Let’s talk about something that trips up even seasoned engineers from time to time: figuring out exactly where National Building Code of Canada (NBCC) Part 9, “Housing and Small Buildings,” hands off the baton to Part 4, “Structural Design.” Get this wrong, and you could be heading for a world of compliance headaches, or worse, an under-designed structure. Get it right, and you’re setting your project up for success from day one.

This isn’t just academic navel-gazing; it’s fundamental to our practice. Whether you’re a fresh EIT or a P.Eng. with decades under your belt, a solid grasp of this distinction is crucial. So, let’s break down the scopes, pinpoint those critical triggers, and even wade into some of those murky “grey areas” where your engineering judgment really earns its keep.

So, What’s Part 9 All About?

Think of Part 9 as the code’s domain for “Housing and Small Buildings.” It’s designed to provide a relatively straightforward, prescriptive path for common, smaller-scale construction.

• Primary Domain: As the name suggests, it covers:
  • Houses (single-family, semis, duplexes, etc.)
  • Secondary suites (with specific floor area limits, often around 80 m² or a percentage of the main unit).
  • Small non-residential occupancies (like business/personal services, mercantile, or low-hazard industrial) provided their combined total floor area is pretty limited – typically not exceeding 300 m².
• Typical Size & Scope: Generally, we’re looking at buildings:
  • Height: Not exceeding 3 storeys.
  • Area: While not a single hard cap in every sentence, the spirit often aligns with buildings having a footprint less than 600 m². However, specific clauses within Part 9 might have their own area limits (e.g., total roof area for simplified snow loads not exceeding 4,550 m²).
• The Vibe: Part 9 is largely prescriptive. It’s packed with “deemed-to-comply” solutions. If your design meets these specific criteria for dimensions, materials, and connections (think span tables for joists, nailing schedules), you’re generally good to go without needing to fire up your full analysis software. It’s based on years of accepted good practice for light-frame construction and similar systems.

Key Takeaway for Part 9: It’s for relatively simple, common building types where empirical data and established practices allow for prescriptive solutions. The goal is safe, functional buildings without requiring bespoke engineering for every joist and stud.

Enter Part 4

When a building steps outside the fairly well-defined box of Part 9, it lands squarely in the realm of Part 4, “Structural Design.”

• Broad Scope: Simply put, Part 4 applies to:
  • All buildings not covered by the prescriptive rules of Part 9.
  • Buildings that start in Part 9 but hit a specific trigger that bumps them up to a Part 4 design requirement (more on those triggers in a moment!).
• Design Philosophy: This is where we roll up our sleeves and do full-blown engineering. Part 4 is rooted in Limit States Design (LSD). You’ll be:
  • Calculating specified loads (dead, live, snow, wind, seismic) as per Section 4.1.
  • Applying load factors and load combinations (Table 4.1.3.2.-A, for instance).
  • Determining factored resistances (ΦR) based on material properties and CSA standards (like CSA O86 for wood, CSA A23.3 for concrete, CSA S16 for steel).
  • Ensuring ΦR ≥ effect of factored loads for Ultimate Limit States (ULS) and checking Serviceability Limit States (SLS) like deflections and vibrations.

It’s a much more detailed and analytical approach, designed to handle a vast range of building types, complexities, materials, and site conditions.

The Triggers: When Part 9 Says, “Not Me, Call Part 4!”

This is where it gets really interesting. Part 9 itself contains numerous “escape hatches” or triggers that mandate a shift to a Part 4 design approach, even if the building seems like a Part 9 candidate at first glance. Ignoring these is a recipe for trouble.

Here are some common triggers you absolutely need to watch out for:

