Description: This report gives details of an investigation on the ultimate strength of simply supported reinforced prestressed concrete box beams subjected to combined bending, shear and torsion. Sixteen specimens, 352 mm wide x 184 mm deep, were tested; two span lengths were used and two densities of orthogonal reinforcing mesh. Two additional tests were carried out on reinforced concrete specimens under pure torsion and subsidiary tests were completed to study behaviour under bending and shear. Basic information only is given on the behaviour of the beams tested under combined loading. Full details are given in a supplementary report. The work described in this report was undertaken as part of a CIRIA project on the design of elevated road structures.