Major axis steel joint with additional plates subjected to torsion: Stiffness characterization

•Major-axis end plate joints with additional plates are tested under torsion.•A parametric study is performed to assess the stiffness by means of FEM.•This research verifies that these joints behave as semi-rigid and full strength.•They improve the behaviour of those without additional plates.•A mechanical model and analytical expressions are proposed and checked. This investigation deals with the characterization of the torsional behaviour of major axis steel joints with doubled extended end-plate and additional plates welded to the column flanges at the height of the beam flanges. One aim of this research is to provide the real joint torsional boundary conditions under which the beam elements behave for frame analysis and stability checks, thus avoiding the need for simplifications such as pinned or rigid joint conditions. Another objective is to characterize the stiffness of 3D joints, which requires knowledge of each of the individual components, such as the ones studied in this investigation. With this purpose two experimental tests were done, followed by a parametric study performed with calibrated finite element models that led to the analytical expressions of the components that characterize the torsional stiffness of this kind of joints. After a detailed analysis, these joints were found to behave as semi-rigid and full-strength. The semi-rigid behaviour is due to the torsional warping effects transmitted by the beam. The results are also compared with those of the same joints but without additional plates (described in a recent publication) and it is shown that the additional plates increase the joint stiffness significantly.