The Garabit Viaduct (Viaduc de Garabit in French) is a railway arch bridge spanning the Truyère river near Ruynes-en-Margeride (Fr), Cantal, France, in the mountainous Massif Central region. The bridge was constructed between 1880 and 1884 by Gustave Eiffel, with structural engineering by Maurice Koechlin, and was opened in 1885. It is 565 m in length and has a principal arch of 165 m.
The French recession of 1864 prematurely ended Eiffel’s tenure at the General Railway Equipment Company, but he used the misfortune to begin independent consulting and eventually, his own engineering firm. Opportunity again came for Eiffel during the late 1870s when European railways attempted to grid the continent. Particularly in France, where a vast mountain range suffocated the southern region from any locomotive transport, Eiffel thrust himself to the forefront of the industry and cemented his place as an engineering legend through his efforts in the Massif Central: home to the wide and windy Garabit Valley.
The undertaking was daunting. In 1878, Eiffel was asked by Leon Boyer to bridge the valley, with a line 120 metres (400 ft) over the Truyère River. Boyer believed this would be considerably less expensive than taking the railway line around or down through the valley. Eiffel accepted the challenge and succeeded because of his recent experience on the very similar Douro bridge. To negate the wind, Eiffel instantly discarded precedents of solid beam construction, as he surmised that “it would be very heavy and the beams would rattle in the wind”. Instead, he adopted the concept of trusses or “a series of open triangles” to assuage wind force that “would blow right through them”. Truss work also provides stability when loads are applied through the theory of tension and compression that states force is exerted on the diagonal and vertical segments causing them to resist one another. Eiffel also improved upon his Douro design, adopting the same two-hinged crescent-arch form but employing an arch visually separated from the thin horizontal girder. The Garabit Viaduct’s arches were engineered to have support hinges, allowing the crescent shape to widen. This method both simplified calculations and improved resistance to wind loads.When it opened with a single track in November 1885, the Garabit Viaduct was 565 metres (1853 ft) long and weighed 3587 tons. The overall project cost was 627,400 USD. Even more impressive was the actual deflection, which was measured at 8 millimetres, a figure precisely anticipated by Eiffel’s calculations. The bridge was also, for many years, the highest in the world.Until 11 September 2009, only one regular passenger train per day in each direction used to pass over the viaduct – a Corail route from Clermont-Ferrand to Béziers. On that date, the viaduct was closed as cracks were discovered in one of the foundation piles. It reopened one month later after a safety inspection and was in service with a speed limit of 10 km/h (6 mph) for all traffic
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