The Millau Viaduct

The World's Biggest Bridge

The Millau Viaduct (French: le Viaduc de Millau) is a cable stayed road bridge that spans the valley of the River Tarn near Millau in southern France. It was formally opened on 14 December 2004 and opened to traffic on 16 December 2004. Designed by British master-architect Lord Foster in collaboration with French bridge engineer Michel Virlogeux, it is the tallest vehicular bridge in the world, with one piers summit at 1,118 ft (341 metres), slightly higher than the Eiffel Tower and only 132 ft (40 m) shorter than the Empire State Building.
The Millau Viaduct consists of an eight-span steel roadway supported by seven concrete piers. The roadway weighs 36,000 metric tons and is 2,460 m (8,071 ft) long, measuring 32 m (105 ft) wide by 4.2 m (13.8 ft) deep. The six central spans each measure 342 m (1,122 ft) with the two outer spans measuring 204 m (670 ft). The roadway has a slight slope of 3% descending from south to north, and curves in plan section on a 20 km (12.4 mile) radius to give drivers better visibility. It carries two lanes of traffic in each direction.


This Cable-Stayed Bridge Project is on schedule to be completed January 2005. (39 month construction contract). This project is being financed privately. This project has the highest bridge piers in world. Thus the title of the Highest Bridge in the World. The tallest will be 240 meters high. Overall height an outstanding 336.4 meters. This project will consist of seven separate cable-stays.

Bridges are often considered to belong to the engineer's realm rather than the architect's. But the architecture of infrastructure has a powerful impact on the environment. The Millau Viaduct, designed in collaboration with engineers, illustrates how the architect can play an integral role in bridge design.

Located in southern France, the bridge will connect the motorway from Paris to Barcelona at the point where it is interrupted by the River Tarn, which runs through a wide gorge between two plateaus. A reading of the topography suggested two possible approaches: to cross the river, the geological generator of the landscape; or there was the challenge of spanning the 2.5 kilometers from one plateau to the other in the most economical manner.

The structural solution follows from the latter philosophical standpoint. The bridge has the optimum span between cable-stayed columns. It is delicate, transparent, and uses the minimum material, which makes it less costly to construct. Each of its sections spans 350 meters and its columns range in height from 75 meters to 235 meters - higher than the Eiffel Tower - with the masts rising a further 90 meters above the road deck. To accommodate the expansion and contraction of the concrete deck, each column splits into two thinner, more flexible columns below the roadway, forming an A-frame above deck level. This structure creates a dramatic silhouette - and crucially it makes the minimum intervention in the landscape.


Electrical installations
The electrical installations of the viaduct are impressive, in proportion to the immensity of the bridge. There are 30 km (19 mi) of high-current cables, 20 km (12 mi) of fibre optics, 10 km (6.2 mi) of low-current cables and 357 telephone sockets allowing maintenance teams to communicate with each other and with the command post. These are situated on the deck, on the pylons and on the masts.
As far as instrumentation is concerned, the viaduct is state of the art. The pylons, deck, masts and stays are equipped with a multitude of sensors. These are designed to detect the slightest movement in the viaduct and measure its resistance to wear-and-tear over time. Anemometers, accelerometers, inclinometers, temperature sensors are all used for the instrumentation network.
Twelve fibre optic extensometers were installed in the base of pylon P2. Being the tallest of all, it is therefore under the most intense stress. These sensors detect movements on the order of a micrometre. Other extensometers — electrical this time — are distributed on top of P2 and P7. This apparatus is capable of taking up to 100 readings per second. In high winds, they continuously monitor the reactions of the viaduct to extreme conditions.

Accelerometers placed strategically on the deck monitor the oscillations that can affect the metal structure. Displacements of the deck on the abutment level are measured to the nearest millimetre. The stays are also instrumented, and their ageing meticulously analysed. Additionally, two piezoelectric sensors gather traffic data: weight of vehicles, average speed, density of the flow of traffic, etc. This system can distinguish between fourteen different types of vehicle.
The data is transmitted by an Ethernet network to a computer in the IT room at the management building situated near the toll plaza.

Description

The Millau Viaduct consists of an eight-span steel roadway supported by seven concrete pylons. The roadway weighs 36,000 tonnes and is 2,460 m (8,100 ft) long, measuring 32 m (100 ft) wide by 4.2 m (14 ft) deep, making it the world's longest cable-stayed deck. The six central spans each measure 342 m (1,120 ft) with the two outer spans measuring 204 m (670 ft). The roadway has a slope of 3% descending from south to north, and curves in a plane section with a 20 km (12 mi) radius to give drivers better visibility. The pylons range in height from 77 m (250 ft) to 246 m (810 ft), and taper in their longitudinal section from 24.5 m (80 ft) at the base to 11 m (36 ft) at the deck. Each pylon is composed of 16 framework sections, each weighing 2,230 tons. These sections were assembled on site from pieces of 60 tons, 4 m (13 ft) wide and 17 m (56 ft) long, made in factories in Lauterbourg and Fos-sur-Mer by Eiffage. The pylons each support 97 m (320 ft) tall masts.
The pylons were assembled first, together with some intermediate temporary pylons which were in themselves a substantial construction project.
The entire length of deck surface (that is to say, the bridge itself, the roadway) was slid out across the pylons from both sides by hydraulic rams that moved the deck 600 mm every 4 minutes. Then the masts were erected on top of the pylons, connected to the deck and the temporary pylons removed.
Construction began on 10 October 2001 and was intended to take three years, but weather conditions put work on the bridge behind schedule. A revised schedule aimed for the bridge to be opened in January 2005. The viaduct was inaugurated by President Chirac on 14 December 2004 to open for traffic on 16 December, several weeks ahead of the revised schedule.
The construction of the bridge was depicted in an episode of the National Geographic Channel MegaStructures series, as well as Discovery Channel's Extreme Engineering.


