Millau Viaduct


The Millau Viaduct is a multi-span cable-stayed bridge completed in 2004 across the gorge valley of the Tarn near Millau in Southern France. In an Anglo-French partnership, it was designed by the French structural engineer Michel Virlogeux and English architect Norman Foster. it is the tallest bridge in the world, having a structural height of.
The Millau Viaduct is part of the A75–A71 autoroute axis from Paris to Béziers and Montpellier. The cost of construction was approximately € 394 million. It was built over three years, formally inaugurated on 14 December 2004, and opened to traffic two days later on 16 December. The bridge has been consistently ranked as one of the greatest engineering achievements of all time, and received the 2006 Outstanding Structure Award from the International Association for Bridge and Structural Engineering.

History

In the 1980s, high levels of road traffic near Millau in the Tarn valley were causing congestion, especially in the summer due to holiday traffic on the route from Paris to Spain. A method of bypassing Millau had long been considered, not only to ease the flow and reduce journey times for long-distance traffic, but also to improve the quality of access to Millau for its local businesses and residents. One of the solutions considered was the construction of a road bridge to span the river and gorge valley. The first plans for a bridge were discussed in 1987 by CETE, and by October 1991 the decision was made to build a high crossing of the Tarn by a structure of around in length. During 1993–1994, the government consulted with seven architects and eight structural engineers. During 1995–1996, a second definition study was made by five associated architect groups and structural engineers. In January 1995, the government issued a declaration of public interest to solicit design approaches for a competition.
In July 1996 the jury decided in favour of a cable-stayed design with multiple spans, as proposed by the Sogelerg consortium led by Michel Virlogeux and Norman Foster. The decision to proceed by grant of contract was made in May 1998; then in June 2000, the contest for the construction contract was launched, open to four consortia. In March 2001, Eiffage established the subsidiary Compagnie Eiffage du Viaduc de Millau, and was declared winner of the contest and awarded the prime contract in August.

Possible routes

In initial studies, four potential options were examined:
  1. Great Eastern — passing east of Millau and crossing the valleys of the Tarn and Dourbie on two very high and long bridges whose construction was acknowledged to be problematic. This option would have allowed access to Millau only from the Larzac plateau, using the long and tortuous descent from La Cavalerie. Although this option was shorter and better suited to through traffic, it did not satisfactorily serve the needs of Millau and its area.
  2. Great Western — longer than the eastern option by, following the Cernon valley. Technically easier, this solution was judged to have negative impacts on the environment, in particular on the picturesque villages of Peyre and Saint-Georges-de-Luzençon. It was more expensive than the preceding option, and served the region badly.
  3. Near RN9 — would have served the town of Millau well, but presented technical difficulties, and would have had a strong impact on existing or planned structures.
  4. Intermediate, west of Millau — was supported by local opinion, but presented geological difficulties, notably on the question of crossing the valley of the Tarn. Expert investigation concluded that these obstacles were not insurmountable.
The fourth option was selected by ministerial decree on 28 June 1989. It encompassed two possibilities:
  1. the high solution, envisaging a viaduct more than above the river;
  2. the low solution, descending into the valley and crossing the river on a bridge, then a viaduct of, extended by a tunnel on the Larzac side.
After long construction studies by the Ministry of Public Works, the low solution was abandoned because it would have intersected the water table, had a negative impact on the town, cost more, and lengthened the driving distance. The choice of the 'high' solution was decided by ministerial decree on 29 October 1991.
After the choice of the high viaduct, five teams of architects and researchers worked on a technical solution. The concept and design for the bridge was devised by French designer and structural engineer Dr Michel Virlogeux. He worked with the Dutch engineering firm Arcadis, responsible for the structural engineering of the bridge.

Choosing the definitive route

The 'high solution' required the construction of a viaduct. From 1991 to 1993, the structures division of Sétra, directed by Michel Virlogeux, carried out preliminary studies, and examined the feasibility of a single structure spanning the valley. Taking into account technical, architectural, and financial issues, the Administration of Roads opened the question for competition among structural engineers and architects to widen the search for realistic designs. By July 1993, seventeen structural engineers and thirty-eight architects applied as candidates for the preliminary studies. With the assistance of a multidisciplinary commission, the Administration of Roads selected eight structural engineers for a technical study, and seven architects for the architectural study.

