Oct. 20, 1899: How to move 120 tons of bridge
- Blog Post by: Ben Welter
- October 25, 2012 - 3:38 PM
Over the past 150 years, five bridges have spanned the Mississippi at Wabasha Street in St. Paul. The first, a wooden Howe truss span known as the St. Paul Bridge, was completed in 1859. The second, built in 1872, was of the same design. The third was built in about 1884. That bridge was, according to a rather dated page on St. Paul’s website, an all-iron Pratt truss, “an innovative version known as a Whipple double-intersection Pratt.” Innovative, perhaps, but not enduring: Five years later it was replaced by an iron cantilever deck-truss that served the city for a century before the high cost of maintenance and repair spelled its doom. The current Wabasha Street Bridge, a concrete segmental box girder bridge, was completed in 1998.The 1889 bridge was built in two parts, first the north section and, 10 years later, the south section. The latter project required that a 120-ton span of wood and iron be moved 50 feet, from temporary wooden piers built downstream to permanent masonry piers. In the story below, the Minneapolis Tribune explained how six men, without the aid of horses or steam power, completed the job in just eight hours. The feat was described in detail in the January 1900 issue of the Journal of the Association of Engineering Societies.
by Modern Methods
Six men moved the 120 tons of wood and iron contained in one span of the Wabasha street bridge, St. Paul, 50 feet Wednesday, and they didn’t make much fuss about it as an expressman would in getting a trunk upstairs. The men were not unusually tired after their feat, for screws and compound levers accomplished what their hands could never have done and completed a task in which it would have been dangerous to have used machinery.
By the aid of screws and rollers the men pulled the bridge the entire distance in eight hours. The lifting and trussing of the bridge requiring a week or more, and it will take almost that long for each of the other two spans that will have to be moved. After all this is done the approaches will have to be shaped up and the connection made with the permanent portion of the bridge.
An attempt was made at first to move the bridge without the use of rollers, but it was found the friction was too great and that it could not be done. The rollers are simply iron bars cut in short sections, and as fast as they roll out from under the plate they are placed in front again. A screw mechanism is employed at each end of the span. In moving the bridge it is necessary to exercise the greatest care to avoid demolishing the old piers.
The contract for moving the old bridge amounts to $7,500 and for building the approach $40,995.
|A photo from the January 1900 issue of the Journal of the Association of Engineering Societies - thanks, Google Books -- shows a 120-ton section of St. Paul's Wabasha Street bridge being maneuvered into place.
|The fourth Wabasha Street bridge, shown here in about 1900, was listed in the National Register of Historic Places in 1989. (Photo courtesy of Minnesota Historical Society)
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