the disclosure concerns a multipurpose structural unit for fastening parts of buildings , or the like , together . the unit comprises a longitudinally elongaged bar , which may be either solid or hollow and which may be comprised of various different materials . the bar has two opposite transverse sides and two opposite end sides which joint the transverse sides . at least one longitudinal groove is defined in each transverse side of the structural unit , and a longitudinal groove may also be defined in each end side of the structural unit . the surfaces between the edge of the groove in the transverse side and the end side of the structural unit slant obliquely inwardly toward the center of the structural unit , whereby the structural unit has a generally octagon shaped exterior profile or cross - section . the cross - section shapes of the longitudinal grooves are disclosed . various possible inserts and configurations for using the structural unit in combination with other structural elements and parts are disclosed , principally through insertion of the margins of the various structural elements and parts into the grooves in the structural unit .