A reinforced concrete slab comprising: a concrete floor having a length, a width and a thickness and a plurality of parallel integral support members or beams running across said floor in a first direction and extending downwardly therefrom with a thickness greater than the thickness of said floor and wherein each of said support members has the structure of a reinforced load bearing structure, comprising: an outer longitudinally extending concrete load bearing structural member and an inner longitudinally extending hollow element (23, 33) fully encased by concrete within said outer longitudinally extending load bearing structural member; a plurality of reinforcing rods extending through said outer longitudinally extending concrete load bearing structural members; a plurality of stirrups (28, 38) spaced longitudinally along said inner longitudinally extending hollow element (23, 33), and around said element, in contact with said reinforcing rods and encased by said outer longitudinally extending load bearing structural member, and wherein said plurality of reinforcing rods and said stirrups (28, 38) are joined together, preferably welded together, at their points of contact, wherein said plurality of reinforcing rods and said plurality of stirrups (28, 38) are fully encased in said concrete of said concrete load bearing structural member; wherein said reinforcing rods and said stirrups (28, 38) are made of metal, in particular made of steel; wherein said concrete load bearing structural members extend along a linear axis and said longitudinal direction lies in a horizontal plane when installed and said stirrups (28, 38) extend helically around said reinforcing rods. a concrete floor having a length, a width and a thickness and a plurality of parallel integral support members or beams running across said floor in a first direction and extending downwardly therefrom with a thickness greater than the thickness of said floor and wherein each of said support members has the structure of a reinforced load bearing structure, comprising: an outer longitudinally extending concrete load bearing structural member and an inner longitudinally extending hollow element (23, 33) fully encased by concrete within said outer longitudinally extending load bearing structural member; a plurality of reinforcing rods extending through said outer longitudinally extending concrete load bearing structural members; a plurality of stirrups (28, 38) spaced longitudinally along said inner longitudinally extending hollow element (23, 33), and around said element, in contact with said reinforcing rods and encased by said outer longitudinally extending load bearing structural member, and wherein said plurality of reinforcing rods and said stirrups (28, 38) are joined together, preferably welded together, at their points of contact, wherein said plurality of reinforcing rods and said plurality of stirrups (28, 38) are fully encased in said concrete of said concrete load bearing structural member; wherein said reinforcing rods and said stirrups (28, 38) are made of metal, in particular made of steel; wherein said concrete load bearing structural members extend along a linear axis and said longitudinal direction lies in a horizontal plane when installed and said stirrups (28, 38) extend helically around said reinforcing rods. an outer longitudinally extending concrete load bearing structural member and an inner longitudinally extending hollow element (23, 33) fully encased by concrete within said outer longitudinally extending load bearing structural member; a plurality of reinforcing rods extending through said outer longitudinally extending concrete load bearing structural members; a plurality of stirrups (28, 38) spaced longitudinally along said inner longitudinally extending hollow element (23, 33), and around said element, in contact with said reinforcing rods and encased by said outer longitudinally extending load bearing structural member, and wherein said plurality of reinforcing rods and said stirrups (28, 38) are joined together, preferably welded together, at their points of contact, wherein said plurality of reinforcing rods and said plurality of stirrups (28, 38) are fully encased in said concrete of said concrete load bearing structural member; wherein said reinforcing rods and said stirrups (28, 38) are made of metal, in particular made of steel; wherein said concrete load bearing structural members extend along a linear axis and said longitudinal direction lies in a horizontal plane when installed and said stirrups (28, 38) extend helically around said reinforcing rods.