As mentioned in the introduction to this chapter, braced frames are essentially pin-jointed structures. Most of their horizontal load-carrying capability is achieved by their members working in either pure compression or tension. Apart from the beams of eccentrically braced frames individual members do not resist significant bending moments. The ability of braced frames to function as vertical trusses lies in their triangulation (Fig. 5.27). Clearly, if any members are required to resist
compression, remembering that seismic forces change direction rapidly, then their cross-sectional dimensions must be sufficient to prevent buckling. The chords or end columns of braced frames also fall into this category for as well as resisting overturning-induced compression they also support gravity forces.
Braced frames, like shear walls, require strong foundations. But since the gravity forces acting on braced frames with their lighter construction are usually less than those of shear walls additional foundation stability might be required. Especially for slender one or two-bay frames, tension piles may be necessary to prevent overturning. The structural engineer determines the type, diameter and length of a tension pile depending on the tension to be resisted and the ground conditions.
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