IAIS Sylvan Slough Bridge

View an article discussing the construction of this bridge

Located in Rock Island, this large through plate girder bridge carries the former Chicago, Rock Island & Pacific Railway over Sylvan Slough. Sylvan Slough is formed by the south channel of the Mississippi River at Arsenal Island. The first bridge at this location likely consisted of a timber truss bridge, set onto stone piers. It is unknown if the bridge was rebuilt with a metal bridge during the 1870s or 1880s. In 1893, the bridge would be reconstructed with a new double track through truss bridge set onto stone substructures. This bridge consisted of two 150-foot quadrangular lattice through truss spans, approached by two 75-foot deck plate girder spans on either side. The truss spans utilized two parallel single track structures, instead of a double track structure. These trusses utilized a standard design for the era, including a combination of built up and solid members and a decorative pedimented portal bracing. As traffic on this line increased in the early 20th Century, the 1893 structure became inadequate. As a result, the bridge would be replaced by the current through girder in 1923. During the reconstruction, the elevation of the tracks was raised. It is likely that portions of the previous structure were reused elsewhere on the Rock Island system. As keeping the old bridge open to traffic was critical, the bridge was built while only impacting one track at a time. Due to the size, the largest girders were transported from Pittsburgh on two 30-foot artillery cars, which proved to be a challenge.

Currently, the bridge consists of five large through plate girder spans, approached by a 75-foot deck plate girder span on the west end. When the bridge was reconstructed, the west span was "twinned" by reusing the adjacent girders. The center three through girder spans each consist of 105-foot spans, set at a heavy skew. The outer spans are also heavily skewed, using girders of 114 feet and 95 feet. Some stone from the previous abutments was reused, although much of the substructure was reconstructed with concrete at the time of construction. The piers of the bridge utilize two separate columns, designed to reduce masonry quantities. The abutments are constructed of a U-shaped abutment, which reuses portions of the old stone. The through girder spans use a heavily constructed floor, a ballast deck and rounded and tapered ends, typical of Rock Island spans. Fort Pitt Bridge Works constructed the superstructure, while Bates & Rogers Construction Company constructed the substructures. Railroad forces were responsible for erection of the bridge, which was designed under the supervision of C. A. Morse, Chief Engineer and I.L. Simmons, Bridge Engineer. The bridge utilizes some decorative features, such as railings on the abutments.

Since the initial construction, this bridge has seen a number of alterations. The north track was removed from the bridge in the 1980s, although the ballast deck remains. In approximately 2015, the west deck plate girder span was replaced by a modern span. It is unknown if the north track span was replaced as well, although the deck appears to have been removed. When constructed, the 114-foot girders were among the largest plate girders ever constructed. Placing of these girders was a significant feat, and was an event that drew large crowds. Larger riveted and bolted spans of a similar design were constructed through the 20th Century, although spans on this bridge continue to remain notable for the length. Overall, the bridge appears to be in good condition, and well maintained. The amount of spalling and cracking in the substructures appears to be minor compared to structures of a similar age. The author has ranked this bridge as being moderately significant, due to the size of the girders.