With this section and the collection ” Les Carnets du Train Jaune ” (Yellow Train Notebook, Ed. Talaia), we offer a retrospective, a journey back in time through the construction of the Yellow Train line and its history. This collection is the result of collaboration between the Parc Naturel Régional des Pyrénées Catalanes and historians Pierre Cazenove and Jean-Louis Blanchon.
In this second part, we take a closer look at one of the technical feats of the Yellow Train line : construction of the Gisclard bridge !
Why a suspension bridge?
In 1904, the French Minister of Public Works, Emile Maruéjouls, appointed a commission to decide on the best way to cross the Têt, upstream and downstream of Fontpédrouse. Made up of experts and chaired by Inspector General Jules Lax, it went on to issue a document whose title left no room for doubt : Construction of two rigid suspension bridges (Gisclard system). The commission’s presentation went on to describe the specific features of the Gisclard system, which then enabled suspension bridges to be used for train traffic.
The Compagnie du Midi, while acknowledging the ingenuity of the planned bridge, had serious reservations about using it for railway purposes. It pointed out that the cable stays would always deflect (whereas trusses need to be made up of straight stays if they are to be dimensionally stable), and argued that this would add to the cost of the specialist personnel needed to adjust them. In addition, the stability of the deck was questioned, and it was pointed out that the action of a vertical wind in the narrow valley hadn’t been taken into account.
Albert Gislcard responded to these observations point by point, and had no trouble refuting them. As for the financial argument, it didn’t hold water : the Lax report favoured the Gisclard project, because « on the one hand, it is much more economical, and on the other, this type of bridge is very easy to put together ».
The Compagnie des Chemins de Fer du Midi would do everything in its power to avoid the use of suspension bridges, as recommended by Inspector General Lax. It went so far as to suggest that the route follow the right bank of the Têt to the right of Fontpédrouse station (in which case, no major bridge would have to be built). Remember that it was the French government that built the infrastructure, while Compagnie du Midi was merely the concession holder for commercial operation of the line. Jules Lax, chairman of the commission, was convinced by the progress made in applying the suspension system to wide-open bridges for both railways and land routes. He felt that the Gisclard system suspension bridge could be used to good effect for both crossings of the Têt valley, and on 22 July 1904, signed the report to be presented to the Minister of Public Works.
A few specialists remained wary of suspension bridges, but the Gislcard Bridge was wonderful !
« Although aesthetics and elegance should occupy only a very minimal place when it comes to a bridge located in open country, it should perhaps not be completely devoid of a degree of attention. In this respect, people are pretty much unanimous in recognising that the suspension bridge, with the lightness of its lines and rational strength of their direction, is the closest thing to the aesthetic we dream of ». In fact, the harmonious proportions that emerge from observing the bridge give its overall appearance a most majestic silhouette in the wild setting into which it blends perfectly. Clearly, Louis Grelot (Inspector General and Professor of Steel Bridges at the Ecole Nationale des Ponts et Chaussées from 1930 to 1958) didn’t think so. He found reasons which, had they all proved true, would have irrevocably doomed the project presented by Albert Gisclard and the Arnodin company for the Têt valley crossing from the outset. It should be remembered that the conclusions of the commission chaired by Jules Lax in 1904, now validated by experience, show that the project was reliable.
Still in service today, the century-old La Cassagne suspension bridge remains a benchmark in its field.
An exceptional construction
The La Cassagne rigid suspension bridge was built to carry the electric railway from Villefranche to Bourg-Madame over the La Cassagne ravine, at the bottom of which flows the Têt River. This section has a 6% ramp 80 m above the riverbed ; the width of the ravine at platform level is 263 m.
- The suspension system : it consists of triangulated trusses exclusively formed by cables hinged at their junction points.
- The deck : in rolled steel, it is continuous from one end of the structure to the other.
- The Ordish suspension system : despite the tensions to which the suspension stays are subjected, their own weight is not a negligible element, and its effect is reflected in a permanent deflection that is all the more appreciable as the cables are more oblique. The passage of rolling loads increases cable tension and reduces their deflection.
- Anchoring galleries : given the rocky nature of the area around the bridge, the cables were anchored in the mountain itself.
- System removability / Cable connections on studs : The cables enter their base plates through a conical opening : their wires are opened out and turned back into hooks in this sort of funnel, inside which a welding alloy is poured. The base plates are drilled with 2 or 4 lateral holes which the threaded ends of the brackets enter. Adjustment and tightening are by nut and locknut.
- Expansion of the structure : the 234 m continuous deck is subject to considerable expansion (0 m 14 for a 50° deviation) due to temperature variations, which are all the more significant as the bridge is located at an altitude of 1,300 m. This expansion takes place very freely, as it is suspended at all points ; all the necessary clearance is provided on the pier-abutment on the Bourg-Madame side.
- Adopted overloads and resistance coefficients : the standard train adopted for the calculations was made up of the heaviest components likely to be used on the line.