Bridge L8803 on Wabasha Street near Terrace Park
Bridge number: L8803
Year built: 1888
Engineer: Andreas W. Munster, St. Paul Engineer’s Office
Contractor: Michael A. O’Brien and E.J. O’Brien
Span length: 70 feet, 6 inches
Length of arch barrel: 58 feet
Adapted from the National Register of Historic Places nomination form prepared by Jeffery A. Hess. The Colorado Street Bridge was listed on the National Register of Historic Places in 1990.
The Colorado Street Bridge, originally constructed as a municipal highway bridge carrying one city street across another, is a skewed, single-span, masonry arch structure that has been closed to vehicular traffic and converted into a pedestrian walkway over a park meadow in the Torre de San Miguel housing development in St. Paul, Minnesota. Although the site has been extensively altered, the bridge retains integrity.
The Colorado Street Bridge is of unorthodox construction. In masonry arch bridges, the voussoirs customarily are laid in courses perpendicular to the longitudinal axis of the arch so that each course forms a continuous ring front abutment to abutment. In the Colorado Street Bridge, only the voussoirs on the face of the arch are laid in the traditional manner. All other voussoirs are laid in courses parallel to the longitudinal axis. Since the intradosal surface of the arch is sheathed in red brick, the unusual construction is concealed from view, except for a few places where the brick has fallen from place. These "holes" in the soffit also reveal that the voussoirs are both brick and limestone, alternating every two courses. The brick matches the soffit. The limestone is grey, locally quarried, rubble stone. The nature of the construction indicates that the arch is sustained by the adhesive power of the mortar rather than by the "pressure" fit of the voussoirs.
The bridge displays a variety of building stone. Red granite is used for the abutments, buff-colored limestone for the ring stones, spandrel walls, railings and coping, and local grey limestone for the curved wing walls that buttress all corners except the southeast. The wing walls are dry rubble masonry, while the railings are dressed coursed ashlar. Otherwise, the elevations display rock-faced, coursed-ashlar stonework. Joints are about one-half inch in the spandrel walls and one-quarter inch between the ring stones, which are ornamented with tooled margins. Although the face voussoirs are cut to simulate the appearance of a segmental arch, the bridge is a skewed structure. Springing approximately 5 feet above grade, the arch rises 11 feet over an oblique span of 70 feet, 6 inches. It is 58 feet in width. A heavy stringcourse marks the roadway level. The roadway itself is bordered by stone railings about 3 feet high and 2 feet thick. At the southwest corner, the railing has been removed. On both elevations, the railings noticeably dip at midpoint, indicating settlement of the arch, which partly occurred immediately after constructions.
In 1927, the bridge was waterproofed by excavating the roadway and sidewalks, removing the dirt fill and sealing the crown of the arch with concrete and tar felt membrane. Repaved with a new roadway and sidewalks, the bridge remained a public highway facility until the early 1970s, when it was included in a parcel of land scheduled for redevelopment as a residential complex. Apart from replacing the roadway and sidewalks with a single sheet of blacktop, the redevelopment project had little structural impact on the bridge, although it considerably altered the general site. The most drastic change concerned the road passing under the bridge, which was vacated, excavated and landscaped as a park meadow. The bridge itself still serves a transportation function as a pedestrian walkway, linking the new housing development on the east with South Wabasha Street on the west.
The Colorado Street Bridge is historically significant for its unorthodox skewed construction and for the length of its span, which at 70 feet, 6 inches, surpasses all other masonry arch highway bridges in the state.
In June 1887, Leonard W. Rundlett, head of the City Engineer's Office, reported to the St. Paul Common Council on the cost of replacing the Colorado Street Bridge, a wooden viaduct over Starkey Street in a rapidly growing section of the city known as West St. Paul. According to Rundlett's estimates recorded in the Proceedings of the St. Paul Common Council, June 7, 1887, "a wooden bridge without any stone abutments, with a 320-foot roadway and two 8-foot walks would cost $3,500; an iron bridge of the same dimensions with stone abutments and cedar block paving would cost about $23,000; a stone arch bridge, same dimensions, with stone sidewalks and cedar block paving would cost about $24,000." Despite the higher cost, Rundlett recommended a stone-arch bridge as being a more permanent structure and "as being better adapted to the location." The common council agreed and the project was put out for bid.
For reasons unknown, the first bids were rejected. Although most proposals were at least $4,000 over the estimated cost, the low bidder, P. Durand, would seem to have been acceptably close at $24,019.96. When the second bids were opened a month later in September, 1887, the project was awarded to O'Brien Brothers, who had reduced their previous offer by about $2,500 to become the new low bidder at $26,893.86. The successful firm was experienced in masonry arch construction. In 1883, Michael A. O'Brien, then working under his own name alone, had been awarded a major city contract for the Seventh Street Improvement project, which entailed the construction of a 320-foot-long, stone-arch, sewer culvert, as well as foundation work for a skewed double-arch, stone, highway bridge known as the "Seventh Street Improvement Arches."
In preparing plans for the Seventh Street Improvement Arches, the City Engineer's Office had thoroughly researched the topic of skewed-arch construction, eventually selecting the innovative "helicoidal, method," which required highly complicated calculations and very precise stone cutting. On the Colorado Street project, Rundlett and his staff attempted to simplify the problems of skewed-arch construction by adopting a still more experimental approach. The actual design work was the responsible of assistant city engineer Andreas W. Munster, a native of Bergen, Norway, and a graduate of the Chalmers Institute in Gothenburg, Sweden. Munster's plan was to lay the voussoirs in lines parallel to the longitudinal] axis of the vault. As he later explained in the Engineering and Building Record, November 23, 1889, "This avoided the curved lines, elaborate computations and warped surfaces involved in the usual construction of the oblique . . . arch, and greatly reduced the labor and expense of stone cutting." Essentially, Munster eliminated the customary pressure fit of stone-arch construction.
Anticipating the future of concrete construction, he devised a method of building a rubble arch held together by the adhesive power of the mortar. The experiment was particularly bold in view of the fact that the structure's 70-foot span was (and still is) the longest of any masonry-arch highway bridge in the state. To ensure the proper bonding of materials, Munster left the centering in place for more than a year. Although the bridge was opened to traffic in the summer of 1888, the centering was not struck until the following spring, at which time it was discovered that the arch had settled about 2 inches. An equal amount of settling occurred over the next few decades without significant effect on the bridge's stability.
In the early 1970s, the Colorado Street Bridge was retired from highway use and slated for demolition to make way for a city-sponsored housing development. Demolition, however, was opposed by the city's bridge engineer, Roy E. Grieder. Greider, who in a letter to the Housing and Redevelopment Authority, urged that "this bridge should be preserved as it is one of few remaining true stone-arch bridges in our vicinity and does show remarkable workmanship." Grieder's position prevailed and the bridge was incorporated into the redevelopment project as a pedestrian walkway for the new residential complex.