The Robert Street Bridge is located in downtown St. Paul, Minnesota, where it carries Robert Street over the Mississippi River. The bridge links the downtown St. Paul business and commercial district at Kellogg Avenue with the city's west side neighborhood and the city of South St. Paul. Aligned on a northwest-southeast axis, the Robert Street Bridge is a reinforced concrete, multiple-arch bridge, with an overall structure length of 1,534.4 feet. Starting at the north end, the bridge includes a reinforced concrete trestle with three spans of varying length, totaling 89 feet; a skew steel deck-girder span of about 53 feet across 2nd St., three flat, open-spandrel, barrel arches of 95.5 feet, 71 feet and 98 feet, with a combined length of about 291 feet; a two-rib, rainbow arch of 264 feet, center to center of piers, with a 244-foot clear span; four five-rib, open-spandrel arch spans of 112 feet each; and a 311 foot concrete-trestle approach. The out-out deck width is 78.5 feet, carrying a 56-foot roadway and 10-foot sidewalks on each side. The main span meets the federal navigation requirements of 62-foot headroom above low water.
Of particular engineering interest in the Robert Street Bridge is the main span. The two main ribs are each 6 feet wide and 8 feet deep at the crown, spaced 64 feet, 8 inches, center to center. Each rib is fundamentally a structural steel frame designed to carry the weight of the steel structure, including the steel floor system and the dead load of the concrete arch proper. The dead load of the concrete roadway and the live loads are carried by the composite concrete and structural steel arch. The arch ribs have heavy steel cross-bracing below the roadway.
Aesthetically, the most important element of the structure is the monumental rainbow arch that dominates the bridge. The overall detailing of the surfaces has been described by Roy Childs Jones, the architectural designer, in the Engineering News-Record, November 11, 1926, as involving "the breaking up of all surfaces with lines of light and shade," with modeling "accomplished by vertical breaks and grooves, by bevels, and by wedge-shaped indentations." According to Jones, "the idea was to make, out of the natural patches of lighter and darker toned material, patterns definitely bounded by strong lines of shadow; and to effect an emphasized interest in light shade in place of the unattainable color interest [which is inherent in concrete]." The railing, a focus of the architect, is comprised of precast perforated panels anchored between poured, heavily reinforced members at top and bottom and between posts from side to side. Although the south railing is erected on a grade, the panels are set vertically. Twelve large medallions mounted on the piers, modeled by the Brioschi-Minuti Company of St. Paul, are the only applied ornament. The original light standards have been replaced with modern light poles.