Ango: Trussed Floors
A French architect named Ango, or Angot, introduced an innovative flooring system known as planchers à fermettes - trussed floors - in the 1780s. The system used iron beams in the form of shallow bowstring girders, set in a filling of plaster, and later of hollow pots and plaster, and offered strength and fire resistance. See here[1].
Ango reported his invention to the Académie Royale d’Architecture in 1782. The Academy appointed four commissioners to examine the construction. Theur report was made public in 1785.
Each iron beam had two wrought iron bars of rectangular cross section, spanning the full width of the floor. The upper bar was curved, and the lower bar was straight. The two were joined at the ends, which were serrated (pairs of buttressed 'teeth'), and held together here by stirrups. There were several spacers in the gaps between the bars. It is not clear whether the lower bar was pre-tensioned to pre-compress the upper bar, or whether the bars only became stressed when loaded. However, pre-stressing is unikely, given the practical difficulties involved.