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Differences between NBCC and ASCE 7 in Seismic Force Resisting Systems

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 seismic force resisting systems differ. This article discusses the distinctions between NBCC and ASCE 7 in terms of seismic force resisting systems, focusing on the fundamental principles that guide the design and construction of structures.

Classification of Seismic Force Resisting Systems

NBCC: The NBCC classifies seismic force resisting systems into a limited number of categories, such as moment-resisting frames, shear walls, and braced frames. The standard focuses on primary force-resisting systems and does not explicitly address secondary systems or specific structural components.

ASCE 7: ASCE 7 provides a more detailed classification of seismic force resisting systems, including a broader range of systems and components, such as special moment-resisting frames, buckling-restrained braced frames, and hybrid systems. This more extensive classification allows for a more accurate representation of the structure's behavior during seismic events.

Design Requirements for Seismic Force Resisting Systems

NBCC: The NBCC provides design requirements for primary force-resisting systems, such as moment-resisting frames, shear walls, and braced frames. These requirements focus on ensuring the safety and performance of the structure during seismic events, with an emphasis on life safety and property protection.

ASCE 7: ASCE 7 offers more detailed design requirements for various seismic force resisting systems, including specific provisions for each type of system and component. The standard addresses factors such as ductility, stability, and redundancy to ensure that structures can withstand the demands of seismic events without compromising safety or performance.

Design Limitations for Seismic Force Resisting Systems

NBCC: The NBCC imposes some limitations on the use of certain seismic force resisting systems in areas with high seismic hazard levels. However, these limitations are relatively broad and do not address the specific needs of different types of structures or occupancy categories.

ASCE 7: ASCE 7 establishes more extensive limitations on the use of specific seismic force resisting systems in areas with high seismic hazard levels. These limitations are based on factors such as the structure's importance, occupancy, and location, ensuring that the most appropriate systems are used for each project.



The American Society of Civil Engineers (ASCE)

The National Building Code of Canada (NBCC)

ASCE 7 (American Society of Civil Engineers 7) provides various design limitations for seismic force-resisting systems (SFRS) to ensure that these systems can effectively resist seismic forces. Some of the key design limitations for different types of SFRS are as follows:

  1. Concentrically Braced Frames (CBF): ASCE 7 limits the brace slenderness ratio to 120 for CBFs in Seismic Design Category (SDC) C and 150 for SDC D, E, and F. The brace buckling strength must also be verified. In addition, ASCE 7 requires that the design of CBFs account for the effect of P-delta moments, which arise due to lateral displacements.

  2. Eccentrically Braced Frames (EBF): ASCE 7 limits the brace slenderness ratio to 120 for EBFs in SDC C and 150 for SDC D, E, and F. The brace buckling strength must also be verified. The EBF design must account for the effect of P-delta moments, as well as the axial load amplification effect that arises due to brace eccentricity.

  3. Special Moment Frames (SMF): ASCE 7 limits the maximum beam-to-column strength ratio to 1.3 and the maximum column slenderness ratio to 20 for SMFs in SDC C and 25 for SDC D, E, and F. The SMF design must also account for the effect of P-delta moments and the effect of beam plastic hinging on the column strength.

  4. Special Structural Walls: ASCE 7 limits the wall slenderness ratio to 12 for walls in SDC C and 10 for walls in SDC D, E, and F. The wall design must also account for the effects of in-plane and out-of-plane forces, as well as the effects of axial loads and P-delta moments.

  5. Dual Systems: ASCE 7 requires that the design of dual systems account for the interaction between the different SFRS types and the transfer of forces between the systems.

In addition to the above limitations, ASCE 7 provides other requirements and design considerations for seismic force-resisting systems, including detailing requirements for connections, reinforcement, and joint design. It is important to note that the specific design limitations and requirements may vary depending on the specific project and design conditions.

The National Building Code of Canada (NBCC) provides various design limitations for seismic force-resisting systems (SFRS) to ensure that these systems can effectively resist seismic forces. Some of the key design limitations for different types of SFRS are as follows:

  1. Moment-Resisting Frames (MRF): NBCC limits the maximum beam-to-column strength ratio to 1.3 for MRFs in all seismic design categories. In addition, the MRF design must account for the effect of P-delta moments, which arise due to lateral displacements.

  2. Braced Frames (BF): NBCC limits the brace slenderness ratio to 120 for BF in seismic design categories D, E, and F. In seismic design category C, a limit of 150 is allowed. The brace buckling strength must also be verified. The BF design must account for the effect of P-delta moments, as well as the axial load amplification effect that arises due to brace eccentricity.

  3. Structural Walls: NBCC limits the wall slenderness ratio to 10 for walls in seismic design categories D, E, and F. In seismic design category C, a limit of 12 is allowed. The wall design must also account for the effects of in-plane and out-of-plane forces, as well as the effects of axial loads and P-delta moments.

