Pont Saint-Thomas (Strasbourg)
This cast iron arch bridge crosses the River Ill in the centre of Strasbourg with a span of 31.1m.
Built in 1841, it is one of the oldest cast iron bridges in France.
Ironwork by Dietrich of Reichshoffen. Constructed under the direction of Nicolas Cadiat, to the design of the engineer Antoine-Rémy Polonceau. It is one of the oldest cast iron bridges preserved in France
Polonceau's arch ribs were of a novel design, having a laminated wooden core inside hollow cast-iron main arches, bonded to the metal by bitumen. See 'The Iron-Wood Composite Section of the Carrousel Bridge in Paris (1834)' by Jorge Bernabeu Larena.
The Pont Saint-Thomas, the Pont de Bourguignon-les-Conflans and the Puente de Isabel II (Triana Bridge), Seville are surviving examples of this type of structure. The pioneering Pont du Carrousel was demolished in 1937.
Two other bridges of this type existed in Strasbourg, the Pont du Corbeau and the Pont Saint-Nicolas, but they were replaced by concrete bridges at the beginning of the 20th century. However, the source of this information also states that the cast iron Pont du Corbeau was recovered and reused for the reconstruction of the Pont Saint-Nicolas, between 1892 and 1895.[1]
Polonceau's arch ribs are nominally of elliptical cross section, but with flanges at the top and bottom to allow the two halves of the tube to be bolted together, and with a rib at the '3 o'clock' and '9 o'clock' positions. The bolted construction was adopted instead of casting the pieces as lengths of 'pipe', one aim being to minimise the risk of having unequal wall thickness resulting from the foundry core shifting in the mould. This type of construction was also necessary to allow the permanent wooden core to be accommodated. The transverse stiffness offered by the tubular section reduced the amount of cross-bracing required between the arch ribs.
The aims of the laminated wooden core in the arch ribs were to reduce the susceptibility to vibration, to improve the load-carrying capacity of the ribs, to improve the resistance to shock loads, and to facilitate the positioning of the iron pieces during construction. Perhaps the latter claim was making a virtue out of necessity. In fact Polonceau listed seven advantages of his system[2], some of which reflected concerns about potential defects in the castings, and he stated that having the wooden core would allow broken or otherwise defective castings to be replaced fairly readily.
Polonceau's expectation was that the bitumen, poured in hot to fill the space, would ensure adhesion between the iron and the wood. The Paper by Jorge Bernabeu Larena provides the results of an assessment showing that the load-carrying capacity of the ribs was improved by the wooden core, regardless of whether it was successfully bonded by the bitumen. However, presumably a close fit was necessary in the absence of adhesion. Polonceau carried out small scale bending tests using thin cast iron pipes 1 metre long, 60 mm diameter, and 5 mm wall thickness. Polonceau himself admitted that the tests were by no means representative, but they did show that the ultimate strength of the tubes was markedly increased by the presence of the bitumen bonded wooden cores. Interestingly, the amount of deflection before failure was also increased.
Each arch rib was assembled from 20 castings of semi-elliptical section, bolted together via the flanges at top and bottom. The joints at the ends of these castings are staggered. At the mid-length of the castings there is a purely decorative 'dummy joint'. The dummy joint on the upstream face corresponds to the actual joint on the downstream face. At the end face of each joint there are three wedges (top, bottom and side). These wedges are carefully fitted to suit recesses in the castings, prepared by chiselling and filing, Driving in these wedges produced compressive loading in the arches. It follows that this must have been done before fully tightening the bolts in the top and bottom flanges.
See here for information on all the bridges in Strasbourg[3]