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Behavior requirements according to Eurocode 8

The behavior requirements described in EC8 (§2.1) are:

• No-collapse requirement: The structure must be designed and constructed to withstand the design seismic action without local or global collapse, thus retaining its structural integrity and a residual load bearing capacity after the seismic events has lapsed.
• Damage limitation requirement: The structure must be designed and constructed to withstand a seismic action having a larger probability of occurrence than the design seismic action, without the occurrence of damage and the associated limitations of use, the cost of which would be dispropotionately high in comparison with the costs of the structure itself.

For common structures the seismic action has a probability of exceedance 10% in 50 years, which means a return period of 475 years. The reference seismic action A_Ek, is marked with the letter R. Reliability differentiation is implemented by classifying structures into different importance classes. An importance factor γ_Ι is assigned to each importance class. Wherever feasible this factor should be derived so as to correspond to a higher or lower value of the return period of the seismic event: A_Ed = γ_Ι A_Ek.

For common structures the seismic action for the damage limitation has a probability of exceedance 10% in 10 years, which means a return period of 95 years. The limitation targets to the reduction of the financial impact in case of a less tense earthquake than of the reference one and the continuation of the operation of buildings important for the national security. EC8 (§2.1 and ­§4.4.3.2) allows the use of a design seismic action A_Ed, multiplied with the reduction factor ν according to the importance class.

These behavior requirements are checked with the compliance criteria that are presented next. Although, EC8 includes a third requirement: the avoid of a total collapse under a rare but undefined seismic action, far bigger than the design seismic action (i.e. with a return period of 2000 years). This requirement targets to avoid total losses, not distinguished victims.

Compliance criteria

The “damage limitation requirement” is considered to have been satisfied, if, under a seismic action having a larger probability of occurrence than the design seismic action corresponding to the “no-collapse requirement” (EC8 §4.4.3.2).

Compliance criteria in any kind of project in the requirement of avoiding (even local) collapse under the design seismic action is satisfying the deformation limits or other relevant limits. This happens because earthquake is a dynamic action that, so that the construction must stand not only specific forces, but also an amount of seismic energy that is inserted through the ground and to the respective deformations. This is the reason why EC8 allows the development of important non-elastic deformations, as long as they do not endanger the integrity of the building or of a part of it.

The resistance and energy-dissipation capacity to be assigned to the structure are related to the extent to which its non-linear response is to be exploited. In operational terms such balance between resistance and energy-dissipation capacity is characterized by the values of the behavior factor q and the associated ductility classification, which are given in the relevant Parts of EN 1998. As a limiting case, for the design of structures classified as low-dissipative, no account is taken of any hysteretic energy dissipation and the behavior factor may not be taken, in general, as being greater than the value of 1.5 considered to account for overstrengths. For steel or composite steel concrete buildings, this limiting value of the q factor may be taken as being between 1.5 and 2.

The main way to design seismic-resistant buildings according to is based on ductility. Specifically, they are designed to:

• Have capacity equal to the horizontal forces inserted by the earthquake, divided by the behavior factor q>1.5 and
• maintain the capacity to transmit the necessary forces and to dissipate energy under cyclic conditions. To this end, the detailing of connections between structural elements and of regions where nonlinear behaviour is foreseeable should receive special care in design.
• In order to accomplish the first of the requirements above, the member parts (beam and column edges, wall base) that are needed to develop plastic seismic deformations are designed for the limit falure condition, so that they have the resistance according to forces R_d, at least equal to the elastic tense, E_d, that is inserted through the horizontal forces:  (EC8 § 4.4.2.2)

R_d ≥ E_d

The design capacity value, R_d, on the limit failure state is calculated as in the design of other actions (i.e. in concrete members with the same values of partial factors, γ_c=1.5, γ_s=1.15).

In order to accomplish the second of the requirements above, the areas where plastic hinges are expected to be formed and their details are designed to have a local plasticity index that ensures the value of the building plasticity, μ_δ. Moreover, It shall be verified that both foundation elements and the foundation soil are able to resist the action effects resulting from the superstructure response without substantial permanent deformations. In determining the reactions, due consideration shall be given to the actual resistance that can be developed by the structural element transmitting the actions.