  4. Dual Systems: NBCC requires that the design of dual systems account for the interaction between the different SFRS types and the transfer of forces between the systems.

In addition to the above limitations, NBCC provides other requirements and design considerations for seismic force-resisting systems, including detailing requirements for connections, reinforcement, and joint design. It is important to note that the specific design limitations and requirements may vary depending on the specific project and design conditions.


Redundancy and Continuity

NBCC: The NBCC addresses redundancy and continuity in seismic force resisting systems to some extent, but it does not provide extensive guidance on these topics. The standard focuses on ensuring the overall stability and performance of the structure during seismic events.

ASCE 7: ASCE 7 places greater emphasis on redundancy and continuity in seismic force resisting systems, providing more detailed guidance on the design, analysis, and construction of these systems. The standard ensures that structures have adequate load paths and can effectively redistribute forces during seismic events, minimizing the risk of structural collapse.

The American Society of Civil Engineers (ASCE)

The National Building Code of Canada (NBCC)

ASCE 7 (American Society of Civil Engineers 7) provides different seismic redundancy requirements depending on the seismic design category and the type of seismic-force-resisting system (SFRS) being used. The seismic redundancy requirements for different SFRS types are summarized below:

  1. Concentrically braced frames (CBF): ASCE 7 requires a minimum of two braces in each orthogonal direction for CBFs in Seismic Design Category (SDC) C and above. In SDC D, E, and F, a minimum of three braces is required in each orthogonal direction. CBFs in SDC E and F must also satisfy additional requirements for member strength and joint detailing.

  2. Eccentrically braced frames (EBF): ASCE 7 requires a minimum of two braces in each orthogonal direction for EBFs in SDC C and above. In SDC D, E, and F, a minimum of three braces is required in each orthogonal direction. EBFs in SDC E and F must also satisfy additional requirements for member strength and joint detailing.

  3. Special moment frames (SMF): ASCE 7 requires that SMFs in SDC C and above have a minimum of two lines of lateral resistance in each orthogonal direction. In SDC D, E, and F, three lines of lateral resistance are required in each orthogonal direction. The SMF must also satisfy additional requirements for beam strength, column strength, and joint detailing.

  4. Special structural walls: ASCE 7 requires that special structural walls have a minimum thickness of 8 in. (203 mm) and a maximum aspect ratio of 3:1 in both directions. Walls in SDC C and above must have a minimum of two lines of lateral resistance in each orthogonal direction. In SDC D, E, and F, three lines of lateral resistance are required in each orthogonal direction.

  5. Dual systems: ASCE 7 allows for the use of a dual system consisting of two different SFRS types. The seismic redundancy requirements for dual systems depend on the combination of SFRS types being used.

It is important to note that the seismic redundancy values presented here are minimum requirements and may be increased depending on the specific design conditions and engineering judgment.

The National Building Code of Canada (NBCC) provides different seismic redundancy requirements depending on the seismic design category and the type of seismic-force-resisting system (SFRS) being used. The seismic redundancy requirements for different SFRS types are summarized below:

  1. Moment-Resisting Frames (MRF): NBCC requires a minimum of two lines of resistance in each orthogonal direction for MRFs in all seismic design categories. In addition, the MRF must satisfy additional requirements for member strength and joint detailing.

  2. Braced Frames (BF): NBCC requires a minimum of two braces in each orthogonal direction for BF in seismic design category D, E, and F. In seismic design category C, a minimum of one brace is permitted in each orthogonal direction.

  3. Structural Walls: NBCC requires that all structural walls have a minimum thickness of 8 in. (203 mm) and a maximum aspect ratio of 3:1 in both directions. Walls in seismic design categories D, E, and F must have a minimum of two lines of resistance in each orthogonal direction. In seismic design category C, one line of resistance is permitted in each orthogonal direction.

  4. Dual Systems: NBCC allows for the use of a dual system consisting of two different SFRS types. The seismic redundancy requirements for dual systems depend on the combination of SFRS types being used.

It is important to note that the seismic redundancy values presented here are minimum requirements and may be increased depending on the specific design conditions and engineering judgment. The NBCC also includes provisions for seismic continuity to ensure that forces can be transmitted through the structure in the event of an earthquake.


The American Society of Civil Engineers (ASCE)

The National Building Code of Canada (NBCC)

​​​The American Society of Civil Engineers (ASCE) has developed a set of guidelines for seismic force resisting systems (SFRS) to help ensure the safety and stability of buildings in seismic zones. These guidelines outline the requirements for the design and construction of SFRS, which are structural elements that are designed to resist the lateral forces generated by earthquakes.

There are several types of SFRS that are commonly used in building design. These include:

  1. Braced Frames: These are vertical and diagonal members that form a rigid frame, which resists lateral forces by transferring them to the foundation.

  2. Shear Walls: These are vertical walls that are designed to resist lateral forces by transferring them to the foundation.

