Summerset Trail - Middle River Bridge

Located at Banner Lakes at Summerset State Park, this unusual pony truss bridge carries a former Chicago, Rock Island & Pacific Railroad (Rock Island) branch line across the Middle River. Little is known about the history of this bridge. The first bridge at this location was likely a timber bridge, consisting of a timber truss main span and timber pile trestle approaches. It is also unclear if the bridge was rebuilt in the 19th Century. If the bridge was rebuilt, it was likely with another wooden truss span. In approximately 1930, the bridge was reconstructed with an unusual secondhand pony truss span, retaining timber pile trestle approaches. Currently, the bridge consists of a 100-foot, 10-panel, riveted Warren pony truss span, approached by four timber pile trestle spans at either end. The entire bridge is set onto timber pile substructures. It is believed that the truss span is an early example of the Warren design, originally fabricated in approximately 1890 for use at an unknown location. An unknown contractor fabricated the main span, while the timber components were likely constructed by railroad company forces.

The truss span appears to be an extremely early example of the Warren pattern, adapted for railroad use in a pony truss span. Similar to earlier double intersection Warren spans, the main span consists of light members and a heavy floor system. The top chord, endposts and bottom chord consist of T-shaped beams, formed out of metal L-shaped bars riveted together. The vertical members and diagonal members are all formed of L-shaped bars which are riveted together to form light T-shapes. The floor system is unusual, and consists of V-laced floorbeams and plate girder stringers. The floorbeams are set at the same elevation as the ties, which is also unusual for the era. It is unclear if the main span is constructed from steel or wrought iron. It is believed the span was fabricated during a time when railroads were switching between iron and steel. The approach spans are typical for the railroad and era, and consist of standard open-deck timber pile trestle spans, set onto timber pile bents. The main piers are composed of two rows of timber piles, which have been partially rebuilt with steel I-beams. Several components of the truss span are typical of early to mid-1880s double intersection Warren truss spans, which were the design of choice for the Rock Island during the era. Other features are unique to this bridge, such as the Warren design, laced floorbeams and the relatively long span length.

Between the late 1870s and late 1880s, the double intersection Warren (lattice) pony truss design was preferred by the Rock Island and some other Midwest railroads. During the late 1880s and early 1890s, some bridge manufactures experimented with other designs, including the Warren pattern. By the turn of the 20th Century, the Warren pattern had become standard for railroad truss bridges, oftentimes using heavy built-up members. This bridge represents an unusual configuration of a standard design, as the truss uses ten panels and light members. This is both unusually long for previous lattice spans, while unusually light for later Warren spans. One major innovator in bridge design was the Lassig Bridge & Iron Works, which worked extensively with the Rock Island in the 1880s and 1890s. Unlike other Lassig spans, this span has no signs of any builders plaque. The Rock Island used relatively few pony truss spans, and it is unclear if this span was reused from a mainline, a branch line or was possibly purchased from another railroad. Pony truss spans were most commonly used for medium-length spans, where a through truss would not have been appropriate but plate girders could not be constructed long enough. Since the initial construction, the bridge has seen few alterations. The only alterations include the reinforcement and partial replacement of the main piers with steel piles. After abandonment, the bridge was retrofitted with a wooden deck and railings, and now serves as part of the Summerset Trail. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted to the main span, approaches or substructures. The author has ranked this bridge as being regionally significant, due to the unique design.