A design method for steel frames equipped with buckling restrained braces

Buckling-restrained braced frames were introduced to enhance the compressive capacity of braces while not affecting their stronger tensile capacity, hence producing a symmetric hysteretic response [1]. A buckling restrained brace (BRB) usually consists of a core steel brace encased in a steel tube that may be filled with concrete or grout. BRBs have been used extensively for seismic applications in Japan and United States due to their simple and efficient behaviour, as testified by several applications [1,2] and by the inclusion in code recommendations such as the AISC seismic provisions [3]. The introduction of BRB members undoubtedly represents a major advancement compared to conventional braces in terms of cyclic inelastic deformation capacity and reduction of design forces. However, buckling restrained braced frames may undergo large inelastic storey drifts without the ability to distribute the ductility demand over the height of multi-storey structures due to possible localizations of inelastic deformations. This latter aspect deserves particular attention due to the bracing system characteristics, i.e., statically determinate structural configuration and limited BRB hardening, especially in steel frames with beams connected to columns by means of pinned joints as often happens in Europe. As a result, the frame global ductility is strongly dependent on the distribution of BRB strength and stiffness at each storey level. In this paper a single degree of freedom (SDOF) based design method for BRB steel frames is examined. The bracing system is modelled as a continuum cantilever beam where BRBs are associated to the shear stiffness and columns are associated to the flexural stiffness. This continuum model allows a more clear identification of the parameters influencing the structural behaviour and a more simple definition of the design procedure as compared to discrete models. Closed form solutions can be obtained for cantilever beams with uniformly distributed mass, resulting in simple analytical expressions that can be adopted for the design of structures with regular mass distribution over the height. An optimal solution for the SDOF system is achieved similarly to other methods based on the same type of approach [4][5]. However the actual dynamic nonlinear response can significantly be influenced by higher vibration modes and by deformation localization at some floor levels. Thus preliminary results of numerical simulated response analyses are illustrated in order to highlight advantages and limitations of the presented design procedure paying particular attention to the differences between linear static analysis based on the first vibration mode and nonlinear dynamic analysis.