ABSTRACT
In case of steel framed buildings located in seismic areas, the selection of an appropriate structural system must satisfy three criteria: strength, stiffness and, in particular, ductility. Moment Frames (MRF) take the benefit of good ductility but they are not efficient for taller buildings, due to large deflection that may lead to large story drifts. On the other hand, Centrically Braced Frames (CBF) have good stiffness and strength, but lower ductility. Eccentrically Braced Frames (EBF) combine the strength and stiffness of a centrically brace system with the ductility of a moment frame. For these systems, there are large expertise and code provisions that generally refer to all design aspects (EN1998-1, AISC 2005). In the recent years, have been developed new structural systems, like Buckling-Restrained Braced Frames (BRBF) or Steel Plate Shear Walls (SPSW). These systems are in use in Japan and in US, both for seismic retrofit of the existing buildings and for new buildings. A major role in this development may be attributed to the introduction of these new systems in the code Provisions (AISC, 2005). Unfortunately, in Europe there are not significant applications, partly due to the lack of design provisions form the design codes, including EN1998-1. One major problem in such cases is the definition of reduction factor q and the overstrength factor Ω. In case of frames of inverted V braces (CBF), in EN1998 q factor is taken equal to 2.5, while for dual frames (MRF + CBF) q can be up to 4.8. However in case of BRB it is expected to obtain a better behaviour, comparable to Moment Resisting Frames (MRF). On the other
hand, if Carbon Mild Steel (S235, S275) is used in the dissipative members, which are the BRB in CBF, and the beams in MRF, while High Strength Steel (S460) is used in “non-dissipative members, beams in CBF and columns, a global plastic mechanism failure can be obtained. For design purpose, according to EN1998-1, a crucial problem is to use a correct value of Ω factor, which in case of these specific structures is still a matter of research. The same MRF stiffened with dissipative steel shear walls (SW) are analyzed as an alternative to CBF systems. The study presented in the paper analyses the seismic performance of four different structural systems, focusing the structural response and global mechanism criteria and the influence of overstrength factors Ω. On this aim, numerical analysis applying N2 method is performed.
