DIAGONAL BRACING CONNECTIONS
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DIAGONAL BRACING CONNECTIONS
If the members in the unbraced frame of Figure 11-4a were connected with simple shear connections, the lateral force Hu acting from the left would cause the building to “rack” or deflect laterally as shown by the dashed lines. In fact, the frame would be unstable under gravity loading. In lieu of a frame with moment connections, frame stability and resistance to lateral loads can be provided by diagonal bracing members. Whereas moment connections resist lateral loads through flexure in the beams and columns which comprise the frame, diagonal bracing members create a vertical truss which transfers the lateral loads through the members of the truss as axial forces. Although a diagonally braced frame is, in general, more efficient than a frame with moment connections, the use of diagonal bracing may be precluded by interference with architectural features such as corridors, windows, and doors. Diagonal bracing may be concentric or eccentric. Eccentrically braced frames are commonly used in seismic regions; their design is beyond the scope of this book; refer to Ishler (1992), Popov, et al. (1989) and Lindsay and Goverdahn (1989). The following discussion is limited to concentric diagonal bracing. The concentric diagonal brace shown in Figure 11-4b will provide for stability and lateral forces acting from the left; the diagonal brace is in tension (+) and induces only axial forces in the other members of the frame. Since the lateral forces may be incident from either the right or the left, two diagonal braces would be used, as shown in Figure 11-4c. As the stiffnesses of the diagonal bracing members increase, lateral forces will divide (not necessarily equally) between the two diagonal braces with one in tension and the other in compression. It is normal practice to neglect the strength of the diagonal in compression and design each diagonal for the tension which results from the lateral loads; this is called tension-only bracing. Figure 11-5a shows the vertical arrangement of X-bracing in a single bay of a multistory building. Figure 11-5b shows a common type of K-bracing. Figures 11-5c and 11-5d show bracing which is composed of members subjected to both tension and compression; as shown, this bracing occupies a single bay of a multistory building. Figure 11-5e is similar except the bracing occupies two adjacent bays of a multistory building. Other arrangements, such as the one shown in Figure 11-5f, are also possible. When possible, diagonal bracing should be located in a bay or bays at the mid-section of a building. In buildings with expansion joints, diagonal bracing should be located in a bay or bays at the mid-section between expansion joints. Furthermore, this bracing
Hu
Hu
(+)
(a)
(b)
Fig. 11-4. Lateral forces and diagonal bracing. AMERICAN INSTITUTE OF STEEL CONSTRUCTION
(c)