Located in the valleys of Millau, France the Millau Viaduct is the tallest bridge in the world with its highest tower being 343m tall, higher than an 80 storey building and often described as a series of Eiffel Towers with a four lane highway slung between them. Construction began in October 2001 and the engineers hope that it would last more than 120 years. However the area was prone to land slides, strong winds and gusts at 130 km/h and massive storms. But then why are they building such a massive scaled structure there in the first place you might ask.
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The reason for this would be that in the 1980s France built a freeway that directly linked Spain and Paris. The freeway headed South across the French countryside while it headed North from the Mediterranean until they stopped dead at one of the deepest valleys of France. The town of Millau is situated in the mountainous regions and is one of the tranquil parts of France. The people there however suffer from poor traffic. The government was late in building the bridge however. 12 years before construction the ministry of transport came up with a plan.
Head Bridge builder Michel Virlogeux put up his idea for the bridge. Concerned authority thought he was crazy. Millau was to have 1 set of cables to hold the road deck. The single set of cables was to withstand a heavy force supporting the 2. 5 km long bridge. French authorities were nervous about the groundbreaking scheme of this multiple span single lined cable stay bridge of such a great scale and so held a competition in which they invited other engineers and architects to design the structure. The winner was Lord Norman Foster, a superstar architect who designed the world’s largest airport terminal.
According to Foster the challenge was making the object fit into the landscape, he wanted to build something strong enough to withstand the forces of nature yet something that seemed fragile. He removed two of the piers from the total 9 piers in the original design and slimmed down the remaining ones as well as the road deck, Pier no. 2 being the highest, 1 likely to pose problems for steep slopes. He wanted to go for a ‘delicate as a butterfly’ feeling yet builds something that would support the weight of five Eiffel towers in gale force winds hundreds of meters above the river.
Construction began in December of 2001. Everything about the project was super sized. 200,000 tons of concrete was required and the factory was built on site to supply such a huge amount. And the formula of the concrete has to be just right, has to have the strength to bear the massive loads, it cannot set too quickly since it has to be hoisted up to its position and poured a 100m above the air and it has to be the right colour to meet Foster’s vision. Foster gave the piers very complicated geometric shapes tapered upwards with vertical grooves to create shadows.
To be build something like this was a major challenge. So the piers were built step by step by pouring concrete into a mould. To give the required strength the mould is framed with steel reinforcing bars and the shape of the mould is altered to to fit the profile of the next 4m section. As a result the mould had to be changed around 250 times. This cycle was repeated by each team in each pier evry 3 days. Each team in each pier set a target height for each pier. They used GPS and multiple satellites to pinpoint their positions to reach each target height; they were one month ahead of schedule.
Phase 2 of the process included putting a 2. 5km road deck weighing 36000 tons on top of the world’s tallest piers. This put the lives of the workers in danger as safety was a major concern. It was then decided that they would fabricate the road deck on solid ground. They had planned to put it on steel instead of lifting it. The challenge was taken up by Mark Buomono of Eiffel. Massive sections that would make up the road deck were fabricated in the steel factories. 2200 separate sections weighing 90 tons and some 22m long were built.
They measured their accuracies with lasers and the process was automated by 2 headed welding robots and plasma cutting machine. However cutting and welding were the easy parts. What was difficult was getting these sections all the way from the factory to Millau. Police coordinated the safety and transport and around 2000 convoys were used. The massive sections were welded together in the valleys to build the two halves of the deck, the two halves were then to be slid from either plateaus and eventually meet above the river.
Hydraulic systems were used to push them. But the deck being slender might get pushed down near the centre. The solution to this was fitting a pylon whose cables would support the deck as it goes out over the valley and constructing temporary steel support towers halfway between adjacent piers. But friction had also reared up as a threat. The piers just might get knocked down as bowling pins. Launching systems that jack up the deck and pushes it forward are used. There are two wedge-shaped blocks.
The upper wedge is pulled forward by a hydraulic flab, slides up the slope of the lower wedge at the same time lifting the deck from its supports and advancing it. The lower wedge retracts dropping the deck onto its supports. Upper wedge then returns to its original position and the cycle repeats. Four of these are placed in each pier and they all work simultaneously. The deck moves forward by 600mm every 4 minutes. The road deck has the shape of an upside down airwing which might just take off.
The weather conditions are studied and only during 3 day weather window and when wind speed is less 85km/h is work conducted. 6 months prior to completion the Teflon in the launcher is ripped during a storm. It is then replaced and the launcher reassembled and the project continued. The two decks are finally joined. Things such as cutting welding are maintained so that accuracy of the union and calculations 99. 9999%. And that is what lead to the completion of the construction of the world’s tallest bridge.
