The structure has two segments: an East Channel bridge consisting of four 175-foot spans and three 240-foot spans crossing from Harrisburg to City Island; and a West Channel bridge, consisting of seven 175-foot spans crossing from City Island to Wormleysburg.
With 15 truss spans totaling 2,820 feet, the Walnut Street Bridge is the finest and largest surviving example of the standardized Phoenix wrought-iron truss bridges produced from 1884 to 1923.
Civil engineer Samuel Reeves founded the Phoenix Bridge Company. Wendell Bollman, inventor of the Bollman Truss, created the company's Phoenix column - a circular compression member assembled from flanged, rolled wrought-iron segments. The superstructures of these bridges were completely prefabricated of wrought iron in the company's plant at Elizabethtown, Pennsylvania, shipped to the site, and erected by local semi-skilled labor.
The Phoenix Bridge Company prided itself on its ability to deliver written estimates based on the answers to five questions regarding span, width, and clearance, and to deliver a finished product to the site. As such, the company was major force in the explosive expansion of the transportation infrastructure in the United States.
- The structure has two segments, an East Channel bridge consisting of four 175-foot spans and three 240-foot spans crossing from Harrisburg to City Island, and a West Channel bridge, consisting of seven 175-foot spans crossing from City Island to Wormleysburg.
- The bridge was closed to vehicular traffic in 1972 as a result of floods during Hurricane Agnes, but remained open as a widely used pedestrian bridge and recreational amenity.
- On January 20, 1996 two spans of the West Channel bridge were lifted by an ice jam during a flood and carried downstream to lodge beneath the Market Street Bridge. A third span collapsed due to flood damage. The surviving spans still comprise the premier example of the Phoenix truss.
- The rapid growth of the iron bridge industry gave rise to an array of truss types and sharpened the analysis and design methods that the civil engineering profession used for the design of bridges in general, and truss bridges in particular.