Plastic Design of Seismic Resistant V-Braced Frames

In this article, a new method for designing chevron concentrically braced steel frames is presented. The aim of the proposed method is the design of concentrically braced steel frames able to guarantee, under seismic horizontal forces, a collapse mechanism of global type. This result is of great importance in the seismic design of structures, because local failure modes give rise to a worsening of the energy dissipation capacity of structures and, therefore, to an higher probability of failure during severe earthquakes. With reference to the examined structural typology, the global mechanism is characterized by the yielding of tensile bracing diagonals and by the buckling of the compressed diagonals of all the stories. The proposed method is rigorously based on capacity design approach which requires that dissipative zones have to be designed to withstand the internal actions due to the seismic design horizontal forces and the vertical loads acting in the seismic load combination; while non dissipative zones have to be designed considering the maximum internal actions that dissipative zones, yielded and strain-hardened, are able to transmit. The new design issue covered by the proposed design procedure is the need to account for the contribution of the compressed diagonals in deriving the design axial force of non dissipative members. The seismic inelastic response of a sample structure is investigated by means of nonlinear dynamic analyses. The results carried out with reference to braced frames designed according to the proposed procedure are compared with those obtained with reference to the same structural schemes designed according to Eurocode 8.