Seismic design and performance of dual structures with BRBs and semi-rigid connections F. Barbagallo ⁎, M. Bosco, E.M. Marino, P.P. Rossi Department of Civil Engineering and Architecture, University of Catania, Via S. Sofia 64, 95123 Catania, Italy abstract article info Article history: Received 6 April 2018 Received in revised form 30 March 2019 Accepted 31 March 2019 Available online xxxx In the past, the use of Buckling Restrained Braces (BRBs) in buildings with braced structure has been proposed to overcome the drawback of steel Concentrically Braced Frames (CBF) caused by the low dissipative cyclic behav- iour of conventional buckling braces. The structure is conceived so that a few braced frames resist the entire seis- mic force and all the other frames sustain gravity loads only. According to the design practice adopted in European countries, all the beam-to-column connections are usually perfectly pinned. The use of these connec- tions leads to low-redundant systems, which tend to form soft storey collapse mechanisms. The concentration of drift demand precludes the full exploitation of the deformation capacity of all the BRBs of the frame and partially reduces the benefit that may derive from these devices. A more effective structural system may be obtained by coupling frames with BRBs with frames with semi-rigid connections. In fact, the frame with semi-rigid connec- tions provides a residual lateral stiffness after yielding of BRBs and thus promotes a more uniform distribution of the drift demand along the height of the building. This paper investigates the seismic performance of dual steel systems with BRBs and semi-rigid connections. A set of frames is designed considering several values of the behaviour factor and the response of the obtained frames is determined by nonlinear dynamic analysis. Then, the behaviour factor that allows the frames to meet the performance objectives of Eurocode 8 is deter- mined. The seismic performance of the frames is assessed in terms of ductility demand-to-capacity ratio of dissi- pative members, in terms of strength demand-to-capacity ratio of non-dissipative members, and residual drifts. © 2019 Elsevier Ltd. All rights reserved. Keywords: Steel frames Behaviour factor Concentrically braced frames Nonlinear dynamic analysis 1. Introduction Steel concentric bracings provide buildings with large lateral stiffness and strength and allow the structural designer to satisfy easily both story drift requirements under serviceability limit state loads and strength re- quirements under ultimate limit state loads. Owing to this, they are com- monly used in earthquake-prone countries [1,2]. However, conventional braces buckle and their resistance in compression severely decreases in the post-buckling range of behaviour [3]. Although conventional braces are the members of concentrically braced frames devoted to the energy dissipation, they develop pinching response and gradual reduction of the dissipated energy, caused by the cumulative plastic deformations ex- perienced in tension. Eventually, local buckling occurs in the plastic hinge when the brace is loaded in compression, and cracks initiate in the yielded region when the force reverses. This leads to the early fracture of the brace [4,5]. Another shortcoming of concentrically braced frames may derive from the type of connection used between members. Indeed, differently from other parts of the world, such us USA or Japan, the design practice adopted in European countries conceives concentrically braced frames as systems in which the seismic force is mainly resisted by mem- bers subjected to axial forces. Transmission of bending moments be- tween members is not needed and thus they are pin-connected. Pin connections are certainly less expensive than rigid moment connections, as they involve much less fabrication work. However, the use of pin con- nections leads to low-redundant structural systems, which are prone to develop soft storey collapse mechanisms [4,6]. The enhancement of the hysteretic behaviour and the energy dissi- pation capacity of conventional braces has been the motivation at the base of the development of the Buckling Restrained Brace (BRB) [7]. The concept of the BRB is based on a buckling restraining mechanism that avoids the instability of a dissipative steel member, which is forced to yield both in tension and compression. The result is a very ductile member that can dissipate energy through stable tension-compression hysteresis cycles [7–15]. Different researches have proposed the inser- tion of BRBs in steel braced frames and developed specific design methods [16–19]. However, the use of BRBs only partially overcomes the limitations of steel concentric bracings. In fact, when BRBs are inserted in concentrically braced structures with pin connections [20,21], the obtained structural system is still low-redundant. Hence, Journal of Constructional Steel Research 158 (2019) 306–316 ⁎ Corresponding author at: Department of Civil Engineering and Architecture, University of Catania, Italy. E-mail addresses: fbarbaga@dica.unict.it (F. Barbagallo), emarino@dica.unict.it (E.M. Marino). https://doi.org/10.1016/j.jcsr.2019.03.030 0143-974X/© 2019 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Constructional Steel Research