Abstract: A composite structural element has a main steel beam having a web and at least two flanges extending therefrom, having oppositely directed outer faces, having outer edges generally defining a plane and defining with the web a recess open away from the web between the outer edges. A mass of concrete fills the recess substantially to the plane, the outer flange faces being exposed and substantially free of concrete. Another profiled steel beam is fixed to the web of the main beam and wholly imbedded in and covered by the concrete mass. Typically the main beam is an H- or I-beam and has two such channels provided with such other beams and filled with respective such masses. The other beams can be of T-section or each other beam can be an I- or H-beam. In addition it is possible to provide longitudinally extending steel reinforcing bars imbedded in the concrete mass. Steel fibers can also be imbedded as reinforcement in the concrete mass, and this mass can be at least partially of colloid concrete.