Elements which are normally vertical and support other elements are classified as walls, if their cross-section has an aspect ratio (ratio of the two sides) above 4. Obviously, if the cross-section consists of rectangular parts, one of which has an aspect ratio greater than 4, the element is also classified as a wall. With this definition, on the basis of the shape of the cross-section alone, a wall differs from a column in that it resists lateral forces primarily in one horizontal direction, namely that of the long side of the cross-section, and, furthermore, that it can be designed for such a unidirectional resistance by assigning flexural resistance to the opposite ends of the section ('flanges', or 'tension and compression chords') and shear resistance to the 'web' in-between, as in a beam. Concentration of longitudinal (i.e. vertical) reinforcement and concrete confinement is needed only at the two ends of the section providing the flexural capacity. If the cross-section is not elongated, the vertical element develops significant lateral force resistance in both horizontal directions; it is then meaningless to distinguish between flanges, where longitudinal reinforcement is concentrated and concrete is confined, and webs, where the aforementioned do not occur

The above definition of walls is consistent with that in EN 1992-1-1 (clause 9.6.1(1)), and may be appropriate as far as dimensioning and detailing at the level of the cross-section is concerned. It is not very meaningful, though, in view of the intended role of walls in the structural system and of their design, dimensioning and detailing as an entire element, and not just at the cross-sectional level. In fact, if at least 50% of the seismic base shear in a horizontal direction is resisted by concrete walls (see the definition of wall-equivalent dual systems below), then EN 1998-1 relies on these walls alone for the prevention of a storey mechanism in that direction, without any additional verification: the check that plastic hinges will form in beams rather than in primary seismic columns, equation (D4.23), is waived. However, walls can meet the objective of enforcing a beam-sway mechanism only if they act as vertical cantilevers (i.e. if their bending moment diagram does not change sign within at least the lower storeys, see Fig. 5.1) and develop plastic hinging only at the base (at their connection to the foundation). The assumption that walls, as defined above, will indeed act as vertical cantilevers and form a plastic hinge only at the base, underlies all the rules in Section 5 for the design and detailing of 'ductile walls'. However, whether this assumption corresponds or not to the real behaviour of the wall depends not so much on the aspect ratio of its section but primarily on how stiff and strong the wall is, compared with the beams it is connected to at storey levels. For concrete walls to play the role intended for them by EN 1998-1 and fulfil its tacit assumptions, the length dimension of their cross-section, /w, should be large, not just relative to its thickness, ¿>w, but in absolute terms. To this end, and

Fig. 5.1. Typical bending moment diagram in a concrete wall from the analysis and according to Eurocode 8

for the beam sizes commonly found in buildings, a value of at least 1.5 buildings or 2 m for medium- or high-rise ones is recommended here for /w.

A distinction is made in Section 5 between 'ductile walls' and 'large lightly reinforced walls'. Ductile walls are further classified as 'coupled' or 'uncoupled'.

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