A New Approach to Longitudinal Connection for Adjacent Box Girder Bridges
Dutta, Animesh Kumar
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Deterioration of longitudinal joints between adjacent box girder bridges is a common problem in the US. Current longitudinal connections involve the use of grouted shear keys between beams and transverse post-tensioning of superstructure spaced out along the span. Precast/Prestressed Concrete Institute presents design charts that suggest transverse post-tensioning amount to maintain the integrity of the longitudinal joint. Despite the design aids, amount of transverse posttension significantly varies from state to state, compression stress at joints fluctuates across span and reflective cracks between box beams are frequently documented, indicating a need for improvement of longitudinal joint performance. Compressive stress across joints of adjacent box girders is a performance measure of the longitudinal connection. This thesis evaluated a new connection technique between adjacent boxes. The new connection involves incorporating additional longitudinal post-tensioning in exterior beams. This additional longitudinal post-tensioning results in deformation of the exterior beam in the horizontal plane of the superstructure, thus generating compressive stress at the interface of beams. The compressive stresses generated with the new connection were compared with those developed with conventional connection. Combining longitudinal post-tensioning and transverse post-tensioning and various sequences of post-tensioning were studied. The results indicate that for the new connection technique, the sequence of post-tensioning, friction properties of beam interfaces and number of transverse post-tensioning locations across span affect the magnitude of compressive stress at the joints of the girder. In case of superstructures where the new connection technique is applied after grouting the shear-keys, tensile stresses are generated at the end of the superstructure that can cause cracking of shear-keys, thus making this construction sequence undesirable. For longitudinal post-tensioning to transverse post-tensioning ratios larger than 5.5, the magnitude of joint compressive stress is larger for the new connection than the one for the conventional connection. Application of transverse post-tensioning at three locations along the span in addition to longitudinal post-tensing results in compressive stresses up to four times larger than the ones created with a conventional connection, suggesting a better performance of the new connection technique compared to conventional connection.