Introduction: Seismic Design Evolution and the Role of Shear Walls in Accordance with U.S. and Canadian Standards (IBC, ASCE-7, NBCC, CSA A23, etc.)

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The practice of seismic design has evolved significantly since its earliest origins in antiquity. Ancient civilizations, such as the Greeks and Romans, developed rudimentary seismic-resistance strategies, primarily through architectural forms that minimized damage from earthquakes. However, it wasn't until the 20th century, particularly following the devastating 1906 San Francisco earthquake, that seismic engineering emerged as a formalized field, driven by the urgent need to protect buildings from seismic forces.

As modern seismology advanced and the understanding of lateral forces grew, seismic design principles began to incorporate not only the need to resist vertical loads but also to mitigate the effects of horizontal forces. Among the most essential innovations for achieving lateral load resistance are shear walls—vertical structural elements that are designed to resist and absorb lateral forces due to wind, seismic loads, or other horizontal pressures.

In both the United States and Canada, seismic design has been codified through national standards that outline rigorous requirements for ensuring structural resilience. These standards reflect the growing understanding of earthquake behavior and the need for buildings to perform adequately during seismic events.

US Standards:

• International Building Code (IBC). The IBC is a comprehensive model building code used throughout the United States. It covers all aspects of building construction including structural, fire protection, egress, accessibility, energy efficiency, and more. It is adopted and modified at the state and local levels. Most recent edition: IBC-2024.

• Building Code Requirements for Structural Concrete and Commentary (ACI-318). ACI 318 provides the minimum requirements for design and construction of structural concrete buildings and other concrete structures. It includes provisions for materials, design, detailing, construction, and inspection:

• Strength and serviceability design (Limit States Design)

• Reinforced and prestressed concrete

• Structural systems including beams, columns, slabs, walls, footings

• Seismic design provisions (when used in conjunction with ASCE 7)

• Durability, fire resistance, and constructability

• Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE-7). This standard defines the minimum load requirements (e.g., dead, live, wind, snow, seismic, rain, and ice loads) for buildings and other structures. It’s widely referenced in the IBC for structural design criteria. Most recent edition: ASCE/SEI 7-22 (referenced in IBC-2024).

• Building Code Requirements for Masonry Structures (TMS 402). This standard was formerly named ACI 530/ASCE 5. TMS 402 provides the minimum requirements for the structural design and construction of masonry in buildings: Concrete masonry units, clay masonry (brick), stone masonry and autoclaved aerated concrete (AAC). Most recent edition: TMS 402-22 (2022 Edition). Since the 2013 edition, TMS is the sole publisher of TMS 402/602, although it maintains compatibility with ACI and ASCE frameworks.

• Others standards:

• ACI 301 – Specifications for Structural Concrete.

• ACI 117 – Tolerances for Concrete Construction.

Canadian Standards:

• National Building Code of Canada (NBCC). This is Canada’s model code for building construction and fire safety. It addresses building design and construction, life safety, structural design, fire protection, energy efficiency, and accessibility. NBCC 2020 (adoption is provincial/territorial)

• CSA A23.3 (Design of Concrete Structures). Governs the structural design of reinforced and prestressed concrete buildings. It’s the Canadian counterpart to ACI 318 in the U.S. Most recent edition: CSA A23.3:19 (2019).

• CSA S16 (Design of Steel Structures). Provides requirements for the design, fabrication, and erection of steel structures and structural steel connections. It parallels AISC standards in the U.S. Most recent edition: CSA S16:19 (2019)

• CSA S304.1 (Design of Masonry Structures). Establishes the requirements for structural design and analysis of masonry elements including walls, piers, and foundations. It is the Canadian equivalent to TMS 402 in the U.S. CSA S304-14 (R2022)—reaffirmed in 2022.

• US Standard Approach:

In the United States, seismic design is governed primarily by the International Building Code (IBC), developed by the International Code Council (ICC), and specifically the ASCE 7 standard, which provides the minimum design loads for buildings and other structures. ASCE 7 incorporates a comprehensive framework for seismic load determination, addressing factors such as seismic hazard, site location, and building type. This standard is not only suitable for concrete structure design but also for steel structure design as well as pressure vessels.

