Fragility assessment and q-behaviour factor of concentrically braced steel frames under seismic sequences based on Cloud Analysis

The structural analysis and design of multi-story buildings subjected to seismic sequences represents a wellknown intriguing research topic, which has been addressed in last decades by several studies and investigations, but still lacks of a more generalized discussion. The primary goal of this paper, in this regard, is to investigate the seismic response of multi-story steel braced frames, in order to assess more in detail their capacity demand and ductility when subjected to seismic sequences, compared to single events. To this aim, two different multi-story steel structures - characterized by the use of concentrically braced frame (CBF) solutions based on cross-X with active tension diagonals (STR1) or inverted V (Chevron) bracings (STR2) respectively - are preliminary designed according to standards and examined for seismic fragility considerations. Their performance assessment is carried out using unscaled real ground motion records and employing a total of 190 non-linear dynamic simulations (95 for each building), within the Cloud Analysis framework. Different intensity measures (IMs) are considered to evaluate the impact of structural features and details on the seismic performance of STR1 and STR2 systems, namely on the associated fragility, ductility and dissipation capacity parameters. Fragility curves are in fact derived for the examined structures, based on conventional linear regression models and lognormal distribution. The corresponding q-behaviour factor is also calculated, by accounting for several influencing parameters. From the critical discussion of parametric numerical results, as shown, emerges that the herein considered fragility method leads to seismic assessment outcomes that highlight the effect of seismic sequences on the examined structural typology, compared to main-shock or first-shock seismic events. Most importantly, the corresponding q-factor is also affected by seismic sequences. The presented approach and results, in conclusion, can offer a robust support towards the definition and/or refinement of specific design recommendations, as well as and practical approaches for structural design applications.