Garabit viaduct: France

The Garabit Viaduct is a railroad arch bridge spanning the Truyere River near Ruynes en Margeride, 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.

Eiffel and his bridge

The French Recession of 1864 prematurely ended Eiffel’s tenure at 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 Massif Central: home to the wide and windy Garabit Valley.

The undertaking was daunting. Eiffel’s mission was to bridge the valley, which seemed ridiculous since one span would cover a gap measuring 541 feet, while suspending a train 400 feet over the Truyere River. The wind was an additional problem. Eiffel accepted the challenge and succeeded because of his innovative ideas. 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 established a design template known as the “crescent-arch form” with his Garabit Viaduct design that features an “arch visually separated from the thing horizontal girder”. The Garabit Viaduct’s arches were engineered to have support hinges to allow the crescent shape to widen. This method, although complex and confusing, actually expedited the construction time-a short five years.

When it opened with a single railroad track in November 1885, the Garabit Viaduct was 1853 feet long and weighed 3587 tons. The overall project cost was $627,400. Even more impressive was the actual deflection, which was measured at 8 millimeters, a figure precisely anticipated by Eiffel’s calculations. The bridge was also the highest in the world.