Camille Polonceau
Jean-Barthélémy Camille Polonceau (born Chambéry, 29 October 1813, died Viry-Châtillon, 21 September 1859), was a French railway engineer, the son of Antoine-Rémy Polonceau. He was the first cousin of Gustave Ernest Polonceau (1832-1900), a railway engineer.
When he left school he worked for Auguste Perdonnet, chief engineer of the Compagnie du chemin de fer de Paris à Versailles-Rive-Gauche. He worked on the construction of the line from Paris to Versailles-Rive-Gauche, and in designing a small railway building in 1837, he intriduced a system of “framework in wood and iron” which would become the "Polonceau Truss".
The above information is condensed from the French Wikipedia entry.
The 'Polonceau Truss', is also known as the Wiegmann Truss or the Wiegmann-Polonceau Truss.
Polonceau patented it in 1837, and described it in 1840 under the heading 'Notice sur nouveau système de charpente en bois et fer' (Note on a new timber framing system) in the Revue générale de l'architecture et des travaux publics [1].
Polonceau proposed that his roof trusses would be made of wood and iron or all iron (using wrought iron for tension members and cast iron for compression members.
In fact lightweight iron roof trusses had already been in use for some years. For example, Boulton and Watt had made iron roof trusses for Chorlton New Mill in Manchester in 1815. These had cast iron rafters and king posts and wrought iron tie bars. What the Chorlton Mill trusses did not have and Polonceau's did, was a method of pre-tensioning or adjusting the tension in the tie bars. In his 1840 article, Polonceau showed two tightening methods - nuts on the end of the threaded tie bars, or tapered cotters - but it is not clear whether these were for pre-tensioning, or merely to facilitate assembly.
Note: Iron trussed floors were in use in France in the 18th century. See Ango: Trussed Floors.