Types of Braces

Braced frames may be grouped into two categories, as either concentric braced frames (CBF) or eccentric braced frames (EBF), depending on their geometric characteristics. In CBFs, the axes of all members—i.e., columns, beams, and braces—intersect at a common point such that the member forces are axial. EBFs utilize axis offsets to deliberately introduce flexure and shear into framing beams. The primary goal is to increase ductility, as discussed later in this chapter.

The CBFs can be configured in various forms, some of which are shown in Fig. 3.8. Depending on the magnitude of force, length, required stiffness, and clearances, the diagonal

Figure 3.8. Typical concentric braced frame (CBF) configurations: (a) one-story X-bracing; (b) single-diagonal bracing; (c) and (d) chevron bracing; (e) two-story X-bracing; (f) single-diagonal, alternate-direction bracing.

Figure 3.8. Typical concentric braced frame (CBF) configurations: (a) one-story X-bracing; (b) single-diagonal bracing; (c) and (d) chevron bracing; (e) two-story X-bracing; (f) single-diagonal, alternate-direction bracing.

Figure 3.9a—g. Brace configurations that allow for door-size openings in interior space layouts. Note: Some configurations are not permitted in areas of high seismicity.

member can be made of double angles, channels, T-sections, tubes, or wide flange shapes. Besides performance, the shape of the diagonal is often based on connection considerations. The least objectionable locations for braces are around service cores and elevators, where frame diagonals may be enclosed within permanent walls. The braces can be joined together to form a closed or partially closed three-dimensional cell for effectively resisting torsional loads.

The most efficient (but also the most obstructive) types of bracing are those that form a fully triangulated vertical truss. Figure 3.9 shows other types of braced bents that pose fewer problems in the architectural organization of internal space as well as in locating door openings, but may cause bending in columns and girders. Historically, bracing has been used to provide lateral resistance to the majority of the world's tallest buildings, from the earliest examples at the end of the 19 th century to the present. An outstanding example is the 1250-ft-high Empire State Building (Fig. 3.9h), completed in 1931.

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