Influence of Design Criteria on the Seismic Reliability of X-braced Frames

According to the most modern trend, performance-based seismic design is aimed at the evaluation of the seismic risk defined as the mean annual frequency of exceeding a threshold level of damage, i.e., a limit state. A procedure for performance-based seismic assessment is herein briefly summarized and applied to concentrically “X” braced steel frames, designed according to different criteria. In particular, two design approaches are examined. The first one corresponds to the provisions suggested by Eurocode 8, while the second approach is based on a rigorous application of the capacity design criteria aiming to the control of the failure mode, as suggested in a companion article. The aim of the presented work is to focus on the seismic reliability obtained through these design methodologies. The probabilistic approach is based on an appropriate combination of probabilistic seismic hazard analysis (PSHA), probabilistic seismic demand analysis (PSDA), and probabilistic seismic capacity analysis (PSCA). Regarding PSDA, nonlinear dynamic analyses have been carried out in order to obtain the parameters describing the probability distributions of demand, conditioned to given values of the earthquake intensity measure. It is demonstrated how the proposed design method leads to a very important improvement of the seismic performances with a negligible increase of the overall building cost.