A hollow structural member constituted of a plurality of wall sections cooperatively laterally enclosing an interior space, wherein adjacent wall sections have straight longitudinal edges aligned and facing each other to define a joint between the sections, with longitudinal recesses respectively formed on the inner surfaces of the wall sections adjacent the joint and opening toward each other across the joint, and wherein the sections are interlocked at the joint by means of an elongated spline inserted in the recesses for bridging the joint and having an outwardly projecting central longitudinal rib for engaging the joint-defining edges of the wall sections to limit the extent of lateral insertion of the spline into either of the recesses. A corner spline is also provided for interlocking wall sections at a joint at which the adjacent portions of the respective sections are substantially perpendicular to each other such that the included angle between their respective recess-bearing inner surfaces is about 270.degree..