Outrigger

[edit] Introduction

Originally a nautical phenomenon, outriggers have been used for millennia as extended floats attached to canoes to provide stability and so avoid overturning. The same principle has been used since the 1950s in construction to provide lateral stability to tall, narrow buildings.

[edit] Structural component

Outriggers are typically horizontal structures (usually trusses or reinforced concrete walls) inserted into buildings at various levels to tie the concrete core to the outer (or perimeter) structural framework. They can be tied to the core and combined with exterior columns, or may be connected to a super- or mega-column.

Outriggers add strength and stiffness to a structure that is far in excess of what may be available at specific locations throughout the building’s height. There is no one-size fits all and so the design of outriggers will usually be unique to suit the specific application.

The structural efficacy of outriggers will depend on their distribution through the building’s height, the number of outrigger levels, their configuration on plan, the depth of the outrigger truss or wall, the materials used and other considerations. But the effect of constructing an outrigger and thereby connecting the core with perimeter columns creates a unified lateral load-resisting system that can reduce the overturning moment at the core by up 40% to 60% in tall and super-tall buildings respectively.

Tall or slender buildings, such as 432 Park Avenue, New York, or 22 Bishopsgate, London, will often feature a core and outrigger system to resist lateral loads. In such cases, the overturning moment is large compared to the shear, and the flexural deformations of the building contribute greatly to lateral deflection e.g drift. Outriggers provide increased stiffness and so can reduce building drift (the sway experienced at the top of very tall buildings), thereby reducing the building accelerations during strong winds which might otherwise cause discomfort to occupants.

[edit] Benefits of outriggers

• Reduce overturning moments and building drift.
• By decreasing building overturning moment, provide more economic use of materials required to construct the core.
• Reduce forces in the core’s foundations to provide further economy.
• Used at different levels, outriggers can allow the creation of service floors for plant etc.

NB Outriggers may also be used to stabilise construction plant, such as mobile cranes, which might otherwise overturn.

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