IAIS 4th Street Bridge (Davenport)

Located in Davenport, this large through truss bridge carries a former Chicago, Rock Island & Pacific Railroad (Rock Island) mainline over 4th Street and Pershing Avenue. An identical bridge with a similar history crosses 3rd Street and Iowa Street nearby. Bridges at this location date to 1856, when the Rock Island was first extended into Davenport. It is believed the first bridge was likely a wooden truss or timber pile trestle bridge. The bridge was replaced with a double track 129-foot pin-connected iron Pratt through truss bridge in approximately 1872, which was set onto stone substructures and fabricated by an unknown contractor. After years of heavy use, the bridge was replaced by a new 129-foot double track riveted iron quadrangular lattice through truss span in 1889, reusing the stone abutments from the previous bridge. This bridge was fabricated by the Lassig Bridge & Iron Works, a firm which worked extensively with the Rock Island in the late 19th Century. The Rock Island extensively used lattice trusses in the 19th Century, as they were thought to be stronger and more resilient in case of a derailment. The 1872-era bridge was narrowed to a single track and reportedly installed at bridge #3001 (then #103) across Soap Creek near Eldon, Iowa. This span remained intact into the 20th Century, before it was ultimately replaced by a heavier through truss span and subsequently scrapped. In 1902, the original stone abutments were replaced with new concrete abutments in conjunction with an adjacent track elevation project. By the late 1920s, the lattice truss had become too light for railroad traffic and the roadway underneath required more space. Construction on a new bridge began in late 1927. During construction of the bridge, significant challenges were encountered. To lengthen the bridge, a temporary trestle had to be constructed at either end to cross the excavations for the new abutments. In addition, the railroad needed to maintain at least one track across the structure at all times. The present structure was completed in late December 1928. It is also likely that the lattice truss span was narrowed to a single track and reused at an unknown location.

Currently, the bridge consists of a 140-foot, 6-panel, riveted subdivided Warren through truss span, approached by a 15-foot steel stringer span on either end. The bridge is set onto concrete abutments and steel bent piers. The truss span follows a typical design for 1920s and 1930s Rock Island truss spans, with heavily constructed members, a heavy ballast deck floor and massive utilitarian-style portal bracings. Both the top chord and endposts consist of massive built-up beams, composed of two U-shaped channels connected by X-lacing on the interior and solid plates on the exterior. The bottom chord is constructed of a similar built-up beam, which uses X-lacing on both sides. All vertical and diagonal members are composed of heavy solid beams. The floor system is constructed using heavy plate girder floorbeams, which connect ten lighter I-beam stringers. Each track uses four heavier stringers with one lighter stringer on the outside edge. The entire deck is covered by a precast concrete ballast deck. The bottom chord is constructed of a "T"-shaped bar. Each portal bracing uses four "X" shaped bays, which also include an additional vertical piece at the bottom of the center of each bay. The portal bracing is connected to the endpost by triangular plates, which form the heel bracing. The entire portal and heel bracing is formed from X-laced beams, typical for trusses from this era. The sway bracing uses a lattice beam with four "X"s, and is formed out of L-shaped bars. The upper lateral bracing consists of X-laced beams. Each end of the truss is approached by a short steel stringer span, formed from several parallel beams and covered by a concrete ballast deck. The piers consist of two heavy steel columns, connected by a plate girder at the top. The truss span is pinned to the tops of the columns, and the bottom of the columns rest on steel castings, supported by concrete pedestals. Both abutments use a modified standard design, with sloped wing walls which extend in different directions.

Several contractors were involved in constructing the bridge. American Bridge Company fabricated the entire superstructure, which weighs an estimated 470 tons. Cedar Rapids Construction Company completed all the concrete work for the bridge, while the F.K. Ketler Company erected the structure and demolished the previous structure. It is unclear if the precast deck panels were constructed by railroad company forces or by the Cedar Rapids Construction Company. The Rock Island utilized several different truss configurations as their standard design between the 1860s and mid-20th Century. While pin-connected Pratt spans were initially popular with the railroad, this design gave way to lattice trusses in the 1880s and 1890s; followed by riveted Warren spans around the turn of the 20th Century. Subdivided Warren trusses became popular with some railroads in the 1910s and early 1920s. These spans allowed for greater strength than traditional Warren spans, while still maintaining a relatively simple design. The design is similar to the Baltimore truss configuration, although it maintains a Warren base configuration. Many similar bridges were constructed for the Rock Island system, and several spans remain intact today. Since the initial construction, the bridge has seen no significant alterations other than minor repairs. Today, only the north track remains in use as part of an Iowa Interstate Railroad mainline. Overall, the bridge appears to be in fair to good condition, with minor deterioration noted to the superstructure and particularly the precast deck panels. The author has ranked this bridge as being moderately significant, due to the truss design.