One of the World’s Tallest Bridges!
The Millau Viaduct is a cable-stayed road-bridge that spans the river Tarn and the Tarn valley near Millau in the central-south region of France. It is one of the world's tallest bridges designed by Norman Foster, a British architect and Michel Virlogeux, the French structural engineer. It features the 12th highest bridge deck in the world with 890 ft distance between the road deck and the ground. The bridge traverses the Tarn valley above its lowest point, connecting two limestone plateaus, the Causse Rouge and the Causse du Larzac. Also the bridge is within the limits of the Grands Causses regional natural park. The viaduct forms the last linkage of the A75 auto-route from Clermont-Ferrand to Pezenas.
At 8071 ft long and 1025 ft tall, it is a remarkable architectural and design feat. Inaugurated on the 14th December, 2004, this bridge is taller than Eiffel Tower! A permanent visitors’ centre near the bridge provides stunning views of the bridge and across the valley which it spans. The viewing point for the Millau Viaduct along with the visitors’ centre can easily be accessed from the A75 route thus giving the bridge both an important functional use and making it into a tourist attraction.
Designing the Millau Viaduct
The construction of a bridge to span the valley was increasingly important due to excessive traffic congestion on the route from Paris to Spain along the stretch passing via the valley, especially during holiday season. The first plans for a bridge were discussed in 1987, and the decision was made to construct a high level bridge crossing of the Tarn River in October 1991. The government consulted with eight structural engineers and seven architects over the period 1993-1994. A second study was made by five associated architect groups along with another group of structural engineers during the period 1995-1996. The government issued a declaration of public interest to solicit design methods for a competition in January 1995.
The panel decided to construct a cable-stayed design with multiple spans in July 1996 inline with a proposal given by the Sogelerg consortium consisting Norman Foster and Michel Virlogeux. In June 2000, the construction contract contest was launched for four consortia. In 2001, Eiffage was given the prime contract for constructing the Millau Viaduct.
The cost of constructing the bridge was reportedly up to €394 million, and an additional toll plaza 6 km north of the viaduct was €20 million. The builders, Eiffage funded the construction in return for a concession to collect the revenues collected from tolls for 75 years, until 2080. However, the French government can take over the control of the bridge if the concession yields high revenues.
The Structure of the Millau Viaduct
The Millau viaduct is a multi cable-stayed bridge, with a slight curve in its path on a radius of 20,000m and interestingly a constant gradient along its length north to south of 3.025%. With such a complicated terrain the structure was designed to contend with a number of difficult environmental and load factors. The basic structure incorporates 7 main piers constructed of concrete (poured in-situ) with the top half of the piers split into two to combat flex and expansion of the material with changes in temperature / wind load etc. Each pier is spaced at 342m intervals with a further 204m span at either end to connect back to the ground. The road deck was predominantly prefabricated (off site) from steel (mainly S355 and S460 structural steel) which not only reduced the overall construction time but also meant that the construction process required far less space on site.
Sitting on top of the concrete piers is the road deck, a trapezoidal profile structure made from structural steel girders and sheet. In some places the sheet steel is up to 80mm thick but for the majority of the structure between 12 - 14mm sheet steel has been used. On top of the road deck, sit 7x steel pylons, 38m tall consisting of stiffened steel box girders. On top of the pylons a 49m mast has been constructed onto which the cables that provide support and tension for the bridge are attached. The final 17m of each vast 87m pylon is an aesthetic addition having no structural significance.
The gigantic steel cables that anchor and support the bridge consist of 15 strands of a super galvanized, sheathed and waxed steel rated to 1,860MPa. On the exterior, each cable is coated with an aerodynamic sheath made from PEHD. Not only does this coat provide a barrier to UV light but by incorporating discontinuous spirals on the sheaths surface, the engineers have managed to mitigate against the negative effects of vibrations / resonance caused by excessive winds passing over and through the structure.
Construction of the Millau Viaduct
Construction of the bridge began on 16 October, 2001, with an estimated completion date of January 2005. The Millau Viaduct features six lanes including a hard shoulder on each side and an extra 1m lane next to the central reservation. Seven tuning-fork shaped hollow reinforced concrete piers with steel pylons and cables support a steel deck that is 13.8 ft deep, around 105 ft. wide and standing 886 ft above ground. The height of these piers starts from 254 ft. up with the second and third rising to around 837 ft. and 725 ft., respectively. The highest point of the bridge reaches a level that is approximately 4% higher than the Eiffel Tower.
The contract for supplying a hydraulic system for performing the challenging task of placing the deck was awarded to Enerpac, a Netherlands-based construction company, which used hydraulic telescopic equipment to push the steel deck over seven temporary steel piers during the placement process. The cranes also assisted in pouring concrete of 850000 m3 and placing form work of 36000 t.
Placement of 36,000 t prefabricated deck began in late February 2003. This process was carried out by inching the deck forward with the help of launching devices placed on the sets of single-acting lock nut cylinders supporting both device and deck, in addition to retractable skates. The deck had progressed to the third pier by March 2004. The third week of April saw the last launching of the deck from the northern side, at which point it was protruding over the river Tarn. The remaining work proceeded rapidly, once the deck was placed. Pylons were completed in July, and the temporary piers were dismantled in November. The Millau Viaduct was opened a month ahead of schedule.
Interesting Facts about the Millau Viaduct
The Millau Viaduct is not straight. The bridge was built with a slight curve having a radius of 20km as a high straight road could induce a floating sensation for drivers. The bridge also holds the record for having the highest piles (803.7 ft deep) of any bridge in the world and the highest mast which extends 1,125 ft above the roadbed of the bridge.
It is also claimed to have the highest roadbed of any bridge in the world with its roadbed extending 885.8 ft above the river beneath. However, the roadbed of the Royal Gorge Bridge in Colorado in the US tops this with its roadbed reaching 1,053 ft above the river below. Hence the Royal Gorge Bridge is the highest in the world based on the height of roadbed and the Millau Viaduct is the highest in the world in terms of mast height.