This study is concerned with the study of the plastic behaviour of overlapped K-joint of lattice girders, made up of elements of rectangular hollow section with very thin walls. An experimental study was carried out on three full-scale truss girders with the same sections and different geometry. According to studies, the main parameters which have defined the strength of overlapped K-joints are the ratio between the width of the diagonals and that of the chord, the ratio between the width of the chord and that of its thickness and the angle of the diagonal with the chord. The latter was the parameter taken in this work. A numerical investigation was combined in order to draw up a comparative assessment of the behaviour. This study will certainly make it possible to better apprehend one of the main parameters governing the behavior of the joints of beams of hollow rectangular sections and to define the analytical model appropriate to this type of beam. It appears that in this study the very thin-walled sections exhibit a different behaviour compared to the trusses with more or less thick or thin sections. Plasticization of the face and walls of the chord was the type of failure observed in this study.