Bracing Systems in High Rise Steel Structures

• Abstract:

Generally, lateral drift is more pronounced in higher and longer structures, such as high-rise buildings and bridges. A typical method employed to control lateral drift is structural bracing, which works by increasing stiffness and stability of structure.

• Introduction:

Dynamic loads can impart significantly greater effect towards the structural response of a structure compared to static loading. As such, the properties of the structure such as lateral stiffness and strength play pivotal role towards achieving efficient structural performance against dynamic loads which include typhoons, earthquakes, blast and many others.

Structural bracings work by providing lateral stiffness and stability to the structure, especially for the multi-story and Highrise buildings. This subsequently increases the lateral resistance of the structure and reduces the internal forces through appropriate bracing arrangement. Thus, for economic reasons, structural bracing has been widely used worldwide.

• History:

One of the earliest buildings to incorporate structural bracing is the Dewitt-Chestnut Building in 1965 in Chicago [Fig.4]. The building used the frame tube system which was developed by Fazlur R. Khan. Later in 1970, Fazlur R. Khan developed the braced tube system for the John Hancock Centre, also in Chicago. The student of Khan, Mikio Sasaki discovered and proposed the first design of diagonally braces tower in 1964 in the seminal thesis writing. In 1968, Robin Hodgkinson developed a brace which uses concrete in outward appearances for the purpose of alternate uses. The concept of exterior bracewith concrete is applied on the Ontario Centrein Chicago in 1985. These ideas contribute significantly to the design of high-rise structure design against the natural forces such as wind load, earthquake and gravity load.

• Systems for the High-Rise steel structures:

The lateral loading due to wind and earthquakes is the major factor that causes the design of high-rise buildings. These lateral loads are resisted by exterior structural system or interior structural system. The lateral load resisting systems that are widely used are mainly rigid frame, shear wall, wall-frame, braced tube system, outrigger system, diagrid system and tubular system.

• Define of the different Types of the Bracing systems:

1) Concentric Bracing System:

1.1) Definition: In this system, the bracing members (usually steel braces) are placed in vertically aligned spans.

1.2) Purpose: Concentric bracing provides lateral strength and stiffness to the structure, preventing lateral instability.

1.3) Effective:

1.3.1) Seismic Load, while it offers good lateral stiffness, it may not be as effective in resisting seismic loads.

1.3.2) Wind Load, Concentric bracing systems tend to be more effective in resisting wind loads.

1.4) Advantages: Stiffness: Provides significant stiffness with minimal added weight, Simplicity: Easier to design and construct.

1.5) Limitation:

1.5.1) Seismic Performance: May require additional retrofitting for seismic resistance.

1.5.2) Lateral Stiffness: May not be sufficient for irregular or tall structures.

2) Eccentric Bracing System:

2.1) Definition: In this system, the bracing members are placed off-center (eccentrically) from the vertical axis.

2.2) Purpose: Eccentric bracing enhances lateral stiffness and provides better seismic performance.

2.3) Effective:

2.3.1) Seismic Load, Eccentric bracing systems are more effective in resisting seismic forces.

2.3.2) Wind Load, may not be as effective as concentric bracing for wind loads

2.4) Advantages:

2.4.1) Seismic Performance: Offers superior seismic resistance

2.4.2) Lateral Stiffness: Suitable for irregular or tall structures.

2.5) Limitation:

2.5.1) Seismic Performance: Complexity: Design and detailing can be more intricate.

2.5.2) Lateral Stiffness: Weight: May add more weight compared to concentric bracing.