Вантовый мост в Окситании: История проектирования и строительства

Cable stayed bridge in Occitanie, France

The 'high solution' required the construction of a viaduct. From 1991 to 1993, the structures division of Sétra, directed by Virlogeux, carried out preliminary studies, and examined the feasibility of a single structure spanning the valley. Taking into account technical, architectural, and financial issues, the Administration of Roads opened the question for competition among structural engineers and architects to widen the search for realistic designs. By July 1993, seventeen structural engineers and thirty eight architects applied as candidates for the preliminary studies. With the assistance of a multidisciplinary commission, the Administration of Roads selected eight structural engineers for a technical study, and seven architects for the architectural study.

Simultaneously, a school of international experts representing a wide spectrum of expertise (technical, architectural, and landscape), chaired by Jean François Coste, was established to clarify the choices that had to be made. In February 1995, on the basis of proposals of the architects and structural engineers, and with support of the school of experts, five general designs were identified. The competition was relaunched: five combinations of architects and structural engineers, drawn from the best candidates of the first phase, were formed; each was to conduct in depth studies of one of the general designs. On 15 July 1996, Bernard Pons, minister of Public Works, announced the decision of the jury, which was constituted of elected artists and experts, and chaired by Christian Leyrit, the director of highways. The solution of a multiple span viaduct cable stayed bridge, presented by the structural engineering group Sogelerg, Europe Etudes Gecti and Serf, and the architects Foster + Partners was declared the best. Detailed studies were carried out by the successful consortium, steered by the highways authority until mid 1998. After undergoing wind tunnel tests, the shape of the road deck was altered, and detailed corrections were made to the design of the pylons. When the details were eventually finalised, the whole design was approved in late 1998.

To allow for deformations of the metal road deck under traffic, a special surface of modified bitumen was installed by research teams from Appia (company) [fr]. The surface is somewhat flexible to adapt to deformations in the steel deck without cracking, but it must nevertheless have sufficient strength to withstand motorway conditions (fatigue, density, texture, adherence, anti rutting etc.). The 'ideal formula' was found after two years of research.

The electrical installations of the viaduct are large in proportion to the size of the bridge. There are of high current cables, of fibre optics, of low current cables, and 357 telephone sockets; allowing maintenance teams to communicate with each other and with the command post. These are situated on the deck, on the pylons, and on the masts. The pylons, road deck, masts, and cable stays are equipped with a multitude of sensors to enable structural health monitoring. These are designed to detect the slightest movement in the Viaduct, and measure its resistance to wear and tear over time. Anemometers, accelerometers, inclinometers, and temperature sensors are all used for the instrumentation network. Twelve fibre optic extensometers were installed in the base of pylon P2. Being the tallest of all, it is therefore under the most intense stress. These sensors detect movements on the order of a micrometre. Other extensometers, electrical this time, are distributed on top of P2 and P7. This apparatus is capable of taking up to 100 readings per second. In high winds, they continuously monitor the reactions of the Viaduct to extreme conditions. Accelerometers placed strategically on the road deck monitor the oscillations that can affect the metal structure. Displacements of the deck on the abutment level are measured to the nearest millimetre. The cable stays are also instrumented, and their ageing meticulously analysed. Additionally, two piezoelectric sensors gather traffic data: weight of vehicles, average speed, density of the flow of traffic, etc. This system can distinguish between fourteen different types of vehicle. The data is transmitted by an Ethernet network to a computer in the IT room at the management building situated near the toll plaza.

The toll plaza is on the A75 autoroute; the bridge toll booths and the buildings for the commercial and technical management teams are situated north of the viaduct. The toll plaza is protected by a canopy in the shape of a leaf, formed from tendrilled concrete, using the ceracem process. Consisting of 53 elements (voussoirs), the canopy is long and wide. It weighs around
The toll plaza can accommodate sixteen lanes of traffic, eight in each direction. At times of low traffic volume, the central booth is capable of servicing vehicles in both directions. A car park and viewing station, equipped with public toilets, is situated at each side of the toll plaza. The total cost was .