Shear walls are explicitly recognized in these codes as essential elements for resisting lateral forces. The IBC and ASCE 7 both provide detailed guidance on the placement, design, and reinforcement of shear walls based on the seismic classification of the site (Seismic Design Categories A to F). The ASCE 7 standard includes specific provisions for the sizing and detailing of shear walls, considering factors like the type of material (reinforced concrete, masonry, etc.) and the building's height and geometry.

 For instance, in high-seismic regions (Seismic Design Categories D, E, and F), shear walls must be designed with greater stiffness and strength to withstand the higher levels of lateral forces expected during significant seismic events. The design provisions also address ductility, ensuring that shear walls can deform without failure, thus enhancing the building’s ability to absorb seismic energy.

Detailed ASCE 7 Provisions for Shear Walls. Shear walls are explicitly recognized in these codes as essential elements for resisting lateral forces. The IBC and ASCE 7 both provide detailed guidance on the placement, design, and reinforcement of shear walls based on the seismic classification of the site (Seismic Design Categories A to F). The ASCE 7 standard includes specific provisions for the sizing and detailing of shear walls, considering factors like the type of material (reinforced concrete, masonry, etc.) and the building's height and geometry.

• Response Modification Factor (R): Ranges from 3.0 to 8.0 depending on the structural system (special reinforced concrete shear walls have R=8.0)

• Overstrength Factor (Ω₀): Typically 2.5 for reinforced concrete shear walls

• Deflection Amplification Factor (Cd): Values range from 4.0 to 5.5 for concrete shear wall systems

• Seismic Base Shear Calculation: V = CsW, where Cs is the seismic response coefficient and W is the effective seismic weight

ACI 318 Specific Requirements.

• Minimum Wall Thickness: 6 inches (152 mm) for seismic applications, or hw/25 (where hw is the unsupported height)

• Reinforcement Ratios: Minimum horizontal reinforcement ratio of 0.0025 and vertical reinforcement ratio of 0.0015

• Boundary Elements: Required when compression strain exceeds 0.003 or when axial load exceeds 0.1f'cAg

• Shear Strength: Nominal shear strength limited to 10√f'c for special structural walls.

Special Seismic Design Requirements:

• Coupling Beams: When designed with diagonal reinforcement, can achieve rotation capacities of 0.08 radians

• Plastic Hinge Regions: Must be detailed for curvature ductility ratios of 10-15

• Lap Splice Restrictions: Prohibited in plastic hinge regions and within twice the wall thickness from the base

• Canadian Standard Approach:

In Canada, seismic design is governed by the National Building Code of Canada (NBCC), which outlines similar principles for seismic load design. Specifically, CSA A23.3 (Design of Concrete Structures) and CSA S16 (Design of Steel Structures) are frequently referenced for the design of shear walls in buildings. The NBCC provides detailed seismic hazard maps that identify areas of varying seismic risk, with design requirements escalating in regions subject to higher seismic activity.

 Like their American counterparts, Canadian standards stress the importance of shear walls in resisting lateral forces, particularly in high-rise and multi-story buildings. The CSA S304.1 (Design of Masonry Structures) also incorporates detailed guidance for shear wall design in masonry structures, while the CSA A23.3 includes provisions for reinforced concrete shear walls. The NBCC mandates that shear walls be proportioned to resist forces calculated based on the building's seismic zone, with explicit guidelines for detailing reinforcement to ensure adequate load transfer and energy dissipation during seismic events.

CSA A23.3 Detailed Provisions:

• Moderately Ductile Walls: Rd = 3.5, Ro = 1.6

• Ductile Walls: Rd = 5.0, Ro = 1.6

• Minimum Reinforcement: 0.30% horizontal, 0.20% vertical for moderately ductile walls

• Confined Boundary Elements: Required when compression strain exceeds 0.002 or under specific axial load conditions.

Like their American counterparts, Canadian standards stress the importance of shear walls in resisting lateral forces, particularly in high-rise and multi-story buildings. The CSA S304.1 (Design of Masonry Structures) also incorporates detailed guidance for shear wall design in masonry structures, while the CSA A23.3 includes provisions for reinforced concrete shear walls. The NBCC mandates that shear walls be proportioned to resist forces calculated based on the building's seismic zone, with explicit guidelines for detailing reinforcement to ensure adequate load transfer and energy dissipation during seismic events.

