Viaduc du Viaur

English translation

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History

Since the second half of the 19th century, commercial relations between Aveyron and Tarn, and notably between Rouergue and the Albigensian region, rendered necessary the opening up of Ségala. The political will of elected officials from Tarn and Aveyron enabled the realization of the railway link between Carmaux and Rodez, declared to be of public utility on 27 December 1879. The definitive route, adopted in 1881, encountered the problem of crossing the Viaur valley at a sufficient height. The construction of a viaduct was put out to tender in 1887. The most interesting solution - and the most economical - was that of the Société de Construction des Batignolles, conceived by its chief engineer Paul Bodin: a bridge designed according to the principle of balanced arches, which spanned 250 metres in a single span. The project was retained on 30 August 1889; work commenced in November 1896. The structural members were initially assembled flat on the drawing board on the Coudénié plateau. The assembly, directed by Jean Compagnon, was carried out in two stages: work commenced on the Carmaux side with the assembly of the half-arch at the bank and of the horizontal beam linking it to the rear abutment by means of a fixed scaffolding which was subsequently dismantled and reinstalled on the north bank to construct the second cantilever. The height of the ravine did not permit the construction of scaffolding, the two half-arches of the central bay were subsequently assembled in cantilever using a device called the "whale". The junction of the two parts of the viaduct took place on 4 July 1902, with great precision: the difference between the planned height at the crown and the actual height was less than 2 millimetres. Put into service on 18 December 1902, the viaduct presented at its completion the longest metal arch built; it remains the only French engineering structure realized according to the cantilever technique.

Description

The central bay of 220 m opening and 53.73 m rise (the height at the apex of the arch is 116 m above the Viaur) is formed of two symmetrical metal frameworks buttressed against one another at the key by means of a hinge forming an articulation. These two frameworks rest on each side of the valley on masonry supports, by means of articulated shoes. During variations in load or temperature, the three articulations arranged at the springings and at the key allow each half of the structure to turn freely on its support, without undergoing additional stress. Each half of the central arch is extended and balanced by another half-arch (or cantilever) of 69.60 m span, connected to the masonry abutments by a horizontal beam forming a counterweight. Each of the four frameworks or half-arches is constituted by two triangulated trusses braced by struts constituted of lattice beams, inclined at 25° to the vertical, so that their spacing which is only 5.89 m at the deck reaches 33.39 m at the level of the supports. This spacing at the base of the metal framework ensures the structure the necessary stability under the action of the most violent winds. The deck, 4.5 m wide, is provided on each side with a guard rail formed of a beam 1.80 m high.