Plastic design of eccentrically braced frames, II: Failure mode control

A design methodology aiming at the development of a collapse mechanism of a global type for eccentrically braced frames is presented in this paper. This result is of primary importance in earthquake resistant design, because partial and local failure modes are responsible of the worsening of the energy dissipation capacity leading to an increased risk of collapse under destructive ground motions.The proposed method is based on the assumption that horizontal member sections are known, as they are designed to resist internal actions due to vertical loads and design horizontal forces. Conversely, column and diagonal sections constitute the unknowns of the design problem. In particular, the presented approach also includes the influence of second order effects which are accounted for by means of the concept of mechanism equilibrium curve. The design requirements are derived by means of the kinematic theorem of plastic collapse. Column and diagonal sections are obtained by imposing that the mechanism equilibrium curve corresponding to the global mechanism has to lie below those corresponding to the undesired mechanisms within a displacement range compatible with the local ductility supply. Moreover, the introduction of the equivalent moment concept presented in a companion paper provides the proposed method with the ability to deal with short, intermediate and long links in the same manner.Aiming at the evaluation of the accuracy of the presented procedure, the inelastic performances of eccentrically braced frames designed by means of the proposed method are investigated, by means of static and dynamic non-linear analyses, in terms of collapse mechanism typology, available ductility and energy dissipation capacity.