The Evolution of the Iron Truss in the Work of John Rennie

Dublin's nineteenth-century Tobacco Store built by John Rennie(1761-1821) is considered significant due to its cast-and wrought-iron roof trusses. Though by no means the first to experiment with iron, Rennie had been a firm believer in the superiority of this material over timber in warehouse construction and used it extensively throughout his portfolio. Less acknowledged is that a similar, yet more advanced truss was used in the New West Stores (demolished 1988), also built as part of the Dublin Docks ensemble by Rennie. That this achievement was overlooked was a result of the roof structure being replaced with a less adventurous timber roof following its collapse in afire in 1833, ten years after its completion.Archival evidence suggests that the original cast-and wrought-iron truss designed by Rennie for the New West Stores mirrored that of the earlier Tobacco Store in profile consisting of queen-post truss, surmounted by a lantern structure and a smaller kingpost truss. However, this later truss far exceeded the earlier structure in span, being 52 foot 6 inches in comparison to the 38 foot 9 inch span of the Tobacco Store. As studies in the evolution of iron truss technology have suggested that the span of early trusses were limited to 40 feet, which was not exceeded until after the 1830swhen such inventions as the Polonceau truss and various arched truss technologies were introduced, this early truss by Rennie was a significant achievement in the 1820s. In addition to its span, it is probable that this truss also represented an evolution in Rennie's connection details. Prior to his work in Dublin Rennie had favoured connections made with slotted assemblies, using wedges and cotter pins to tighten and hold pieces together, which were of-ten dependent on the structure's own self weight to provide stability –a strategy employed in the London Tobacco Dock, the West India Dock sheds and even the Southwark Bridge. The truss work in the Dublin Tobacco Store mirrored this tendency, until the connections were re-worked during a recent renovation. Evidence from Rennie's resident engineer John Aird (1760-1832) and later work by his sons at the Royal William Victualing Yard, suggest that the connections used in the New West Stores truss may have represented an evolution in this thinking, with the introduction of bolted connections to the assembly.