This bridge is a rare example of Massillon Bridge Company bowstring. The bridge features a unique design that the original bridge company founder, Joseph Davenport designed, which is his unique built-up beams. There are two patents which include his form of built-up beam that is composed of plates with a pipe webbing. It is this pipe webbing that makes the design unique, as well as the Howe truss configuration formed by these pipes contained within the built-up beam. This pipe and plate format was commonly used by the company for built-up top chords on bowstrings, as well as for the composition of a truss girder as seen in the Longman Road Bridge. The design was also used for floorbeams, sway bracing, and portal bracing. The company's use of this built-up beam form seems to have declined and ended in the 1880s in favor of more traditional beams.
The Washington Mill Bridge uses the pipe and plate built-up beam for the top chord. In contrast, the sway bracing on this bridge uses a more traditional v-laced built-up beam. The bridge has very few alterations and remains in excellent condition for its age. Its location on a dead-end road serving a single residence means that the bridge carries very little traffic and can likely continue to serve in this location for the foreseeable future.
The Washington Mill Bridge is a rare example of a Massillon Bridge Company bowstring. It is particularly noteworthy because of its exceptionally high degree of historic integrity because of its unaltered condition. The bridge is significant as a bridge type that is today very rare and for its representation of this key period in bridge construction and an unaltered representative example of one of the most prolific bridge companies of the period.
Historic American Engineering Record created a large and very informative historical overview and context for Iowa's bridges, and it is offered here by HistoricBridges.org in convenient PDF format for easy printing or offline viewing. The HAER source for the documents composing the PDF is here.View Bowstring Arch Bridges of Iowa, An Online Book By Michael Finn (PDF)
Michael Finn has composed a concise and detailed overview of Iowa's beautiful historic bowstring bridges. It has been made available for free by Iowa Department of Transportation.View Historic American Engineering Record's Structural Analysis of Iron Bowstring Bridges (PDF)
Historic American Engineering Record created a large and very informative structural analysis of how bowstring truss/arch bridges function. Everything from basic discussion of the engineering behind the bridges to advanced mathematical equations are available. The HAER source for the documents composing the PDF is here.
Information and Findings From Iowa's Historic Bridge Inventory
Discussion of Bridge
The Dubuque County Board of Supervisors first contracted for a timber bridge over Lytle Creek, next to the Washington Mill, in 1865. That August the board hired local contractors Ambrose and Joel Higgins to build a small-scale timber stringer structure for this crossing in Washington Township, along with a bridge at Higgins Ford, for $295. Predictably short-lived, the first Washington Mill Bridge required frequent repairs to keep it serviceable. It was rebuilt in 1873 and again in 1876. Finally, in the fall of 1877 the board "ordered that a bridge be built at Washington Mill in Washington Township." The proposed new span would feature an all-iron superstructure in the hopes of eliminating the maintenance problems of its predecessors. In October the supervisors hired local mason T.J. Donahue to build new stone abutments for $765. At that time they contracted with the Massillon Bridge Company of Ohio to design, fabricate and erect a 113-foot bowstring arch-truss span for $1,824. Donahue began work on the substructure immediately, completing the massive abutments by January. Massillon fabricated the arch-truss using its standard, latticed double-plate design, completing the bridge in April 1878. The Washington Mill Bridge proved only slightly more durable than the structures it replaced; it required repairs in at least 17 of its 32 years of service. Nevertheless, the bridge has continued to function in place, with only maintenance-related alterations. Once on a main road through the southern part of Dubuque County, it now services a single farm.
The bowstring arch-truss was the iron span of choice for Iowa counties in the late 1860s and 1870s. Marketed extensively by such Ohio-based industry giants as the Wrought Iron Bridge Company, the King Iron Bridge Company and the Massillon Bridge Company, these often-patented bridge forms featured a wide range of span lengths, economical fabrication cost and relatively quick erection. The proliferation of the bowstring coincided with the initial development of Iowa's road system, and as a result, perhaps thousands of these prototypical iron spans were erected throughout the state. The bowstring design had some rather severe structural flaws, however, and it was superseded by the pin-connected truss in the early 1880s. Through subsequent attrition, almost all of Iowa's bowstrings have since been replaced and demolished. The Washington Mill Bridge is distinguished among those that remain by its pristine state of preservation. One of the oldest originally placed roadway bridges in the state, it is an outstanding early transportation-related resource [adapted from Fraser 1992].
Bridge Considered Historic By Survey: Yes
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