Types of braced frames

Often concealed within the walls of building cores braced frames cantilever vertically from their foundations to resist the seismic forces transferred from diaphragms. The basic types of braced frames are illustrated in Fig. 5.23 .

Tension and compression

Tension and compression

(Inverted U) (Diagonal)

Tension-only Eccentric

▲ 5.23 Common types of braced frames.

Seismic Forces Building
▲ 5.24 Tension-only bracing. Light industrial building, Wellington.

The diagonal members for all frame types except for tension-only braced frames are designed to resist tension and compression. Member cross-sections are therefore quite large to resist buckling. For multi-storey steel construction typically steel tubes or steel universal column sections are used for bracing members subject to compression.

Tension-only bracing is particularly common in low-rise and light-industrial buildings. It is usually very cost-effective since it utilizes steel in its most efficient structural mode - tension. The bracing members are usually very slender, such as steel rods or flats, so their compressive strengths are negligible. Depending on the building weight and the number of braced frames bracing member cross-section diameters can be as little as 20 to 30 mm.The advantage of this type of bracing from an architectural perspective is its economy and transparency. Vertical tension-only bracing is often used in conjunction with similar roof diaphragm bracing (Fig. 5.24).

▲ 5.25 The deformation of an eccentrically braced frame showing the distortion of the structural fuses.

▲ 5.26 An eccentrically braced frame with stiffener plates and fly-braces to stabilize the beam bottom flange in the structural fuse region. Apartment building, Wellington.

▲ 5.25 The deformation of an eccentrically braced frame showing the distortion of the structural fuses.

▲ 5.27 Two tension and compression fully triangulated frames. The types of axial force are indicated.

Eccentrically braced frames are the most ductile of all braced frames. The eccentricity between the inclined braces or between columns and braces ignores the centuries-old best-practice of concentric connections. Consequently, severe seismic bending moments and shear forces form in the beam fuse region located between braces. Plastic bending and shear deformation of the steel beam absorbs earthquake energy (Fig. 5.25). Special welded stiffener plates in the fuse region prevent the beam web from buckling (Fig. 5.26) . The beams of eccentrically braced frames may be pin-joined to columns but they must be continuous over the braces. The floor slab supported by the beam will suffer damage unless it is separated from the fuse region and its potentially severe distortions.

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