channel section is then positioned so that its recess 24 fits over the exposed side of the spline and is pressed toward the center of the spline in the direction of arrow 64 fig6 so as to advance the shoulder 40 of the latter recess over the free edge 60 of the leg 58 on that side of the spline the force thus exerted tends to deform the side of the spline already fully inserted in the first mentioned recess 24 but such deformation is limited by abutment of the rib 62 on the first inserted spline side with the adjacent recess shoulder 40 fig7 enabling completion of the assembly operation if in place of the spline 28 a spline h lacking ribs 62 but otherwise identical to spline 28 were used as shown in fig8 the deforming effect of the later assembly steps on the first inserted spline side would cause it to collapse and might consequently dislodge it from the recess in which it had previously been inserted preferably as indicated in fig6 and 7 the tip of each rib 62 is formed with a short transverse flangelike projection 65 extending along its length to provide an enlarged bearing surface for the shoulder 40 during assembly and to assure proper register between the rib and shoulder the projection is perpendicular to the rib and oriented toward the center of the spline as shown in fig3 the two channel sections 20 of the member 18.