Launching the Millau Viaduct's Bridge Deck Sections
The deck of what is going to be the world’s highest bridge is being launched over the Tarn valley, in the South of France, with hydraulic technology from Enerpac, a US multinational that specializes in Hydraulic System Integration for large-scale construction projects. The hydraulic system was designed and built by Enerpac's Construction Centre of Excellence in Madrid, Spain.
The technically advanced hydraulic system is designed to push the 27,35 m wide deck (with a capacity for six lanes plus hard shoulders) from both sides onto the seven concrete piers. During the launching process, the deck will be supported by seven temporary metal piers (pier T7 to T1). The first of these temporary piers was raised using cranes, but all other temporary piers will be raised using a hydraulic telescopic system also designed and built by Enerpac. Once put in place, the deck will be 245 metres high and 2460 metres long. The enormous yet at the same time “light” deck is pushed by means of hydraulic launching devices on each pier, which first lift and then push the deck. An adjustable nose structure at the end of the deck, allows the deck to land on each pier as it approaches it.
Basically, each system consists of a lifting cylinder, with a capacity of 250 ton, lifting the deck off the supporting structure of the pier, and two or four skates, each equipped with two 60 tons cylinders, which retract to launch the deck a maximum of 600 mm. All of this rests on a system of single-acting lock nut cylinders supporting both the launching device and the deck.
The launching process was started on the western slope (C8) with two launching devices, each with two 120 ton cylinders. In total, in the last phase of the launch, there will be 5280 tons pushing capacity from the southern slope (1752m of deck) and 2400 tons from the northern one (708m of deck, making up a total length of 2460m). Each push-cycle moves the deck 600 mm and takes 4 minutes, which means that there will be 3280 pushes from the west and 1540 from the east.
As you can see, each launching system rests on a system of cylinders which allows the load of the skates to be balanced right and left on each pier, to compensate for the rotation of the deck during the launch phase, and correct or modify the height of the skate and thus of the deck where necessary.
Manufactured by Enerpac, these cylinders are of different capacities according to the needs of the deck. The most demanding system having an absolute capacity of 14.400 tons, supporting four skates and 24 support cylinders of 600 tons each, with a stroke of 500 mm. Other piers use cylinders of 280 tons with a stroke of 300 mm. Valves are used to make the different groups of cylinders independent of each other and so control the height and angle of the deck, each system having an independent control centre for the skates and the jacking cylinders.
The nose of the deck
The weight of the deck means that, as it is pushed along and gets further from its support, it curves downwards, so that it approaches the next pier below the proper level. In order to compensate for this deviation, a nose recovery system is constructed at the end of the deck. This independent system, consisting of a hydraulic group of four 270 tons cylinders, pulls the nose upwards to the level of the skate. Another hydraulic system allows the nose-end to pivot.
Illustration of the nose recovery process.
All hydraulic systems for pushing the deck are operated from the Control Centre on the bridgehead. This control centre receives data via a PROFI-BUS cable, where it is automatically handled so that the system can follow the parameters established when programming the cycle. Although all hydraulic systems installed on each pier are controlled from this centre, each single hydraulic system has a local control panel, which allows local movement of the skates to be made from that pier independently, as long as this is allowed by the Control Centre, which in turn must receive the approval of each local control centre in order to make synchronised pushing movements from all the pushing cylinders of all the piers. The outer cylinders on each pier have a positional transducer that indicates the amount of travel, and each hydraulic system has its independent hydraulic control centre. Movement of the deck can be made in three modes: manual, semi-automatic and automatic. The manual mode is used for adjusting the system and, if necessary, to make instant corrections. In semi-automatic mode, each movement is made step by step: raise, push, lower, withdraw cylinders. Automatic mode completes the entire cycle.
Hydraulic system integration, of high force hydraulics and advanced control technology, nowadays play and important role in the controlled movement of large-scale civil engineering projects, like the launching of the Millau Viaduct.
(Deck launch was accomplished on May 28, 2004)