Toll plaza

Toll StationAn 18-lane toll station 6km north of the Millau Viaduct is housed under a structure made of a special concrete patented by the group Eiffage. The toll plaza includes a CCTV connection to the viaduct and the highway. It also accommodates technical and administrative services.
The Gare de péage (toll plaza)
The only toll plaza on the A75 autoroute, the bridge toll booths and the buildings for the commercial and technical management teams are situated 4 km (2.5 mi) north of the viaduct. The toll plaza is protected by a canopy in the shape of a leaf (formed from tendrilled concrete, using the ceracem process). Consisting of 53 elements (voussoirs), the canopy is 100 m (330 ft) long and 28 m (92 ft) wide. It weighs around 2,500 metric tons (2,800 short tons).
The toll plaza can accommodate sixteen lanes of traffic, eight in each direction. At times of low traffic volume, the central booth is capable of servicing vehicles in both directions. A car park and viewing station, equipped with public toilets, is situated each side of the toll plaza. The total cost was €20 million.


Service area
The Visitor Centre and Farm, Aire de Viaduc de Millau
In 2005 temporary provision had been made to access the viewing point from junction 45. By 2006, there were separate exits from both carriageways of the A75, to Aire de Viaduc de Millau. Here there are three separated car parks for northbound, southbound and non-motorway traffic so cross over is not possible. There is an exhibition centre, and the existing buildings of the Farm of Brocuejouls are being restored.


Statistics

2,460 m (8,071 ft): total length of the roadway
7: number of piers
77 m (253 ft): height of Pier 7, the shortest
343 m (1,125 ft): height of Pier 2, the tallest (245 m/804 ft at the roadway's level)
87 m (285 ft): height of a mast
154: number of shrouds
270 m (886 ft): average height of the roadway
4.20 m (13 ft 9 in): thickness of the roadway
32.05 m (105 ft 2 in): width of the roadway
85,000 m3 (111,000 cu yd): total volume of concrete used
290,000 metric tons (320,000 short tons): total weight of the bridge
10,000–25,000 vehicles: estimated daily traffic
€5.40–7.00: typical automobile toll, as of July 2007
20 km (12 mi): horizontal radius of curvature of the road deck

The Visitor Centre and Farm, Aire de Viaduc de Millau
In 2005 temporary provision had been made to access the viewing point from junction 45. By 2006, there were separate exits from both carriageways of the A75, to Aire de Viaduc de Millau. Here there are three separated car parks for northbound, southbound and non-motorway traffic so cross over is not possible. Each mast of the viaduct is equipped with a monoaxial layer of eleven pairs of stays laid face to face. Depending on their length, the stays were made of 55 to 91 high tensile steel cables, or strands, themselves formed of seven strands of steel (a central strand with six intertwined strands). Each strand has triple protection against corrosion (galvanisation, a coating of petroleum wax and an extruded polyethylene sheath). The exterior envelope of the stays is itself coated along its entire length with a double helical weatherstrip. The idea is to avoid running water which, in high winds, could cause vibration in the stays and compromise the stability of the viaduct.
The stays were installed by the Freyssinet company.




Surface

To allow for deformations of the metal deck under traffic, a special surface of modified bitumen was installed by research teams from Appia.

The surface is somewhat flexible to adapt to deformations in the steel deck without cracking, but it must nevertheless have sufficient strength to withstand motorway conditions (fatigue, density, texture, adherence, anti-rutting, etc.). The "ideal formula" was found only after ten years of research.

Choice of technical design

Simultaneously, a school of international experts representing a wide spectrum of expertise (technical, architectural and landscape), chaired by Jean-François Coste, was established to clarify the choices which had to be made. In February 1995, on the basis of proposals of the architects and structural engineers, and with support of the school of experts, five general designs were identified.
The competition was relaunched: five combinations of architects and structural engineers, drawn from the best candidates of the first phase, were formed to each conduct in-depth studies of one of the general designs. On 15 July 1996, Bernard Pons, minister of Public Works, announced the decision of the jury constituted of elected artists and experts and chaired by the director of highways, Christian Leyrit at the time. The solution of a cable-stayed bridge, presented by the structural engineering group Sogelerg, Europe Etudes Gecti and Serf and the architects Foster + Partners was declared the best.
Detailed studies were carried out by the successful consortium, steered by the highways authority until mid-1998. After wind tunnel tests, the shape of the road deck was altered and detailed corrections were made to the design of the pylons. When the details were eventually finalised, the whole design was approved in late 1998.