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 design criteria differ in several aspects. This article discusses the distinctions between NBCC and ASCE 7 in terms of seismic design criteria, focusing on the factors that influence the design and construction of structures.
Seismic Hazard Levels
NBCC: The NBCC uses a probabilistic seismic hazard assessment (PSHA) methodology to define seismic hazard levels across Canada. The seismic hazard is expressed in terms of spectral acceleration at different periods, which are used to define the design ground motions for structures.
ASCE 7: ASCE 7 also uses a PSHA methodology, but it defines the seismic hazard in terms of spectral response acceleration parameters (Ss and S1) for short and long periods, respectively. These parameters are then used to determine the design ground motions for structures.
Site Classifications and Soil Factors
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, affecting 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 differences can lead to variations in the design response spectra and impact the seismic demands on structures.
Structural Performance Categories and Design Requirements
NBCC: The NBCC uses a single set of performance criteria to achieve life safety and property protection goals through force-based design methods. The design objectives are met by controlling structural and nonstructural damage under various levels of seismic events.
ASCE 7: ASCE 7 defines different structural performance categories based on occupancy and structural importance, with varying design requirements for each category. This approach allows for a more tailored design based on the intended use and importance of the structure, resulting in different levels of structural performance expectations.
Design Spectra and Seismic Response Analysis
NBCC: The NBCC uses design response spectra for seismic response analysis, constructed based on spectral acceleration at different periods. The design spectra are utilized to estimate the seismic demands on structures, which are then used in the design process.
ASCE 7: ASCE 7 also uses design response spectra for seismic response analysis but constructs the spectra based on spectral response acceleration parameters (Ss and S1) for short and long periods. These differences in design spectra construction can impact the seismic demands on structures and influence the overall design process.
Seismic Force Resisting Systems
NBCC: The NBCC provides guidelines for selecting and designing seismic force resisting systems (SFRS) and includes various types of SFRS with associated design requirements. The SFRS selection is based on factors such as building type, seismic hazard, and structural configuration.
ASCE 7: ASCE 7 also provides guidance on selecting and designing SFRS, but it offers a more extensive range of SFRS options and more detailed design requirements. This difference allows for greater flexibility in selecting appropriate systems based on the specific requirements of a project and site conditions.
Conclusion
Understanding the differences between NBCC and ASCE 7 in seismic design criteria is crucial for engineers working on seismic design projects. The distinctions in seismic hazard levels, site classifications, structural performance categories, design spectra, and seismic force resisting systems can significantly impact the design and performance of structures during seismic events.