Interacting System Of Braced And Rigid Frames

Even for buildings in the of 10- to 15-story range, unreasonably heavy columns may result if the lateral bracing is confined to the building's service core because the depth available for bracing is usually limited. Additionally, high uplift forces may occur at the bottom of core columns, presenting foundation problems. For such buildings, an economical structural system can be devised, using a combination of rigid frames with a core bracing system. Although relatively deep girders are required for a substantial frame action, rigid frames are often architecturally preferred because they are least objectionable from the interior space planning considerations. When used on the building exterior, deep spandrels and closely spaced columns may be permissible because columns usually will not interfere with the space planning, and the depth of spandrels need not be shallow as for interior beams, for the passage of air conditioning and other utility ducts. A schematic floor plan of a building using this concept is shown in Fig. 3.15a.

As an alternative to perimeter frames, a set of interior frames can be used with the core bracing, as shown in Fig. 3.15b, in which frames on grid lines 1, 2, 6, and 7 interact with core bracing on lines 3, 4, and 5. Yet another option is to moment-connect the girders between the braced core and perimeter columns, as shown in Fig. 3.15c. In this example, the moment-connected girders act as outriggers connecting the exterior columns to the braced core.

Suspension Structure Building

Figure 3.15. Schematic plans showing interacting braced and rigid frames: (a) braced core and perimeter frames; (b) braced core and interior and exterior frames; (c) braced core and interior frames; (d) full-depth interior bracing and exterior frames; (e) transverse cross section showing primary interior bracing, secondary bracing, and basement construction.

Figure 3.15. Schematic plans showing interacting braced and rigid frames: (a) braced core and perimeter frames; (b) braced core and interior and exterior frames; (c) braced core and interior frames; (d) full-depth interior bracing and exterior frames; (e) transverse cross section showing primary interior bracing, secondary bracing, and basement construction.

Figure 3.15. (Continued).

For slender buildings with height-to-width ratios in excess of 5, an interacting system of moment frames and braces becomes uneconomical if braces are placed only within the building core. A good structural solution, if architecturally acceptable, is to spread the bracing for the full width of the building along the facades.

Another possibility is to move the full-depth bracing to the interior of the building, as shown in Fig. 3.15d. The braces stretched out for the full width of the building form giant K-braces, resisting overturning and shear forces by developing predominantly axial forces. A transverse cross section of the building is shown in Fig. 3.15e, which identifies a secondary system of braces required to transfer the lateral loads to the panel points of the K-braces. The diagonals of the K-braces running through the interior of the building result in sloping columns whose presence, must be architecturally acknowledged as a trade-off for structural efficiency.

All of the aforementioned bracing systems or any number of their variations can be used singly or in combination with one another, depending on the layout of the building

Figure 3.15. (Continued).

and architectural requirements. The lateral resisting system can be turned by varying the relative stiffness of braces and frames to achieve an economical and sound structural system.

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