Influence Analysis on Prestressed Reinforcement Tensioning Modes of Cast-in-place Box Beam

In order to study the influence of prestressed reinforcement tensioning modes on deformation,stress and effective prestress of bridge beam,and to ensure the safety during construction process and the reasonable structure stress at completion status,according to the construction of the first span(3×20 m) of the ramp bridge located in Dongguan section of Conghua-Dongguan expressway project,which is a cast-in-place concrete continuous box beam,using the finite element analysis software MIDAS/CIVIL,considering plane curve,longitudinal slope of the bridge,shrinkage creep and strength development of concrete,and the 5 kinds of prestress loss of prestressed reinforcement,keeping the prestressed curve and the controlling stress,3 tensioning modes,including single end tensioning span by span(scheme 1),single end tensioning on single span at first and then 2 spans simultaneously(scheme 2),and 2-end tensioning on 3 spans(scheme 3),are simulated.The stresses of the beam during construction,the stresses and deformations of the beam,and the effective prestresses of the prestressed reinforcement in completion stage are analysed and compared.The results show that(1) the concrete tensile stresses at top surface of box beam at a distance of 46.71 m from the beam end nearby abutment are both greater than the design value in the construction stage of scheme 1 and scheme 2;(2) in completion stage,the beam deformation of scheme 1 is the maximum,the beam deformation of scheme 3 is the least,while there is the maximum tensile stress whose value is 0.17 MPa at top surface of box beam at a distance of 31.71 m from the beam end nearby abutment;(3) tensioning mode has the direct influence on the value and layout of the effective prestress,compared with scheme 1,the effective prestresses near the middle section of the second span are smaller,which should be paid attention to in design checking calculation.