Plastic design of seismic resistant knee braced frames

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, push-over 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, available ductility and energy dissipation capacity.