Initial rotational stiffness of tubular joints with axial force in chord

In the frame analysis, the local model of the joint must follow the behavior of the joint. When completing the elastic global analysis, the initial rotational stiffness of joints should be known to obtain reliable moment distributions in frames. This paper consists of two parts. The first one evaluates the existing calculation approach for the initial rotational stiffness of welded rectangular hollow section T joints. The validation with the experimental data shows that the current approach significantly underestimates the initial rotational stiffness. An improvement for determining the initial stiffness of T joints is proposed. The second part deals with the influence of the axial force in the main member on the rotational stiffness of the joint. The conducted numerical study shows the extreme reduction of the initial stiffness, when the main member is loaded by axial loads. To consider this effect in the frame analysis, the paper proposes a chord stress function for the initial rotational stiffness for square hollow section T joints, using the curve fitting technique.