John A. Roebling’s 1845 Allegheny Aqueduct: Design, History, and Performance

Abstract

The Allegheny Aqueduct (1845–1861) in Pittsburgh was the first suspension bridge designed and constructed by John A. Roebling (1806–1869), who would go on to design some of the most prominent and longest-spanning suspension bridges of the 19th century. The Allegheny Aqueduct was a seven-span suspension bridge built on the existing unreinforced masonry piers of the prior wooden-covered aqueduct. To evaluate the progression of Roebling’s innovations throughout his career, it is important to document his expertise and experience in suspension bridge design and construction at the onset of his career. This research used the published writings of Roebling and other contemporary engineers, archival documents, and modern nonlinear structural analyses to evaluate the design and performance of the Allegheny Aqueduct as built, as well as several possible design alternatives. The structural design of the multispan bridge required Roebling to accommodate unbalanced loading conditions from water within the aqueduct and from repair or removal of any single span, all while maintaining the stability of the masonry piers. Roebling developed a unique design, analysis, and construction approach to safely meet these criteria, and modern structural analyses demonstrated the soundness of Roebling’s approach. Roebling patented and constructed a new style of anchorage that was simpler to build, prevented ingress of moisture and corrosion of the iron bars, and allowed for straightforward rigid-body stability analysis. Roebling also patented, developed the mechanical equipment for, and implemented a new wire-by-wire cable fabrication (spinning) technique that resulted in a nearly uniform stress state amongst all wires of the cable and prevented ingress of moisture and corrosion of the wires. Roebling’s developments in anchorages and cables represented significant advancements over existing European techniques, and fundamental aspects of his techniques remain in use to this day.