The popularity of the Pratt through-truss bridge resulted from its simple and reliable design, durable iron construction, and availability at the time Wisconsin embarked on a broad program to improve bridges on local roads. Nineteenth-century engineers had long known that bridges could be made longer and stronger if they were built of short structural pieces, attached in repeated triangular forms. But triangular trusses required that some structural pieces carry tensile loads, and wood, the most common building material at the time, is weaker in tension than in compression. Truss bridges were impractical until wrought- and cast-iron materials, equally strong in tension and compression, became more widely available in the second half of the nineteenth century. Iron truss bridges then became ubiquitous for roads and railroads.
The most popular design for iron through-truss bridges (where the roadway runs through, rather than above, the truss) was the Pratt design. These had heavy vertical members in compression and lighter diagonal members in tension. The pieces were connected with pins rather than rivets, the top and bottom chords of the bridge were built parallel to one another, and the end posts inclined toward the top, forming a trapezoid. The Mill Street Bridge is an early single-span bridge with a Pratt through-truss and a wooden deck. It crosses the Manitowoc River to link the town of Manitowoc Rapids to the city of Manitowoc. The road itself dates from 1839, built as part of a military route connecting Fort Howard, near Green Bay, with Fort Dearborn, at present-day Chicago. By 1887, heavy traffic made it necessary to replace the earlier wooden bridge. The resulting bridge is about 150 feet long and has a roadway 15 feet wide. Vertical and diagonal posts are arranged in a lattice fashion. The lighter struts running overhead across the bridge, and tying the upper chords, also form a lattice. The struts at either portal end are more decorative, with the latticed bars forming eight-pointed stars. At the center of these end struts, a triangular plaque, stamped with a sunburst motif, displays the date and the name of the manufacturer. The portals rest on massive abutments of locally quarried dolomite, a type of limestone.
Wider vehicles and heavier truckloads made these bridges obsolete, so a once-ordinary bridge type has become quite rare. This one was spared in 1956, when County Road R bypassed this portion of the historic road. Now, only pedestrians can use it, due to structural weakening.