Designing by “Expérience”: Lecreulx Model Tests for the Design of the Abutments of the Bridge of Fouchard

In the 1770s the French engineer J.-R. Perronet introduced a new type of masonry bridge, with very slender piers and extremely surbased segmental arches. Circa 20 bridges of these kind were designed by Perronet and his disciples. The ratio height of arch to span was between 1/9 and 1/18. The flat arches would have exerted a great inclined thrust. The piers were equilibrated, but the abutments at both ends of the bridge had to support the thrust. The danger of failure by some kind of fracture of the buttress on the upper part was evident; in particular then danger of sliding failure. The thrust could be calculated following the La Hire theory, which, though incorrect, was safe, i.e., led to greater, more inclined thrust. However, to know the effect of the thrust in the massive abutments it was necessary to make tests on models. These were carried out in 1774 by F.-M. Lecreulx during the construction of the bridge of Fouchard. The results demonstrated the enormous danger of a catastrophic failure by sliding. He then proposed some dispositions to avoid this danger, prolonging the voussoirs of the arch inside the mass of the arch. Fouchard’s test must have been influential in the great increase of the size of buttresses from the original designs of the 1770s in all the bridges built (most completed after 1780). Eventually, this type of bridge was abandoned, but their short history (about 30 years) demonstrated the audacity, courage and faith in reason and experiment of Perronet and his disciples.