In the design and construction of multi-storey steel structures, bracing systems play a crucial role in ensuring the structural integrity and stability of the building. These systems are designed to transfer horizontal forces, such as wind loads and seismic forces, and resist the sway or movement of the structure. Bracing systems can be broadly classified into two main categories: vertical bracing systems and horizontal bracing systems. This article explores the different types of bracing systems used in multi-storey steel structures and their importance in maintaining structural safety.
Types of Bracing Systems
Bracing systems are essential to ensure the strength and stability of multi-storey steel structures, especially against lateral forces like wind or earthquakes. These forces exert pressure on the structure, potentially causing it to sway or deform. Bracing systems are designed to counteract these forces and prevent excessive movement. The two major types of bracing systems used are vertical bracing systems and horizontal bracing systems.
Vertical Bracing System
Vertical bracing systems are commonly employed in multi-storey steel structures to resist horizontal forces and prevent sway. As shown in Figure 2, vertical bracing typically involves diagonal braces installed between two lines of columns. This system serves two primary functions: transferring horizontal loads to the foundations and providing lateral stability by reducing overall sway.
Configurations of Vertical Bracing
Vertical bracing systems can be designed in different configurations, the most common being cross diagonal bracing and single diagonal bracing.
- Cross Diagonal Bracing (Cross Bracing):
Cross bracing consists of two diagonal braces that cross each other, forming an ‘X’ shape between the columns. These braces are typically designed to resist tension forces only, as they cannot effectively resist compression forces. Because of their tension-only nature, cross braces provide a high degree of lateral stability. The placement of these bracings between two lines of columns helps ensure that the structure remains stable under lateral loads. - Single Diagonal Bracing:
Single diagonal bracing consists of a single diagonal brace placed between two columns. This system is designed to resist both tension and compression forces. The braces are typically positioned at a 45° angle, which offers an efficient load transfer mechanism and strong, compact connections between the bracing members and the beam-column juncture. A 45° angle provides optimal structural efficiency, as it balances the forces of tension and compression. If the bracing angle is smaller than 45°, the structure becomes more sensitive to sway, while a wider angle provides better structural stability.
Forces Resisted by Vertical Bracing Systems
Vertical bracing systems must be designed to resist several forces, including wind forces and the equivalent horizontal forces that account for the effects of initial imperfections and second-order effects (due to frame sway). These forces are critical in maintaining the integrity of the structure under dynamic loads such as high winds or seismic activity.
Number of Vertical Bracing Planes
It is recommended to incorporate at least three vertical bracing planes to ensure the structure’s resistance to forces in both directions of the plan, as well as torsional forces around the vertical axis. In practice, a minimum of two vertical bracing planes in each orthogonal direction is advised to avoid disproportionate collapse. By installing multiple planes of vertical bracing, the structure gains enhanced stability, and the risk of progressive collapse is minimized.
Placement of Vertical Bracing
The location of vertical bracing should be carefully considered to ensure maximum stability. It is ideal to place vertical bracing planes at the furthest points of the structure, as this will help resist torsional forces that may arise due to horizontal loads. Proper placement ensures that the bracing system can efficiently counteract forces from multiple directions, improving the overall stability of the building.
Horizontal Bracing System
Horizontal bracing systems serve the important role of transferring horizontal loads from the perimeter columns to the vertical bracing planes. These horizontal forces typically arise from wind pressure acting on the cladding of the structure. Horizontal bracing systems are designed to resist these forces and provide a path for them to be transmitted to the vertical bracing systems.
There are two main types of horizontal bracing systems: diaphragms and discrete triangulated bracing.
1. Diaphragms
Diaphragms are horizontal floor systems that can act as bracing elements. Some floor systems, such as composite floors, provide excellent horizontal diaphragm action, meaning they can transfer horizontal forces effectively. However, other types of floor systems, such as precast concrete slabs, may require additional measures to ensure that they perform adequately as horizontal bracing systems. For example, the connection between steelwork and precast concrete slabs must be designed to avoid relative movements between the two materials, ensuring they work together as an effective bracing system.
2. Discrete Triangulated Bracing
When the floor system is unable to act as a horizontal bracing element, discrete triangulated bracing is used. This system consists of a network of steel bracing arranged in a triangulated pattern between the vertical bracing supports. These bracing members are positioned in each orthogonal direction to resist horizontal forces. Discrete triangulated bracing is particularly useful when the floor system cannot provide the necessary horizontal support.
Bracing at Roof Level
In multi-storey steel structures, horizontal bracing is often required at the roof level as well. A wind girder is typically used at the top of the columns to resist the horizontal forces that act on the structure from above. This additional layer of horizontal bracing at the roof level helps to ensure that the building remains stable under high wind pressures and other dynamic loads.
Conclusion
Bracing systems are a vital component of multi-storey steel structures, ensuring that the building can withstand lateral forces and maintain its stability under a variety of conditions. Vertical bracing systems, such as cross diagonal and single diagonal braces, provide essential resistance to horizontal forces and prevent excessive sway. Horizontal bracing systems, including diaphragms and discrete triangulated bracing, transfer horizontal loads to vertical bracing and help distribute forces throughout the structure.
By carefully designing and placing these bracing systems, engineers can ensure that multi-storey steel structures are safe, stable, and capable of withstanding the forces they will encounter throughout their lifespan. Properly implemented bracing systems not only enhance structural performance but also contribute to the overall safety and longevity of the building.