BDS – Việt Nam

Steel structures have become a popular choice in construction due to their strength, flexibility, and durability. One of the critical aspects that influence the design and integrity of these structures is their seismic resistance. Earthquakes pose a significant risk to buildings and infrastructure, and as a result, engineers must incorporate seismic resistance into the design of steel structures to ensure safety during seismic events. This article explores the concept of seismic resistance in steel structures, the factors that affect it, and the design strategies used to enhance the earthquake resilience of steel buildings.

Understanding Seismic Resistance in Steel Structures

Seismic resistance refers to the ability of a structure to withstand the forces generated by an earthquake, ensuring that it does not collapse or suffer catastrophic damage. In an earthquake, the ground motion produces horizontal and vertical forces on buildings, which can cause structures to sway, twist, or even fail. The main goal of seismic resistance is to control and mitigate the effects of these forces to prevent structural failure and ensure the safety of occupants.

Steel structures are inherently advantageous in seismic conditions due to the material’s high strength-to-weight ratio, flexibility, and ductility. Unlike brittle materials such as concrete, steel can absorb and dissipate the energy produced during an earthquake without breaking, making it ideal for earthquake-resistant design.

Factors Affecting Seismic Resistance in Steel Structures

Several factors contribute to a steel structure’s ability to resist seismic forces effectively. Understanding these elements helps in designing buildings that are not only safe but also capable of withstanding the intense forces generated during an earthquake.

1. Building Shape and Configuration

The shape and layout of a building play a crucial role in its seismic performance. Irregular shapes or complex configurations can lead to torsional effects, where parts of the building experience different movements during an earthquake. This can cause uneven distribution of forces and increase the risk of structural failure.

• Rectangular or Regular Shapes: Simple, symmetrical designs with regular floor plans tend to perform better in seismic conditions. These designs help distribute seismic forces evenly across the structure.

• Torsional Effects: Buildings with asymmetrical shapes or irregular floor plans may experience twisting during an earthquake, which can lead to structural instability. Careful design can minimize these effects.

2. Building Height and Mass Distribution

The height of a building significantly impacts its seismic response. Taller buildings are more likely to experience greater sway, and their design must account for this movement to avoid damage.

• Height Considerations: High-rise steel buildings require a more robust seismic design to resist the sway and vibrations caused by earthquakes.

• Mass Distribution: Uneven distribution of mass within a building can lead to uneven forces during an earthquake. For example, if a building’s upper floors are much heavier than the lower floors, the structure may be more prone to toppling.

3. Structural Connections and Joints

The joints and connections between different components of the steel structure are critical for seismic resistance. Steel is a flexible material, and the connections between beams, columns, and braces must allow for movement while maintaining structural integrity.

• Ductile Connections: Connections should be designed to be ductile, meaning they can deform without failing. This allows the structure to absorb and dissipate energy during an earthquake.

• Bolted vs. Welded Joints: The choice between bolted and welded connections can influence the performance of the structure during seismic events. While welded connections are stronger, bolted connections may provide better flexibility and are easier to inspect and repair.

4. Foundation Design

The foundation of a building plays a vital role in ensuring seismic resistance. If the foundation is not designed to withstand seismic forces, the entire structure may be at risk.

• Seismic Base Isolation: This technique involves placing a building on isolators or bearings that can absorb seismic energy and reduce the transmission of forces to the superstructure. This allows the building to move independently of the ground motion.

• Deep Foundations: In areas prone to earthquakes, deep foundations that extend into more stable layers of soil can provide better stability and prevent excessive settlement during an earthquake.

5. Ductility and Flexibility of Steel

One of the primary reasons why steel structures perform well during earthquakes is their ability to bend and flex without breaking. Steel has a high degree of ductility, which means it can undergo significant deformation while maintaining its load-carrying capacity.

• Ductile Behavior: Steel’s ability to deform without fracturing allows it to absorb seismic energy, reducing the risk of sudden failure. This flexibility also enables steel buildings to sway during an earthquake without collapsing.

• Material Selection: The type of steel used in construction can affect the material’s ductility and overall seismic performance. High-strength steel alloys with good ductile properties are often preferred for seismic-resistant structures.

Seismic Design Strategies for Steel Structures

To ensure the seismic resistance of steel structures, engineers employ various design strategies and building codes that account for the effects of earthquakes. These strategies focus on enhancing the building’s ability to absorb seismic energy, minimize damage, and maintain structural integrity.

1. Moment-Resisting Frames

Moment-resisting frames are a common design strategy used in steel buildings to resist lateral forces caused by earthquakes. These frames consist of beams and columns that are connected in such a way that they can resist bending and deformation during seismic events.

• Rigid Connections: The moment-resisting frame relies on rigid connections between beams and columns, which allow the structure to resist lateral forces while remaining stable.

• Energy Dissipation: By resisting lateral forces, moment-resisting frames help to dissipate the energy generated by the earthquake, reducing the risk of damage to the structure.

2. Braced Frames

Braced frames are another common seismic-resistant design used in steel buildings. These frames consist of steel braces that help to resist lateral forces and prevent the structure from swaying excessively during an earthquake.

• Cross-Bracing: In cross-bracing, diagonal braces are installed in a crisscross pattern between columns and beams. These braces form triangular shapes that distribute seismic forces more effectively.

• K-Bracing and Chevron Bracing: Different types of bracing systems, such as K-bracing and Chevron bracing, are used to enhance the building’s seismic performance. These systems provide additional support and help to prevent sway and buckling.

3. Seismic Dampers

Seismic dampers are devices installed in steel structures to absorb and dissipate the energy produced during an earthquake. These dampers help to reduce the amount of motion experienced by the building and minimize the risk of damage.

• Viscous Dampers: These dampers use a fluid to resist motion and dissipate energy. They are effective in reducing the amount of sway and vibrations during an earthquake.

• Friction Dampers: Friction dampers use the resistance of friction to absorb seismic energy. They are commonly used in buildings with higher seismic risk.

4. Base Isolation

As mentioned earlier, base isolation involves placing the building on bearings or isolators that allow it to move independently of the ground motion during an earthquake. This technique reduces the transmission of seismic forces to the structure, significantly improving its seismic resistance.

• Rubber Bearings: Base isolation often involves the use of rubber bearings that can flex and absorb seismic energy. These bearings allow the building to move in response to ground motion, without transmitting the forces to the structure.

Steel structures offer excellent seismic resistance due to their flexibility, ductility, and strength. By incorporating seismic-resistant design strategies such as moment-resisting frames, braced frames, seismic dampers, and base isolation, engineers can ensure that steel buildings remain stable and safe during earthquakes. As the demand for earthquake-resistant buildings increases in seismically active regions, it is essential for architects, engineers, and contractors to prioritize seismic safety in the design and construction of steel structures. Through careful planning, innovative design, and the right materials, steel structures can continue to protect lives and property in earthquake-prone areas.