The National Building Code of Canada (NBCC) and the American Society of Civil Engineers' ASCE 7 Standard are two widely adopted guidelines for seismic design. Both standards aim to ensure the safety and performance of structures during seismic events, but they define seismic hazard levels differently. This article discusses the distinctions between NBCC and ASCE 7 in terms of seismic hazard levels, focusing on the factors that influence the design and construction of structures.
Seismic Hazard Assessment Methodology
NBCC: The NBCC uses a probabilistic seismic hazard assessment (PSHA) methodology to define seismic hazard levels across Canada. This methodology takes into account local geology, historical seismicity, and site conditions to provide a comprehensive representation of the seismic hazard for each region.
ASCE 7: ASCE 7 also employs a PSHA methodology, but the seismic source models and ground motion prediction equations (GMPEs) used in the assessment differ from those used in the NBCC. These differences can lead to variations in the assessed seismic hazard levels for specific locations, affecting the design ground motions for structures.
Seismic Hazard Maps
NBCC: The NBCC provides seismic hazard maps that display spectral acceleration values at different periods across Canada. These maps allow engineers to visualize the distribution of seismic hazard levels and estimate the seismic hazard for specific locations.
ASCE 7: ASCE 7 also provides seismic hazard maps, but these maps display spectral response acceleration parameters (Ss and S1) for short and long periods, respectively. The differences in parameters can lead to variations in the estimated seismic hazard levels for specific locations.
Seismic Hazard Levels and Design Categories
NBCC: The NBCC does not differentiate between seismic hazard levels in terms of design categories. Instead, it uses a single set of performance criteria to achieve life safety and property protection goals for structures in all regions.
ASCE 7: ASCE 7 defines different seismic design categories (SDCs) based on the seismic hazard levels and the importance of the structure. These SDCs determine the applicable design requirements, with more stringent criteria for structures located in areas with higher seismic hazard levels or those with critical functions.
Design Ground Motion Return Periods
NBCC: The NBCC establishes design ground motion parameters for a return period of 1 in 2,475 years for most structures, which is equivalent to a 2% probability of exceedance in 50 years. This return period ensures an appropriate level of safety for structures designed according to the NBCC provisions.
ASCE 7: ASCE 7 sets the design ground motion parameters for a return period of 1 in 2,500 years for most structures, equivalent to a 2% probability of exceedance in 50 years. While this return period is similar to the NBCC, differences in seismic hazard assessment methodology can still result in variations in the design ground motions for specific locations.
Conclusion
Understanding the differences between NBCC and ASCE 7 in seismic hazard levels is essential for engineers working on seismic design projects. The distinctions in seismic hazard assessment methodologies, seismic hazard maps, design categories, and design ground motion return periods can significantly impact the design and performance of structures during seismic events. By recognizing these differences, engineers can select the most appropriate standard for their projects and ensure the safety and performance of structures in earthquake-prone regions.