Outrigger technology in construction

OUTRIGGER TECHNOLOGY

Outriggers are rigid horizontal structures designed to improve building overturning stiffness and strength by connecting the building core or spine to distant columns. Outriggers have been used in tall, narrow buildings for nearly half a century, but the design principle has been used for millennia.The oldest “outriggers” are horizontal beams connecting the main canoe-shaped hulls of Polynesian ocean going boats to outer stabilizing floats or “amas”.

Features of Outrigger Structural System:

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.

Outrigger systems function by tying together two structural systems – typically a core system and a fringe system – to yield whole-building structural behaviors that are far better than those of the component systems. They do this by creating a positive interaction between the 2 tied systems. The beneficial effect is most pronounced where the responses of the component systems under lateral loads are most disparate. Outrigger system performance is suffering from outrigger locations through the peak of a building, the amount of levels of outriggers provided, their plan locations, the presence of belt trusses to engage adjacent perimeter columns versus stand alone mega columns, outrigger truss depths, and therefore the primary structural materials used.

The main types of outrigger structural systems are:

1. Conventional outrigger system

2. Offset outrigger system

3. Virtual outrigger system

Benefits of an Outrigger

SystemDeformation

ReductionIn a building with a central core braced frame or shear walls, an outrigger system engages perimeter columns to efficiently reduce building deformations from overturning moments and the resulting lateral displacements at upper floors. A tall building structure which incorporates an outrigger system can experience a reduction in core overturning moment up to 40% compared to a free cantilever, as well as a significant reduction in drift depending on the relative rigidities of the core and the outrigger system.

Efficiency

For systems with belt trusses that engage all perimeter columns, columns already sized for gravity load may be ca-capable of resisting outrigger forces with minimal changes in size or reinforcement, as diff erent load factors apply to design combinations with and without lateral loads. In the event that additional overall fl exural stiffness is required, the greater lever arm at outrigger columns makes additional material more effective than in the core. Outriggers may also permit optimization of the overall building system using techniques such as the unit load method to identify the best locations for additional material.

Foundation Forces

A separate but related advantage is force reduction at core foundations. Outrigger systems help to effectively distribute overturning loads on foundations. Even where a foundation mat is extended over the full tower footprint, a core-only lateral system applying large local forces from overturning can generate such large mat shear and fl exural demands, as well as net tension in piles or loss of bearing, that the design becomes uneconomical or impractical. Reducing core overturning and involving perimeter column axial

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