"The Sixth Street Viaduct is actually comprised of two separate structures: a pin-connected Pratt truss built over the tracks of the Chicago, Burlington and Quincy Railroad in 1886 and a girder overpass that was built as an approach in 1903-04. Both bridges are technologically significant as rare survivors of their type. The 1886 span is one of only three pinned through trusses remaining in Iowa's urban areas. Similarly, the open-web design of the 1903 girders is the only example of this uncommon configuration known to remain in the state. The viaduct as a whole is historically significant for its role in the development of downtown Burlington and its representation of the sometimes uneasy relationship between the city and the railroad that supported it."The new [1886] iron viaduct erected by a railroad construction crew that summer, contrasted sharply with its predecessor. The 235-foot-long structure stretched from Valley Street on the north to Market Street on the south, sloping slightly uphill along its length. The viaduct featured a pin-connected Pratt through truss over the railroad tracks, flanked on both sides by 60-foot and 43-foot plate girder deck approach spans. Fabricated by the Lassig Bridge & Iron Works of Chicago, using members rolled in Pittsburgh by Carnegie, the truss was made up entirely of built-up wrought iron components.
"City Engineer Emmet Steece determined that the steep approach on the viaduct's norht end needed to be reconfigured. Steece had drawn a steel structure comprised of three 36-foot girder spans supported by steel columns on concrete pedestals. With their built-up flanges, through-deck configuration and upper chords radiused continuously to form the end posts, the structures resmbled most girders of the time. What made these girders noteworthy was their patticed webs. Instead of solid plates, which was the industry standards, the webs of Steece's girders were made up of angle sections, riveted into a series of triangles with curved gusset plates - resembling a Vierendeel Warren truss...the Sixth Street Viaduct is the only such open-web girder known to exist in Iowa."
Carl W. McWilliams
Fraserdesign
Loveland, Colorado
January 1992
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