BNSF Des Moines River Bridge (Ottumwa)

Located west of Ottumwa, this through truss and deck plate girder bridge carries a former Chicago, Burlington & Quincy Railroad mainline over the Des Moines River. This bridge has an extensive history, including numerous reconstructions, alterations and partial replacements. The first bridge at this location was likely a single track wooden through truss bridge, constructed when the line was first built. In 1880, the bridge was rebuilt with five 140-foot iron Pratt through truss spans, set onto new stone substructures and likely approached with timber pile trestle spans on either end. In 1899, the CB&Q double tracked the line at this location. Instead of completely reconstructing the bridge, it was decided to extend the piers north with new stone and place five new 140-foot double track Pratt through truss spans on the piers. It is unclear if the previous bridge spans were reused at another location. In 1904, the bridge was further altered when a series of deck plate girder spans were installed on each end, set onto concrete substructures. The truss spans were strengthened with additional bracing at the corners of the portals in 1928, and the main span piers were completely encased in concrete in 1937 and 1943.

Between 1904 and 2001, the bridge consisted of one 50-foot deck plate girder span (originally span #1), four 60-foot deck plate girder spans (spans #2-#5), five 140-foot, 7-panel, pin-connected Pratt through truss spans (spans #6 through #10) and an additional seven 60-foot deck plate girder spans (spans #11 through #17). Both abutments (originally piers #1 and #18) and the approach piers (piers #2 through #5 and #12 through #17) were entirely constructed of concrete, while piers #6 through #11 were constructed of stone and later encased in concrete. Lassig Bridge & Iron Works fabricated the five truss spans, while King Bridge Company fabricated the remaining deck plate girder spans. The original stonework of piers #6 through #11 was constructed by an unknown contractor, and the additional stone extensions to those piers in 1899 was also likely constructed by an unknown contractor. It is unclear if the remaining piers and abutments were constructed by an unknown contractor or by railroad company forces. It is also unclear if the various alterations to the bridge were made by unknown contractors or by railroad company forces.

The truss spans all followed an unusual variation of a standard CB&Q design, with heavy built-up members, an unusual portal bracing and a standard floor system. Both the top chord and endpost used a built-up construction, consisting of a box shaped beam with a solid plate on the exterior and V-lacing on the interior. The bottom chord used two different designs, with a V-laced built-up box beam at the outer two panels and eyebars for the center three panels. The vertical members mainly consisted of V-laced beams, while the diagonal members mainly consisted of eyebars of different thicknesses. The floor system consisted of two heavy plate girder stringers per track and plate girder floorbeams. The lower lateral bracing was composed of L-shaped bars, while the upper lateral lacing used a set of two L-shaped bars connected by thin plates. The portal bracing used an unusual modified M-frame design, constructed using V-laced beams. The sway bracing consisted of an unusual X-laced beam. At the joints with the top chord, large angled brackets were installed during the 1928 strengthening to provide greater rigidity between the portal and sway bracing. The deck plate girder spans used a standard design, consisting of two heavy plate girders and an open deck. The main piers (piers #6 through #11) used a standard diamond shaped design, meant to protect the bridge from river ice. The approach piers all used a standard rounded rectangle shape, while the abutments used a standard rectangular shape with no wing walls.

By the end of the 20th Century, the main truss spans had become too light for modern railroad traffic, and plans were made to reconstruct the bridge. It was decided to replace the truss spans with spans of different designs and sizes, retaining almost all of the deck plate girder spans. Work began on the bridge in 2000, and the project was completed at the end of 2001. As part of the work, spans #1 through #4 were retained and span #5 was shortened to 30 feet. Piers #1 through #5 were also retained with no alterations. Two new 190-foot, 8-panel, riveted Warren through truss spans (spans #6 and #7) were installed on new concrete piers (piers #6, #7 and #8). These spans are flanked by five 70-foot steel stringer spans on the west (spans #8 through #12), which are set onto two new concrete piers (piers #10 and #12) and reused three old piers (piers #9, #11 and #13; formerly piers #9, #10 and #11). The entire west approach (renumbered spans #13 through #19) and west approach substructure (renumbered piers #14 through #20) were retained with minimal alterations. The truss spans use a standard design for the era, with members constructed from heavy punch-plate style beams, high strength bolts, a ballast deck and an M-frame portal bracing. The steel stringer spans also follow a standard design, with six beams per track and a ballast deck. All new steel consists of a pre-weathered material. The new piers consist of drilled shafts connected by a concrete beam and diamond shaped piers. Unknown contractors fabricated and constructed the new spans and piers, and Hanson Professional Services, Inc. oversaw the design.

While replacement plans were underway, an intensive historic survey was conducted on the bridge between 1999 and 2000. It was determined that the bridge was eligible for the National Register of Historic Places under Criterion "A" and "C". As part of a Memorandum of Understanding, extensive documentation of the bridge was required before it could be reconstructed. The Pratt truss design were arguably the most popular railroad truss pattern in the second half of the 19th Century, as it was simple, economical and easy to construct. This design was largely superseded by Warren spans around the turn of the 20th Century. This design provided greater rigidity while maintaining the simplicity and economy. Deck plate girder spans were popular for railroad use, as they were durable and easy to construct. Since the initial construction, the bridge has seen no significant alterations, and remains in regular use. Overall, the bridge appears to be in fair to good condition, with some spalling noted to the original substructures and deterioration noted in the deck plate girder spans. The author has ranked this bridge as being locally significant, as it has been heavily altered and no longer contains the historic through truss spans.