1870s innovation in London Canada's Blackfriars Bridge

Blackfriars Bridge in London, Canada, is a wrought-iron bowstring-arch-truss bridge fabricated by the Wrought Iron Bridge Company of Canton, Ohio and erected in 1875. Although typical of the 19 km of wrought-iron bowstrings constructed by the company in this period, the bridge is one of the first to feature a ‘double-panel’ web diagonal arrangement that was patented in the USA one year after the bridge was constructed. Considering the state of structural engineering knowledge in the 1870s, this paper investigates whether the structural response of the double-panel diagonal could have been correctly analysed by the original designers or whether the bridge may have been constructed as a prototype to validate this innovative feature. Design live loads are determined from values specified in the company's 1874 book of designs that depend on whether the bridge is located in the country, a town, a second-class city or a first-class city. The truss, statically indeterminate to the thirteenth degree, can be manually analysed neglecting slender web diagonals in compression if the participation of vertical members above the double-diagonals is ignored. The patent highlights the structural contribution of these members, however, and because their participation can only be accurately assessed using modern computer analyses, Blackfriars Bridge may have been built as a prototype to validate the double-panel design feature.