The paper is devoted to the study of constrained torsion in joints of flat thin-walled rod systems. The goal of the work is to analyze the stress state of finite stiffness joints made of two thin-walled open-profile rods with different parameters, which are under conditions of bending torsion (The object of research), and to determine the corresponding rotation coefficient. Method. The main research method in the paper is the finite element method. The software program ABAQUS is used in order to design and calculate 1-D and 3-D finite element models of various joints and structures made of thin-walled profiles. Results. The areas of application of previously known hypotheses were determined: the hypothesis of A.R. Tusnin; the hypothesis of A.S. Gorodetsky and others. By the design and calculations of 3-D finite element models, the behaviour of the bimoment transmission was investigated in the joints made of thin-walled profiles: with different stiffness; connected at different angles; taking into account structural elements (gusset plate, corners, bolted connections). The recommended values of the “coefficient of the cross-sectional axis rotation” in the joints made of thin-walled profiles connected at the angle of 90˚ were determined. It is shown the existence of an angle between the profiles of the Г-shaped connection, at which the rotation coefficient is equal to zero, which in turn demonstrates the absence of the bimoment transmission in the joint.