From obstacles to exploits.
A complex structure designed in three dimensions
Due to its complex geometry and the multiple interfaces between the civil engineering and electrical and mechanical equipment, the Jacques Chaban-Delmas Bridge was conceived using a complete 3D digital model right from the design phase. This process helped to provide a volumetric vision of the final structure down to the finest of details. In particular, it helped detect the mechanical clashes with extreme precision, and provide the solutions to resolve them.
A remarkable construction process
To pre-empt the complications of manufacturing in the bed of the River Garonne, which is subject to strong tidal flows, the concrete structures (protective islands, bases, pylons, intermediate piers, etc.) were prefabricated, but merely 5 km away from the site, thus saving time and making possible the construction of the two main booms (2 x 300 m). The prefabricated elements were then towed and erected using different methods that took into consideration the mixed geological conditions.
The spans, meanwhile, each travelled 5,500 km by barge: a seabound journey lasting three weeks from the Cimolai factories (near Venice) to the Port de la Lune. It took several hours to place them on their bases, factoring in the tides and to an accuracy of several centimetres. Weighing 2,500 tonnes and measuring 117 m long and 45 m wide, the central lift span has the same surface area as a football pitch!
The rise of a giant
The lifting span is raised and lowered by a simple and proven lifting mechanism similar to that of a giant elevator. Each end of the mobile deck is suspended from each of the pylons by ten cables wound around a 4-metre diameter pulley wheel installed atop the pylons. These suspension cables are connected to the tops of counterweights fitted in each of the four pylons. The counterweights rise and fall inside the pylons with the help of winch cables attached to their underside, themselves entwined around motorised winches fixed to each base. The machinery therefore only needs to supply a lifting force of approximately 100 tonnes, corresponding to the weight difference between the lift span and the counterweights. As a result, the lifting operation only requires reduced, energy efficient motorisation.
The exceptional size of the lift span made the structure vulnerable to the effects of the wind, in particular when the deck is raised. From the earliest design drawings, the shape of the lift span was streamlined to reduce the aerodynamic thrusts on the bridge. Detailed studies were then conducted, drawing on local data and harnessing the results of tests carried out on a scale model (1/70 of the final size), subjected to a range of weather conditions in the Nantes laboratories of the Scientific and Technical Centre for Building (CSTB).