As noted in earlier chapters, EC8 aims to ensure life safety in a large earthquake together with damage limitation following a more frequent event. Whilst the code allows these events to be resisted by either dissipative (ductile) or non-dissipative (essentially elastic) behaviour, there is a clear preference for resisting larger events through dissipative behaviour. ence, much of the code is framed with the aim of ensuring stable, reliable dissipative performance in predefined 'critical regions', which limit the inertial loads experienced by other parts of the structure. he design and detailing rules are formulated to reflect the extent of the intended plasticity in these critical regions, with the benefits of reduced inertial loads being obtained through the penalty of more stringent layout, design and detailing requirements.
his is particularly the case for reinforced concrete structures here such performance can only be achieved if strength degradation during hysteretic cycling is suppressed by appropriate detailing of these critical zones to ensure that stable plastic behaviour is not undermined by the occurrence of brittle failure modes such as shear or compression in the concrete or buckling of reinforcing steel.
ith this in ind, three dissipation classes are introduced:
• Low (ductility class low (DCL)) in which virtually no hysteretic ductility is intended and the resistance to earthquake loading is achieved through the strength of the structure rather than its ductility.
• Medium (DCM) in which quite high levels of plasticity are permitted and corresponding design and detailing requirements are imposed.
• High (DCH) where very large inelastic excursions are permitted accompanied by even more onerous and complex design and detailing requirements.
In this chapter, the primary focus is on DCM structures, which are likely to form the most commonly used group in practice. However, the limited provisions for DCL structures and the additional requirements for DCH
structures are briefly introduced. Only the design of in-situ reinforced concrete buildings to EC 8 Part 1 is addressed here. Rules for the design of precast concrete structures are included in Section 5.11 of the code and guidance on their use in standard building structures is given in the Institution of Structural Engineers' manual on the application of EC8 (Institution of Structural Engineers/SECED/AFPS, 2009). Prestressed concrete structures, although not explicitly excluded from the scope of EC 8 Part 1, are implicitly excluded as dissipative structures since the rules for detailing of critical regions are limited to reinforced concrete elements. Prestressed components could still be used ithin dissipative structures but should then be designed as protected elements, as discussed later.
5.2 Design concepts
E8 is not a stand-alone code but relies heavily on the aterial Eurocodes to calculate resistance to seismic actions. EC2 (BS EN 1992-1-1:2004 in the UK) fulfils this function for concrete structures. For DCL structures, EC 8 imposes very limited material requirements in addition to the EC2 provisions, hereas for and structures, increasingly ore onerous aterial requirements are imposed, together with geometrical constraints, capacity design provisions and detailing rules tied to local ductility demand.
These rules are aimed at the suppression of brittle failure modes, provision of capacity to ithstand non-linear load cycles ithout significant strength degradation, and improving the ability of defined critical regions to undergo very high local rotational ductility demands in order to achieve the lower global demands. Typically, this includes:
• Ensuring flexural yielding prior to shear failure.
• Providing stronger columns than beams to promote a more efficient beam sidesway mode of response and avoid soft storey failure.
• Retention of an intact concrete core within confining links.
• Prevention of buckling of longitudinal reinforcement.
• Limiting flexural tension reinforcement to suppress concrete crushing in the compression zone.
These detailed requirements build upon the guidelines in Section 4 of EC8 Part 1 on:
• Regularity of structural arrangement, aiming to promote an even distribution of ductility demand throughout the structure.
• Providing adequate stiffness, both to limit damage in events smaller than the design earthquake and to reduce the potential for significant secondary P-5 effects.
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