Design Considerations for Outrigger Systems

All multi-story buildings require at least one core to accommodate elevators, stairs, mechanical shafts, and other common services. Because views are a significant part of the intrinsic value in tall buildings, it is most common for their core or cores to be centrally located within the floor plan to place occupants along exterior walls. A central core also locates the center of lateral stiffness close to the center of lateral wind load and center of mass for lateral seismic loads, minimizing torsional forces. In high-seismic regions many tall buildings have a dual system, sometimes called “core and frame” or “tube in tube,” with a perimeter moment frame providing significant torsional stiffness but a smaller contribution to overturning stiffness.

When the core is relatively large in plan it may be wide enough to provide strength against overturning and stiffness against drift. However, a core becomes less efficient as the height/ core width aspect ratio increases. For an aspect ratio exceeding eight or so, the structural premium to control drift and resist overturning is large enough to consider introducing outriggers. The building height for which this occurs is typically lower for residential buildings with small cores for isolated stairwells and elevator shafts than for office buildings with larger cores including washrooms and mechanical rooms. Some residential tower designs include cores enlarged by enclosing occupied rooms as well as elevator banks for this reason. For constant core properties, drift from flexural or overturning behavior will increase approximately as the cube of building height (Lame 2008). To maintain the building drift/height ratio below a particular criterion, as building height doubles, core stiffness would have to quadruple. But simply thickening core walls for more stiffness would reduce rentable area. Introducing outriggers can alleviate the dependence on the core system and maximize useful space between the core and exterior columns.

When direct or conventional outrigger walls or trusses are not acceptable for the building due to space limitations or a column layout which is not aligned with the core walls, an indirect, “virtual”outrigger or belt truss system may be used. Behavior of the exterior columns is tied to behavior of the core through stiff belts and strong, stiff floor diaphragms at upper and lower levels of each belt. This approach eliminates complicated outrigger connections at columns and at the core. It minimizes concerns about inadvertent load transfers between core and perimeter from differential shortening. Alternatively, a belt truss can be used together with direct, conventional outriggers to engage more, smaller columns rather than’requiring fewer, larger mega columns. This results in more uniform perimeter column sizes where desired.

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Text: Hi Sun Choi , Thornton Tomaseti, Inc.;
Goman Ho, Arup Honh Kong Ltd.;
Leonard Joseph, Thornton Tomasseti, Inc.;
Neville Mathias, Skidmore, Owings & Merrill