The Robert Street Bridge is historically significant as an outstanding example of an unaltered, monumental, multi-span, reinforced concrete arch bridge. It is the product of a very complex engineering design process to enable this bridge to be built in this location with its established vehicular, railroad, streetcar and river-navigation demands. The resulting bridge includes a monumental reinforced concrete rainbow arch, by far the largest in Minnesota, which is outstanding not only for its engineering, but for its aesthetic effect in the overall design of the bridge. In addition, the bridge received special architectural treatment by the architect assigned to the design team.
Work on the bridge was begun on June 19, 1924. The bridge was completed and dedicated on August 6, 1926. It was a joint undertaking of Ramsey County and St. Paul. Plans and specifications were prepared by Toltz, King & Day, Inc. The Toltz, King & Day, Inc. design team included Max Toltz, mechanical engineer; W.E. King, structural engineer; B.W. Day, architect; Roy Childs Jones, architectural designer; P.E. Stevens, office engineer; W.A. Thomas, electrical engineer; and John F. Greene, in charge of arch design and resident engineer. The contractor was Fegles Construction Company, Ltd.
The Robert Street Bridge was built to replace an 1884-1885 wrought-iron span that, by the 1920s, had proved inadequate for drastically increased traffic and streetcar demands. The original structure was designed for horse-drawn vehicles with no provision for streetcars. Streetcar tracks were added in 1893. By 1920, the bridge was carrying 2,730 vehicles and 400 streetcars every 12 hours. Two years later the vehicular traffic had increased 55 percent. This traffic increase had been caused by widening Robert Street in 1912-1914 and by connecting Robert Street with University Avenue, a major artery linking St. Paul with Minneapolis. This brought traffic to and from Minneapolis and downtown St. Paul on the north, and St. Paul's west side neighborhood and South St. Paul on the south. In fact, cities as far south as Winona, Minnesota, viewed the new bridge as a needed "capitol highway" to give them greater access to the state capitol.
The engineering firm commissioned to design the new bridge, not only had to provide a span with adequate vehicular and streetcar capacity, but had to accommodate the congested local conditions, with the location of nearly every pier being determined by the clearances required by existing structures and railroad property. The engineers had to reckon with Second Street, the freight shed and tracks of the C.St.P.M.&O. Railway, the tracks of the St. Paul Union Depot, which handles the entire passenger traffic of the city, the main line of the Chicago Great Western Railroad, the river channel of the Mississippi as defined by the War Department and the south end of the bridge then terminating in a busy manufacturing district. These factors and their various requisite clearances dictated the exact location of the roadway. They came together with foundation conditions and the existing Chicago Great Western railroad lift bridge which strictly defined the navigation channel, to dictate the location, size and design of the piers. The net result is the combination of barrel-arch and rib-arch flanking spans and especially the rainbow arch main span over the navigation channel.
Because of the many factors dictating elements of the main span, a rainbow arch was the only solution if an arch was to be used. The solution was an unusual rainbow arch. Instead of the usual compound curves resembling a basket handle, with the long radius at the crown and the shorter radii at the haunches, the radius is 122.16 feet at the crown and 191.60 feet at the haunch. The structural-steel-arch inside each concrete rib also is a significant feature. The steel arch is designed to carry the dead load of the steel arch, floor and concrete rib.
According to the bridge architect Roy Childs Jones in the Engineering News-Record of November 4, 1926, "The Robert Street Bridge is unique in that its designers included in their own permanent organization both architects and engineers." This design team, Jones wrote, allowed the bridge to avoid "applied ornament" on a predetermined structure. Instead, the team could "select and control the structural features so as to secure for the bridge an inherent beauty of form and proportion." The design team faced "the complicated requirements of street grades and of railroad and channel clearances," which precluded "any simple and regular composition of arches and piers." For the most part, then, architectural treatment in this bridge involved working with "shapes and proportions and relations of the structural members" and employing shadow and line. There also was a conscious effort to deal aesthetically with concrete as a material and Jones felt that unbroken surfaces and lines did not work well in concrete. Instead, a choice was made to create a totality out of a series of "definitely bounded segments," produced by "the breaking up of all surfaces with lines of light and shade." This was accomplished by using "vertical breaks and grooves, by bevels, and by wedge-shaped indentations." The result of this practice is readily seen in the surface treatment of the massive rainbow-arch ribs.