Bridge Design and Construction
The project was demanding, with a line 120 metres (400 ft) over the River Truyère. Boyer believed this would be considerably less expensive than taking the railway line around or down through the valley. To resist the wind, Eiffel instantly discarded the principle of solid beam construction, thinking 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 m (1,854 ft) long and weighed 3587 tons. 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, when built, the highest in the world. The overall project cost was 3,100,000 francs. 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.
On 15 June 2011, the Garabit closed for extensive work. It is due to reopen on 15 December 2011. During the works, the train from Béziers to Clermont-Ferrand terminates at St Chély d'Apcher and a bus continues to Clermont-Ferrand. There is an excellent view of the Garabit viaduct from the bus.
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