You’ve got a project that, at first glance, looks like a straightforward Part 9 building – a “house or small building” as the NBCC puts it. But then you start digging into the details, the architect throws in a few “features,” or the site conditions are less than ideal, and suddenly you’re wondering if those prescriptive Part 9 solutions are going to cut it.

That’s what we’re diving into today: those increasingly common situations where our Part 9 projects need a little (or a lot of) help from Part 4 of the National Building Code of Canada. It’s not about making things more complicated than they need to be; it’s about ensuring safety, performance, and sometimes, just making that ambitious design actually stand up. This post is for you, whether you’re just starting out or you’ve been navigating these codes for years. We’ll look at common triggers, specific examples, and the crucial role your engineering judgment plays.

Part 9: The Go-To for “Simple” Structures

NBCC Part 9, “Housing and Small Buildings,” is fantastic for a reason. It provides a set of prescriptive requirements – essentially a “deemed-to-comply” pathway – for buildings that fit within certain size and occupancy limits. Generally, we’re talking about buildings up to 3 storeys in height and not exceeding 600 m² in building area. For many standard designs, if you follow Part 9, you’re meeting the Code.

It covers a lot, from basic framing with wood (hello, Section 9.23!) to simplified load considerations and even some details for masonry and Insulating Concrete Forms (ICFs). The idea is that for these “small buildings,” often residential, the built-in conservatism and simplified approaches in Part 9 are sufficient.

Pro-Tip : Get really familiar with the scope and application of Part 9 (Article 9.1.1. and associated Division A references). Knowing its limitations is the first step to knowing when to look beyond it. The User’s Guides are your friend!

But “small” doesn’t always mean “simple,” does it?

Common Triggers for Part 4 Engineering

As designs get more adventurous or site conditions throw us curveballs, we hit points where Part 9 explicitly tells us to pack our bags and head over to Part 4, “Structural Design.” Let’s break down some of the usual suspects.

1. Beyond Basic Ground Conditions

Part 9 offers prescriptive solutions for foundations, largely assuming decent soil conditions (e.g., allowable bearing pressure of at least 75 kPa, as often cited in foundation sections like 9.15.1.1.). But what happens when you encounter:

• Poor Soil Conditions: If your geotechnical report comes back with low bearing capacities, highly compressible soils, or expansive clays, the standard Part 9 footing sizes and designs might not be adequate. Section 9.15.3. for footings, for instance, will quickly point you to Section 4.2 (Foundations) if you’re outside its simplified design parameters, such as when specified live loads on supported floors exceed 2.4 kPa or joist spans exceed 4.9m.
• High Loads: Speaking of loads, if the building design results in column loads or wall loads that are higher than what Part 9 prescriptively covers (often linked to those span and live load limits), you’re into Part 4 territory for foundation design.
• Permafrost: This is a big one for our colleagues working in northern regions. Article 9.15.1.2. is crystal clear: “Foundations for buildings constructed on permafrost shall be designed by a competent designer in accordance with Part 4.” There’s no prescriptive shortcut here.
• Surcharge or Hydrostatic Pressure: If foundation walls are supporting more than drained earth or are subject to significant hydrostatic pressure, Article 9.15.2.3. might send you to Section 4.2 for a more detailed analysis.

2. Those “Special” Structural Elements

Architects love their grand openings and unique features. While Part 9 provides tables for lintels (like in Section 9.20 for masonry or Section 9.23 for wood-frame), these have limits.

• Lintels Over Large Openings: If you’ve got a massive window or a wide garage door, the spans might exceed what’s covered in Part 9 tables (e.g., Tables A-12 to A-15 for wood-frame construction often have span limits). Article 9.20.7.2. concerning masonry lintels (other than veneer) often requires design to Part 4. You’ll need to do a proper Part 4 analysis considering all loads, including potential stress concentrations.
• Columns Exceeding Part 9 Limits: Part 9 provides some guidance for columns (e.g., Section 9.17 for wood columns, Section 9.20 for masonry columns). However, if your columns are supporting more than two wood-frame floors, larger spans (often around 5m for joists they support), or specified live loads greater than 2.4 kPa, Article 9.17.1.1. and its counterparts for other materials will direct you to Part 4. The same goes for columns with smaller dimensions than prescribed minimums.
• Complex Roof Truss Connections: While Part 9 allows for pre-engineered trusses (often designed by the manufacturer to Part 4 principles referencing CSA O86), the overall roof system integration or connections for particularly complex roof geometries or heavy loads might need a site-specific Part 4 review. Article 9.23.14.2. points out that wood roof trusses and their joint connections need to conform to Subsection 4.3.1 (Wood Design in Part 4).

3. Anchorage in High Wind & Seismic Zones

This is a critical one. Part 9 provides some prescriptive anchorage details (e.g., Article 9.23.6. for anchoring wood-frame buildings). However, as wind pressures or seismic forces ramp up, these prescriptive solutions hit their limits.

