It is a lateral load resisting system that is located within the building. Outriggers are interior lateral structural systems provided to improve the overturning stiffness and strength of high-rise buildings. The outriggers can be in the form of horizontal beams, truss, or walls. The whole system consists of a core structure connected to the perimeter columns of the building by means of structural members called outriggers.
Outriggers are categorized as interior structural systems that can work efficiently for up to 150 floors. Outrigger structural system is popular in construction since the 1980s due to its unique combination of architectural flexibility and structural effectiveness. It is one of the successful and stable configurations used in high-rise construction.
This article explores the working principle and important features of outrigger structural systems.
An outrigger framework functions in a high-rise construction by tying together two systems, namely,
As shown in Figure-1, the core framework is the most critical unit of a high-rise construction. 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. Other than outriggers, the framework is accompanied by belt trusses around the perimeter to provide extra lateral resistance. Figure-2 shows a truss type outrigger connected to the perimeter column.
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.
High-rise buildings are subjected to large lateral deformation either due to wind or seismic loads. The core framework of a high-rise building is subjected to cantilever deformation, as shown in Figure-3, while the frame frameworks surrounding the core are subjected to portal type deformation. This causes high drift and overturning effects on the framework.
The incorporation of outriggers effectively controls the excessive drift due to lateral loads. This, hence, allows to prevent structural and non-structural damage in the framework. 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.
Here, the outriggers act as a stiff arm that connects with the perimeter column. Figure-5 and 6 show a high-rise structure with an outrigger structural system. It engages the outer columns and the central core for secure load transfer. This way, the overall overturning moment is reduced. 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.
The performance of outrigger structural system is dependent on the:
Based on the location and arrangement of outriggers in tall buildings, the outrigger system can be classified into the following types:
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 frameworks that has incorporated the conventional outrigger system at eight different building levels.
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 effective 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 construction 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 framework.
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.
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.
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:
Some of the benefits of outrigger structural systems are:
It consists of a core framework connected to the perimeter columns of the construction by means of structural members called outriggers. Outrigger structural system is an interior lateral load resisting system that is located within the interior of the construction. The outriggers can be in the form of horizontal beams, walls, or truss.
Both serve the purpose of improving the turning stiffness and strength of the building. Outriggers and belt truss are two lateral load resisting systems used in high-rise buildings. Outriggers connect the core framework of the building to the perimeter columns, while belt truss tie up the perimeter columns around the core framework.