Carpentersville Railroad Bridge

Once located across the Fox River in Carpentersville, this historic through truss bridge carried a Chicago & North Western Railway spur serving the Illinois Iron & Bolt Company, which operated a sprawling facility on both banks of the river. The first bridge at this location was constructed in 1879 and consisted of a lengthy pile trestle. By the 20th Century, this bridge had become outdated for railroad traffic, and the narrow clearance between bents proved costly to maintain. Reconstruction of the bridge was authorized in 1924 and completed in March 1926. As part of the work, the railroad installed a secondhand truss span and six new steel stringer spans, and rebuilt the remainder of the bridge in kind. The bridge consisted of a single timber pile trestle span on the east end; a 98-foot, 9-inch riveted quadrangular lattice through truss span; two 36-foot steel stringer spans; 16 spans of timber pile trestle; four additional 36-foot steel stringer spans; and four additional timber pile trestle spans. The bridge was set entirely on timber pile substructures and included a significant curve at the west end, which resulted in the western set of steel stringers utilizing a heavy skew.

This truss span followed a typical design for the first generation of quadrangular lattice through truss spans constructed for the C&NW, with light members and an attractive arched lattice portal bracing. The top chords of the spans were composed of a lightly built up beam, composed of two L-shaped plates covered by a solid plate on top. The bottom chord was constructed of two parallel L-shaped plates. The end posts used a built-up design commonly seen on early truss spans, with two channels connected by Z-bars between the channels and a solid plate on the exterior. The diagonal members used a combination of L-shaped bars and tightly X-laced built-up beams, with the crash bars at each end using a lighter X-laced beam. The floor system used a standard design, with four light stringers connected to plate girder floor beams. These floor beams used an unusual shape, which is deeper in the middle with the ends sitting on the bottom chord. Both the upper lateral bracing and the lower lateral bracing were constructed of L-shaped iron bars, which are riveted to the top and bottom chords. The sway bracing of the bridge consisted of light L-shaped bars, while the portal bracing used a unique curved lattice design. The approach spans all used a standard design, consisting of six shallow beams arranged into two sets of three. It is believed that the truss span was either fabricated by the Leighton Bridge & Iron Works (prior to mid-1881) or the Alden and Lassig Bridge & Iron Works (after mid-1881). An unknown contractor fabricated the approach steel stringer spans, and the timber components were constructed by railroad company forces.

The truss span was originally fabricated in approximately 1880 for use at an unknown location. After the span was replaced at its original location in 1917-1918, it was installed at Bridge #726 across the White River near Glen, Nebraska in 1918. The span was short lived at this location, as it had become too light for traffic and was replaced with a girder span in the early 20th Century. The span was then shipped to the bridge yard in Chicago, before ultimately being installed here. While the original location of the truss is unknown, the unusual size may give some clues to its origins. Annual Reports of the Chicago & North Western Railway indicate that a 98-foot 9-inch truss was built at Peshtigo, Wisconsin in 1881. In a period between 1875 and 1885, this was the only span of this size listed in these reports. While other truss spans of this size may have been constructed, the C&NW seemed intent listing major iron bridges constructed each year. It is believed that the Peshtigo Bridge was replaced sometime around 1917, which may provide further evidence to its reuse at Bridge #726 and ultimate reuse here. Research on the original location of this span is ongoing.

While a few other railroads, such as Union Pacific, Rock Island and Erie all used the quadrangular lattice through trusses at various times, no railroad relied on the design as much as the C&NW. Between 1878 and 1930, through trusses constructed by the C&NW nearly exclusively used this design. Although lattice trusses were not popular with American railroads after the 1880s, the C&NW relied heavily on this design, as it had proven to be resilient towards derailments and damage. The main drawback of the design was the unpredictable nature, as the structure was not statically determinant. Despite this, numerous examples of this design continue to serve traffic today. The lattice truss design appears to have first been introduced to the C&NW by Leighton Bridge & Iron Works, which constructed some of the first iron lattice trusses for railroad use. Early designs of this truss utilized members constructed with L-shaped bars, light plates and tightly laced built-up beams. These early designs also used boxy end posts and upper chords, and a portal consisting of an arched lattice, with lattice heel bracings. After Leighton sold the company to Alden and Lassig Bridge & Iron Works in 1881, this company continued to produce a modified version of the design, using a pedimented lattice portal. In 1886, John Alden and Moritz Lassig split partnership, and Lassig Bridge & Iron Works continued to construct the design. Spans constructed from approximately 1885 until the mid-1890s utilized heavier members, a pedimented portal bracing with lattice on the sides and decorative cutout heel bracing. 20th Century versions of this design included much heavier members and heavily constructed portals with no decorative features.

The bridge at Carpentersville was an excellent remaining example of the earliest generation of the quadrangular lattice through truss design, constructed between approximately 1878 and 1883. Upon relocation, the bridge received some alterations, including the apparent replacement of the original stringers. As a spur bridge, this bridge received relatively little maintenance. Timber components of trestle bridges generally need to be replaced on a regular schedule. It is likely that the timber components of the bridge were last updated in the 1940s or 1950s. By the late 20th Century, the bridge had begun to significantly deteriorate. Flooding, ice and age had all weakened the substructures and timber stringers of the bridge. High water in 2012 or 2013 caused the pier between the middle of the eastern two stringers to collapse, sending the spans into the water. All of the timber piers contained significant structural deterioration, with piers #6 and #7 under the west approach partially collapsing. The eastern two piers holding the truss had significant rotting of nearly all piles, and some piles which had completely failed. Due to concerns about the structural integrity and the impending removal of the dam upstream, the bridge was demolished in early 2024. At the time of demolition, the bridge was in exceptionally poor condition, and on the verge of failure. Rotted timbers in the piers were causing steel spans to shift and tilt, distorting the truss span in particular. In addition, the floor of the truss span had severe section loss at locations which were reconstructed in the early 1920s. While there had been some community involvement asking for the preservation of the truss span, deterioration to the span and substructures it rested on likely made it unfeasible to attempt reusing the span. The author has ranked this bridge as being highly significant, due to the age of the truss.