BNSF Aurora Viaduct

View an article discussing this track elevation project (digitized by Google)
View an article discussing this track elevation project (digitized by Google)
View an article discussing this track elevation project (digitized by Google)
View an article discussing this viaduct (digitized by Internet Archive)

Located in Aurora, this lengthy concrete slab viaduct carries the former Chicago, Burlington & Quincy Railroad over a number of city streets and parking lots. Since the CB&Q reached Aurora in the 1849, the city had served as a major terminal for the CB&Q. In the early 20th Century, railroad congestion became a serious concern for residents in Aurora. The City of Chicago had successfully ordered elevations of various railroads throughout the city, resulting in the construction of retaining walls, with underpasses constructed at the street crossings. Understanding the concerns of the public, the CB&Q sought to improve operations and fix the traffic problem by realigning and elevating the tracks through Aurora. Construction on the project began in 1914, although much of the work was delayed due to World War I. Work on the project resumed in 1919, and the new line was opened in November 1922. The entire project was completed at a cost of $4,500,000. The railroad spared no expense, constructing new facilities and including decorative features in the bridges. The use of retaining walls on the project was minimized, as discoloration and cracking proved to be visually unappealing on earlier track elevation projects in Chicago. Design and construction of the project was under A.W. Newton, Chief Engineer of the railroad. Company forces constructed all of the concrete work, and contractors were used for the earthwork.

One of the primary structures of the track elevation project was the large viaduct, extending from Benton Street to New York Street. Because the line was located in the heart of the business district, the railroad decided to use a continuous viaduct instead of traditional retaining walls. A cost analysis of the two designs found that the space under the viaduct would be valuable for parking, cellars or storage. In this area, the new viaduct would be constructed parallel to the original ground level tracks. The viaduct was planned to contain three tracks, with room to add an additional two tracks on the north side at a later date. Construction on the viaduct began in 1920, with work beginning with the foundations. Favorable conditions for foundations minimized the amount of excavation, and the columns were generally constructed to bedrock, located between 10 and 30 feet below street level. After the columns were constructed, falsework was erected, and the new spans constructed. The superstructure was constructed as a continuous slab, with joints only placed at expansion joints. Due to the length of the structure, work progressed from one end to the other; with substructure crews continuously staying ahead of superstructure crews. Upon completion, the viaduct would consist of 78 spans, and generally run at a curve. In addition, some spans of the bridge were constructed at a slight skew, to maximize space available underneath the viaduct. An estimated 15,000 cubic yards of concrete were used in the viaduct, and the viaduct cost approximately $220,000 to construct.

Since the initial construction, the viaduct has remained largely unchanged. The viaduct is generally 44 feet wide, and different sized spans were used throughout the structure. 25-foot spans were used at New York Street, Galena Boulevard, Downer Place and Benton Street, with 10-foot spans on the east end at New York Street and west end at Benton Street. The remainder of the viaduct consists of slab spans, which are primarily 19 feet long. 22 19-foot spans are used between New York Street and Galena Boulevard, 22 between Galena Boulevard and Downer Place, and 24 between Downer Place and Benton Street. The superstructure of the viaduct was constructed with continuous reinforcement, and generally used a thickness of 23 inches, with a thickness of 27 inches at the 25-foot spans. Expansion joints were placed at intervals of approximately 225 feet. The concrete columns of the bridge are primarily cylindrical, with rectangular columns at street crossings, and a double rectangular column at expansion joints. These columns are placed at an 18-foot transverse spacing, and consist of three columns per pier. The tops of the cylindrical columns is mushroomed, and is connected to the slab spans by use of a drop slab. The slab spans are roughly finished, although decorative paneling imprints were used at street crossings. A system of downspouts was used on the bridge to facilitate drainage, and much of the original drainage system appears to be intact.

When the bridge was constructed, this type of construction was considered a departure from normal practice. Continuous concrete structures such as this were not often used for railroad use, and were more difficult to construct than simple span bridges. The continuous nature of the structure also allows for a thinner than normal cross section, minimizing material costs. Other concrete slab structures from this era also used column piers, although these columns were typically connected by a concrete beam. This bridge also diverted from traditional designs in the use of individual columns and drop slabs. Despite the success of this structure, very few other continuous viaduct track elevations were constructed. Today, two of the three tracks remain in service, and the space under the viaduct is primarily used for parking lots. A number of buildings butt up to the viaduct. Overall, the bridge appears to be in good condition, with minimal spalling noted throughout the structure. Portions of the east end of the viaduct appear to have the most deterioration, and additional steel bents have been installed at pier #7 and pier #9. The author has ranked this bridge as being regionally significant, due to the unconventional design, construction methods used and excellent historic integrity.