Seismic performance reduction in steel braced frames due to uneven brace over-strength distributions

Concentrically braced frames (CBFs), eccentrically braced frames (EBFs), and more recently buckling-restrained braced frames (BRBFs), have been proved being efficient seismic resistant systems with adequate dissipation capacity and elastic lateral stiffness. Seismic codes generally allow to design dissipative structures at the ultimate limit state by means of linear analyses based on a reduced spectrum (design spectrum) and provide values of the reduction factor for different structural systems. The values suggested are associated to a homogeneously distributed mechanism involving all the potentially dissipative structural elements (global ductility) and to a sufficient ductility of critical yielded zones (local ductility). The local ductility is usually controlled by means of specific design and detailing provisions, while global ductility is ensured by applying capacity design procedures and by limiting the variation of the over-strength ratios (the ratios between the seismic demand and the strength capacity) of the yielding portions. This last issue is often very difficult to obtain and irregularity has significant negative effects on the seismic performance. In fact, CBFs as well as EBFs and BRBFs suffer from low post-elastic stiffness, especially in the case of frames with non-moment resisting beam-to-column connections, due to the low post-yield stiffness of their braces [1][2][3]. Thus, these structural systems might show significant tendency to soft storey formation during seismic events, due to the irregularity of the brace over-strength distributions over the building height. Overall, the studies available in the technical literature delineate the need to further examine the influence of the brace over-strength on the structural seismic performance as well as the effectiveness of current code recommendations. In this paper a general methodology to investigate the relation between seismic performance reduction and regularity of brace over-strength distributions is presented. The influence of brace over-strength distributions on the seismic motion are studied through gradient methods based on sensitivity computation [4]. A range of variation of the brace over-strengths is defined and the brace over-strength patterns that are the most unfavourable for each response parameter are identified. Finally, the relationship between the brace over-strength used in the seismic design and the maximum seismic performance reduction is obtained. Results for a case study considering a realistic 4-storey BRBFs with pinned beam-to-column connections and hinge restraints at the base are presented and discussed.