Boundary elements at section ends in the critical region of ductile walls

As noted in the definition of walls in Section 5.2.2, what mainly differentiates the design and Clauses detailing of a wall as a concrete member from that of a column is that for a wall, flexural 5.4.3.4.2(6), resistance is assigned to the opposite ends of the section (flanges, or tension and compression 5.5.3.4.5(6) chords) and shear resistance to the web in between. This is accomplished by concentrating the vertical reinforcement and limiting the confinement of the concrete only at the two ends of the section, in the form of boundary elements (Fig. 5.10).

Confined boundary elements need to extend only over the part of the section where at ultimate curvature conditions the concrete strain exceeds the ultimate strain of unconfined concrete ecu2 = 0.0035. This means that the centreline of the hoop enclosing a boundary element should have a length ofxu(l - ecil2/ccu2jc) in the direction of the wall length, Zw, with the neutral axis depth after concrete spalling, xu, estimated from equation (D5.46) for the value of awwd provided in the boundary element. The length of the confined boundary element from the extreme compression fibres, lc >xa(\ - £cu2/£cu2jC) + 2(c + dbJ2), should respect the prescriptive minimum value of 0.15/w and 1.5£>w.

Boundary elements with the confinement specified above are required only in the critical region at the base of DCM and DCH walls. In DCH walls they should be continued for one more storey with half of the confining reinforcement required in the critical region. Although not required by Eurocode 8, it is advisable to extend boundary elements to the top of the wall, with their minimum length and reinforcement. This is particularly so in barbelled walls, in which the barbells have to be detailed anyway as column-like elements.

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