• Exceeding Dimensional or Load Limits:
  • Spans: If any structural member span exceeds a certain limit (e.g., 12.2 m is a common one mentioned for applicability of simplified snow loads), Part 9 might bow out.
  • Live Loads: Specified live loads exceeding Part 9’s assumptions (e.g., a floor live load greater than 2.4 kPa) will often push you to a Part 4 design for those elements or the whole structure.
  • Wall Heights/Thicknesses: Exceeding specified unsupported heights for foundation walls (e.g., >3.0m for unreinforced concrete block) or not meeting minimum thicknesses for masonry walls often requires Part 4.
• Higher Environmental Loads:
  • Seismic: In regions where seismic spectral acceleration Sa(0.2) gets high (e.g., > 1.8), Part 9 explicitly states that bracing and anchorage must be designed according to Part 4. Even at moderate levels (e.g., Sa(0.2) > 0.70), more stringent bracing or Part 4 design is invoked.
  • Wind: Similarly, high wind pressures (e.g., 1-in-50 hourly wind pressure HWP ≥ 1.2 kPa) will typically require a Part 4 design for anchorage and key structural elements. Thresholds like HWP ≥ 0.8 kPa trigger more robust fastening even within Part 9.
• Specific Materials or Systems Not Covered Prescriptively:
  • Reinforced Concrete (Beyond Basics): While Part 9 covers some basic concrete elements and flat ICF walls prescriptively, more complex reinforced concrete structures (or those exceeding Part 9’s ICF limits) need a Part 4 design based on CSA A23.3.
  • Structural Steel (Loadbearing): The design of loadbearing steel studs or more complex steel frames generally falls under Part 4 and CSA S16.
  • Engineered Wood Products (Complex Cases): While Part 9 has tables for sawn lumber and some generic EWP, complex EWP applications or proprietary systems often need specific engineering or deferral to Part 4.
  • Wood Roof Trusses: Most wood roof trusses require joint connections to be designed according to Subsection 4.3.1 (which references CSA O86), effectively making them an engineered element.
• Foundations Under Specific Conditions:
  • Poor Soil: Foundations on very weak soils (e.g., allowable bearing pressure < 75 kPa for some prescriptive solutions) or permafrost demand a Part 4 geotechnical and structural design.
  • Complex Foundation Walls: Foundation walls supporting significant surcharge or hydrostatic pressure, or those not meeting prescriptive height/reinforcement, will need a Part 4 approach.
  • Foundation Cripple Walls: If these don’t meet strict prescriptive height and bracing rules, they become a Part 4 design issue.
• Complex Geometries or Structural Systems:
  • Buildings with irregular shapes, large openings, or non-standard load paths often don’t fit neatly into Part 9’s prescriptive boxes.
  • Post, beam, and plank construction, or log construction, are examples where Part 9 typically points towards Part 4.

Pro-Tip: Don’t just assume a building is Part 9 based on its size. Dig into the specific conditions and look for these triggers. The NBCC Part 9, Section 9.4, “Structural Requirements,” and Section 9.23, “Wood-Frame Construction,” are riddled with these conditional statements.

Navigating the “Grey Areas”

Sometimes, a building might technically squeak by under Part 9’s prescriptive rules, but your gut (backed by experience, of course!) tells you a more robust Part 4 approach might be warranted. These are the “grey areas.”

• Unique Site Conditions: Maybe the seismic hazard is just below a hard Part 4 trigger, but the soil conditions are a bit suspect (hello, \(V_{s30}\) uncertainties or potential liquefaction at slightly lower \(S_a(T)\) values!). Or perhaps it’s a site exposed to unusual wind effects not fully captured by simplified Part 9 approaches (like a building perched on a bluff where the topographic factor, \(C_t\), would be significant in a Part 4 analysis).
• Unusual Building Configurations: An open-concept design with very few interior braced walls, even if technically meeting minimum Part 9 bracing lengths, might make you consider a more rigorous Part 4 lateral analysis.
• Desired Performance or Robustness: The client might want a building that performs better than the minimum Part 9 standard, perhaps aiming for reduced deflections, better vibration control, or enhanced resilience.
• Long-Term Durability Concerns: For structures with unique exposure conditions or materials where long-term performance under prescriptive rules might be questionable.
• Addressing the “Protection Gap”: We know from documents like the ICLR report that a “protection gap” can exist, particularly with seismic provisions where Part 9 didn’t fully align with the updated seismic hazard values used in Part 4. While proposed changes aim to address this (like introducing Smax parameters and more stringent prescriptive solutions in higher seismic zones for BC’s adaptation of the 2020 code), it highlights that just meeting minimum Part 9 might not always align with the latest understanding of risk, especially in areas of moderate to high seismicity.

This is where your professional responsibility kicks in. Documenting your rationale if you choose to apply Part 4 principles to a “Part 9 building” is always a smart move.

Key Differences in Approach

Parting Thoughts

Nailing down whether your project falls under Part 9 or Part 4 is step one in any structural design. It dictates your entire approach, from how you calculate loads to the level of detail in your analysis and documentation. While Part 9 offers a streamlined path for simpler structures, it’s loaded with conditions that can quickly punt you over to a full Part 4 engineering design.

And remember, those “grey areas” are where your experience and judgment as an engineer truly come into play. Don’t be afraid to apply a higher standard if the situation warrants it.

What are your experiences? Have you encountered projects that straddled this line, or triggers that surprised you? Share your thoughts and questions in the comments below – let’s learn from each other!

References:

• National Building Code of Canada 2020 (NBCC 2020), Part 4 and Part 9.
• CSA O86 “Engineering design in wood.”
• CSA A23.3 “Design of concrete structures.”
• CSA S16 “Design of steel structures.”
• Structural Commentaries (User’s Guide – NBC 2020: Part 4 of Division B).
• Illustrated User’s Guide – NBC 2020: Part 9 of Division B, Housing and Small Buildings.

Disclaimer: This blog post is for informational purposes only and should not be taken as specific engineering advice. Always consult the latest edition of the National Building Code of Canada and relevant CSA standards for your projects.