John A. Roebling’s 1846 Smithfield Street Bridge: History, Design, and Performance

The Smithfield Street Bridge (1846–1883) in Pittsburgh was the first roadway suspension bridge designed and constructed by John A. Roebling (1806–1869). The eight-span bridge was built on the surviving masonry piers of a previous wooden covered bridge. Roebling's design combined main suspension cables, pendulum cable supports, stiffening trusses, inclined stays, and underfloor bracing to create a complex, indeterminate structural system. The multispan bridge needed to accommodate heavy moving loads and unbalanced loads between adjacent spans without excessive deflections. In the mid-19th century, theoretical analysis of suspension bridges was limited to the calculation of tension in an unstiffened cable, yet Roebling applied sound engineering principles to design a bridge that was well beyond the structural analysis capabilities of the day. The design, performance, and end-of-life of the bridge were examined through Roebling's archived design calculations and published articles, other contemporary published literature works, and modern nonlinear structural analyses. The research revealed that the overall design of the bridge was sound, with typical stresses and deflections well within allowable limits, and it performed successfully as originally designed and constructed. The underfloor bracing system carried a substantial portion of the live load and helped to limit displacements. However, the pendulum cable hangers created a very flexible cable system, and the stays were likely ineffective. At the time of its replacement, the Smithfield Street Bridge was carrying loads much heavier than it was designed for, and its stiffening truss suffered from substantial degradation. A comparison of structural analysis results with contemporary descriptions indicated that it was functioning largely as an unstiffened suspension bridge. Over the remainder of his career, Roebling would refine his understanding of, and design methods for, suspension bridges with deck stiffening and inclined stays, ultimately leading to his designs for several of the most prominent and sophisticated bridges of the 19th century.