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The Tallest Bridge in the World: Millau Viaduct

The Millau Viaduct is a cable-stayed road-bridge that spans the valley of the river Tarn near Millau in southern France.

It is the tallest bridge in the world with one mast's summit at 343.0 meters (1,125 ft) above the base of the structure. It is the 12th highest bridge deck in the world, being 270 meters (890 ft) between the road deck and the ground below. The bridge is part of the A75-A71 autoroute axis from Paris to Montpellier. Construction cost was approximately $496 million. It was formally dedicated on 14 December 2004, inaugurated on the 15th, and opened to traffic on the 16th. The bridge received the 2006 IABSE Outstanding Structure Award. It was designed by the French structural engineer Michel Virlogeux and British architect Norman Foster.

Costs and resources

The bridge's construction cost was approximately $471 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 profit, the French government can assume control of the bridge as early as 2044.

The project required about 127,000 cubic meters (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 asserts that the lifetime of the bridge will be at least 120 years.

Construction

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.

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: 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 600 mm per cycle which was roughly four minutes long.

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.

Construction records

The bridge's construction broke several records: The highest pylons in the world: pylons P2 and P3, 244.96 meters (803 ft 8 in) and 221.05 meters (725 ft 3 in) in height respectively, broke the French record previously held by the Tulle and Verrieres viaducts (141 m/463 ft), and the world record previously held by the Kochertal Viaduct (Germany), which is 181 meters (594 ft) at its highest. The highest bridge tower in the world: the mast atop pylon P2 peaks at 343 meters (1,125 ft).

The highest road bridge deck in Europe, 270 m (890 ft) above the Tarn River at its highest point. It is nearly twice as tall as the previous tallest vehicular bridges in Europe, the Europabrucke in Austria and the Italia Viaduct in Italy. It is slightly higher than the New River Gorge Bridge in West Virginia in the United States, which is 267 m (876 ft) above the New River. Since opening in 2004, the deck height of Millau has been surpassed by several suspension bridges in China, including Siduhe, Balinghe and two spans (Beipanjiang River 2003 Bridge and Beipanjiang River 2009 Bridge) over the Beipanjiang 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 291 meters (955 ft) over the Arkansas River.

Location

The Millau Viaduct is located on the territory of the communes of Millau and Creissels, France, in the departement 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 holiday 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 Meridienne), from Clermont-Ferrand to Pezenas (to be extended to Beziers 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 traveling along this route. Many tourists heading to southern France and Spain follow this route because it is direct and without tolls for the 340 kilometers (210 mi) between Clermont-Ferrand and Pezenas, 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 $8.36 for light automobiles ($9.67 during the peak months of July and August).

Structure

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


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

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 meters (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 downforce in strong wind conditions.

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

Each mast of the viaduct is equipped with a mono-axial 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 (galvanization, 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.

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 two years of research.

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