The behaviour of concentrically braced steel frames under strong seismic actions

The present paper deals with the seismic behaviour of steel structures which are designed in the attempt of exploiting the dual characteristics of moment resisting frames (MRFs) and concentrically braced frames (CBFs) as lateral force resisting systems. Two prototype frames (identical for dimensions and design loads) are studied within the context of Eurocode 8 (EC8) provisions; the first (named CS1) is a MRF which is traditionally designed, against ultimate seismic actions (ULS), without checking serviceability limit state rules (SLS-interstory drift limits). To fulfill these requirements concentrical braces are inserted in the frame, which are not considered in the collapse resistance of the structural system. The second system (CS2) is a traditional CBF, which differs from standard design practice only in that the column section orientation is such that maximum stiffness and resistance lie in the frame plane. The non-linear behaviour of both systems under ultimate seismic environment is studied; the CS1 system response is compared to the one of the unbraced MRF. The CS2 response is studied in order to assess the influence of the column flexural resistance on the overall behaviour. The influence of second order effects is investigated for both cases.