Abstract: A composite floor structure comprising a floor slab of concrete or other compression resistant material; and at least one reinforced beam under the floor slab. The beam comprises permanent shuttering including an open-mouthed, upwardly facing channel formation of steel or other tension resistant material; and a plurality of transverse members spaced apart along the length of the channel formation and extending transversely to the channel formation across substantially the entire width of the mouth of the channel formation. Each transverse member is fast with the channel formation in the region of its mouth on opposite sides of the mouth and at least part of at least certain of the transverse members extend upwardly from the region of the mouth of the channel formation into the overlying floor slab. Concrete or other compression resistant material is located in the channel formation and is formed integrally with the compression resistant material of the floor slab.