Choice of technical design

Simultaneously, a school of international experts representing a wide spectrum of expertise, chaired by Jean-François Coste, was established to clarify the choices that 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; each was to 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, which was constituted of elected artists and experts, and chaired by Christian Leyrit, the director of highways. The solution of a multiple-span viaduct 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 undergoing 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.

Contractors

Once the Ministry of Public Works had taken the decision to offer the construction and operation of the viaduct as a grant of contract, an international call for tenders was issued in 1999. Five consortia tendered:
  1. Compagnie Eiffage du Viaduc de Millau, a new subsidiary created by Eiffage;
  2. PAECH Construction Enterprise, Poland;
  3. a consortium led by the Spanish company Dragados, with Skanska, Sweden, and Bec, France;
  4. Société du Viaduc de Millau, including the French companies ASF, Egis Projects, GTM Construction, Bouygues Travaux Publics, SGE, CDC Projets, Tofinso, and the Italian company Autostrade;
  5. a consortium led by Générale Routière, with Via GTI and Cintra, Nesco, Acciona, and Ferrovial Agroman.
Piers were built with Lafarge high performance concrete. The pylons of the Millau Viaduct, which are the tallest elements were produced and mounted by PAECH Construction Enterprise from Poland.
The Compagnie Eiffage du Viaduc de Millau, working with the architect Norman Foster, was successful in obtaining the tender. Because the government had already taken the design work to an advanced stage, the technical uncertainties were considerably reduced. A further advantage of this process was to make negotiating the contract easier, reducing public expense, and speeding up construction, while minimising such design work as remained for the contractor.
All the member companies of the Eiffage group had some role in the construction work. The construction consortium was made up of the Eiffage TP company for the concrete part, the Eiffel company for the steel roadway, and the Enerpac company for the roadway's hydraulic supports. The engineering group Setec has authority in the project, with SNCF engineering having partial control. was responsible for the job of the bituminous road surface on the bridge deck, and Forclum for electrical installations. Management was handled by Eiffage Concessions.
The only other business that had a notable role on the building site was Freyssinet, a subsidiary of the Vinci Group specialising in prestressing. It installed the cable stays and put them under tension, while the prestress division of Eiffage was responsible for prestressing the pillar heads.
The steel road deck, and the hydraulic action of the road deck were designed by the Walloon engineering firm Greisch from Liège, Belgium, also an information and communication technologies company of the Walloon Region. They carried out the general calculations and the resistance calculations for winds of up to. They also applied the launching technology.
The sliding shutter technology for the bridge piers came from PERI.

Costs and resources

The bridge's construction cost up to €394 million, with a toll plaza north of the viaduct, costing an additional €20 million. The builders, Eiffage, financed the construction in return for a concession to collect the tolls for 75 years, until 2080. However, if the concession yields high revenues, the French government can assume control of the bridge as early as 2044.
The project required about of concrete, of steel for the reinforced concrete, and of pre-stressed steel for the cables and shrouds. The builder claims that the lifetime of the bridge will be at least 120 years.

Opposition

Numerous organisations opposed the project, including the World Wildlife Fund, France Nature Environnement, the national federation of motorway users, and Environmental Action. Opponents advanced several arguments:
Two weeks after the laying of the first stone on December 14, 2001, the workers started digging the deep shafts. There were four shafts per pylon; deep and in diameter, assuring the stability of the pylons. At the bottom of each pylon, a tread of in thickness was installed to reinforce the effect of the deep shafts. The of concrete necessary for the treads was poured at the same time.
In March 2002, the pylons emerged from the ground. The speed of construction then rapidly increased. Every three days, each pylon increased in height by. This performance was mainly due to sliding shuttering. Thanks to a system of shoe anchorages and fixed rails in the heart of the pylons, a new layer of concrete could be poured every 20 minutes.
The bridge road deck was constructed on land at the ends of the Viaduct, and rolled lengthwise from one pylon to the next, with eight temporary towers providing additional support. The movement was accomplished by a computer-controlled system of pairs of wedges under the deck; the upper and lower wedges of each pair pointing in opposite directions. These were hydraulically operated, and moved repeatedly in the following sequence: the lower wedge slides under the upper wedge, raising it to the roadway above, and then forcing the upper wedge still higher to lift the roadway. Both wedges move forward together, advancing the roadway a short distance. The lower wedge retracts from under the upper wedge, lowering the roadway and allowing the upper wedge to drop away from the roadway; the lower wedge then moves back all the way to its starting position. There is now a linear distance between the two wedges equal to the distance forward the roadway has just moved. The upper wedge moves backward, placing it further back along the roadway, adjacent to the front tip of the lower wedge and ready to repeat the cycle and advance the roadway by another increment. It worked at per cycle which was roughly four minutes long.
The mast pieces were driven over the new road deck lying down horizontally. The pieces were joined to form the one complete mast, still lying horizontally. The mast was then tilted upwards, as one piece, at one time in a tricky operation. In this way, each mast was erected on top of the corresponding concrete pylon. The stays connecting the masts and the deck were then installed, and the bridge was tensioned overall, and weight tested. After this, the temporary pylons could be removed.

