Recommendations for Optimum Locations of Belt and Outrigger Trusses

Based on the approximate method presented thus for, the following recommendations are made for the general arrangement of the structure, particularly for estimating the optimum levels of the outriggers for minimizing the drift:

• The optimum location for a single outrigger is, perhaps unexpectedly, not at the top. The reduction in the drift is about 50%, as compared to a maximum of 75% achievable by placing it at approximately half-height. However, since other architectural requirements take precedence in a structural layout, the benefits of placing a truss at the top are still worth pursuing.

• A two-outrigger structure appears to offer more options in the placements of outriggers. Reductions in building deflections close to the optimum results may be achieved with outriggers placed at levels entirely different from optimum locations. Thus, the engineer and architect have some leeway in choosing the outrigger locations. However, as a rule of thumb, the optimum location for a two-outrigger structure is at one-third and two-third heights. And for a three-outrigger system, they should be at the one-quarter, one-half, and three-quarter heights, and so on. Therefore, for the optimum performance of an «-outrigger structure, the outriggers should be placed at 1/n + 1, 2/n + 1, 3/n + 1, 4/n + 1,., n/n + 1 height locations. For example, in an 80-story building with four outriggers (i.e., n = 4), the optimum locations are at the 16th, 32nd, 48th, and 64th levels. A summary of the recommendations is shown in Fig. 3.30.

Figure 3.30. Optimum locations of outriggers: (a) single outrigger; (b) two outriggers; (c) three outriggers; (d) four outriggers.

Figure 3.30. Optimum locations of outriggers: (a) single outrigger; (b) two outriggers; (c) three outriggers; (d) four outriggers.

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