A proposal for improving the elastic behaviour of dogbone

The Reduced Beam Section (RBS) connections are one of the prequalified connections approved by international standards and adopted to improve the seismic behaviour of steel Moment Resisting Frames (MRF) also protecting the joint between beams and columns. This goal is reached by locally reducing the resistance of a selected portion of the beam element by suitably trimming the flanges, to promote in that portion the onset of plastic deformations. In the framework of RBS, the most important typology proposed is the so-called dogbone (DB) connections that are widely diffused in seismic engineering. The essential advantages of DB are related to the protection of the welded cross section between beam and column, to the increasing of the total energy dissipation and to the high rotational capacity of the portion interested by the flange cutting. The basic drawbacks are represented by the local stiffness decrease, the reduced lateral strength and the risk of web buckling. The present paper is devoted to proposing an improvement of the dogbone features regarding its stiffness. Specifically, in the proposed approach suitably steel elements are added to the beam equipped with DB in such a way that the section reduction related to the dogbone does not cause any global reduction for the beam element stiffness. Some numerical examples are carried out for different flange reductions and the related stiffness interventions are evaluated by means of the solution to simple optimization problem. The goodness of the results is verified by analyzing the mechanical and the displacement response of frames equipped with both classical dogbones and improved beam.