A hinge joint assembly for flexibly connecting rigid structural components in the construction of modular buildings. More specifically, the joint permits the fastening of struts (rods, tubes, or the like) and/or panel studs with three possible degrees of rotational freedom for each strut or stud connected. Each joint assembly includes a plurality of interacting fastening units having a connecting member to which a longitudinally extending strut or stud is rotatably connected about its longitudinal axis, the connecting member in turn being rotatable about either of two axes which are perpendicular to each other and to the longitudinal axis of the strut or stud. Several forms of joint assembly are shown and described.