THE SEISMIC RESPONSE OF CONCENTRICALLY BRACED MOMENT-RESISTING STEEL FRAMES

The paper deals with the dynamic non-linear behaviour of concentricall y braced steel moment-resisting frames under severe earthquake conditi ons. The first part of the paper is devoted to the description of a no n-linear beam element which has been developed starting from the class ical '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 P olitecnico di Milano. The non-linear brace element used to model the d iagonal bars and the procedure for step-by-step dynamic analysis are t hen briefly summarized. In the second part of the paper two examples o f application are presented; the first one concerns a six-storey MRF d esigned 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 co nditions. The numerically computed seismic behaviour of the system is analysed and compared to that of the unbraced frame. The second exampl e is a six-storey CBF designed according to the Eurocode 8 specificati ons; the non-linear behaviour under seismic loading is studied especia lly to the aim of analysing the effect of the columns flexural stiffne ss and resistance, which was not considered in the design, on the CBF response.