Canadian Seismic Design Factors:

• Seismic Force Reduction: V = S(Ta)MvIEW/RdRo

• Site Classification: Ranges from Site Class A (hard rock) to Site Class F (special soils)

• Importance Factor (IE): 1.0 for normal importance, 1.3 for high importance structures

• Higher Mode Effects: Must be considered for walls with height-to-length ratios greater than 2.0

The Role of Shear Walls in Seismic Design

Shear walls are integral to mitigating the effects of lateral forces, ensuring that buildings can maintain stability under the pressure of seismic or wind-induced loads. Their design is critical for preventing the catastrophic displacement of the building's superstructure. Shear walls help control drift, which is the lateral displacement of the structure during an earthquake, and reduce the risk of torsional motion, which occurs when buildings twist due to uneven distribution of lateral forces.

 The placement of shear walls is often concentrated around the core of the building, such as around elevator shafts and stairwells, as well as at the building's perimeter. These locations maximize the resistance to lateral forces while minimizing the impact on the usable space within the structure. The use of shear walls also enhances the building's stiffness, improving its overall dynamic response during seismic events.

In both the U.S. and Canada, the incorporation of shear walls in building design is not only a best practice but a regulatory necessity, driven by the imperative to protect public safety and mitigate the risks associated with seismic events. Adhering to standards like IBC, ASCE 7, NBCC, and CSA A23.3 ensures that buildings are designed to perform adequately under lateral loading conditions, including those induced by earthquakes. As seismic risk continues to be a major concern in high-risk areas, the strategic use of shear walls remains one of the most effective methods of ensuring that structures can withstand these dynamic forces and safeguard human lives

Key Functions of Shear Walls

1. Resist Lateral Loads: Shear walls help resist forces that push or pull horizontally on the structure. These forces could be from wind, seismic activity, or uneven settlement. These forces could be from wind, seismic activity, or uneven settlement. Design wind pressures typically range from 15-40 psf depending on exposure and building height.

2. Provide Rigidity: By distributing and transferring lateral loads to the foundation, shear walls reduce the building's sway and ensure its stability. Fundamental period can be reduced by 30-50% with proper shear wall placement.

3. Increase Safety: In seismic-prone areas, shear walls are critical for preventing structural damage and maintaining the building’s integrity during earthquakes. Energy dissipation through controlled yielding prevents catastrophic failure.

4. Strengthening: They provide additional strength to the building structure, particularly in high-rise buildings or buildings with large open spaces. Lateral stiffness increases proportionally to the cube of wall thickness.

 Types of Shear Walls (Bearing shall be understood floor weight bearing)

1. Bearing Shear Wall: Resists lateral loads and transfers them to the foundation. It’s often part of the overall structural system. Load combinations include 1.2D + 1.0E + 0.5L for seismic design.

2. Non-Bearing Shear Wall: Primarily intended for lateral load resistance, not for bearing the weight of floors or roofs. Minimum thickness requirements still apply for stability.

Specialized Shear Wall Systems

• Coupled Shear Walls: Connected by coupling beams, providing enhanced ductility

• Outrigger Walls: Used in super-tall buildings to engage perimeter columns

• Core Walls: Integrated with elevator and stair systems for efficiency

• Buttressed Walls: With perpendicular walls for enhanced stability

Placement

Shear walls are usually placed:

1. Around the building’s core (for example, in elevator shafts or stairwells).

2. At strategic points around the perimeter or within the building to ensure an even distribution of forces.

In taller buildings, shear walls may be placed symmetrically to prevent torsional effects and maintain stability.

 Advantages

1. Efficient in resisting lateral forces: Particularly effective in resisting wind and seismic forces.

2. Reduces building sway: Helps in reducing vibrations and movement in the structure, enhancing comfort for occupants.

3. Drift Limitations: Typically limited to 0.020hsx for structural walls (where hsx is the story height)

4. P-Delta Effects: Must be considered when the stability coefficient θ exceeds 0.10

5. Torsional Irregularity: Occurs when maximum story drift exceeds 1.2 times the average drift

6. Accidental Torsion: Minimum 5% eccentricity must be applied to account for construction tolerances

Disadvantages

1. Space Constraints: They take up valuable space inside the building, especially in residential or commercial projects.

2. Aesthetic Considerations: The large, solid walls may not always fit well with the architectural design or desired aesthetics.

