Meena Rezkallah, P.Eng.
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 their approaches to load combinations differ. This article discusses the distinctions between NBCC and ASCE 7 in terms of load combinations, focusing on the fundamental principles that guide the design and construction of structures.
NBCC: The NBCC provides basic load combinations that include dead loads, live loads, snow loads, wind loads, and earthquake loads. These load combinations consider the primary load effects on the structure, but they do not explicitly address the combined effects of multiple loads acting simultaneously.
ASCE 7: ASCE 7 provides a more comprehensive set of basic load combinations that consider the simultaneous effects of multiple loads, such as dead loads, live loads, snow loads, wind loads, and earthquake loads. The standard also includes load combinations with temperature, soil, and hydrostatic loads, providing a more accurate representation of the structure's response to various loading conditions.
NBCC: The NBCC assigns load factors and importance factors to various loads in the load combinations. These factors are used to adjust the nominal loads to account for uncertainties in the design and construction process, as well as the structure's importance and expected performance during seismic events.
ASCE 7: ASCE 7 provides a more detailed approach to load factors and importance factors, assigning different values to loads based on factors such as the structure's performance category, seismic design category, and importance factor. This approach results in more accurate estimates of the structure's response to various loads and ensures that the structure can withstand the demands of seismic events without compromising safety or performance.
NBCC: The NBCC provides seismic load combinations that include the effects of dead loads, live loads, and earthquake loads. These combinations consider the primary load effects during seismic events, but they do not explicitly address the combined effects of multiple loads acting simultaneously.
ASCE 7: ASCE 7 provides a more comprehensive set of seismic load combinations that consider the simultaneous effects of multiple loads, such as dead loads, live loads, snow loads, wind loads, and earthquake loads. The standard also includes load combinations with temperature, soil, and hydrostatic loads, providing a more accurate representation of the structure's response to various loading conditions during seismic events.
NBCC: The NBCC incorporates overstrength factors in the load combinations to account for the additional capacity of structural elements beyond their nominal strength. The standard primarily relies on the limit states design approach, which does not explicitly address allowable stress design.
ASCE 7: ASCE 7 provides overstrength factors for various structural elements and systems, ensuring that the structure can withstand the demands of seismic events without compromising safety or performance. The standard also includes load combinations for both limit states design and allowable stress design, providing engineers with the flexibility to choose the most appropriate design approach for their projects.
Understanding the differences between NBCC and ASCE 7 in load combinations is essential for engineers working on seismic design projects. The distinctions in basic load combinations, load factors and importance factors, seismic load combinations, and overstrength factors and allowable stress design 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.