Bundled Tube

A bundled tube typically consists of a number of individual tubes interconnected to form a multicell tube, in which the frames in the lateral load direction resist the shears, while the flange frames carry most of the overturning moments. The cells can be curtailed at different heights without diminishing structural integrity. The torsional loads are readily resisted by the closed form of the modules. The greater spacing of the columns and shallower spandrels permitted by the more efficient bundled tube structure provide for larger window openings than are allowed in a single-tube structure.

The shear lag experienced by conventional framed tubes is greatly reduced by the addition of interior framed web panels across the entire width of the building. When the building is subjected to bending under the action of lateral forces, the high in-plane rigidity of the floor slabs constrains the interior web frames to deflect equally with the exterior web frames. Thus, the shear carried by each web frame is proportional to its lateral stiffness. Since the end columns of the interior webs are activated directly by the webs, they are more highly stressed than in a single tube where they are activated indirectly by the exterior web through the flange frame spandrels. Consequently, the presence of the interior webs reduces substantially the nonuniformity of column forces caused by the shear lag. The vertical

Figure 3.38A. Bundled tube, schematic plan.

Figure 3.38B. Bundled tube concept: (a) perimeter diagonal bracing; (b) X-bracing with moment-connected spandrels; (c) perimeter moment frames.

1454 ft

1454 ft

Figure 3.39. Bundled tube structural system; Sears Tower, Chicago. Building height: 1454 ft.

stresses in the normal panels are more nearly uniform, and the structural behavior is much closer to that of a braced tube than a framed tube.

Because a bundled tube is configured from a layout of individual tubes, it is possible to achieve a variety of floor configurations by simply terminating a given tube at any desired level. See Fig. 3.38A. Schematic elevations of structural systems consisting of perimeter diagonal bracing and moment-connected frames are shown in Figs. 3.38Ba, b, and c. Figure 3.39 shows the bundled tube system for the 1454-ft tall Sears Tower consisting of nine tubes at the bottom, with only two of the constituent tubes rising to the top. The building designed by the Chicago office of Skidmore, Owings, and Merrill was completed in 1974.

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