IAIS Rock River Bridge (Milan, North)

Located at Vandruff Island in Milan, this large through truss bridge is the northern 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 1884, the bridge was reportedly replaced with three 155-foot riveted quadrangular lattice through truss spans, set onto stone substructures. These trusses utilized a pedimented lattice portal bracing with four lattices and bars for heel bracing, an unusual portal for this type of lattice span. The remainder of the truss components utilized a standard design, including laced compression members and bars for tension members. The stone substructures would be replaced with concrete substructures in the early 20th Century. In approximately 1890, the bridge would be replaced again, reusing secondhand spans of the same design.

Currently, the bridge consists of three 155-foot riveted quadrangular lattice through truss spans, set onto concrete substructures. The truss spans utilize a standard design, including laced compression members, bars for tension members and a floor constructed of two stringers, built of plate girders. The portal bracing of these spans utilizes a standard Lassig Bridge & Iron Works design, including a pedimented portal bracing with six lattices and solid heel bracing with decorative cutouts. The truss spans are all identical, and appear to have been fabricated between approximately 1885 and 1895. These spans were reused from unknown locations, and it is unknown if any of the spans were reused from the same location. 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.

At 155 feet long, the truss spans on this bridge are notably long for the design. 155-foot spans were a standard span size for the Rock Island, and a number of such spans were constructed. 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 truss spans, indicating that these spans were largely unaltered at the time of relocation. The truss spans still utilize the original floor system and all truss members. In addition, the trusses still retain the original portal bracing, and there is no evidence of other changes to the features of the truss. 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.