Successful Conversion of a Suspension Bridge into a Cable-Stayed Bridge
Over 150 after its erection, the suspension bridge across the Durance river near Volonne, France has been converted into a cable-stayed bridge. The original structure’s last service was to serve as falsework for the central steel box girder of the new bridge.
The bridge was first opened to traffic in 1846, has a varied history. The original bridge was demolished in the mid-1920s and replaced by a more stable structure that went into service in 1928. The bridge was destroyed in the confusion of the Second World War in 1944, then reconstructed in the following years and reopened in 1949.
More than 150 years after its first construction, the bridge shows a completely different face. The two new, 28m high pylons were constructed directly on the existing bridge piers, with the new pylon crossbeams enclosing the old bridge piers. For this purpose, the tendons to absorb the transversal expansion forces of the new pylons were inserted into the old piers by core drilling through the piers. After completion of the bridge, the old piers projecting beyond the crossbeams were removed flush with the new transverse beams.
To construct the new bridge deck with a main span of 102 m, the steel box girders were welded on top of the old deck. A special challenge of this project was that the DYNA Grip Stay Cables could initially only be prestressed with a maximum force of about 650 kg per strand due to the low weight of the steel box girders. It was only by the subsequent welding of the outer transverse girders and concreting of the roadway slab that the weight of the new bridge deck was increased to the point where the prestressing force in the cables could be gradually increased to their final values. To ensure equal forces between the individual strands, the ConTen method was used to apply the last stressing level.
After completion of the new bridge in autumn 2006, the old bridge deck was disassembled into individual parts and lowered from the new bridge in segments to pontoons.