Behaviour of double prestress system in prestressed concrete girders

Prestress generally refers to the compressive prestress that is introduced by a compressive force beforehand to prevent tensile stress on the lower fibre of the bending member, and is often used in concrete girders. In girders with a smaller height compared to span length, compressive stress, which has a harmful impact on structural behaviour, is applied excessively to the girder's upper edge, limiting the reduction in girder height of the prestressed concrete girder. In this study, a prestress system integrating a tendon and two bars to support the compressive force in the girder's side ends was devised to offset the compressive force generated in the girder as a result of the introduction of prestress. To manage the jacking force of this system, formulae are proposed by theoretically clarifying the behaviour manifested during jacking in relation to the friction coefficient of the tendon and bars. A 10-m long rectangle-shaped girder is fabricated to check the friction coefficient of the bar and to verify the moment prestress method using the proposed double prestress system. Applicability to real structures is validated by measuring the behaviour manifested during the jacking of 35-m long real bridge girders.