Extradosed Bridge establishes new Landmark of South Osaka

The city of Osaka is an important location for business in Japan. In order to extend the city’s transportation system, a new railway line between Shin-Osaka and Kyuhoji has been constructed. The Osaka Higashi Line was created by converting an existing freight line into a double-track passenger railway line.

At the same time, a viaduct was built in the Hirano district, south-east of the city of Osaka. This viaduct will facilitate future maintenance services on the Osaka Higashi Line and will be used to carry vehicular traffic across the railway. The new viaduct will replace two existing grade crossings that used to cause traffic obstructions in the district of Hirano on a regular basis. The new structure will also improve the infrastructure in all of Osaka’s eastern districts.

The Hirano Bridge has a main span length of 63m and was built as part of the viaduct in the Hirano district. Due to an intersection of two public roads in the immediate vicinity of the bridge, space for the construction site was limited. Taking into consideration various site constraints, the owner opted for an extradosed concrete bridge, the aesthetic design of which turns it into a regional landmark.

The pylon, in which extradosed tendons are deviated by means of a saddle system, deserves special attention. A system consisting of two tubes, one inside the other, and a shear nose ensures that differing forces in the extradosed tendons are transferred safely into the pylon so that the replaceable extradosed tendons cannot slide over the saddle, as in the case of ordinary external tendons.

For the design of the saddle system, detailed calculations were done, taking into account a tolerance of +/-1° for deformations of the structure and a sagging of stay cables in the vertical position. Thus, bending stress in the parallel strand bundles at the transition of the saddles can be minimized and wear due to friction can be prevented.

The extradosed tendons consist of double corrosion protected epoxy coated strands and polyethylene (PE) sheathing inside an HDPE duct. The stay cables were prefabricated in a factory and installed at the site using a crane.

In the anchorage and saddle region, the PE sheathing on the strands was removed. These tendons were grouted after tensioning to establish triple corrosion protection and provide sufficient bond in the saddle. In addition, all wedges of the individual strands were post-blocked using a hydraulic jack to ensure that the wedges grip each strand firmly, particularly in the case of strands with low initial tension.

For the anchorages at the main girder, the DYWIDAG Post-Tensioning System for external tendons was used. Since the orientation of each anchor pipe was different owing to the curved profile of the bridge, the exact orientations of the anchorages was established by using laser technology.