The Structural Systems oF Tall Buildings - Tall Buildings: Structural Systems and Aerodynamic Form - page 41

Core

Outrigger

Belt

Figure 3.29 Outriggered frame system

outrigger, distributes the axial load effect of the outrigger to other columns by means
of the belt. A belt consists of a horizontal shear truss or shear wall (or deep beam)
adequately stiff in flexure and shear, and of equal depth to the outrigger ( Figure 3.29 ).
In this way, all perimeter columns are connected together to participate in supporting
the outriggers. Belts are used not only in the abovementioned conventional outrigger
systems, but also used in the “virtual” outrigger systems. Virtual outrigger concept
takes advantage of floor diaphragms to eliminate direct connection of core and
perimeter columns by outriggers. A virtual outrigger consists of belt, and floor slabs
engaged by belt. In this manner, the problem associated with the space occupied by
the conventional outriggers is avoided. Efficiency of the virtual outriggers depends on
the rigidity of the belt and floor slabs at belt levels.
In cases where an outrigger is used at a single level throughout the height of the
building, the most effective, and for this reason the optimum location for the outrigger
is approximately 40-60 per cent of the building height (Smith and Coull, 1991;
Taranath, 1998) ( Section 3.9.2 ).
There is a relation between the number of levels where outriggers are used
throughout the height of the building and their optimum locations. The optimum
location of “n” number of outriggers used at levels throughout the height of the
building can be given approximately by the formula 1/(n+1), 2/(n+1) … n/(n+1) (Smith
and Coull, 1991) ( Section 3.9.2 ) . The optimum locations for outriggers at one or two
levels throughout the height of the building using various assumptions are calculated
in Sections 3.9.1 and 3.9.2 .