Pont de Cubzac

From The Engineer, 5 July 1867

There are four bridges crossing the River Dordogne at Cubzac-les-Ponts, Gironde, France.

The most northerly is a railway bridge, the Pont ferroviaire de Cubzac.

The next bridge upstream (south) is the most interesting of the four. It carries the D1010 road, and has commemorative plaques which identify it as the Pont de Cubzac.

Next is a concrete bridge carrying the A10 road, the Pont autoroutier de Cubzac, or Pont de Guyenne

The last bridge is a steel box girder railway bridge (LGV), opened in 2017.

The Historic Pont de Cubzac

This road viaduct crosses the river on lattice girders, with multi-arch masonry approach viaducts on each side of the river.

A series of five suspension bridge spans originally crossed the river here, Construction of the remarkable multi-span suspension bridge started in September 1835. Jean-Baptiste Billaudel (designer). Contractors: Marie Fortuné de Vergès (engineer) and Ferdinand Bayard de la Vingtrie. The bridge was severely damaged in a storm in 1869, leading to its demolition. The piers were modified and re-used for the present lattice girder bridge in 1879-1883. Engineer: Charles de Sansac. Constructors: Compagnie des établissements Eiffel

See Structurae entry for the suspension bridge and the article below transcribed from The Engineer for information on the suspension bridge.

See also Structurae entry for the present bridge and the Bridgemeister entry.

1867 Description from The Engineer 1867/07/05

The article is transcribed below:-

'The Pont de Cubzac, constructed in 1835-39 from the designs of M. Vergès over the Dordogne, a short distance above the town from which it takes its name, is one of the finest suspension bridges in Europe, and from the difficulties involved in its construction - difficulties which have been successfully surmounted - is well worthy of notice.
The Dordogne rises in the mountains of Auvergne, and joins the Garonne below Bordeaux. At the point where it is crossed by the bridge it is about 1780ft. wide, and 12ft. deep at low water, with a strong current liable to be violently agitated by a westerly wind, whilst from careful borings it was ascertained that the bed of the river consisted of fine sand for a depth of 82ft. It was moreover requisite to provide sufficient headway for the passage of vessels of large burthen up the river to Libourne.
These considerations decided M. Vergès on adopting the suspension principle as affording a means of crossing the river with as large spans and consequently as few piers, as possible. With so treacherous a foundation it was necessary to reduce the weight of the piers to a minimum ; for this purpose the towers supporting the suspension cables were constructed in cast iron, by which means a saving in weight of 3900 tons over piers constructed entirely m masonry was effected, and the number of bearing piles reduced by two-thirds.

'The total length of bridge and approaches is 6966.16ft., made up as follows:- Bridge proper, 1787.6ft.; two viaducts in masonry, 820 ft. each, 1640; right embanked approach, 2133.64; left, 1404.92, making total of 6966.16.

'The distance between abutment towers of 1787.6ft. is divided into five spans of 357.52 ft. each, with a headway above low water of 93.48ft. at the centre of the bridge, and 83.64ft. at the abutments. The piers are of masonry for a height of 42·64ft. above low water, their shape in plan being that of a rectangle 46ft. long and 16ft. broad terminated at either end by two intersecting arcs of a circle. From this masonry base rise two towers in cast iron, 84ft. high, connected just below platform level by an arch of an elliptical shape. (See Figs 1 and 2). Each tower is composed of two truncated cones of different diameters, connected together at platform level by a junction piece (Fig 4,) which, by breaking the outline, relieves the elevation; the whole is surmounted by a cupola-shaped casting, to which is bolted the saddle supporting the cables.

