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Minnesota's Historic Bridges
Colorado Street Bridge
Historic Significance

 Colorado Street Bridge

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.

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