2.3 Frames, Vertical Trusses, Belt and Outrigger Trusses
The exterior fascia shear frames and the vertical trusses in the core can be tied together by a system of outrigger and belt trusses which are provided at plant room levels, where the trusses will not interfere with the interior space planning. Figure 4 shows the arrangement of trusses. The primary result of the outrigger trusses is the development of axial forces in the exterior columns due to wind action. This behaviour significantly improves the lateral stiffness under wind forces. The use of belt trusses on the facades, at the same level and perpendicular to the outrigger trusses, further enhances participation of exterior frames in the cantilever behaviour. The belt trusses transform the two-dimensional frame system into a three-dimensional frame system which resists wind action. The building sway under wind is significantly reduced by the introduction of these trusses. A review of the deflection curve indicates two stiffening effects: one related to the participation of the external columns in a total-building-width cantilever mode; the other related to the stiffening of the facade frame by the belt trusses. Improvements in overall stiffness of up to 25% can result as compared to the Shear Truss and Frame System without such outrigger-belt trusses (Figure 5). The effectiveness of the system depends on the number of trussed levels and the depth of the truss at each level.