Millau Bridge

MILLAU BRIDGE The Millau Viaduct is a cable-stayed road-bridge that spans the valley of the river Tarn near Millau in southern France. In February 1995, on the basis of proposals of the architects and structural engineersengineers,, five general designs were identified.identified. ThenThen,, The solution of a cable-stayed bridge,bridge, presented by architects Norman Foster was declared the best. 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 above the base of the structure. It is the 12th highest bridge deck in the world, being 270 metres between the road deck and the ground below. The viaduct is part of the A75-A71 autoroute axis from Paris to Montpellier. Construction cost was approximately €400 million. It was formally dedicated on 14 December 2004, inaugurated on the 15th, and opened to traffic on the 16th. Problems with traffic on the route from Paris to Spain along the stretch passing through the valley near the town of Millau, especially during the summer when the roads became jammed with holiday traffic, necessitated the building of a bridge across the valley. Construction Records The bridge’s construction broke several records: •The highest pylons in the world: pylons P2 and P3, 244.96 metres and 221.05 metres in height respectively, broke the French record previously held by the Tulle and Verrières Viaducts and the world record previously held by the Kochertal Viaduct (Germany), which is 181 metres at its highest; •The highest bridge tower in the world: the mast atop pylon P2 peaks at 343 metres Costs and Resources The bridge's construction cost up to €394 million,with a toll plaza 6 km north of the viaduct costing an additional €20 million. The project required about 127,000 cubic metres of concrete, 19,000 tonnes of steel for the reinforced concrete and 5,000 tonnes of pre-stressed steel for the cables and shrouds. The builder claims that the lifetime of the bridge will be at least 120 years. Pylons and Abutments Each pylon is supported by four deep shafts, 15 m deep and 5 m in diameter. Heights of the piers P1 P2 P3 P4 P5 P6 P7 94.501 244.96 221.05 144.21 136.42 111.94 77.56 m m m m m m m 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, is 2,460 m long and 32 m wide. It comprises eight spans. The six central spans measure 342 m, and the two outer spans are 204 metres. 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 cross-section and a length of 15–22 m for a total weight of 90 metric tons. The deck has an inverse Airfoil shape, providing negative lift in strong wind conditions. Masts The seven masts, each 87 m high and weighing around 700 tonnes 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 steelcables, 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. 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. The "ideal formula" was found only after two years of research. 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 deep and 5 m in diameter, assuring the stability of the pylons. At the bottom of each pylon, a tread of 3–5 m in thickness was installed to reinforce the effect of the deep shafts. The 2,000 m3 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. .

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