Omaha Bridge #15

View an article discussing the construction of this bridge (digitalized by Google)

Located near the Lilydale area of St. Paul, this unique swing bridge carries the former Chicago, St. Paul, Minneapolis & Omaha Railway (Omaha Road) and Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) mainlines across the Mississippi River. The bridge is named as it is the fifteenth bridge on the Western Division of the Omaha Road, and has the unique feature of being the only counterbalanced swing span to cross the Mississippi River. In 1855, authority for the construction of a bridge at this location was granted by the territorial legislature of Minnesota. Work on the bridge began in the late 1860s, and was reportedly completed in 1869. The first bridge at this location consisted of a trestle approach on the south end, a 151-foot wooden Howe through truss span, a 270-foot wooden Howe through truss swing span, and six additional 148-foot wooden Howe through truss spans. All piers of the initial bridge were constructed of stone masonry, which rested on both timber piles and bedrock. The entirety of the bridge was rebuilt with similar wooden trusses between 1876 and 1877. In 1885, the swing span was replaced with a 270-foot riveted quadrangular lattice through truss swing span, fabricated by the Lassig Bridge & Iron Works. In 1877, the southernmost 150-foot truss was replaced with a riveted quadrangular lattice through truss span, also fabricated by Lassig. Additional work was contracted by the Milwaukee Road in 1888 when Keystone Bridge Company replaced the six 150-foot east approach spans with 6-panel, pin-connected Pratt Through Trusses.

By the early 20th Century, the 1880s-era bridge had become too light for traffic. In 1914, plans were made to construct a new bridge, with the Department of War requiring a new 160-foot clear navigation channel approximately 200 feet south of the existing channel. Due to the navigation channel moving closer to the south bank, the chosen design would use a counterbalanced ("bobtail") swing span with new piers placed between the existing piers. Work on the substructures began in August 1915, and were completed in early February 1916. Work on the superstructure began prior to the completion of the final substructures. Demolition of the previous bridge and installation of the girder spans was handled by bridge crews employed by the two railroads. Each girder was moved to its relative position, placed on temporary bents while the floor of the trusses was cut out, and lowered into place after the old floor was removed. To accommodate the height of the girder spans, the stone piers were notched in the middle. It took an average of just over 2 hours to place a girder, plus an additional hour to notch the existing piers. Installation of the swing span was handled by the Strobel Steel Construction Company, and was erected using standard falsework. The entire project was completed in March 1916. As part of the construction, 48,000 cubic yards of earth were dredged from the river bed and the approach trestles filled by the LaCrosse Dredging Company of Minneapolis, Minnesota. The only significant to the bridge alteration came in approximately 1930, when the south approach trestle was replaced with a new deck plate girder span, giving the structure its present configuration. Milwaukee Bridge Company fabricated the swing span, while the girder spans were fabricated by the Chicago Bridge & Iron Works. A. Guthrie & Company completed the concrete substructures. The south approach superstructure was fabricated by an unknown firm, while an unknown company constructed the south abutment. The bridge was jointly designed by C.F. Loweth of the Milwaukee Road and H. Rettinghouse of the Omaha Road. The bridge was completed under the direction of J.D. Moen, with I.F. Stern the consulting engineer for the swing span and T.E. Van Meter as the assistant engineer.

Currently, the bridge consists of a 260-foot riveted counterbalanced Pratt through truss swing span, with a 175-foot, 7-panel channel arm, a 65-foot, 4-panel counterweight arm and a 20-foot center panel. The south approach consists of a single 55-foot deck plate girder span. The north approach consists of a 90-foot, a 100-foot, six 80-foot and one 70-foot deck plate girder spans. All plate girder spans use a standard design, with two heavy girders per span and heavy internal bracing. The girders are all approximately the same depth, with the exception of the 100-foot span, which is approximately 6 inches deeper. The entire bridge is set onto concrete substructures. The north and south approach piers consist of standard rectangular structures, with an angled upstream face. The swing pier consists of a standard octagon shape. The north abutment uses a standard design with sloped wing walls extending perpendicular from the bridge, while the south abutment uses a U-shaped design. All substructures are founded on timber piles, and have been extensively reinforced with rip-rap.

The swing span uses an unusual design, with heavy members, a unique concrete counterweight and a rim-bearing swing span design. Both endposts of the channel arm and the interior endpost of the counterweight arm consist of two channels, covered by a solid plate on the inside and connected by V-lacing on the back. The top and bottom chords both consist of built-up beams, which use V-lacing on both sides. The tops of the top chord on the channel arm are covered by a solid plate instead of V-lacing. The vertical and diagonal members both use a combination of V-laced beams and solid beams. The floor of the span is heavily constructed, with two plate girder stringers and plate girder floorbeams placed at the panel points. The two arms of the span are joined over a center pier by a rectangular tower, which is composed of V-laced and X-laced members. The center tower is connected to each arm by pin-connected eyebars. The exterior portal bracing consists of an M-frame design, while the interior portal bracing uses a large X-frame design. The sway bracing of the channel arm consists of X-frames of various heights, while there is no sway bracing on the counterweight arm. The upper lateral bracing consists of X-laced lattice bars, while the lower lateral bracing uses solid bars.

The positioning of the counterweight is unique to this bridge. During the late 1890s and early 20th Century, the Milwaukee Road designed and constructed several counterbalanced swing spans to meet a wide variety of unique situations. Several variations of counterweight placement were developed, including overhead, underneath and on-the-side designs. The counterweight of this bridge consists of a large triangular reinforced concrete structure, which forms a portal of the counterweight arm. The counterweight is formed around an additional vertical and diagonal member at the end of the counterweight arm, and is connected by a large V-laced top chord. The span follows a rim-bearing design, where the superstructure is placed onto a metal drum, which rotates around a track of rollers. The center panel uses a deeper floorbeam, which is connected to a round plate girder drum. The end lifts use a standard cam-like design, developed by the Milwaukee Road. These unique end lifts both serve as bearing blocks and allow for the span to lift.

Bobtail spans were rarely used by railroads, due to the additional engineering required to properly counterbalance the span. Several bobtail spans were constructed by the Milwaukee Road, particularly in Chicago and Milwaukee. It is believed this is the only span of this design that has ever been constructed over the Mississippi River. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked this bridge as being highly significant, due to the unique design. Few alternations have been made to the structure since the original construction.