A design method for steel frames equipped with buckling restrained braces

In this paper a design method for buckling-restrained braced steel frames is examined. The proposed method aims to obtain an uniform yielding of all Buckling Restrained Braces (BRBs), thus avoiding concentration of plastic deformation at some storey level. This is a critical point in the design since BRBs usually have very low hardening in post-elastic response, especially if they are used in steel frames with pinned joints. An optimal distribution of column stiffness and of brace stiffness can be preliminarily defined from the free vibration equations of an equivalent shear-deformable cantilever. This continuum model permits to obtain closed form solutions that can be adopted for the design of structures with regular mass distribution over the height. A similar procedure is directly applied to discrete models. An optimal solution for the equivalent SDOF system is achieved by using force-based or displacement based design approaches. Being the proposed method based on the first vibration mode, dynamic analysis are performed and discussed in order to evaluate the reduction of performance in the nonlinear range deriving from higher vibration modes.