Steel Outrigger
Stiff arms that tie a tall building's core to its outer columns.
Quick Answer
A steel outrigger is a stiff truss, or sometimes a wall, that extends from a tall building's central core to its perimeter columns. Linking the two makes the columns help resist overturning from wind or earthquakes, which reduces core bending and sway. A belt truss often wraps the perimeter to engage the outer columns.
The Full Picture
In a tall building, the central core of elevator shafts and stairs is often the main lateral system. As the building gets taller, the core alone becomes too flexible, because it behaves like a tall slender beam fixed at the base. An outrigger is a deep, stiff element that reaches from the core out to columns near the perimeter. When wind pushes the building, the core tries to rotate, and the outrigger pulls the outer columns into action, so one side goes into tension and the other into compression. This resists overturning and cuts the building's sway.
Outriggers are usually steel trusses, typically one or two stories deep so that they are stiff, and they are commonly placed at mechanical floors or refuge floors where the diagonals are less obtrusive. A belt truss, a band of trusses around the perimeter, may connect the outer columns so they act together. The system is designed by the structural engineer and checked against the code, including the combined effect of wind and seismic loads.
Constructing an outrigger is a coordination problem. Columns shorten under gravity load at different rates than the core, which can lock unintended forces into a stiff outrigger. Designers sometimes delay the final connection or use adjustable details until much of the building weight is in place, and the contractor must follow the sequence on the drawings. The steel is heavy, with large connections, so crane capacity, shop splices and field welding or bolting sequences affect the schedule.
For an estimator, outriggers are a concentrated steel item. Truss members, large gusset plates and heavy built-up connections are taken off from the structural elevations and details, and the delayed-connection sequence and temporary supports are carried as separate scope. Because they sit on mechanical levels, the surrounding architectural and MEP work must be coordinated with the large diagonals.
Real Examples
Common Misconceptions
People assume: Outriggers are just extra bracing.
Actually: They work with the core and the perimeter columns as a combined system. Their job is to engage the outer columns in resisting overturning, which ordinary bracing in a single frame does not do.
People assume: Outrigger connections are made like any other steel connection.
Actually: Because column shortening can load a stiff outrigger, the sequence and details of the connection may be special, and the engineer's instructions for timing have to be followed exactly.
People assume: Outriggers only matter for the structural trades.
Actually: Their size affects mechanical floor layouts, ceilings, shafts and the architectural plan around them, so several trades coordinate openings and clearances.
Frequently Asked Questions
What does an outrigger do in a skyscraper?
It connects the central core to perimeter columns so they resist overturning together. That reduces the bending in the core and limits how far the building sways in wind or an earthquake.
What is a belt truss?
A belt truss is a band of trusses around the perimeter of a building, usually at the same level as an outrigger. It ties the outer columns together so they share the load the outrigger delivers.
Where are outriggers located in a building?
Commonly at one or more intermediate levels, often mechanical floors, where the deep diagonals fit more easily. The engineer chooses the number and location based on the building's height and stiffness needs.
Why is the outrigger connection sometimes delayed?
Columns and the core shorten by different amounts as load builds up. Delaying or adjusting the final connection avoids locking those movements into a stiff element, and the drawings define the timing.