The beginning of the summer saw the last major stage in the construction of the world’s highest bridge, the Millau viaduct, which spans France’s southern Tarn valley. After the casting of the seven concrete piles (a world height record of 245 metres for Pile P2) and the laying of the deck (2,460m long), five of the seven pylons had to be raised. The first two pylons, at the north and south ends of the viaduct, were installed at the beginning of the works in order to allow the deck to be laid.

Once shored up, they will ensure that the structure holds together. Each of these steel pylons takes the form of an inverted Y, 77 metres high and weighing 650 tonnes. They were prefabricated on site and then raised. ‘We preferred the prefabrication solution rather than assembly on site with a mobile crane, as it was quicker and reduced not only the risk of interruption because of strong winds but also the problems of safety,’ said Jean-Pierre Garner, works director at Eiffel Construction Metallique, the subcontractor responsible for the deck and pylons. Main construction contractor for the bridge is Eiffage, which cast the concrete piles.

Once the assembly and welding work was complete, the pylon is transported flat onto the viaduct deck by means of a special multi-axle lowered Kamag trailer weighing 200 tonnes. Once alongside the pile, the pylon is manoeuvred into a vertical position using Belgian lifting and transport company Sarens Group’s patented Sarens Tower. The team used lifting towers rather than mobile cranes because of the speed of erection, narrow confines of the deck, weather risks and safety requirements.

First, the pylon is laid flat between the two masts of the lifting tower. The central part of the pylon is then fixed to a crosspiece, the ends of which rest on the tower masts. This crosspiece is pushed up the masts by a rack of hydraulic jacks with 500mm stroke. The base of the pylon is supported by the six-axle Kamag trailer which approaches the tower as the pylon is gradually lifted upright. ‘The originality of our Sarens Tower system allows the lifting to be done without any of the vertical load being put onto the lifting tower, which instead is simply used to guide the pylon,’ explains Karl Sarens of the Sarens Group.

Sarens acknowledges that the Millau site is a more demanding location because of its narrowness – the width of the viaduct deck leaves a free space of just nine metres on each side of the pylon.

The time taken to lift a pylon is around 6-7 hours. All the operations involving the pylon, including loading, transportation, lifting and welding to the deck, and the lifting tower (taking down and setting up) mean that the rate of erection is just one pylon per week. The last of the five pylons was installed at the end of July 2004. The bridge is expected to open for vehicle traffic later this year.

Photo captions

Photo 174: Each pylon, 77 metres high and weighing 650 tonnes, is raised by a lifting tower using a system developed and patented by Sarens

Photo 154: The pylon is raised in steps of 500 mm using a hydraulic rack

Photo 151: The base of the pylon rests on a trailer during the raising operation