Millau Viaduct

The Millau Viaduct
Official name Le Viaduc de Millau
Carries 4 lanes of the A75 autoroute
Crosses Valley of the River Tarn
Design Cable-Stayed
Total length 2,460 m (8,071 ft)
Width 32 m (105 ft)
Height 343.0 m (1,125 ft)
Longest span 342 m (1,122 ft)
Clearance below 270 m (886 ft) at maximum
Opened 14 December 2004

The Millau Viaduct (French: le Viaduc de Millau, Occitan: lo Viaducte de Milhau) is an enormous cable-stayed road-bridge that spans the valley of the river Tarn near Millau in southern France.

Designed by the French structural engineer Michel Virlogeux and British architect Norman Foster, it is the tallest bridge in the world, with one mast's summit at 343.0 metres (1,125 ft) — slightly taller than the Eiffel Tower and only 37 m (121 ft) shorter than the Empire State Building.

The viaduct is part of the A75-A71 autoroute axis from Paris to Montpellier. Construction cost was around €400 million.

It was formally dedicated on 14 December 2004, inaugurated the day after and opened to traffic two days later.

The bridge won the 2006 IABSE Outstanding Structure Award.[1]

Contents

Construction records

The bridge’s construction broke three world records:

Location

The Millau viaduct, and the town of Millau on the right

The Millau Viaduct is located 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 River valley and pass along the route nationale N9 near the town of Millau, causing heavy congestion at the beginning and end of the July and August vacation season. The bridge now traverses the Tarn valley above its lowest point, linking two limestone plateaus, the Causse du Larzac and the Causse Rouge, and is inside the perimeter of the Grands Causses regional natural park.

The bridge forms the last link of the A75 autoroute, (la Méridienne) from Clermont-Ferrand to Pézenas (to be extended to Béziers by 2010). The A75, with the A10 and A71, provides a continuous high-speed route south from Paris through Clermont-Ferrand to the Languedoc region and through to Spain, considerably reducing the cost 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 340 kilometres (210 mi) between Clermont-Ferrand and Pézenas, except for the bridge itself.

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. The toll bridge costs €5.60 for light automobiles (€7.40 during the peak months of July and August).

North-South axes

The four route options for Perpignan–Paris

As of 2007, there are four north-south routes, or axes, traversing France:

A75 autoroute

Construction started in 1975 and was finished in 2004 when the Millau Viaduct went into service.

The new A75 autoroute, complementing the A71 from Orléans to Clermont-Ferrand, created a fourth route through France and has several advantages:

Bypassing Tarn at Millau

The Tarn flows from the east to the west of France, south of the Massif Central, bisecting the country's North-South axis.

For nearly thirty years prior to the construction of the Millau Viaduct, the A75 autoroute had remained unfinished. Before the bridge, a crossing of the River Tarn was provided by a bridge situated in the valley bottom, in the town of Millau. Millau was then known and dreaded as a ‘great black spot’ of motoring. Kilometres of congestion and hours of waiting to transit the town occurred each year with the great surge in traffic in summer months. These slowdowns meant that the advantages of the A75 were lost. The A75 was meant to be a positive example of spatial planning, a modern, direct highway entirely free along its 340 km (210 mi) length. As it was, the traffic from the autoroute brought pollution and danger to the town of Millau.

Design and construction of the bridge took a long time. In this region, climatic conditions are tough, with violent winds. Geological characteristics of the high plateaus of Larzac are peculiar, and, because the Tarn Valley is so deep, crossing is difficult. Different approaches were investigated, and all of them were found to be very technically demanding. Ten years of research and four years of implementation were required for completion of the Millau Viaduct.

Description

The Millau Viaduct consists of an eight-span steel roadway supported by seven concrete pylons. The roadway weighs 36,000 tonnes (40,000 short tons) and is 2,460 m (8,070 ft) long, measuring 32 m (105 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 (1,122 ft) with the two outer spans measuring 204 m (669 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 (253 ft) to 246 m (807 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 tonnes (2,460 short tons). These sections were assembled on site from pieces of 60 tonnes (66 short 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 87 m (285 ft) tall masts.

The enormous pylons were built first, together with intermediate temporary pylons which were in themselves a massive record-breaking construction project.

Remarkably, the entire length of deck surface (that is to say, the bridge itself, the actual kilometres of roadway) was slid out, into the valley, across the pylons from both sides.

This feat was achieved using hydraulic rams that moved the deck about 600 mm every 4 minutes, over the course of many days.

While the kilometres of roadway was being slid-out through space, it was supported by both the final pylons and the temporary pylons.