'The towers are built up in piers of open panel work, ten tiers below the level of the roadway and five above. Figs. 5 and 8 show a front and back view with a vertical and horizontal section of the lower tier. The panels in each tier are bolted to each other laterally through the flanges, six bolts to a panel, but the tiers are connected by an intervening ring of a T section, as shown in Fig. 11, the vertical flange preventing any lateral spreading.
In the centre of each tower rises a vertical column of a cruciform section, which forms as it were the backbone of the structure. The exterior casing is connected with this column by means of horizontal bracing in flat cast iron bars, and diagonal bracing in round iron. The general arrangement is shown in Fig. 3, and the details in Fig. 10. The base plate is shown in plan in Fig. 7, and in section in Fig. 9; the thickness of metal is 2 3/8in., that of the lowest tier of panels being 1 3/16in.

'The strains which the towers have to resist are of two kinds, one vertical and composed of the permanent load 221.4 tons, of the proof load 147.6, making a total of 369 tons, half of which, 184·5 tons, is the maximum vertical weight on each tower. The smallest sectional area of the tower being 540 square inches, the strain per square inch is only 6.83 cwt., an amount much less than cast iron is capable of supporting if direct pressure alone is taken into account; but the varying strains to which the tower may be subjected, and the provision for flanges &c., necessitates an increased thickness of metal.

'The second strain is horizontal, and arises from the friction of the cables on the rollers; its amount is 24 tons, and produces no appreciable effect on the towers. During the raising of the cables the tower tilted 3/8 in., not being then fixed to the masonry, but returned afterwards to its former bearing.

'The arrangement of the cables is peculiar, as will be perceived from Fig. 1. The roadway is supported by twelve wire cables, which hang in catenary curve, the deflection being 42ft. At the point of suspension, 135 ft. above low water, the cables pass through blocks and are led back in a straight line to the opposite tower at platform level, where they are connected to a horizontal cable stretching along the platform of the bridge. By this means a series of triangles is formed, the summits of which are the points of suspension. The object of the above arrangement is to obtain rigidity, but this result would hardly appear to have been arrived at, for in the recent discussion of the paper on the Clifton Bridge at the Institution of Civil Engineers, Mr. Barlow stated that he had experienced considerable vibration when standing on the Pont de Cubzac from the passage of but a moderate load.

'The roadway is of planking laid on cross joists, which are arranged in pairs, a suspension rod passing between each pair. The width between handrails is 24.6ft. From the great headway given to the bridge the approaches, as already stated, are of great length. and consist on either side of a viaduct of twenty-nine arches and a long embankment. Owing to the uncertain character of the ground the first four piers of the viaducts are founded on piles.

'The towers were erected in the following manner: The base plates having been placed on the masonry, and brought to a true level by the insertion of wedges of dry oak, were tied down by bolts 16ft. long. On this base was placed the first tier of ten frames, felt being inserted between the joints. The T-shaped ring connects all the frames of the first tier together at the top, and having been brought to a true level as before, and the flange covered with felt, was ready to receive the base of the second tier, and so on for the rest of the structure. For raising the various pieces a movable crane was used, consisting of a mast and cross beam, which was lifted as the work progressed, that part of the tower already completed serving as its support. Th greatest weight raised at one time was about two tons.

'To obviate the danger of the bolt-holes in the horizontal flanges not tallying with each other the upper holes were drilled in place; the bolts having been inserted the joints were made good in iron cement.

'The exterior casing was erected first and next the central column; the two were then connected by the horizontal and diagonal bracing. The cupola surmounting the tower was afterwards placed in position. This cupola can be made to bear entirely on tho external casing, on the central column, or on both together. The cables being in place the cupola was wedged up so as to throw the weight on the central column. The proof load was then placed on the bridge and the joint between the cupola, and the casing made good. The object of this proceeding was to throw the weight equally on the casing and central column.

'The bridge was commenced on the 5th September, 1835, and finished in September, 1839, M. Emile Martin being the contractor for the ironwork.

'[The above article is prepared chiefly from an interesting account of the structure by M. Emile Martin.]'

Pont ferroviaire de Cubzac

This was built in 1885-1886 by Daydé & Pillé. Destroyed by the Germans in August 1943

Reconstructed in 1945-6 by Entreprise Générale Industrielle du Sud-Ouest. Brief description of the work in The Engineer .

See Also

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