introduction of a new seismic design parameter, often referred to as Smax.

The aim is to:

Better align Part 9 with the current understanding of seismic hazard.
Introduce more nuanced prescriptive solutions, especially for wood-frame construction in higher seismic regions.
Reduce the “protection gap” between Part 4 and Part 9.

For instance, some proposed changes suggest that in regions where \(S_a(0.2)\) is greater than a certain threshold (and eventually where the new Smax parameter dictates), more stringent prescriptive solutions for wood-frame construction would apply, or a Part 4 design would be explicitly required sooner.

These changes are vital, especially when you consider the increasing trend towards open-concept designs and larger window openings in Part 9 buildings, which can reduce inherent redundancy for lateral loads.

Implications Across Canada’s Seismic Zones

So, what does this all mean for your projects?

Western Canada (e.g., BC, Yukon): In high seismic zones, you’re likely already working extensively with Part 4 for most multi-storey or complex structures. For smaller buildings, be acutely aware of those Part 9 triggers. The difference between using NBCC 2015 seismic values for a Part 9 design versus current CanadaSHM6 values for a Part 4 design can be substantial.
Eastern Canada (e.g., Quebec, parts of Ontario & New Brunswick): While seismicity might be lower overall than in the West, there are significant seismic zones. Here, the Part 9 vs. Part 4 debate can become even more critical for “standard” housing. A Part 9 building designed to older seismic data might not offer the same protection level as one even marginally pushed into a Part 4 engineered design using current data.
Prairies & Central Canada: Even in traditionally lower seismic zones, Part 4 now mandates no exemptions from lateral load considerations. While Part 9 might still have some exemptions, it’s a good reminder that seismic design isn’t just a coastal concern.

The Bottom Line

The seismic provisions in NBCC 2020 Part 4 represent a significant step forward, aligning Canadian practice with the latest understanding of seismic hazards. Part 9, while serving its purpose for simpler structures in lower-risk scenarios, currently lags in adopting these advancements.

As engineers, it’s our responsibility to understand these differences:

Know your site’s seismicity based on the latest data (CanadaSHM6 for Part 4 context).
Understand the limitations and triggers within Part 9 that might necessitate a Part 4 approach.
Advise clients appropriately when a more robust, Part 4 engineered design might be prudent, even if a building technically squeaks by under Part 9.

The evolution of our codes is all about enhancing safety and resilience. Staying on top of these changes, especially in critical areas like seismic design, is paramount.

What are your thoughts? How is this Part 4 / Part 9 seismic divergence impacting your projects? Drop a comment below!

References:

National Building Code of Canada 2020 (NBCC 2020), Part 4 and Part 9.
Structural Commentaries (User’s Guide – NBC 2020: Part 4 of Division B).
Geological Survey of Canada – NBC 2020 Seismic Hazard Tool.
Relevant CSA Standards (CSA O86 for wood, etc.).