BCBC 2024 to bring big changes for structural design.

February 20th, 2024

The BC Government has ratified the newest edition of the BC Building Code, which means there are some big changes coming for the structural design of Part 4 buildings in BC. The majority of BCBC 2024 will take effect in March 2024, except for seismic and adaptable unit provisions which have been delayed to March 2025 to provide developers and designers with time to adjust to the new requirements. You can find the announcement and PDF copy of BCBC 2024 on the BC Government’s website here.

Building codes in Canada generally follow a hierarchy from a national model code (National Building Code of Canada) to a provincial code (BC Building Code) to a local municipal code in some cases like Vancouver). Although the bulk of code provisions are similar, especially in structural provisions, this system provides the chance for regional tweaks to better align the code with local practices and concerns. But it also introduces a significant lag between when the national model code is updated and the local code takes effect. In this code cycle, we got our first peak of upcoming changes more than two years ago when NBC was released. In that time, Kor has been investigating the effects on our projects to understand how structural schemes will need to change to meet new code requirements.

In general, there are four significant structural changes that will take effect in March 2025. These are:

Changes to the seismic hazard design values
Changes to how site amplification (Site Class) can be determined
New requirements for Tall, High-Importance, and Post-Disaster Buildings
Introduction of a new Sloped Column irregularity and design requirements for these elements

Seismic Hazard Goes Up

In general, the seismic hazard values have gone up in most regions of the province. The table below shows some comparisons of the BCBC 2018 and BCBC 2024 spectral acceleration design values of different municipalities in the Lower Mainland, Vancouver Island, and Okanagan. You can think of these values as the peak acceleration that a building would experience in the design earthquake, as a ratio of earth’s gravity (g). Since a force is mass multiplied by acceleration, this parameter is proportional to the seismic design forces for a building.

The values do not increase uniformly, but rather vary by geography, site class, and building properties. Generally, tall & flexible buildings are less affected than short & stiff buildings. The bulk of new construction in the province is taking place in the Lower Mainland, and seismic forces are expected to increase by approximately 20-30% for shorter buildings and 0-10% for Tall Buildings.

What does this mean for a development project? Well, more seismic force generally means more structure needed to resist it. In wood-frame, this might mean more shear walls, double-sided sheathing, larger hold-downs. In a concrete shear wall building, this might mean more reinforcing, thicker walls, higher concrete strength, and larger foundations. Ultimately there is no one-size fits all approach to buildings and each structure needs to be looked at holistically for the most efficient structural scheme.

New Approach to Site Class Brings New Benefits

The second major change relates to site amplification, or what happens to earthquake waves as they travel through the soil directly under a building. Whether a vibration travels through a loose or stiff soil can change the characteristics of the motion, which then alters the affect it will have on the structure above. In former codes this was accounted for with Site Class, which grouped all soils into one of six classes, namely A, B, C, D, E, or F. Site Classes still exist, but BCBC 2024 also provides the opportunity to do a more detailed investigation and rewards the developer for doing so in the form of more refined site amplification calculations. In order to take advantage of this, new projects will have to directly measure shear wave velocity (Vs30) such as by using Seismic CPT as part of the geotechnical investigation. Using shear wave velocity for refined site amplification calculation instead of site class can bring down seismic forces, and in some cases completely negate the increases to the seismic hazard.

New Requirements for Tall, High-Importance, and Post-Disaster Buildings

The third major change introduces new requirements for some buildings in high seismic regions. You can think of these requirements as additional checks of these buildings under a serviceability earthquake – an earthquake that is more likely to occur over the life of the building than the design earthquake. It’s more nuanced than that, but that description is a good mental model for what the code is getting at. The design earthquake has a 2% probability of exceedance in 50 years, but it also has a performance objective of life-safety. In the design earthquake, some damage might occur to dissipate earthquake energy but the building will not collapse and everyone can get out safely. The new code has added this second check

Buildings taller than 30m will require additional checks of the columns and gravity framing to ensure they remain essentially undamaged in the serviceability earthquake (an event with 10% probability of exceedance in 50 years). High importance buildings will need to be designed so that the whole structure remains elastic in the same serviceability earthquake (10% in 50 years). Post-disaster buildings will need to be designed to remain undamaged for an even rarer earthquake, with 5% probability of exceedance in 50 years. The effect of these new checks mean higher seismic design forces in most cases. For high-importance and post-disaster buildings, the effects are quite dramatic with a possible 150 – 200% increase in seismic design loads.

New Sloped Column Irregularity

The final major change in BCBC 2024 is the introduction of a sloped column irregularity and new requirements for designing these elements. The code defines any column sloped more than 2% as a sloped column, and these elements (as well as all elements above & below the sloped column) will be subjected to additional design requirements that consider vertical earthquake acceleration. This doesn’t necessarily mean sloped columns are unfeasible, but you should expect to see larger column sizes than would traditionally be utilized in prior codes.

Summary

BCBC 2024 will bring significant changes to the structural design of buildings due to increased seismic hazard and new design requirements for certain buildings. In some cases, changes can be lessened by doing additional geotechnical investigation. Efficient scheming of the structure can also help mitigate the changes, such as by placing structural elements where they are most effective, minimizing discontinuities, and eliminating irregularities. We recommend getting preliminary structural input as early as possible to get your project on the right path.