In steel-concrete composite beams, the lower flange is of prestressed concrete, and enhanced loadbearing capacity compared with conventional beams is obtained by the use of head-shear-connectors attached to the upper flange. The lower steel flange is tensioned up to 95% of its elastic limit while the concrete is poured and set. Deflection under load may be half that of traditional beams. In construction, floor slabs may be placed on the upper flange of the beam. The beams are most suited for bridging wide spans with limited construction heights, and where minimum beam deflection is required. Alternative designs have been developed, in which the beam is combined with a reinforced concrete floor slab, bonded using steel or reinforced concrete dowels. 9 refs.