Assessment of the effect of seismic sequences on steel X-CBF for industrial buildings

This work concerns about the study of the effect of the seismic sequences on steel mono-storey industrial buildings equipped with X-CBFs, with the aim to evaluate a code change to adequately consider this issue. Indeed, current technical regulations (CEN, 2004; MIT, 2018) do not take into account this phenomenon, thus the structure are designed only to withstand a single main-shock, without considering the possible accumulation of damage due to the after-shocks. In this work it is instead shown how this effect has to be properly considered and evaluated. First, a mono-storey industrial building is analysed as preliminary case study. Fragility curves are built for both sequences and single main events, thus obtaining an important comparison. Then the study focuses on a single X-CBF, validated through an experimental test from literature. The calibrated system is subjected to both seismic sequences and corresponding mainshocks. Analyses are carried out also by varying brace profiles. The results show a significant influence of the seismic sequences on the increase of the ductility request of the structure. Therefore, with the aim to properly represent the effect of the sequences, it is considered necessary to require a reduction of the available behavior factor, providing a precautionary estimate. This operation wants to give a first preliminary estimate of the increase of the seismic risk only on the seismic vulnerability side of this kind of structures.