IAIS Rock River Bridge (Milan, South)

Located at Vandruff Island in Milan, this large through truss bridge is the southern of two similar bridges carrying the former Chicago, Rock Island & Pacific Railway over the Rock River. The first bridge at this location was likely a timber truss bridge, set onto stone or timber piers. In 1881, the bridge was reportedly replaced with three 134-foot, 9-panel, pin-connected Pratt through truss spans, set onto stone substructures. Little is known about the history of these spans, or what they looked like. Further repairs were made to the bridge in the early 20th Century, including concrete repairs to the original stone piers. In 1930, the bridge would be reconstructed with new truss spans, and the substructures encased in concrete. To accommodate a longer south span, a new south abutment would be constructed.

Currently, the bridge consists of one 150-foot and two 134-foot riveted quadrangular lattice through truss spans, set onto concrete substructures. The truss spans utilize variations of a standard design, including laced compression members, bars for tension members and a floor constructed of two stringers, built of plate girders. However, all three spans are visibly different, likely indicating different construction dates. The south span utilizes a portal bracing consisting of a standard Lassig Bridge & Iron Works design, including a pedimented portal bracing with six lattices and solid heel bracing with decorative cutouts. The north spans utilized a similar design, with four lattices, although the heel bracing was removed at an unknown time. The south span utilizes a tightly laced "crash bar" at the end panels, while the north spans utilize a lighter version. All three spans appear to have different floor depths, and utilize slightly different member designs. These spans appear to have been fabricated in between approximately 1885 and 1890, and were reused from unknown locations. The northernmost span appears to be the lightest and possibly the oldest span, while the center span appears to be the heaviest and possibly newest. Numerous examples of this type of span were constructed for the Rock Island in the 1880s and 1890s, with many of the spans being replaced and reused elsewhere in the early 20th Century. The concrete substructures appear irregular, possibly indicating that older stonework was encased. The truss spans were likely fabricated by Lassig Bridge & Iron Works, although this is unconfirmed.

Although the lattice truss design was popular in Europe, few American railroads utilized the design. The Rock Island utilized the design extensively between the late 1870s and the early 1900s, and it appears to have been the standard truss for the railroad. Unlike railroads such as the Chicago & North Western Railway which used the design near exclusively, the Rock Island also used other truss designs where practical. The lattice truss design appears to have first been introduced to the Rock Island by Leighton Bridge & Iron Works, which constructed some of the first iron lattice trusses for railroad use. After the dissolution of Leighton Bridge & Iron Works, Alden and Lassig Bridge & Iron Works and later the Lassig Bridge & Iron Works adopted and tweaked the design. It is unknown if other fabricators also constructed these types of spans for the Rock Island. This type of truss span offers two main drawbacks. The span length was generally limited to 150 feet, and the design was not statically determinant under loading conditions. Despite this, this type of lattice truss was generally noted for its durability and resilience in a derailment.

Railroads often reused steel spans, as it provided a cost effective way to replace bridges while minimizing new material required. The Rock Island ran a sophisticated bridge program in the early 20th Century, electing to strengthen, rebuild and relocate spans to minimize orders of new material. No evidence such as empty rivet holes can be found on the south span, indicating that this span was largely unaltered at the time of relocation. This span still contains the original floor system and all truss members. The northern truss spans appear to have been strengthened more, and have had modifications to the portal and heel bracing. Since the last reconstruction, there have been little changes to the bridge. Minor repairs to the superstructure appear to have been made in the 20th Century. Overall, the bridge appears to be in fair condition, with no significant deterioration noted. The author has ranked this bridge as being regionally significant, due to the design and age.