Large lightly reinforced walls

Walls with a large horizontal dimension compared with their height cannot be designed effectively for energy dissipation through plastic hinging at the base, as they cannot be easily fixed there against rotation relative to the rest of the structural system. Design of such a wall for plastic hinging at the base is even more difficult if the wall is monolithically connected with one or more transverse walls also large enough not to be considered merely as flange(s) or rib(s) of the first wall. Section 5 recognizes that such walls, due to their large dimensions, will most likely develop limited cracking and inelastic behaviour in the seismic design situation. Cracking is expected to be mainly horizontal and to coincide with construction joints at floor levels. Flexural yielding, if it occurs, will also take place mainly at these locations. Then, the lateral deflections of large walls, acting as vertical cantilevers, will be produced through a combination of (1) a rotation of the foundation element of the wall relative to the ground, most often with partial uplifting from the ground, and (2) similar rotations concentrated at the locations of horizontal cracking and possibly flexural yielding at one or more floor levels, with the wall swaying in a multi-rigid-block fashion. Due to the relatively low axial load level in large walls, all these rotations will take place about a 'neutral axis' very close to the compressed tip of the foundation element or the compressed edge of the wall section at the locations of cracking and (possibly) yielding. Such rotations induce significant uplift of the centroid of the sections, raising the floor masses which are tributary to the wall and the ends of beams framing into it, to the benefit of the global response and the stability of the system. For example, part of the input seismic energy is - be it temporarily - harmlessly transformed to potential energy of these tributary masses, in lieu of damaging deformation energy of the wall itself. Moreover, rigid-body rocking of the wall promotes radiation damping, which is particularly effective for reducing the high-frequency components of the input motion.

Section 5 recognizes the capability of large walls to withstand strong seismic demands, through their geometry, rather than via the strength and hysteretic dissipation capacity provided by reinforcement. It defines a 'large lightly reinforced wall' as a wall with horizontal dimension, /w, at least equal to 4.0 m or to two-thirds of its height, h„ (whichever is less), and provides it with a special role and special design and detailing rules (that result in much less reinforcement than for ductile walls), under the condition that this type of wall is used in a lateral-force-resisting system consisting mainly of such walls (see the definition of the system of large reinforced walls in Section 5.3).

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