Outrigger Structural Systems in Tall Buildings

The outrigger system is one of the most effective and widely used structural arrangements for tall buildings. It solves a basic limitation of a central core: on its own, a core resists overturning only across its own narrow width, which becomes inefficient as a tower grows taller. By connecting the core to the perimeter columns with stiff horizontal members, an outrigger system recruits the full width of the building to resist wind and seismic overturning, sharply reducing both the demand on the core and the sway at the top.

This article explains how an outrigger system works mechanically, the role of belt trusses, where outriggers are placed, the practical challenges of building them, and the real towers that use them. It is written for owners, developers and designers who want to understand why this approach appears so often in super-tall construction.

What an outrigger system is

An outrigger system consists of stiff horizontal elements — deep trusses or solid concrete walls one or more storeys deep — that span from the central core out to the perimeter columns. The outriggers are rigidly connected to the core and engage the outer columns, so the core and the perimeter no longer act independently but are tied together into a single, much wider, lateral-resisting unit.

The simplest way to picture the benefit is to imagine the core as a tall cantilever trying to bend over in the wind. Left alone, it resists that bending only through its own section. Add an outrigger near the top, connected to perimeter columns, and the columns on the windward side are pulled into tension while those on the leeward side are pushed into compression. That push-pull couple acts across the full width of the building and resists a large part of the overturning moment.

How it resists overturning and drift

The mechanics come down to a lever arm. The core alone has a short internal lever arm equal to its own width. The perimeter columns sit much further from the building’s centre, so when the outrigger forces them into axial push-pull, they resist overturning with a far longer lever arm. The result is a substantial reduction in the bending moment the core has to carry and a marked reduction in lateral drift at the top of the building.

This matters because, in tall buildings, drift and occupant comfort, rather than raw strength, usually govern the design. By stiffening the structure so efficiently, an outrigger system lets engineers control sway without resorting to a much larger core or filling the facade with bracing. The improvement comes from working the perimeter columns in axial action, which is a very efficient way to use material. Our overview of tall building structural systems sets the outrigger approach in the context of the alternatives.

Belt trusses and perimeter engagement

An outrigger only directly engages the few columns it reaches. To spread its effect to the rest of the perimeter, engineers add a belt truss — a deep truss or wall running horizontally around the facade at the outrigger level, tying the perimeter columns together. The belt truss collects the push-pull forces and distributes them among many columns, so a larger share of the perimeter participates in resisting overturning.

Belt trusses also help even out the forces and reduce the demand on any single column. Together, the outrigger and belt truss form a stiff horizontal band around the building at that level. Because these elements are deep and occupy whole storeys, they are normally located where they cause the least disruption to usable space.

Placement and number of outriggers

Outrigger levels are typically placed at mechanical or plant floors, where the storey-deep trusses do not interfere with lettable or saleable space. The number and position of outrigger levels is an optimisation problem: a single outrigger gives a useful reduction in drift, while two or three outriggers distributed up the height of the building can be considerably more effective, with diminishing returns beyond a point.

Single outrigger — often near the upper-middle of the building, giving a meaningful stiffness gain for one structural intervention.
Multiple outriggers — placed at intervals up the height, each engaging the perimeter at its level and together controlling drift across the full structure.
Position optimisation — engineers tune the heights to extract the most drift reduction for the least disruption and cost.

Real super-tall towers illustrate the approach. Jin Mao Tower, Taipei 101 and Shanghai Tower all use outrigger trusses, in some cases combined with belt trusses, to tie their cores to perimeter mega-columns. These are widely documented examples of the system working at extreme height.

Construction challenges

The same stiffness that makes outriggers so effective also creates a practical difficulty: differential shortening. The core and the perimeter columns carry different stress levels and, in concrete buildings, shorten by different amounts over time through elastic deformation, shrinkage and creep. Because the outrigger rigidly links the two, that differential movement induces forces in the outrigger that have nothing to do with wind — it gets dragged by the relative settlement of the elements it connects.

Engineers manage this through careful construction sequencing. The outrigger-to-column connections are often left unlocked, or made with delay joints, until much of the differential shortening has taken place, and only then fixed to complete the system. This staged connection is a defining feature of outrigger construction and requires close coordination between the design team and the builder. It is the kind of detail that belongs in a thorough structural engineer’s report.

Advantages and limitations

The advantages of an outrigger system are clear: a large gain in lateral stiffness and a large reduction in core bending and drift, achieved with relatively little material and without cluttering the facade. It keeps floor plates open and works well alongside a reinforced concrete core, which is why it is so common in tall residential and commercial towers, including in Singapore’s high-rise sector.

The limitations are equally real. Outrigger floors are structurally congested and occupy whole levels, the connections are complex, and differential shortening demands disciplined sequencing. The system also concentrates force at discrete levels, so the columns and connections there must be designed for high local demands. These trade-offs are exactly why outrigger design is specialist work. If you are planning a tall building and weighing lateral systems, our guide on whether you need a structural engineer explains when to bring one in, and our consultancy can advise on real schemes.

Frequently asked questions

What is an outrigger system in a building?

An outrigger system uses stiff horizontal trusses or walls to connect a building’s central core to its perimeter columns, engaging those columns in axial push-pull action to resist overturning and reduce lateral sway.

How does an outrigger reduce drift?

By forcing the perimeter columns into tension and compression across the full width of the building, the outrigger resists overturning with a much longer lever arm than the core alone, which sharply reduces the bending in the core and the sway at the top.

What is a belt truss?

A belt truss is a deep truss or wall running around the perimeter at an outrigger level, tying the perimeter columns together so the outrigger’s effect is spread to many columns rather than only the few it directly reaches.

Why are outriggers placed at mechanical floors?

Outriggers are storey-deep elements that would obstruct usable space, so they are located at mechanical or plant floors where they cause the least disruption to lettable or saleable area.

What is differential shortening in outrigger buildings?

It is the unequal vertical shortening of the core and perimeter columns over time due to elastic deformation, shrinkage and creep. Because the outrigger links the two, this movement induces forces in it, which engineers manage through staged or delayed connections.

Need accountable structural advice on an outrigger system or lateral design for your tower? Talk to our consultancy or get in touch.