Fragility of Steel Frames with DYBs Designed in the Framework of EC8

For a long time, the steel concentric bracing has been considered an effective structural system to sustain seismic forces. Unfortunately, today, it is widely recognized that the concentric bracing presents also serious limitations: the poor cyclic response determined by the buckling of the brace in compression and the low redundancy of the structural type. Hence, in a previous paper, the authors proposed the use of the Double-stage Yield BRB (DYB) as an alternative to the conventional steel brace. The DYB yields in tension and compression exhibiting fat hysteresis loops and large ductility capacity. In addition, yielding occurs in two stages. The double-stage yielding mitigates the lack of redundancy of the concentric bracing and promotes a global collapse mechanism. Steel frames equipped by DYBs can be designed by a procedure that is formally identical to that stipulated in Eurocode 8 for conventional braced frames. Since the cyclic response of the DYB is controlled by the ratio ρYS between first and second yield forces of the DYBs, the behaviour factor q is provided by a linear function of ρYS calibrated to fulfil the performance objectives of Eurocode 8. The goal of this paper is to investigate on the seismic fragility of steel frames equipped by DYBs characterized by different values of ρYS, by means of fragility curves.