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The Garabit Viaduct

Updated: Sep 8, 2019


The Garabit Viaduct - original

Context:


The Garabit Viaduct is a two-hinged arch truss bridge which forms part of a rail line, built from 1880 - 1884 and opened in 1888. The rail line the viaduct is passes from Paris to Béziers, and the viaduct itself is found between the Gambit and Ruynes-en-Margeride stations. Part of the Viaducts’ claim to fame, apart from having been both the tallest and longest viaduct in the world until 1886, is that the structural engineer behind the project, Léon Boyer, entrusted the realisation of the bridge to Gustave Eiffel, the man famous for designing the Eiffel Tower and the frame of the Statue of Liberty, earning him the title of the “Iron Magician”. Léon Boyer, an engineer from the École des Ponts et Chaussées, had the idea to make the rail line cross the gorges of the Truyére river in a straight line. This meant that instead of bringing the rail line down to a lower level and creating a more basic solution to cross the gorge, Boyer proposed the innovative solution of constructing across the gorge. The Garabit viaduct has since been included in many films due to its pleasant aesthetics most notably “un Homme de Trop” - 1967, One Man Too Many - featuring Bruno Cremer.



Structure:


The Garabit Viaduct is a two hinged truss arch bridge built at 835m of altitude, with an apex 95m above the water level of the Truyère river, although this used to be 123m before the building of the dam in the area. It has a total span of 565m, with the arch having a span of 165m. There are seven cast iron piers of a range of different heights, with a maximum of 80m which support the deck, of which 5 rest on masonry supports. Two of the seven piers are connected to the truss arch which spans the river. The arch of the viaduct rests on two large masonry piers, constituted of 20,370m3 of stone. The Garabit Viaduct makes use of 3169 tonnes of machined cast iron, 41 tonnes of steel and 23 tonnes of cast iron. The viaduct makes use of 678,768 rivets, and only has a deflection under its dead load and live loads combined - roughly 4000 tonnes - of 8mm. The Viaduct was originally the colour of the iron - dark grey - used in construction however it was since painted red using 34 tonnes of paint, to protect the metal from corrosion.



Structural behaviour:


The apex of the arch - original


The Garabit Viaduct is known as a two-hinged arch because it is fixed at thrust points at its base on either side and is fixed in other locations along its arch, however not at its apex, meaning it cannot be termed a three-hinged arch, but a two-hinged arch. The arch of the viaduct is used to transfer the downward forces created by the dead and live loads to horizontal and vertical forces at the foundations. Most of the viaduct was constructed using cast iron. This is iron with a carbon content of usually between 2%-4%. Cast iron has very good vibration dampening qualities due to the graphite formations in the material, which is very useful in rail bridges due to the large amounts of vibration caused by the passing of heavy vehicles. In addition, cast iron is strong in compression and tension and so is ideal in the use of truss structures in which beams are in theory put only in either tension or compression. Although cast Iron is brittle, due to the function and location of the viaduct, the structure is unlikely to encounter any large dynamic forces, making its use relatively safe. The most important structural aspect of the Garabit Viaduct is the trussing used throughout the entirety of the structure.


Trussing - original

Trusses are triangulated structures composed, in this case, of beams of cast iron and connected at their joints by pins, in this case rivets, which allow for the free movement of the beams. Trussed structures are such that the load is shared equally between all their members, such that the trussed structure acts as one giving it a very high torsional and shear strength. It may seem surprising that the beams should be connected in a way which allows them to move relatively freely, given the rigidity of the overall structure. However, this is so that none of the beams are subject to bending moments, which considering the brittleness of the cast iron, would result in structural failure as fissures would appear in the iron. Whilst the pins at the joints technically allow for free movement, because of the way in which the beams are arranged and the extensive calculations to measure the resultant forces in both horizontal and vertical directions, the beams are immobile, with the viaduct only having a deflection under both dead and live loads of 8mm, the precise amount which Eiffel had calculated at the time.


Trussing is advantageous because trussed structures are often lighter than their masonry or concrete counterparts, and have a more transparent profile, thus making them more suited to structures built in the depths of nature as is the case with the Garabit Viaduct. However whilst trussing has many advantages, it can cause premature wear in its members especially if there is a slight miscalculation leading to one of the beams carrying significantly more load than its counterparts. Moreover, truss bridges are difficult to maintain due to the necessity of the routine inspection of all components, as the failure of one could significantly impact all other components immediately.



Construction:


The construction of the Viaduct began in 1880, with the establishment of the large masonry foundations, for both the arch, and the piers supporting the deck. In 1882, the construction of the cast iron piers, atop the masonry foundations began. The arch was then installed through the semi-arch method. This consisted of cranes attached at either end of the growing semi-arches, extending each gradually. The semi-arches were supported with cables attached to back-stay towers, which in turn were supported by back-stay cables connected to the ground. Eventually the two semi-arches met and the arch was formed. The deck girders were eventually launched from both sides, and were united in June of 1884. After the laying of the rail lines in September 1884, the viaduct was only fully inaugurated in 1888, as the rest of the rail line had yet to be completely opened.


Large masonry abutments - original

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