Millau Sky Bridge - France

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• published: 05 Jan 2011
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The Millau Viaduct is a 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). 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 the day after and opened to traffic two days later. 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. Construction began on 10 October 2001 and was intended to take three years, but weather conditions put work on the bridge behind schedule. The enormous pylons were built first, together with intermediate temporary pylons which were in themselves a massive record-breaking construction project. 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 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 were 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.