Bridges

Australian bridge design code AS 5100.5 in Section 8.6.2 prescribes simplified criteria to meet the requirements for crack control applicable to a partially prestressed concrete beam. The incremental stress permitted in bonded reinforcement after decompression is 200 MPa. As this requirement dominates critical design dimensions, Roads and Maritime Services of New South Wales lowered this limit to 160 MPa in individual projects and further issued instructions to restrict maximum span lengths for its standard Super-T girder bridges. In the new draft code this limit is proposed to increase to 330 MPa. What actually controls the critical design parameter needs to be carefully reviewed. There are other factors not explicitly covered by the simplified criteria in the Australian Standards. This paper discusses the influence of Clause 8.6.2(a) of the concrete bridge design code on the performance of partially prestressed composite concrete beams, particularly the Super-T girder. The issues discussed intend to draw attention to the application of this clause by further exploring the allowable stresses. A proposal for the rewrite of this clause is put forward by introducing a progressive limit on maximum tensile stress in concrete together with the tensile stress in steel.