IAIS Big Bear Creek Bridge

Located along US Highway 6 on the west side of Marengo, this through truss bridge carries a former Chicago, Rock Island & Pacific Railroad (Rock Island) mainline over Big Bear Creek. The first bridge at this location was likely a timber pile trestle bridge, constructed when the line was first built. It is believed that this bridge was replaced by an iron quadrangular lattice through truss span, set onto stone abutments in approximately 1888. Between 1899 and 1902, the Rock Island invested significant capital into this line, replacing aging bridges with new steel and concrete structures. In 1901, the bridge was reconstructed with a through truss span, approached by three 44-foot deck plate girder spans on the west end. As part of the work, new concrete piers and a new west abutment were constructed, and the east abutment and east pier were modified with concrete. In 1926, the bridge was completely rebuilt, retaining the through truss span and substituting a large through plate girder span in place of the original west approach. In addition, all of the substructures were completely rebuilt with concrete, and a timber pile trestle approach was added to the east end. The most recent alteration to the bridge came in 2012, when the east approach was replaced by concrete modular girder spans, giving the structure its present configuration. Currently, the bridge consists of a 113-foot riveted quadrangular lattice through truss span, approached by a 113-foot through plate girder span on the west end and three concrete modular girder spans on the east end. The entire bridge is set onto concrete substructures, except for the east approach, which is set onto steel pile bents.

The truss span follows a standard design for the era, with heavy solid and built-up members, a standard floor system and unusual portal bracing. The endpost and top chord both consist of built-up beams, composed of two U-shaped channels joined by V-lacing on the interior and solid plates on the exterior. The bottom chord is composed of two U-shaped channels, which are connected by thin plates spaced throughout. The diagonal members are composed of a number of different designs, including light V-laced beams, solid beams and L-shaped bars. The vertical members at the end of the span are composed of four L-shaped bars, arranged into two sets of two and connected by thin plates. In addition, V-laced crash bars are present at each end of the truss. The floor system is traditionally composed, using heavy plate girder floorbeams at the panel points and two plate girder stringers. Both the upper lateral bracing and lower lateral bracing are constructed of standard L-shaped bars. The portal bracing follows a unique design, consisting of a solid plate with decorative cutouts and a date cutout. This plate is braced by an additional triangular heel bracing, which also has a decorative triangular cutout. The sway bracing is composed of V-laced beams. The through girder span follows a standard design, with two massive plate girders and a standard floor. At the west end, the girder ends are both rounded and tapered, while the span uses straight ends at the east end. The east approach follows a typical design for modern railroad bridges, with ballast deck concrete beams set onto concrete capped steel pile piers. Both the main piers use a rounded diamond shape, which is reinforced with a piece of scrap rail in the nose. The west abutment uses a standard U-shaped design, typical for Rock Island bridges.

Lassig Bridge & Iron Works fabricated the truss span immediately prior to their merger into American Bridge Company, while the west span was fabricated by American Bridge Company. An unknown contractor completed the concrete substructures, and an additional unknown contractor constructed the east approach. This type of truss design is relatively uncommon throughout the United States. However, a few railroads preferred the design, such as the Chicago, Rock Island & Pacific Railway (Rock Island), the Chicago & North Western Railway (C&NW) and the Chicago, St. Paul, Minneapolis & Omaha Railway (Omaha Road). The Rock Island began using the design in the early 1880s as an alternative to iron pin-connected spans. Spans constructed in the 1880s and 1890s used lightly built-up members and a decorative pedimented portal bracing. At the turn of the 20th Century, the Rock Island constructed a handful of significantly heavier trusses of this design, with a functional portal bracing and more solid members. It is believed the last spans of this design were fabricated for the Rock Island in approximately 1901 or 1902, when the railroad switched to using a more traditional riveted Warren design. The main detraction of the design was its statically indeterminate nature, while the design performed well in case of a derailment. Since the 1926 reconstruction, the bridge has seen minor alterations, including strengthening of the truss span with high strength bolts. Today, it remains in use as part of an Iowa Interstate Railroad mainline. Overall, the bridge appears to be in fair condition, with no significant deterioration noted. This bridge will likely be replaced in the coming years, as IAIS is seeking to replace all truss bridges along their lines. The author has ranked this bridge as being moderately significant, due to the truss design.