06/13/2026
John Roebling Part 2 of 2
Once John Roebling had established his reputation as a manufacturer of wire cable, he turned his attention to the application of wire to the building of suspension bridges. In 1844 the old 1092-foot long wooden aqueduct, which brought the Pennsylvania Canal into downtown Pittsburgh across the Allegheny River, was rated as unsafe by canal engineers. The aqueduct had all sorts of problems. Once, a section of the channel bottom dropped out, draining water out of good portions of the canal system on both sides of the river. Flood damaged various individual spans. In 1845, a fire hastened the decision of the canal commissioners to build a new aqueduct. They contacted John Roebling for advice.
Roebling felt the problem could solved with a "bundled" wire cable suspension structure and laid his plans before the canal engineers. There was considerable opposition but Roebling was finally told to proceed. He rebuilt the seven-span structure with 162-foot slack spans of two 7-inch diameter bundles of 1900 wires each, laid parallel to each other, taking great care to ensure equal tension in each wire. Each cable was protected and tightly bound together by an external wrapping of annealed wire.
The success of Roebling's first structure in Pittsburgh led immediately to another. A suspension replacement to the Smithfield Street Bridge over the Monongahela in 1847, on the piers of the old wooden structure destroyed by the Great Fire of 1845. The bridge had eight suspension spans of 188 feet each, supported by two 4 ½ inch diameter cables. It had cast iron suspension towers 16 feet high and a 35 foot roadway which carried two lines of car tracks, pedestrian promenades on both sides, and the heaviest kind of street traffic, for the next 35 years. -Three Hundred Years with the Pennsylvania Traveler by William H. Shank. (jh)
Image: Smithfield Street Bridge.
