Invention of the thin iron wire cable

Among the innovative practices of the Tournon-Tain bridge, that of fine iron wire cables attracts particular attention.

Seguin also widely highlights it as the key to the success of the suspension bridge in his book Des ponts en fil de fer (Wire Suspension Bridges).

This is one of the bases of the project, because the French civil engineer understood the weaknesses of English forged chains as well as their great difficulty of manufacturing in a country like France in the 1820s.

Using iron wire from Franche-Comté, of which this is one of the specialties, he assembles them into bundles of numerous parallel wires with a section of 3 mm. It thus reduces the risk of breakage and benefits from their better resistance compared to forged bars.

He presented this discovery of resistance of materials to the Academy of Sciences in September 1823.

This scientific success led to the acceptance of the proposal by the ‘Ponts et Chaussées’ (Bridges and Road administration), until then opposed to the suspension wire cable which they judged to be too uncertain and never used before by the English.

The parapet’s rigidity structure

If the idea of the fine wire for the suspension cables appeared at the origins of the project, other innovations were gradually dictated by the reality of the test footbridges on the Cance in Annonay, then on the Galaure in Saint-Vallier: the suspension bridge oscillates as its users pass by!

A particularly difficult problem to control and understand physically (the mechanical resonance of elastic structures).

With the help of his collaborators, notably the master carpenter Barjot, he gradually imagined an apron with a rigid structure; and this is a field where the Ardèche engineer is once again doing pioneering work on an international scale.