With the release of the 2024 BC Building Code (BCBC 2024), there have been notable changes to seismic design parameters, resulting in increased seismic loads engineers are required to apply to buildings. The increased loading is intended to provide engineers with the most accurate data available. 

Key Changes and Load Increases 

The seismic load requirements have been notably increased in the new code. For instance, low-rise buildings in Vancouver will see an approximate 25% increase in seismic loads, while Victoria will experience a more substantial 45% increase. This adjustment comes in response to new analyses of ground motion models, which indicated that previous load requirements were lower than the probable load the building will experience. 

Currently, BCBC 2024 has an exemption allowing the use of BCBC 2018 seismic loads until March 2025. After this transition period, the new load requirements will be mandatory for BCBC 2024 projects. Vancouver is anticipated to adopt the increased loading requirements in its 2025 Vancouver Building Bylaw, though at the time of writing this article a specific release date has not been announced. 

Concepts to Understand 

1. How Earthquake Forces are Generated: Earthquake loads are generated by the acceleration and deceleration of the ground below the building. This movement shifts the foundation which then translates loading to the building itself. 
2. Soil Types affect Loading: The forces applied to a building vary based on soil types below the building, which can either amplify or mitigate the effects of an earthquake. The top 30 meters of soil is analyzed for its type by a Geotechnical Engineer. The Geotechnical Engineer will either provide a Site Class, or a parameter that is referred to as the Shear Wave Velocity or Vs30 to the Structural Engineer. 
3. Soil Type and Its Impact on Design: Soil types play a critical role in how earthquake forces are transmitted to a building. Vibrations from earthquakes are transmitted through the soil. Rock and stiff soil displace less under seismic loads and transmit less load to buildings. Soft or liquefiable soils act more like Jello and transmit more load to the building through larger displacements. Sites with rock (Site Class A or B) provide the best conditions and the highest Vs30 values, while liquefiable soils (Site Class E and F), such as those in Richmond, present the highest amplification of seismic loads and lowest Vs30 values.  
4. Geotechnical Involvement: BCBC 2024 requires Structural Engineers to assume the worst-case soil conditions (Site Class E or low Vs30) unless a Geotechnical engineer provides better data. Most sites will have significantly better soil conditions, and a Geotechnical Engineers analysis will help reduce the loads that Structural Engineers are required to apply to the building. The extent of analysis conducted by the Geotechnical Engineer generally depends on project type. For residential and smaller commercial buildings providing a Site Classification in the geotechnical report is standard. For larger projects a drilling program will allow the Geotechnical Engineer to provide a site specific Vs30 value. The site specific Vs30 value is the most accurate but comes with a significant cost. The project must be of a scale that justifies the investment in the analysis, particularly in terms of potential reductions in structural systems based on the lower criteria. 
5. Building Mass: The load a building experiences during an earthquake is proportional to its mass. Lighter buildings will experience less force compared to heavier ones. One effective way to reduce the mass of wood frame buildings is by avoiding concrete topping on floors, or heavy exterior veneers. 

Implications for Building Design 

1. Geometry and Construction Costs: Buildings with regular geometry will perform better and be more cost-effective to construct. Aligning shear walls from roof to foundation is ideal and will be required more frequently under the new code. 
2. Steel Moment Frames: Buildings with significant amounts of glazing and minimal wall area will most likely need to incorporate steel moment frames to ensure adequate seismic performance. 
3. Shear Wall Alignment: Misaligned shear walls will increase costs due to the need for additional load transfer solutions. 
4. Innovative Seismic Systems: New seismic systems, such as mass timber (CLT) shear walls, or Mid Ply framed shear walls have higher capacities than traditional wood-framed shear walls. These systems may be considered and integrated. 
5. Mass Reduction: Removing concrete topping from floors is an easy way to reduce the mass of a wood frame building. It has the added benefit of reducing the carbon footprint of the building at the same time. 
6. Geotechnical Expertise: Involving a Geotechnical Engineer early in the project is crucial for ensuring both efficiency and cost-effectiveness in addressing site-specific conditions. 

Key Takeaways 

1. Early Collaboration: Engage with a Structural Engineer early in the design process to ensure the building remains efficient and cost effective while meeting the architectural intent. 
2. Increased Costs: Anticipate higher costs associated with seismic systems and the potential need for what are now considered non-standard seismic solutions. 
3. Geotechnical Input: Early involvement of a Geotechnical Engineer is vital for optimizing design and managing costs effectively. 

The BCBC 2024 seismic load increase will change the way buildings are designed. While current layouts may still work, they will likely incur a cost premium. Early planning and adaptation are key to managing these changes effectively. 

by Scott Ash-Anderson, P.Eng.