Only after the roadway was completely slid-out in to the final position, were the masts erected on top of the deck (that is to say, over the pylons). To be clear, the masts on top are not continuing elements of the pylons underneath, although they appear to be. The masts are separate constructions which were built on land, wheeled out to position only after the pylons and roadway were complete, raised (with difficulty) and emplaced.

The construction of the massive cable-stay system between the masts and deck then followed.

Finally, the massive temporary pylons in the valley were 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, both of which included time-lapse footage of the ultimate astonishing feat of sliding the roadway out over the valley, on to the plyons, to create the bridge.

Pylons and abutments

Each pylon is supported by four deep shafts, 15 m (49 ft) deep and 5 m (16 ft) in diameter.

Heights of the piers
P1 P2 P3 P4 P5 P6 P7
94.501 m (310 ft 0.5 in) 244.96 m (803 ft 8 in) 221.05 m (725 ft 3 in) 144.21 m (473 ft 2 in) 136.42 m (447 ft 7 in) 111.94 m (367 ft 3 in) 77.56 m (254 ft 6 in)
A pylon under construction

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

Deck

The metallic deck, which appears very light despite its total mass of around 36,000 tonnes (40,000 short tons), is 2,460 m (8,070 ft) long and 32 m (105 ft) wide. It comprises eight spans. The six central spans measure 342 m (1,122 ft), and the two outer spans are 204 metres (669 ft). 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 4 m (13 ft) cross-section and a length of 15–22 m (49–72 ft) for a total weight of 90 metric tons (99 short tons). The deck has an inverse Airfoil shape, providing negative lift in strong wind conditions.

Masts

The seven masts, each 87 m (285 ft) high and weighing around 700 tonnes (690 LT; 770 ST), are set on top of the pylons. Between each of them, eleven stays (metal cables) are anchored, providing support for the road deck.

Stays

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.

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.

Toll plaza

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 tonnes (2,500 LT; 2,800 ST).

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

Preliminary studies

Chronology

Possible routes

Routes of the four projects of the A75 autoroute around Millau

In initial studies, four options were examined:

  1. An option called Great Eastern (grand Est) ( yellow route ) passing east of Millau and crossing the valleys of the Tarn and Dourbie on two very high and long bridges (spans of 800 m/2,600 ft and 1,000 m/3,300 ft) 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 the through traffic, it was abandoned because it did not serve the needs of Millau and its area satisfactorily.
  2. An option called the Great Western (grand Ouest) ( black route ), longer than the eastern option by 12 km (7.5 mi), following the Cernon valley. Technically easier (requiring four viaducts), this solution was judged to have negative impacts on the environment, in particular on the picturesque villages of Peyre and Saint-Georges-de-Luzençon. More expensive than the preceding option, and serving the region badly, this option was also abandoned.
  3. An option called near RN9 (proche de la RN9) ( red route ), would have served the town of Millau well, but presented technical difficulties and would have had a strong impact on existing or planned structures. This option was also abandoned.
  4. An option called intermediate (médiane), west of Millau ( blue route ) had the blessing of 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 the ministerial decree on 28 June 1989.[5] It encompassed two possibilities:

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 October 29, 1991.[6]

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 Michel Virlogeux. He worked with the Dutch engineering firm ARCADIS, responsible for the structural engineering of the bridge.

Choosing the definitive route

Satellite image of the route before construction of the bridge.

The "high solution" required the construction of a 2,500 m (8,200 ft) long viaduct. Obviously, this would be the crown jewel of the entire A75 autoroute project. 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 then opened the question for competition between structural engineers and architects to widen the search for realistic designs. By July 1993, 17 structural engineers and 38 architects presented themselves as candidates for the preliminary studies. With the assistance of a multidisciplinary commission, the Administration of Roads selected 8 structural engineers for technical study and 7 architects for the architectural study.

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.

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. Four consortia tendered:

The pylons of the Viaduc de Millau, which are the tallest elements (the tallest pylon- 244.96 meters)were produced and mounted by PAECH Construction Enterprise from Poland.

The Compagnie Eiffage du Viaduc de Millau, working with the architect Sir Norman Foster, was successful in obtaining the tender[7]. The fact that the government had already taken the design work to an advanced stage meant that the technical uncertainties were considerably reduced. A further advantage was that it made the process of 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 (Gustave Eiffel built the Garabit viaduct in 1884, a railway bridge in the neighboring Cantal département), 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.

Appia was responsible for the job of the bituminous coating 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, which was entrusted with installing the cable stays and putting them under tension, while the prestress division of Eiffage was responsible for prestressing the pillar heads.

