Abstract: A building structural element (1) having a pair of beams (5)joined by a plate member (13) or alternatively a generally U-shaped channel having a pair of opposed side walls (33, 34) joined by a further portion (35). In either case an interior space (57) is defined by either the pair of beams (5) and plate member (13) or by the pair of side walls (33, 34) and the further portion (35). A cementitious material, such as concrete, occupies a substantial volume of the interior space (57) and the building structural element (1) has a post-tensioned pre-stressing force applied thereto.