Design concepts design for strength or for ductility and energy dissipation ductility classes

Clause 5.2.1 As already mentioned in Section 2.2.2, Eurocode 8 gives the option to design concrete buildings for more strength and less ductility, or vice versa. This option is exercised through the ductility classification of concrete buildings: Eurocode 8 permits trading ductility and dissipation capacity for strength by providing for three alternative ductility classes: low (DCL), medium (DCM) and high (DCH).

Buildings of DCM or DCH have q factors higher than the value of 1.5 considered to be available owing to overstrength alone. DCH buildings are allowed to have higher values of q than DCM ones. They also have to meet more stringent detailing requirements for members and to provide higher safety margins in capacity design calculations aiming at ensuring ductile global behaviour. The two upper ductility classes represent two different possible combinations of strength and ductility, approximately equivalent in terms of total material cost and achieved performance under the design seismic action. DCM is slightly easier to design for and achieve at the construction site, and may provide better performance in moderate earthquakes. DCH is believed to provide higher safety margins against local or global collapse under earthquakes (much) stronger than the design seismic action.

Section 5 itself does not link selection between the two higher ductility classes to seismicity of the site or importance of the structure, nor puts any limit to their application. It is up to a

CEN member state to make a choice for the various parts of its territory, or - preferably - to leave the choice to the designer, depending on the particular design project.

Buildings of DCL are designed not for dissipation capacity and ductility but only for Clauses 5.2.1 (2), strength: they have to follow, in practice, only the dimensioning and detailing rules of 5.3.1,5.3.3 Eurocode 2, and are designed to accommodate earthquakes in exactly the same way as for other lateral actions, such as wind. Although design to Eurocode 2 alone implies that the structure essentially remains elastic under its design actions, the members of DCL concrete buildings are dimensioned for internal forces derived by dividing the elastic response spectrum by a q factor of 1.5 instead of 1.0. This value of q is considered not to be due to any presumed energy dissipation capacity of the so-designed buildings, but only to overstrength of its members with respect to the seismic internal forces they are dimensioned for. This overstrength is a result of:

• the systematic difference between the expected strength of steel and concrete in situ from the corresponding design values (mean strength is considered to exceed the nominal value by 8 MPa for concrete or by about 15% for reinforcing steel - on top of that difference, nominal strengths are divided by the partial factors for materials to arrive at the design values)

• rounding-up of the number and the diameter of rebars

• placement of the same reinforcing bars at the two cross-sections of a beam or column across a joint, determined by the maximum required steel area at these two sections

• the frequent control of the amount of reinforcement by non-seismic actions and/or minimum reinforcement requirements, etc.

In moderate-to-high-seismicity regions, DCL buildings may not be cost-effective. Moreover, as they do not possess engineered ductility and energy dissipation capacity, they may not have a reliable safety margin against an earthquake significantly stronger than their design seismic action. So, they are not considered appropriate for regions of moderate or high seismicity. Eurocode 8 recommends the use of DCL only in cases of low seismicity, but it will be up to a CEN member state to decide whether it will follow this recommendation or not. It should be recalled that the definition of what constitutes a low-seismicity case is also left to member states, with Eurocode 8 recommending a ceiling for low-seismicity cases of 0.08g for the design ground acceleration on rock, a , or of O.lg for the design ground acceleration on the type of ground of the site, a S, with a including the importance factor 7,.

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