The steel deck and the hydraulic action of the deck were designed by the Walloon engineering firm Greisch from Liège in Belgium[8], also an Information and communication technologies company of the Walloon Region[9] . They carried out the general calculations and the resistance calculations for winds of up to 225 km/h (140 mph). They also applied the launching technology[10].

The sliding shutter technology for the bridge piers came from PERI.

Costs and resources

The bridge's construction cost up to €394 million,[11] with a toll plaza 6 km (3.7 mi) 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.[12] However, if the concession is very profitable, the French government can assume control of the bridge in 2044.

The project required about 127,000 cubic metres (166,000 cu yd) of concrete, 19,000 tonnes (21,000 short tons) of steel for the reinforced concrete and 5,000 tonnes (5,500 short tons) 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 organizations opposed the project, including the WWF, France Nature Environnement, the national federation of motorway users, and Environmental Action. Opponents put forward several arguments:

Construction

Project timeline

The viaduct under construction, seen from the south in early 2004

Pylons and abutments

Two weeks after the laying of the first stone on 14 December 2001, the workers started to dig the deep shafts. There were 4 per pylon; 15 m (49 ft) deep and 5 m (16 ft) in diameter, assuring the stability of the pylons. At the bottom of each pylon, a tread of 3–5 m (10–16 ft) in thickness was installed to reinforce the effect of the deep shafts. The 2,000 m3 (2,600 cu yd) 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 4 m (13 ft). 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.

Rolling out of the deck

The bridge 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:

  1. 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.
  2. Both wedges move forward together, advancing the roadway a short distance.
  3. 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.
  4. 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 600mm per cycle (roughly 4mins long).

Erection of masts

The mast pieces were driven over the new 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 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.

Impact and events

Pedestrian sporting events

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

Famous visitors

During construction, various personalities flocked to the bridge.[13] Amongst those:

Miscellanea

Gallery

Single segment of the construction on the small exhibition under viaduct

Large numbers of people stop to observe the viaduct from the viewing area on the northern side

Front view of the toll plaza

See also

References

  1. ↑ "Millau Viaduct, France". 2006-09-13. http://www.iabse.org/association/awards/ostrac/Millau.php. Retrieved 2008-12-27. 
  2. ↑ Décret du 10 janvier 1995 déclarant d'utilité publique les travaux de construction des sections de l'autoroute A 75 comprises entre Engayresque et Lasparets (mise aux normes autoroutières du P.R. 23,520 au P.R. 26,580), entre Lasparets et La Cavalerie Sud (du P.R. 26,580 au P.R. 66,820) y compris les voies de raccordement à Saint-Germain (R.D. 911), à la Côte rouge (R.D. 999) et à La Cavalerie (R.N. 9), de l'échangeur d'Engayresque, des aires de repos, de la section de route à créer pour assurer la continuité de l'itinéraire de substitution d'Engayresque à Lasparets ainsi que des mesures d'accompagnement sur cet itinéraire à Aguessac et à Millau, classant dans la catégorie des autoroutes l'ensemble de la voie comprise entre l'échangeur d'Engayresque et La Cavalerie Sud (du P.R. 22,700 au P.R. 66,820) dans le département de l'Aveyron et portant mise en compatibilité des plans d'occupation des sols des communes d'Aguessac, Millau, Creissels et Saint-Georges-de-Luzençon
  3. ↑ [1]
  4. ↑ Décret n°2001-923 du 8 octobre 2001 approuvant la convention de concession passée entre l'Etat et la Compagnie EIFFAGE du viaduc de Millau pour le financement, la conception, la construction, l'exploitation et l'entretien du viaduc de Millau et le cahier des charges annexé à cette convention
  5. ↑ Le viaduc de Millau : un ouvrage exceptionnel initié par le ministère de l’équipement,op. cit., p. 4
  6. ↑ Le viaduc de Millau : un ouvrage exceptionnel initié par le ministère de l’équipement,op. cit., p. 4
  7. ↑ Millau Viaduct on Structurae database
  8. ↑ Cable-stayed bridges, by Greisch
  9. ↑ Database of ICT companies in the Walloon Region
  10. ↑ The art of cable-stayed bridges on the Meuse and all over Europe This French-language video illustrates the launching technique
  11. ↑ France shows off tallest bridge BBC News Online. 14 December, 2004. Retrieved 2007-08-03.
  12. ↑ France 'completes' tallest bridge BBC News Online, 29 May, 2004. Retrieved 2007-08-03.
  13. ↑ (French) Visiteurs célèbres sur ViaducMillau.com, le site du Midi Libre viaduc.midilibre.com

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