Plastic Design of Moment Resisting Frames with Low Ductility

The Theory of Plastic Mechanism Control (TPMC) is an advanced method for the seismic design of steel structures that derives from the extension of the kinematic theorem to the concept of the equilibrium curve of the mechanism. Its original formulation was developed in the alpha-delta plane, recently the design procedure has been improved in the alpha-theta plane, indicated as TPMC(theta), for the purposes of structural optimization. The aim of this work is to adapt the TPMC(theta), originally created to design structures in high ductility class, to the new provisions contained in the new Eurocode 8 that allows to choose 3 ductility classes in which it is possible to design a structure depending on the reference seismic zone. In the in low (DC1) and medium (DC2) ductility classes it is possible to obtain a partial mechanism because a high seismic performance of the structure is not required, as is instead required in the high ductility class (DC3). This work proposes a new design method to involve at least two storeys in the collapse mechanism in a new perspective of DC2 effectiveness. In this way it is possible to design structures that manifest a partial mechanism when it is not necessary to recall all the ductility resources of the structure. A case study is proposed and the comparison with the 2-TPMC(theta) designed with the aim of avoiding only the soft-storey mechanism is reported. The effectiveness of the design method is tested using pushover analysis.