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Outriggers are interior lateral structural systems provided to improve the overturning stiffness and strength of high-rise buildings. It is a lateral load resisting system that is located within the building. The whole system consists of a core structure connected to the perimeter columns of the building by means of structural members called outriggers. The outriggers can be in the form of horizontal beams, truss, or walls.
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Outriggers are categorized as interior structural systems that can work efficiently for up to 150 floors. It is one of the successful and stable configurations used in high-rise construction. Outrigger structural system is popular in construction since the 1980s due to its unique combination of architectural flexibility and structural efficiency.
This article explores the working principle and important features of outrigger structural systems.
Outrigger Structural Systems
An outrigger structure functions in a high-rise building by tying together two systems (see Figure-2), namely,
- Core system
- Perimeter system
As shown in Figure-1, the core structure is the most critical unit of a high-rise building. It is a combination of units like lifts, staircases, ducts, etc. Whereas the perimeter system is a combination of mega columns. The core system and mega columns located in the perimeter are connected using outriggers. Figure-2 shows a truss type outrigger connected to the perimeter column. Other than outriggers, the structure is accompanied by belt trusses around the perimeter to provide extra lateral resistance.
Both the core and perimeter systems together with the outrigger control the behavior of the whole building. This is performed by the positive interaction between the core and the perimeter system through outriggers.
Principle of Outrigger Structural Systems
High-rise buildings are subjected to large lateral deformation either due to wind or seismic loads. The core structure of a high-rise building is subjected to cantilever deformation, as shown in Figure-3, while the frame structures surrounding the core are subjected to portal type deformation. This causes high drift and overturning effects on the structure.
The incorporation of outriggers efficiently controls the excessive drift due to lateral loads. This, hence, allows to prevent structural and non-structural damage in the structure. As shown below, outriggers reduce the core moment from top to down under the lateral load.
The structural action of outrigger systems in high-rise buildings is based on the tension-compression couple induced in the outer columns, as shown in Figure-5. Under the action of lateral loads, a structure without any lateral support system is subjected to overturning.
Figure-5 and 6 show a high-rise structure with an outrigger structural system. Here, the outriggers act as a stiff arm that connects with the perimeter column. It engages the outer columns and the central core for safe load transfer. When a structure with an outrigger system is subjected to lateral load, the core structure's lateral force is transferred to the perimeter columns via outriggers and then to the ground. This way, the overall overturning moment is reduced.
The performance of outrigger structural system is dependent on the:
- Height of the building
- Depth of outrigger members
- Primary structural material of the structure
- The number of levels of outriggers provided
Types of Outrigger System
Based on the location and arrangement of outriggers in tall buildings, the outrigger system can be classified into the following types:
1. Conventional Outrigger System
In a conventional outrigger structural system, the outriggers are directly connected to the core structure, and the columns located at the periphery of the structure.
The 492 m-high, 101-story Shanghai World Financial Centre, designed by Leslie E Roberston Associates in 2008, is one of the famous structures that has incorporated the conventional outrigger system at eight different building levels.
2. Offset Outrigger Structural System
In this system, the outriggers are situated in locations other than the planes of the core walls. This arrangement retains all the advantages of outriggers and is observed to reduce the lateral deflection of tall buildings than conventional outrigger system.
For an efficient offset outrigger system, the outrigger arm and the floor slab must be rigid enough to transfer the horizontal plane shear in the slabs between the outrigger and the core. When a building with an offset outrigger system is subjected to lateral loads, the slab forces cause a couple in the outrigger. To counteract this, a restraining couple is created by the perimeter columns. This restraining couple from the perimeter columns goes back through the slabs to the core structure.
Offset outrigger system has been used in the construction of Dubai Tower, Doha, which is 438 m-high with 90 floors. It is a combination of conventional and offset outrigger structural system.
3. Virtual Outrigger System
In a virtual outrigger system, the overturning moment is transferred from the core structure to the columns without a direct connection between the core and the peripheral columns. This is achieved using floor diaphragms, which transfer the overturning moment from the core to the outboard using a horizontal couple.
Another type of virtual outrigger system is belt trusses. Belt trusses are constructed by tying the peripheral columns as shown in Figure-1 and Figure-10.
Features of Outrigger Structural System
The outriggers tie together the core and the perimeter structural system creating a unique design providing solutions to high-rise construction problems. Some of the features of the outrigger structural systems are:
- Outrigger system is adopted for buildings that are subjected to large overturning moments compared to shear and lateral deflections.
- Outrigger in structures reduces the overall drift and core wind moments.
- The outrigger system reduces the overall building acceleration due to high winds and improves occupant comfort.
Benefits of Outrigger Structural System
Some of the benefits of outrigger structural systems are:
- A tall building that employs an outrigger structural system can reduce overturning moment by 40%.
- It reduces the overall drift of the building during lateral load action. The reduction is dependent on the relative rigidity of the core and the outrigger.
- The outrigger system engages the perimeter columns of the structure to reduce the overturning moment efficiently.
- Outrigger structural system helps to distribute the overturning loads to the foundation effectively. This hence reduces the force at core foundations.
Outrigger structural system is an interior lateral load resisting system that is located within the interior of the building. It consists of a core structure connected to the perimeter columns of the building by means of structural members called outriggers. The outriggers can be in the form of horizontal beams, walls, or truss.
Outriggers and belt truss are two lateral load resisting systems used in high-rise buildings. Both serve the purpose of improving the turning stiffness and strength of the building. Outriggers connect the core structure of the building to the perimeter columns, while belt truss tie up the perimeter columns around the core structure.
The main types of outrigger structural systems are:
1. Conventional outrigger system
2. Offset outrigger system
3. Virtual outrigger system
Outrigger structural system provides the following benefits to the building structure:
1. Reduction in overturning moment
2. Reduction in lateral drift
3. Safe reduction and transfer of core structure forces to the foundation