In earthquake-prone areas, the safety and stability of buildings depend heavily on the incorporation of seismic design elements that allow them to withstand seismic forces. Engineers have developed various structural systems that can absorb and dissipate the energy from seismic events, preventing structural failure and ensuring the safety of occupants. This lesson will explore three of the most widely used earthquake-resistant structural systems: braced frames, shear walls, and moment frames. Each of these systems plays a crucial role in making buildings more resilient to seismic forces.
Learning Objectives
By the end of this lesson, you will:
Understand the purpose and function of braced frames, shear walls, and moment frames in resisting seismic forces.
Identify the strengths and limitations of each structural system.
Explore how these systems are used in combination to maximize building safety during an earthquake.
Understand the role of material selection in the effectiveness of these structural systems.
What Are Earthquake-Resistant Structures?
Earthquake-resistant structures are designed to endure the forces generated by seismic waves. These structures incorporate features that allow them to absorb or redirect the energy from an earthquake, reducing the stress on critical components such as walls, columns, and floors. The main goal is to prevent catastrophic failure, such as a collapse, and to ensure that the building remains usable after the earthquake.
Three of the most common systems for earthquake-resistant design are braced frames, shear walls, and moment frames. Each system behaves differently under seismic forces, and engineers often use them in combination to provide comprehensive protection.
Braced Frames: Enhancing Lateral Stability
Definition and Function
Braced frames are structural systems that use diagonal members to resist lateral forces, such as those generated by earthquakes. These diagonal braces connect the beams and columns of a structure, creating a triangulated framework that helps distribute seismic forces evenly across the building. This system helps prevent the building from swaying excessively, which could lead to structural failure.
Braced frames are typically used in steel or reinforced concrete buildings, as these materials provide the strength and ductility required to withstand seismic forces. The braces themselves can be made of steel or other strong materials that are capable of absorbing significant amounts of energy without breaking.
Types of Bracing
There are several types of bracing used in braced frame systems, each offering different advantages depending on the design and intended use of the building.
Concentric Braced Frames (CBFs): In a concentric braced frame, the diagonal braces meet at a single point with the beams and columns, forming a triangle. This configuration provides excellent lateral stiffness and is effective in reducing the swaying of the building during an earthquake. CBFs are commonly used in mid-rise and high-rise buildings.
Eccentric Braced Frames (EBFs): In an eccentric braced frame, the braces are offset from the intersection of the beams and columns, creating a small gap. This gap acts as a fuse during an earthquake, absorbing energy and preventing the building from collapsing. EBFs offer greater ductility than CBFs, making them ideal for buildings in regions with high seismic risk.
K-Bracing and V-Bracing: These are configurations in which diagonal braces form either a "K" or a "V" shape within the frame. K-bracing is rarely used in modern designs because it can concentrate forces in undesirable ways, potentially leading to local failures. V-bracing, however, is more common and can provide both stability and flexibility.
Strengths and Limitations of Braced Frames
Braced frames offer several advantages for earthquake-resistant design:
Strength: The triangulated design of braced frames allows them to efficiently resist lateral forces, making them highly effective in seismic zones.
Simplicity: Braced frames are relatively easy to design and construct, making them cost-effective solutions for many types of buildings.
However, braced frames also have some limitations:
Stiffness: While the system is excellent at reducing sway, it can make the building too rigid, potentially leading to localized damage in non-structural components like walls and windows.
Aesthetic Limitations: The diagonal braces can sometimes interfere with the architectural design of a building, limiting window placement or creating less flexible interior spaces.
Shear Walls: Resisting Lateral Forces
Definition and Function
Shear walls are vertical elements that provide lateral stability to a building by resisting horizontal seismic forces. Typically made of reinforced concrete or masonry, shear walls are strategically placed throughout the building to absorb and redistribute the energy from an earthquake. These walls help prevent the building from tilting or collapsing by providing a rigid, unyielding structure that opposes lateral movement.
