side of the web the flanges which are of similar construction are generally rectangular in form and are open to the opposite side of the web preferably the structural member is roll formed from a single sheet of material that possesses relatively high strength but which is resilient enough to permit the member to deform sufficiently so that it can be interlocked or snapped together with similar members in a manner which shall be explained in greater detail below each rectangular flange includes a bottom wall or base 15 that is positioned in parallel alignment with the web the base is connected to the longitudinal edge of the web by means of an inner side wall 17 that is normal to the plane of the web a second outer side wall 18 depends upwardly from the outer edge of the base the outer side wall is positioned in parallel alignment with the inner side wall and teminates in the plane of the web a lip 20 depends inwardly from the distal end of the outer wall and is located in coplanar relationship with the web in practice the lip extends inwardly into the opening 23 of the flange to restrict the size of the opening as best seen in reference to fig2.