Figure 4.9. Bundled tube: (a) schematic plan; (b) framed bundled tube; (c) diagonally braced bundled tube.
The underlying principle is to connect two or more individual tubes in a bundle with the object of decreasing shear lag effects (Fig. 4.9a). Two basic versions are possible using either framed or diagonally braced tubes as shown in Fig. 4.9b and c. A mixture of the two is, of course, feasible.
Figure 4.10 shows a schematic plan of a building with a cap wall consisting of a 1- or 2-story-high outrigger wall that connects the core to the perimeter columns. A 1- or 2-story
wall at the perimeter acting as a belt wall is typically used in the system to tie the exterior columns together. The cap wall at the top tends to reverse the bending curvature of the cantilever shear core. A substantial portion of moment in the core is thus transferred to the perimeter columns by inducing tension in the windward columns and compression in the leeward columns (Fig. 4.10b). Optimum locations discussed in Chapter 3, for single and multiple outriggers related to steel systems, are also relevant to concrete systems (Fig. 4.11). In high seismic zones it is prudent to use a 1- or 2-story-deep vierendeel-type ductile frame for outriggers and belt trusses instead of walls. A schematic elevation of a building with a two-story vierendeel outrigger is shown in Fig. 4.12.
Buildings with high plan aspect ratios tend to be inefficient in resisting lateral loads because of shear lag effects. By introducing interior columns (three at every other floor in the example building shown in Fig. 4.13), it is possible to reduce the effect of shear lag, and thus increase the bending efficiency. A 2-story haunch girder vierendeel frame at every other floor effectively ties the building exterior columns to the interior shear walls thus mobilizing the entire flange-frames in resisting overturning moments.
One concept of full-depth interior bracing interacting with the building's perimeter frame is shown in Fig. 4.14. The interior diagonal bracing consists of a series of wall
panels interconnected between interior columns to form a giant K-brace stretched out for the full width of the building.
A system suitable for super-tall buildings—taller than, say, 80 stories—is shown in Fig. 4.15. It consists of a service core located at each corner of the building interconnected by a super diagonal in-fill bracing. The service core at each corner acts as a giant column carrying most of the gravity load and overturning moments. The eccentricity between the super diagonals and exterior columns is a deliberate design strategy to enhance the ductility of the lateral bracing systems. The ductile response of the links helps in dissipating seismic energy, thus assuring the gravity-carrying capacity of the building during and after a large earthquake.
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