NS Sangamon River Bridge (Decatur)

View an article discussing the reconstruction of this line and bridge

Located on the south side of Decatur, this massive deck plate girder bridge carries the former Wabash Railroad over the Sangamon River and Illinois Route 48. It is believed that the first bridge at this location was a wooden truss with timber trestle approaches. In 1889, the first bridge would be replaced by a large iron bridge, which was fabricated by the Detroit Bridge & Iron Works. The bridge consisted of three 123-foot, 6-panel, pin-connected Pratt deck trusses, set onto stone piers. The south approach consisted of a lighter 7-panel, pin-connected span and the north approach consisted of 730 feet of deck plate girder viaduct, supported by iron towers. These early trusses were extremely light, using lightly laced vertical members and eyebars for tension members. By the early 1910s, the Wabash double tracked the line between Decatur and St. Louis, and this bridge remained one of the few single track bottlenecks. Repairs were made to the bridge in 1912, which included strengthening the structure and converting the bridge to a gauntlet bridge. By converting the structure to a gauntlet bridge, the bridge could serve two tracks in the space of one.

By the 1920s, the iron bridge had become too light for traffic, and the Wabash began to plan its replacement. In May 1926, contracts were awarded for the reconstruction of the bridge, as well as the rebuilding of an underpass at Sunset (St. Louis) Avenue and constructing two new underpasses at Forest Avenue and Decatur Avenue. As part of the project, a large fill would be constructed, and the elevation of the bridge raised significantly. To maintain railroad traffic over the old bridge, it was decided to brace the old structure with timber falsework, and construct the new substructures offset of the old stone piers and iron towers. To clear the old trusses, a column type pier was used for most of the piers. This allowed a notch to be created to avoid disrupting the old bridge. Because the new bridge was taller than the old bridge, it was required to construct one track at the new elevation before demolishing the old bridge. Once one track was installed, the old bridge was removed and the second track constructed. Iron from the old bridge was disassembled, and moved to storage in Decatur for possible reuse. It is unknown if any spans of the old bridge were reused. Piers from the old bridge were demolished, and the stonework used for rip-rap to protect the new bridge. The project was started in 1926 and completed in 1927, costing over $850,000. American Bridge Company fabricated the superstructure, while Strobel Steel Construction Company completed the erection. The substructures and approaches were constructed by the Carmichael-Cryder Company.

As originally constructed, the bridge consisted of, from north to south: one 18-foot concrete slab span, three 50-foot deck plate girder spans, four 90-foot spans, four 106-foot spans and a 55-foot span, with an 18-foot and two 16-foot concrete slab spans approaching the south end. The entire substructure is constructed of concrete, which is founded on timber piles. With the exception of piers #9, #10 and #11, all piers use a concrete column design. These piers are constructed using two large columns, which are connected by concrete beams on the tops and bottoms. By using these piers, a considerable cost savings was achieved and less interruptions to railroad traffic were required. The remaining piers use a solid design, which support slightly skewed spans. These spans were skewed to accommodate the angle of the river. The deck plate girder spans are all heavily constructed, with deep girders and heavy bracing. Originally, these bridges used a precast concrete deck, which was specially designed to allow for the construction of half of the bridge at a time.

Deck plate girder spans were popular for railroad use, as they were durable and easy to construct. Most spans of this design did not exceed 90 feet long. However, by 1920s larger spans were able to be constructed. The 106-foot spans across the river were among the largest and heaviest spans when constructed. Concrete slabs were used for an approach, which allowed the embankments to settle without potentially damaging the main steel superstructure. These types of spans were popular, as they could be constructed offsite and installed with minimal interruption to traffic. Since the initial construction, the bridge has seen a number of changes. The original slab approaches and concrete deck were replaced in approximately 1995 with modern versions of the same design. At this time, a modernized drainage system was installed on the bridge. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The bridge remains among the largest steel and concrete railroad viaducts in Illinois. The author has ranked the bridge as being moderately significant, due to the size of the structure.

Because the abutments from the previous bridge were too narrow, it was impossible to raise the bridge while keeping the abutments at the same location. As a result, it was decided to construct piers on the old abutments, and add concrete slab spans on either end. To improve the visual appearance of the structure, a concrete curtain wall would be constructed along the approaches, appearing as a solid abutment. Currently, the bridge consists of a 45-foot through plate girder span, approached by two 40-foot concrete slab spans on either end. The entire bridge is set onto concrete substructures, and the main span runs at a slight skew. The girder span uses a standard Wabash design, including a ballast floor consisting of parallel I-beams covered by a timber deck and square girder ends. American Bridge Company fabricated the girder span, while an unknown contractor constructed the original concrete abutments. The new approaches and substructures were constructed by the Carmichael-Cryder Company, which had the contract for concrete work on the entire project.

Through plate girder spans were popular for railroad use, as they were durable and easy to construct. The Wabash preferred solid floor spans, as it reduced the amount of maintenance required. The process of raising the bridge and constructing new approaches was an ingenious solution to a complicated problem. During construction, traffic over the bridge needed to be maintained. A temporary trestle was constructed on the west side of the bridge, which allowed uninterrupted work on the rebuilding of the bridge. Since the 1927 reconstruction, the bridge has seen little changes. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked this bridge as being locally significant, due to the common design.