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ABSTRACT:In this paper a design methodology aiming at the development of a collapse mechanism of global type for seismic resistant knee braced frames is presented. The proposed methodology is based on the assumption that the beam, brace and knee sections are known, while the column sections constitute the unknowns of the design problem. The design requirements are derived by means of the kinematic theorem of plastic collapse. In particular, column sections are obtained by imposing that the mechanism equilibrium curve corresponding to the global mechanism has to lie below those corresponding to all the undesired mechanisms within a displacement range compatible with the local ductility supply of knee elements.The proposed design procedure has been implemented in a computer program and applied to design some knee braced frames. Successively, static and dynamic non-linear analyses have been carried out aiming at the evaluation of the performance of the designed braced frames in terms of collapse mechanism developed under seismic forces, energy dissipation capacity and global and local ductility demands.

The paper is devoted to the investigation of the seismic response of eccentrically braced frames characterised by links having different length. In addition, the analysed structures have been designed according to a methodology, already proposed by the authors, aiming to guarantee a collapse mechanism of global type. Therefore, the results of the nonlinear analyses herein presented provide the validation of the proposed design procedure, by testifying that all the designed structures exhibit a global failure mode where all the links are yielded while all the columns remain in elastic range with the exception of the base section of first storey columns, leading to high energy dissipation capacity and global ductility. Furthermore, two different distributions of the link lengths are examined. The first one is characterised by short links with uniform lengths along the height of the structure. The second one is characterised by the use of link elements having different length at the different storeys which are selected to assure the same value of the non-dimensional link length. The seismic response of EB-Frames with such distributions of the link length is investigated by means of both push-over analyses and dynamic non-linear analyses. The comparison of the performances is mainly carried out in terms of plastic hinges distribution, local ductility demand and frame lateral stiffness.