Trussed Tube

A trussed tube system improves on the efficiency of the framed tube by increasing its potential for use in taller buildings and allowing greater spacing between the columns. This is achieved by adding diagonal bracing at the faces of the tube to virtually eliminate the shear lag in both the flange and web frames.

The framed tube, as discussed previously, even with its close spacing of columns is somewhat flexible because the high axial stresses in the columns parallel to the lateral loads cannot be transferred effectively around the corners. For maximum efficiency, the tube should respond to lateral loads with the purity of a cantilever, with compression and tension forces spread uniformly across the windward and leeward faces. The framed tube, however, behaves more like a thin-walled tube with openings. The axial forces tend to diminish as they travel around the corners, with the result that the columns in the middle

Figure 3.36. (a) Tube building with multistory diagonal bracing; (b) rotated square tube with super diagonals. (Adapted from an article by Mahjoub Elnimeiri, published in Civil Engineering Journal.)

Figure 3.36. (a) Tube building with multistory diagonal bracing; (b) rotated square tube with super diagonals. (Adapted from an article by Mahjoub Elnimeiri, published in Civil Engineering Journal.)

of the windward and leeward faces may not sustain their fair share of compressive and tensile forces. This effect, referred to previously as the shear lag, limits the framed tube application to 50- or 60-story buildings unless the column spacing is very close, as was the case with the 109-story World Trade Center Towers, New York, which had columns at 3.8 ft (1.0 m).

Addition of diagonal braces, as shown in Fig. 3.36 is by far the most usual method of increasing the efficiency of a framed tube. The fascia diagonals interact with the trusses on the perpendicular faces to achieve a three-dimensional behavior, virtually eliminating the effects of shear lag in both the flange and web frames. Consequently, the spacing of the columns can be greater and the size of the columns and spandrels less, thereby allowing larger windows than in a conventional tube structure. The bracing also contributes to the improved performance of the tube in carrying gravity loading. Differences between gravity load stresses in the columns are evened out by the braces transferring axial loading from the more highly to the less stressed columns.

An example of an exterior braced steel building is shown in Fig. 3.37. The building has six eight-story deep chevron braces on each facade that collect about half the gravity loads and resist the entire lateral load above the transfer level.

Figure 3.37. Citicorp Center (Structural Engineers, LeMessurier Consultants, Inc.): (a) typical floor framing plan; (b) elevation; (c) lateral bracing system.

Figure 3.37c. (Continued).

Figure 3.37c. (Continued).

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