Timeline

Construction records

The construction Millau Viaduct broke several records:
Since opening in 2004, the deck height of Millau has been surpassed by several suspension bridges in China, including Sidu River Bridge, Baling River Bridge, and two spans over the Beipan River. In 2012, Mexico's Baluarte Bridge surpassed Millau as the world's highest cable-stayed bridge. The Royal Gorge suspension bridge in the U.S. state of Colorado is also higher, with a bridge deck approximately over the Arkansas River.

Location

The Millau Viaduct is on the territory of the communes of Millau and Creissels, France, in the département of Aveyron. Before the bridge was constructed, traffic had to descend into the Tarn valley and pass along the route nationale N9 near the town of Millau, causing much traffic congestion at the beginning and end of the July and August holiday season. The bridge now traverses the Tarn valley above its lowest point, linking two limestone plateaus, the Causse du Larzac and the, and is inside the perimeter of the Grands Causses regional natural park.
The Millau Viaduct forms the last link of the existing A75 autoroute, from Clermont-Ferrand to Pézenas. The A75, with the A10 and A71, provides a continuous high-speed route south from Paris through Clermont-Ferrand to the Languedoc region, thence to Spain, considerably reducing the cost and time of vehicle traffic travelling along this route. Many tourists heading to southern France and Spain follow this route because it is direct and without tolls for the between Clermont-Ferrand and Pézenas, except for the bridge.
The Eiffage group, which constructed the Viaduct also operates it, under a government contract, which allows the company to collect tolls for up to 75 years. As of 2018, the toll bridge costs €8.30 for light automobiles.

Structure

Pylons and abutments

Each of the seven pylons is supported by four deep shafts, deep and in diameter.
P1P2P3P4P5P6P7

The abutments are concrete structures that provide anchorage for the road deck to the ground in the Causse du Larzac and the Causse Rouge.

Road deck

The metallic road deck, which appears very light despite its total mass of around, is long and wide. It comprises eight spans. The six central spans measure, and the two outer spans are. These are composed of 173 central box beams, the spinal column of the construction, onto which the lateral floors and the lateral box beams were welded. The central box beams have a cross-section, and a length of for a total weight of. The deck has an inverse airfoil shape, providing negative lift in strong wind conditions.

Masts

The seven masts, each high, and weighing around, are set on top of the concrete pylons. Between each of them, eleven stays are anchored, providing support for the road deck.

Cable stays

Each mast of the Viaduct is equipped with a monoaxial layer of eleven pairs of cable-stays; laid face to face. Depending on their length, the cable stays were made of 55 to 91 high tensile steel cables, or strands, themselves formed of seven strands of steel. Each strand has triple protection against corrosion. 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.

Road surface

To allow for deformations of the metal road deck under traffic, a special surface of modified bitumen was installed by research teams from. 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. The 'ideal formula' was found after two years of research.

Electrical installations

The electrical installations of the viaduct are large in proportion to the size of the bridge. There are of high-current cables, of fibre optics, 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.
The pylons, road deck, masts, and cable stays are equipped with a multitude of sensors to enable structural health monitoring. 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 road 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 cable 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.

Toll plaza

The toll plaza is on the A75 autoroute; the bridge toll booths and the buildings for the commercial and technical management teams are situated 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, the canopy is long and wide. It weighs around.
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 at each side of the toll plaza. The total cost was €20 million.

Rest area of Brocuéjouls

The rest area of Brocuéjouls, named Aire du Viaduc de Millau, is situated just north of the viaduct, and is centred on an old farm building named 'Ferme de Brocuéjouls'. It was inaugurated by the prefect of Aveyron, Chantal Jourdan, on 30 June 2006, after 7 months of works. The farm and its surroundings can accommodate entertainment and tourism promotion activities.
The cost of this work amounted to € 5.8 million:

Pedestrian sporting events

Unusually for a bridge closed to pedestrians, a run took place in 2004, and another on 13 May 2007: