The Seismic Response of Concentrically Braced Moment-Resisting Frames

The paper deals with the dynamic non-linear behaviour of concentrically braced steel moment-resisting frames under severe earthquake conditions. The first part of the paper is devoted to the description of a non-linear beam element which has been developed starting from the classical one-component formulation, having concentrated inelastic hinges at the ends. The hysteretic behaviour of the end hinges is calibrated on the basis of a set of experimental tests previously performed at Politecnico di Milano. The non-linear brace element used to model the diagonal bars and the procedure for step-by-step dynamic analysis are then briefly summarized. In the second part of the paper two examples of application are presented; the first one concerns a six-storey MRF designed in accordance with Eurocode 8, in which concentric braces are introduced only to satisfy the serviceability limit state requirements (interstory drifts) but are not taken into account in the ultimate conditions. The numerically computed seismic behaviour of the system is analysed and compared to that of the unbraced frame. The second example is a six-storey CBF designed according to the Eurocode 8 specifications; the non-linear behaviour under seismic loading is studied especially to the aim of analysing the effect of the columns flexural stiffness and resistance, which was not considered in the design, on the CBF response.