  3. Moment Frames: These are structural elements that are designed to resist lateral forces by using rigid connections between the beams and columns to transfer the forces to the foundation.

  4. Buckling-Restrained Braces: These are special types of braces that are designed to resist lateral forces by using a combination of steel and concrete to prevent buckling.

The ASCE guidelines also specify the design criteria and performance objectives for SFRS. These criteria include the maximum allowable drift and displacement, as well as the minimum strength and stiffness requirements for the SFRS components.

In addition to the design and construction of SFRS, the ASCE guidelines also address the importance of regular maintenance and inspection of these systems to ensure their continued effectiveness in resisting seismic forces.

The National Building Code of Canada (NBCC) provides guidelines for seismic force resisting systems (SFRS) that are intended to ensure the safety and stability of buildings in seismic zones. These guidelines provide requirements for the design, construction, and maintenance of SFRS to ensure they can withstand seismic forces. The NBCC identifies several types of SFRS that are commonly used in building design. These include:

  1. Braced Frames: These are vertical and diagonal members that form a rigid frame, which resists lateral forces by transferring them to the foundation.

  2. Shear Walls: These are vertical walls that are designed to resist lateral forces by transferring them to the foundation.

  3. Moment Frames: These are structural elements that are designed to resist lateral forces by using rigid connections between the beams and columns to transfer the forces to the foundation.

  4. Dual Systems: These are combinations of different SFRS, such as a combination of moment frames and braced frames, which can provide additional strength and redundancy.

The NBCC guidelines also specify the design criteria and performance objectives for SFRS. These criteria include the maximum allowable drift and displacement, as well as the minimum strength and stiffness requirements for the SFRS components. In addition to the design and construction of SFRS, the NBCC guidelines emphasize the importance of regular maintenance and inspection of these systems to ensure their continued effectiveness in resisting seismic forces. The guidelines also provide guidance on retrofitting existing buildings with SFRS to improve their seismic resistance.


​The American Society of Civil Engineers (ASCE) has established design limitations for seismic force resisting systems (SFRS) to ensure that buildings are designed and constructed in a safe and stable manner in seismic zones. These limitations are outlined in the ASCE 7 Standard, Minimum Design Loads for Buildings and Other Structures.

The ASCE design limitations for SFRS include:

  1. Maximum Allowable Drift: The SFRS must be designed to limit the maximum allowable drift, which is the lateral displacement of the building during a seismic event. The maximum allowable drift is based on the building occupancy and the importance of the structure.

  2. Strength Limitations: The SFRS must be designed to have sufficient strength to resist the anticipated seismic forces without exceeding the allowable stress limits for the building components.

  3. Deflection Limitations: The SFRS must be designed to limit the deflection of the building components to prevent excessive deformation during a seismic event.

  4. Ductility Limitations: The SFRS must be designed to exhibit ductility, which is the ability to deform and absorb energy during a seismic event. The level of ductility required depends on the type of SFRS used and the expected seismic forces.

  5. Interaction Limitations: The SFRS must be designed to consider the interaction between the SFRS and other building components, such as the foundation and non-structural elements.

The ASCE design limitations for SFRS are intended to ensure that the building components are designed and constructed to withstand the anticipated seismic forces while limiting the potential for damage or collapse. It is important to note that these limitations are subject to change as new research and developments are made in the field of earthquake engineering.

The National Building Code of Canada (NBCC) has established design limitations for seismic force resisting systems (SFRS) to ensure the safety and stability of buildings in seismic zones. These limitations are outlined in the NBCC, Part 4, Structural Design.

The NBCC design limitations for SFRS include:

  1. Maximum Allowable Drift: The SFRS must be designed to limit the maximum allowable drift, which is the lateral displacement of the building during a seismic event. The maximum allowable drift is based on the building occupancy and the importance of the structure.

  2. Strength Limitations: The SFRS must be designed to have sufficient strength to resist the anticipated seismic forces without exceeding the allowable stress limits for the building components.

  3. Deflection Limitations: The SFRS must be designed to limit the deflection of the building components to prevent excessive deformation during a seismic event.

  4. Ductility Limitations: The SFRS must be designed to exhibit ductility, which is the ability to deform and absorb energy during a seismic event. The level of ductility required depends on the type of SFRS used and the expected seismic forces.

  5. Interaction Limitations: The SFRS must be designed to consider the interaction between the SFRS and other building components, such as the foundation and non-structural elements.

The NBCC design limitations for SFRS are intended to ensure that the building components are designed and constructed to withstand the anticipated seismic forces while limiting the potential for damage or collapse. It is important to note that these limitations are subject to change as new research and developments are made in the field of earthquake engineering.


Conclusion

Understanding the differences between NBCC and ASCE 7 in seismic force resisting systems is essential for engineers working on seismic design projects. The distinctions in classification of seismic force resisting systems, design requirements for seismic force resisting systems, design limitations for seismic force resisting systems, and redundancy and continuity 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.



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