The National Building Code of Canada (NBCC) and the American Society of Civil Engineers' ASCE 7 Standard are two prominent guidelines for seismic design. Both standards aim to ensure the safety and performance of structures during seismic events, but their seismic hazard assessment and ground motion parameters differ. This article discusses the distinctions between NBCC and ASCE 7 in terms of seismic hazard assessment and ground motion parameters, 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.
Ground Motion Parameters
NBCC: The NBCC expresses seismic hazard in terms of spectral acceleration at different periods, which are used to define the design ground motions for structures. These ground motion parameters are then used to construct the design response spectra for seismic response analysis.
ASCE 7: ASCE 7 defines the seismic hazard in terms of spectral response acceleration parameters (Ss and S1) for short and long periods, respectively. These parameters are used to determine the design ground motions for structures and are then used to construct the design response spectra for seismic response analysis.
Site Classifications and Site Coefficients
NBCC: The NBCC classifies sites based on soil properties and assigns a site classification, which influences the design ground motion parameters. The site coefficients are used to adjust the ground motion parameters, accounting for the site's soil conditions and their impact on the design response spectra.
ASCE 7: ASCE 7 also classifies sites based on soil properties, but the classification system and site coefficients used to adjust ground motion parameters differ from those in the NBCC. These distinctions can lead to variations in the design response spectra and impact the seismic demands placed on structures.
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 assessment and ground motion parameters is crucial for engineers working on seismic design projects. The distinctions in seismic hazard assessment methodologies, ground motion parameters, site classifications, 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.