Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Major axis steel joint with additional plates subjected to torsion: Stiffness characterization Beatriz Gil ⁎ , Rufino Goñi, Eduardo Bayo University of Navarra. Department of Construction, Services and Structures, 31009 Pamplona, Spain ARTICLEINFO Keywords: Semi-rigid steel connections Finite element models Experimental research Torsion Warping 3D steel joint characterization Out-of-plane behavior ABSTRACT 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 cali- brated 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. 1. Introduction and objectives The research on semi-rigid steel joints has been mainly focussed on 2D joints with in-plane loads and followed the component method pro- posedinEurocode3Part1-8 [1].Newcombinationsorvariationsofthe springs that represent every component, and the way of assembling themhavebeenproposedbymanyauthors.Thereaderisreferredtothe recently published Ref. [2] for a description on the progress of the 2D component methods contained in the literature. However, the char- acterization of 3D steel joints is a complex field in which many issues still remain to be solved. Recent investigations have dealt with different experiments as well as providing numerical and analytical solutions for some of the components involved in this broad problem. The work presented in [3] was focused on the experimental behaviour of 3D end- plate joints with the aim of assessing the effect of the minor axis on the behaviour of the major axis, as well as the interaction of the two minor axis end-plate connections. In Ref. [4] a new 3D design for composite joints was proposed and tested. It was found that due to the presence of the concrete slab, the interactions between the loads in the beams did not affect the overall joint behaviour in a significant way. Refs. [5] and [6] describe the research of 3D joints with additional plates initially using experimentation and later characterizing the newly called E-Stub. 3D joints under seismic loading were dealt with in Ref. [7], where a new semi-rigid joint was proposed with the minor beam attached to the column web by an end-plate connection and the major beam attached to the column flange by a T-stub connection. The interaction between the loadings in the minor and the major axes was also considered. As mentioned before, most of reported studies are based on the component method, but this method is still lacking when it comes to defining the characteristics of 3D dimensional joints under out-of-plane loads. In addition, the above mentioned interactions among the many components that integrate the joint and the different loading conditions (such as torsion) that may take place pose a serious challenge for a direct application of the component method, especially for 3D cases. Torsion, warping and lateral buckling in beam elements are usually considered assuming simplified boundary conditions, such as pinned or rigid, for frame analyses. Current codes allow advanced methods to include more accurate boundary conditions; however, it is necessary to previously characterize them properly. Ref. [8] presents a summary of the different methods that have been developed to assess warping in beams with restrained end supports, and also includes a method based on bending analogy for the case of partially restrained supports. The research reported in this paper is aimed at expanding the scope of the component method and to define more accurate boundary condi- tions for 2D and 3D joints under torsion. This work is a continuation of the research reported in Refs. [9–13], which describe and analyse this https://doi.org/10.1016/j.engstruct.2020.111021 Received 14 February 2020; Received in revised form 18 May 2020; Accepted 24 June 2020 ⁎ Corresponding author. E-mail address: bgilr@unav.es (B. Gil). Engineering Structures 220 (2020) 111021 0141-0296/ © 2020 Elsevier Ltd. All rights reserved. T