Shear walls are commonly used in both low-rise and high-rise buildings, and they are especially effective in regions that experience frequent or severe seismic activity. They are often located around stairwells, elevator shafts, and other central core areas of the building, where they can maximize stability without taking up too much interior space.
Types of Shear Walls
Shear walls can be classified based on the material used and the configuration of the wall:
Reinforced Concrete Shear Walls: These are the most common type of shear walls, and they are highly effective at resisting both vertical and lateral loads. The steel reinforcement embedded in the concrete allows these walls to flex slightly without breaking, giving them the ductility needed to withstand seismic forces.
Masonry Shear Walls: These walls are made of brick or concrete masonry units (CMUs). While they are less ductile than reinforced concrete walls, they can still provide significant lateral stability when properly designed and reinforced.
Wood Shear Walls: In low-rise residential buildings, wood shear walls are often used. These walls consist of wooden studs covered with plywood or oriented strand board (OSB) sheathing, which acts as the primary load-bearing element. While not as strong as concrete or masonry shear walls, wood shear walls can still provide adequate resistance in areas with moderate seismic risk.
Strengths and Limitations of Shear Walls
Shear walls have several strengths:
Excellent Lateral Resistance: Shear walls are among the most effective structural systems for resisting lateral forces, making them ideal for buildings in high-risk seismic zones.
Versatility: Shear walls can be used in a wide range of building types, from residential homes to skyscrapers.
However, there are also some limitations:
Design Constraints: Shear walls are rigid elements, which can limit the flexibility of a building's design. Large windows or open spaces are often difficult to incorporate near shear walls.
Weight: Reinforced concrete shear walls can be quite heavy, which may increase the overall weight of the building and require a stronger foundation.
Moment Frames: Flexibility and Energy Dissipation
Definition and Function
Moment frames are a type of structural system that resists lateral forces through the bending, or moment, of beams and columns. Unlike braced frames or shear walls, moment frames do not rely on diagonal members or solid walls to provide stability. Instead, they use rigid connections between beams and columns to resist seismic forces.
Moment frames are often used in buildings where flexibility and open interior spaces are important, such as office buildings, shopping centers, and hotels. The absence of diagonal braces or shear walls allows for large windows and open floor plans, giving architects more design freedom.
Types of Moment Frames
There are two primary types of moment frames used in seismic design:
Ordinary Moment Frames (OMFs): OMFs are designed to resist moderate lateral forces and are typically used in areas with low to moderate seismic risk. These frames provide a balance between flexibility and strength, but they may not perform as well as other systems in areas with high seismic activity.
Special Moment Frames (SMFs): SMFs are designed for high-seismic regions and are capable of withstanding large lateral forces. They are more ductile than OMFs, meaning they can bend and flex without breaking. SMFs are often used in tall buildings or structures that need to remain operational after a major earthquake.
Strengths and Limitations of Moment Frames
Moment frames offer several advantages:
Design Flexibility: Moment frames allow for open floor plans and large windows, making them popular in commercial and residential buildings where aesthetics and functionality are important.
Energy Dissipation: The flexible connections in moment frames allow them to absorb and dissipate seismic energy, reducing the forces transmitted to the rest of the building.
However, moment frames also have some limitations:
Lower Stiffness: Moment frames are less stiff than braced frames or shear walls, meaning they may allow more sway during an earthquake.
Complexity and Cost: The design and construction of moment frames can be more complex and costly than other systems, particularly in high-seismic areas where SMFs are required.
Conclusion
In earthquake-prone regions, the design of earthquake-resistant structures is essential for protecting both people and property. Braced frames, shear walls, and moment frames each offer distinct advantages in resisting seismic forces, and the choice of system depends on factors such as building height, location, and intended use. By understanding the strengths and limitations of each system, engineers can create buildings that are both safe and functional, ensuring resilience in the face of seismic activity.
