Reliability‐based partial factors for seismic design and assessment consistent with second‐generation Eurocode 8

Second-generation Eurocode 8 introduces displacement-based design alongside the traditional force-based one with reduced equivalent forces; it provides resistance models needed to apply the approach to reinforced concrete, steel and composite frame or wall-frame structures: chord rotation for members, and shear strength for members and joints; finally, it declares a target reliability level for near collapse. This paper discusses the formulation of partial factors to achieve a target reliability when using the displacement-based approach. The factors are derived following a traditional, time-invariant way of performing code-calibration for other non-seismic design situations, starting from the probability distributions for the seismic action effect and the resistance. It is shown that the problem can be simplified to one with two Lognormal variables, a uniform reliability can be achieved over the calibration space when both a load-side and a resistance-side factor are used, and constant sensitivity coefficients can be used, with values different from the original König and Hosser ones and reflecting 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 European practice, thus a corrected site-independent resistance-side partial factor is proposed, which is compatible with the framework of Eurocode 8. As a further advantage, the proposed format coincides with that proposed in the second-generation EN1990/EN1992 for the assessment of existing structures under non-seismic design situations.