Partial Factors for Displacement-based Seismic Design and Assessment Consistent with the Second-generation of Eurocodes

Abstract

The second generation of Eurocode 8 introduces the displacement-based approach (for both design and assessment) alongside the traditional force-based one, which employs linear analysis and equivalent forces reduced to account for ductility, redundancy and overstrength. On the contrary, in displacement-based design and assessment, the unreduced seismic action is used in a nonlinear analysis to determine action effects, in terms of deformations for ductile failure modes and forces for brittle ones. The new Eurocode 8 thus provides the necessary resistance models to apply the approach to reinforced concrete, steel and composite frame and wall-frame structures: chord rotation models for members, and shear strength models for both members and joints. Finally, it also declares, for the first time, a target reliability level for collapse.

This paper discusses the formulation of partial factors to achieve that target reliability, or any other desired target reliability, when using the displacement-based approach. The partial factors are derived for the seismic design situation following a traditional way of performing code-calibration for other non-seismic design situations. Probability distributions for the seismic action effect and the resistance are introduced and discussed. It is shown how the problem can be eventually simplified to one with only two lognormal variables and a uniform reliability can be achieved over the calibration space when two factors are used, one on the resistance-side and the other on the load side. It is also shown that the well-known K�nig and Hosser formulation of partial factors with constant sensitivity coefficients can be used also in the seismic design situation, with constant values, tailored to the seismic case, that are not dramatically different from the original ones but reflect the larger weight of the action-related uncertainty in the seismic case. The problem with this otherwise good result is that a partial factor on the load side must be used and, further, that its value is site dependent. This approach is not in line with current practice, thus a corrected site-independent resistance-side partial factor is proposed, which is compatible with the framework of Eurocode 8. Notably, the proposed format coincides with that proposed in the second-generation EN1990 and EN1992 for the assessment of existing (concrete) structures under non-seismic design situations, resulting in sure benefit for the final user.