Materials Used:

• Reinforced Concrete: Common in modern buildings for its strength and durability. Typical concrete strengths range from 4,000 to 8,000 psi (28-55 MPa), with reinforcement grades of 60,000 psi (420 MPa) or higher.

• Steel: Often used in high-rise buildings where weight is a concern. Steel plate walls can achieve strengths of 50 ksi (345 MPa) with buckling-restrained systems.

• Masonry: Sometimes used in smaller buildings or as part of a hybrid system. Grouted masonry with f'm values of 1,500-3,000 psi (10-21 MPa).

Advanced Material Considerations:

• High-Strength Concrete: Up to 12,000 psi (83 MPa) for high-rise applications

• Fiber-Reinforced Concrete: Enhanced ductility with steel or synthetic fibers

• Post-Tensioned Systems: Improved crack control and reduced wall thickness

• Composite Systems: Steel-concrete composite walls for optimal performance.

Conclusion:

In both the U.S. and Canada, the incorporation of shear walls in building design is not only a best practice but a regulatory necessity, driven by the imperative to protect public safety and mitigate the risks associated with seismic events. Adhering to standards like IBC, ASCE 7, NBCC, and CSA A23.3 ensures that buildings are designed to perform adequately under lateral loading conditions, including those induced by earthquakes. As seismic risk continues to be a major concern in high-risk areas, the strategic use of shear walls remains one of the most effective methods of ensuring that structures can withstand these dynamic forces and safeguard human lives.

REFERENCES used in this article

• ASCE/SEI 7-22: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. American Society of Civil Engineers, Reston, VA, 2022.

• ACI 318-19: Building Code Requirements for Structural Concrete and Commentary. American Concrete Institute, Farmington Hills, MI, 2019.

• International Building Code (IBC) 2021. International Code Council, Washington, DC, 2021.

• National Building Code of Canada (NBCC) 2020. Canadian Commission on Building and Fire Codes, National Research Council of Canada, Ottawa, ON, 2020.

• CSA A23.3-19: Design of Concrete Structures. Canadian Standards Association, Toronto, ON, 2019.

• CSA S16-19: Design of Steel Structures. Canadian Standards Association, Toronto, ON, 2019.

• CSA S304.1-14: Design of Masonry Structures. Canadian Standards Association, Toronto, ON, 2014.

Technical References and Research

• Paulay, T., & Priestley, M. J. N. (1992). "Seismic Design of Reinforced Concrete and Masonry Buildings." John Wiley & Sons, New York.

• Moehle, J. P. (2015). "Seismic Design of Reinforced Concrete Buildings." McGraw-Hill Education, New York.

• Fintel, M. (1991). "Handbook of Concrete Engineering, 2nd Edition." Van Nostrand Reinhold, New York.

• Priestley, M. J. N., Calvi, G. M., & Kowalsky, M. J. (2007). "Displacement-Based Seismic Design of Structures." IUSS Press, Pavia, Italy.

• Chopra, A. K. (2017). "Dynamics of Structures: Theory and Applications to Earthquake Engineering, 5th Edition." Pearson Education, Boston.

Performance-Based Design References

• PEER/ATC-72-1 (2010). "Modeling and Acceptance Criteria for Seismic Design and Analysis of Tall Buildings." Applied Technology Council, Redwood City, CA.

• ASCE/SEI 41-17: Seismic Evaluation and Retrofit of Existing Buildings. American Society of Civil Engineers, Reston, VA, 2017.

• FEMA P-58-1 (2012). "Seismic Performance Assessment of Buildings - Volume 1: Methodology." Federal Emergency Management Agency, Washington, DC.

Specialized Topics and Advanced Applications

1. Harries, K. A., & Mitchell, D. (2002). "Seismic Design of Coupled Wall Systems." Canadian Journal of Civil Engineering, 29(4), 615-629.

Software and Analysis Tools References

1. CSI (2021). "ETABS - Integrated Building Design Software." Computers and Structures, Inc., Berkeley, CA.

2. Dassault Systèmes (2021). "SIMULIA Abaqus - Finite Element Analysis Software." Dassault Systèmes, Vélizy-Villacoublay, France.

3. OpenSees (2021). "Open System for Earthquake Engineering Simulation." Pacific Earthquake Engineering Research Center, University of California, Berkeley.