Engineering places: Millau Viaduct
Image credit: Dreamstime
In celebration of IET@150 we look at feats of engineering from around the world. Here we explore the tallest bridge in Europe.
Spanning a gorge valley near a small town in the south of France stands an incredible bridge. Called the Millau Viaduct, it is the tallest bridge structure in Europe. Eight consecutive cable-stayed spans total 2,460m and its pier and mast are a lofty 343m above ground – around 19m taller than the Eiffel Tower. With a roadway 277m above France’s Tarn river, it was the highest cable-stayed bridge in the world until 2012, when Mexico’s Baluarte Bridge opened.
Plans to build a bridge to span the Tarn Valley were first discussed as early as 1987, as eager holidaymakers from Paris descending into the valley to get to Spain often caused heavy congestion and much disruption for locals in the town of Millau. In 1991, the French government decided to build the viaduct to relieve pressure on local roads.
Between 1993 and 2001, the government held consultations with architects and engineers, and held competitions until they found the best design. An engineering and architectural group led by French engineer Michel Virlogeux and British architect Sir Norman Foster won the bid to design and build the structure.
A committee of experts, politicians, and specialists officially approved the group’s eight-span cable-stayed scheme in April 1996. A final set of plans would take two more years of design and testing.
Construction began in October 2001. The bridge took around three years to build and came with some hefty challenges, the most significant being the unusual launching of the roadway. Instead of cantilevering the road deck outward in small sections from each of the seven pylons, they constructed the deck on flat land on the north and south sides of the bridge, in two large sections. Builders synchronised hydraulic jacks on the tops of the piers to move the entire deck out over the valley in small increments of about 600mm. However, to make this method work, they built massive, temporary intermediate piers to prevent bending of the thin deck as they would not assemble the cable stays and masts above the roadway until the entire deck was in its ultimate position. As a result, both sides eventually met over the Tarn river.
At the peak of construction, 600 master craftworkers were hired to make the viaduct a reality, and it cost around €300m (£260m) to build. Construction company Eiffage financed the bridge’s construction in return for being granted the right to charge tolls for the next 75 years.
Construction workers built the viaduct with around 127,000 cubic metres of concrete, 19,000 tonnes of steel for reinforcing the concrete, and a further 5,000 tonnes of pre-stressed steel for cables and shrouds. The viaduct’s builders have claimed the bridge should last for at least 120 years. The seven concrete piers support the viaduct’s superstructure of 16 steel roadway sections. It also has six carriageways with hard shoulders, all held in place with 154 steel cable stays.
The bridge also features other technological advances that aren’t obvious to the eye. These include an array of movement and motion sensors including anemometers, accelerometers, inclinometers, and extensometers. Data collected from these devices helps engineers pinpoint potential trouble spots and wear-and-tear that could shorten the lifespan of bridge components.
While most of the visual impact of the viaduct comes from the seven cable-stayed ‘sails’ that appear to ‘float’ across the valley, it also features tall, slender piers that spectators say have an ‘unusual elegance’ – a narrow opening that gradually splits the column into two before closing up again within the mast above the road deck. The roadway also has a slight curve, adding a third dimension of shape to the structure and offsetting the repetition.
The viaduct has certainly relieved a severe congestion zone that slowed down thousands of tourists driving from northern France to the Mediterranean coast. Before the bridge was built, motorists would take up to four hours to cross the valley when traffic was heavy. Now they can cross in minutes.
The bridge has also become a tourist attraction – the designers modified the approaching motorway so drivers from the north catch an impressive vision of the viaduct and the valley before they arrive.
Many consider the mast summit, which spans across the Massif Central mountains, as one of the world’s greatest engineering and architectural marvels, providing travellers with a spectacular sight in typical French fashion.
Timeline: Millau Viaduct
1991: French structural engineer Michel Virlogeux begins bridge design.
April 1996: A committee of experts, politicians and specialists officially approve the eight-span cable-stayed scheme designed by Virlogeux and British architect Sir Norman Foster.
16 October 2001: Construction of bridge begins.
14 December 2001: The first stone is laid.
January 2002: Laying of pier foundations.
March 2002: Work begins on the pier support C8.
June 2002: Support C8 is completed, and work on the piers starts.
July 2002: Work starts on the foundations of temporary height-adjustable roadway supports.
August 2002: Start of work on pier support C0.
September 2002: Assembly of roadway begins.
November 2002: First piers complete.
25-26 February 2003: First pieces of roadway are laid.
November 2003: The last piers are completed (piers P2 at 245m and P3 at 221m are the highest piers in the world).
28 May 2004: The pieces of roadway are several centimetres apart, their juncture to be accomplished within two weeks. In the second half of 2004, installation of the pylons and shrouds occurs, and the temporary roadway supports are removed.
14 December 2004: Official inauguration.
16 December 2004: The viaduct opens ahead of schedule.
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