• High Hourly Wind Pressure (HWP): If your 1-in-50-year hourly wind pressure (HWP) gets too high – typically the trigger is HWP ≥ 1.20 kPa – Articles like 9.23.6.1. or 9.23.3.5. (for sheathing fastening) will mandate that anchorage and bracing be designed according to Part 4.
• High Seismic Hazard (Sa(0.2)): Similarly, if the 5%-damped spectral response acceleration at a 0.2-second period, $S_a(0.2)$, is high – the magic number is often $S_a(0.2) > 1.8$ – you’re in Part 4 land for anchorage and lateral load resisting system design. Part 9, in its NBCC 2020 version, notably didn’t update its seismic hazard values in line with Part 4’s adoption of the 6th Generation Seismic Hazard Model (CanadaSHM6), creating a known “protection gap” discussed in the industry. While some provincial codes might address this, it’s a key area where Part 4 engineering is vital.

Key Takeaway: Lateral loads are a major driver for Part 4 intervention in Part 9 buildings. Don’t just assume Part 9 bracing details are sufficient without checking your site-specific HWP and $S_a(0.2)$ values against the triggers. The values from Appendix C of the NBCC are your starting point for these.

4. Those Tricky Foundation Cripple Walls

Foundation cripple walls, those short stud walls between the foundation and the first floor, can be a weak link if not handled properly, especially seismically. Part 9 (e.g., in its proposed changes for some jurisdictions, and in principle) offers prescriptive solutions, but these come with strict height and load limitations.

• If cripple walls are taller than prescribed (e.g., often around 1.2m, though this can vary by seismic hazard level like \(S_{max}\) in some proposed changes), or if they support heavier loads or more storeys than anticipated in the prescriptive tables, they often need to be designed as an additional storey or designed in accordance with Part 4. This ensures they have adequate bracing and connections to transfer lateral loads.

5. Irregularities and Non-Standard Systems

Part 9 thrives on regularity and common construction systems.

• Irregular Configurations: If your building has significant geometric irregularities (think complex shapes, large offsets in load paths, or significant torsional sensitivity that Part 9 bracing can’t handle), a Part 4 analysis becomes necessary to ensure overall stability and proper load distribution. Part 4, in Subsection 4.1.8., has detailed definitions and requirements for various irregularity types.
• Non-Standard Materials/Systems: If you’re using materials or structural systems not explicitly covered by the prescriptive tables in Part 9 (e.g., novel composite materials, unique framing systems, or even just wood species not listed in CSA O86 span tables used by Part 9), you’ll need to use Part 4 principles and the relevant material design standards (like CSA S16 for steel, CSA A23.3 for concrete, CSA O86 for wood, etc.). Article 9.4.1.1. indicates that structural members and their connections shall conform to Part 9, or be designed according to good engineering practice (like specific guides), or be designed according to Part 4. This gives an opening but also a responsibility.
• Exceeding Prescriptive Limits: Many Part 9 tables for joist spans, beam sizes, or wall heights are based on specific load assumptions (e.g., live load not exceeding 2.4 kPa for floors, or span of any structural member not exceeding 12.2m, as noted in 9.4.1.1.). If your design exceeds these fundamental limits, Part 4 design is required.

The Indispensable Role of Engineering Judgment

Sometimes, even if you could technically squeeze a design into Part 9’s prescriptive requirements, your engineering judgment might tell you that a Part 4 approach is warranted. Why?

• Enhanced Performance: The client might desire a building that performs better than the minimum Part 9 standard, perhaps for reduced deflections, better vibration control, or increased resilience to extreme events beyond code minimums.
• Complex Architectural Features: Highly complex rooflines, large cantilevers, or unique load paths, even if individual elements seem to fit Part 9, might benefit from a holistic Part 4 analysis to understand system interactions.
• Long-Term Durability & Maintenance: For some structures, especially those with unique exposure conditions or where long-term maintenance is a concern, using Part 4 principles could lead to a more robust and durable design.
• Peace of Mind: Let’s be honest, sometimes the complexity just reaches a point where a detailed engineering analysis following Part 4 principles gives you, the engineer, greater confidence in the design.

A Word from the Wise: Don’t let the “small building” label lull you into a false sense of security. Document your decisions. If you’re opting for Part 4 principles when Part 9 might seem applicable, or if you’re pushing the boundaries of Part 9, make sure your rationale is clear. It’s all about due diligence.

A Few Final Thoughts

The relationship between Part 9 and Part 4 isn’t always a hard line; it’s more of a well-managed handover. Part 9 provides an excellent baseline for a vast majority of housing and small buildings in Canada. But as structural engineers, our job is to know when that baseline needs to be elevated.

Understanding the triggers that necessitate Part 4 engineering is crucial. It means:

• Thoroughly reviewing architectural plans for complex features.
• Always obtaining and understanding the geotechnical report.
• Accurately determining site-specific wind and seismic loads (using Appendix C and tools like the NRC’s Seismic Hazard Calculator for \(S_a(T)\) values compatible with NBCC 2020 Part 4).
• Being conservative when in doubt, especially regarding life safety.

The goal isn’t to over-engineer but to right-engineer. Knowing when Part 9 leans on Part 4 is a key part of that for any Canadian structural engineer dealing with buildings that are small in name, but sometimes quite complex in nature.

What are some of the trickiest Part 9-to-Part-4 transitions you’ve encountered? Share your experiences in the comments below – let’s learn from each other!

References:

• National Building Code of Canada 2020 (NBCC 2020)
• 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
• CSA O86, “Engineering design in wood”
• CSA A23.3, “Design of concrete structures”
• CSA S16, “Design of steel structures”
• CSA S304, “Design of masonry structures”

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.