NS Lake Decatur Bridge

View an article discussing the reconstruction of this line and bridge

Located on the east side of Decatur, this massive iconic concrete arch bridge carries the former Wabash Railroad over Lake Decatur, which is formed by the impounded Sangamon River. Little is known about the first bridges at this location. The first bridges were likely constructed of timber trusses, set onto stone piers. It is assumed that the bridge was upgraded a number of times between the initial construction and the 1884 reconstruction. In 1884, the bridge was replaced by a new iron truss bridge, set onto stone substructures. This bridge consisted of a 158-foot, pin-connected Whipple deck truss span, two 116-foot, 8-panel spans of the same design. These spans were approached by a 40-foot deck plate girder span on either end. Detroit Bridge & Iron Works, which constructed a number of bridges for the Wabash, fabricated the iron for the bridge.

By the early 20th Century, the Wabash was upgrading and double tracking the line heading east from Decatur. In 1906, a contract was awarded for the construction of a new double track concrete bridge, located approximately 900 feet north of the original bridge. As part of the project, a new embankment was constructed to carry the new alignment. Work on the new bridge commenced in July 1906, and was completed in the summer of 1907. During construction, work on each arch ring was commenced and completed without interruption, requiring continuous around-the-clock effort. After completion of the new bridge, the old structure was removed. At least one span is known to have been rebuilt and reused on a branch line near Malvern, Iowa. The new bridge was designed by Wabash engineer A.O. Cunningham, and the bridge was completed by William P. Carmichael Company. This company constructed many of the concrete bridges and substructures for the Wabash during the early 20th Century. The entire project cost approximately $117,000.

The bridge consists of four 100-foot closed spandrel concrete arch spans, set at a 45-degree skew. Each arch was designed with a clearance of 58 feet at the crown of the arch, and used radii of 55 and 75 feet. The arches are set onto concrete piers, and are approached by hollow concrete abutments. The piers are each supported by 259 piles, which were driven to a bed of gravel and reinforced with steel rods. Flat footings were used on the piers to help prevent scouring. The abutments use a two layered design, with vertical ribbed walls connected by cross walls supporting a slab deck, which is covered by a curtain wall. The abutments contain no wing walls, and extends straight back, parallel to the track. Unlike many arches, the bridge contains no parapet walls, and the ties rest slightly above the top of the structure. The arches and piers are heavily reinforced with steel reinforcement, mainly consisting of Johnson bars. The arches have both longitudinal reinforcement, as well as transverse reinforcement which overlaps the longitudinal reinforcement. The bridge was designed for a loading of 9,000 pounds per linear foot.

While concrete arch bridges became popular for railroad use in the early 20th Century, this structure represents an unusual and important evolution in the design of reinforced concrete arch bridges for railroad use. Open spandrel arches were preferred for large arches, as they minimized the amount of material required for a span and reduced the deadweight of the structure. Closed spandrel arches had mainly been used for smaller spans, particularly spans which were not as tall. The skew of the bridge is another unusual feature of the structure, as most engineers at the time preferred to make arch bridges square to minimize the complexity of the design. The use of a hollow abutment is another unusual feature of the structure. The Wabash used a number of hollow abutments on their bridges, as it was more cost effective than constructing a solid concrete abutment. The Sangamon River was impounded in 1922, creating Lake Decatur and giving the location its current appearance. Since the initial construction, the bridge has seen one major rehabilitation in 1943. It is believed this rehabilitation consisted of concrete repairs, and installed a drainage system on the bridge. This drainage system consists of pipes that are placed at low points inside the structure and protrude through the outside walls of the bridge. Overall, the bridge appears to be in fair condition, with some spalling noted throughout the bridge. Portions of the stone substructures from the previous bridge remain intact south of this bridge. The author has ranked this bridge as being highly significant, due to the design, size and age of the bridge.