Plastic design of CB-frames with reduced section solution for bracing members

The seismic behaviour of concentrically braced frames (CBFs) designed according to the current European provisions is unsatisfactory due to the premature out-of-plane buckling of columns. For this reason, a new design methodology, based on a rigorous application of “capacity design” criteria has been recently proposed. In addition, aiming at a reduction of the plastic out of plane deformations of gusset plates due to brace buckling and at the prevention of sudden impact load affecting connections at the end of the straightening phase, Eurocode 8 requires the limitation of the brace slenderness. This limitation leads to the oversizing of diagonals and, consequently, of beams and columns. Therefore, to avoid this problem a new design strategy for bracing members is suggested: the Reduced Section Solution (RSS). It allows the calibration of the diagonal yielding resistance, leaving the brace slenderness practically unchanged.The results of dynamic inelastic analyses carried out with reference to braced frames designed according to the proposed procedure, both with and without RSS, are compared with those obtained with reference to the same structural schemes designed according to Eurocode 8. The obtained results show that the proposed design approaches are able to assure a significant improvement of the seismic performance.