Global and local ductility through capacity design and member detailing overview

As already noted in Section 4.11.2.2, to achieve a value of the global displacement ductility Clause 5.2.3

factor, that corresponds according to equations (D2.1) and (D2.2) to the value of the q factor used in the design of multi-storey buildings, a stiff and strong vertical spine should be provided up the height of the building, to spread the inelastic deformation demands throughout the structural system. As shown in Figs 4.4b and 4.4d, in concrete buildings this is accomplished either by using a wall system (or a wall-equivalent dual system), or by designing the columns of frames (and of frame-equivalent dual systems) to be stronger than their beams, so that they do not hinge except at the base of the building.

Wall systems (or wall-equivalent dual systems) are indirectly promoted not only through the strict interstorey drift limits for the damage limitation seismic action (see Section 4.11.2.1), which are difficult to meet with concrete frames alone, but also through their q factors. The q factors of dual and coupled-wall systems are the same as in frames, while those of uncoupled-wall systems are only 10-20% lower.

In frame systems (and frame-equivalent dual systems), strong columns are promoted, indirectly through the interstorey drift limits of Section 4.11.2.1, and directly through the capacity design of columns in flexure in accordance with Section 4.11.2.3 and equation (D4.23), so that formation of plastic hinges in columns before beam hinging is prevented.

Further to the control of the global inelastic response mechanism through selection of the structural configuration and dimensioning of vertical members to remain elastic above the base, the design strategy aims at ensuring that those individual members where the demand for global ductility and energy dissipation is spread possess the necessary local capacity to sustain this demand. As concrete members can dissipate energy and develop significant cyclic ductility only in flexure - and this only if certain conditions on material ductility and detailing are met - failure of members in shear before they yield in flexure should be precluded. To this end, prevention of pre-emptive shear failure is pursued by establishing the shear force demands on primary seismic beams, columns and walls in DCM and DCH buildings and beam-column joints in DCH frames not from the analysis for the seismic design situation but through capacity design calculations, as outlined in Section 5.6.4. In addition, the aforementioned conditions for the development of flexural ductility should be met, at least in those element zones where it is expected that inelastic deformations will be concentrated and energy dissipation will take place (plastic hinges - 'critical regions'). Section 5.6.3 outlines the conditions imposed by Section 5 on the ductility of materials used in plastic hinge zones and on the curvature ductility required from these zones; it also presents the rationale and background of these ductility conditions.

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