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The Green Mill Ford Bridge is an excellent example of the King Iron Bridge Company's bowstring truss design. Not only is it rare as a surviving bowstring, it is further significant as an example with little to no alterations noted on the superstructure. In addition, the bridge is significant as a rare multi-span example of its type. As a result, the bridge provides a clear look at exactly what the king bowstring design looked like. The bridge includes typical King bowstring design including a built-up box beam top chord. Poles/pipes are present on the bridge acting as sway bracing. The bottom chord of the bridge are long eyebars whose pin points at the eyes serve only as a flexible splice for the eyebar, and do not hold any other members of the bridge. At the ends of the bridge, the bars taper into threaded rods which are bolted at the bridge shoe. Additional significance arises from the cast iron components on this bridge all of which remain intact. Most noteworthy are the bottom chord connection assemblies. These beautifully crafted and complex castings provide attachment points for floorbeams, lateral bracing rods, and diagonal members. The bridge shoe is also cast iron and is made to fit into a cast iron bearing plate on the pier, which are both beautiful cast iron details. Cast iron washers are also present on the top chord connections. There are also beautiful star-iron members also known as cruciform members, which are a beautiful type of wrought iron rods that only appear in bridges built in the 1870s and the early 1880s.
The Green Mill Ford Bridge's two bowstring spans were salvaged from a three span bowstring bridge crossing Cedar River in Waverly, Iowa that was replaced in 1898. In 1902, these two spans were relocated to the Green Mill Ford site to serve as the main spans for a bridge that also included a series of approach spans. In 1989 the bridge was permanently closed with no plans for replacement or reopening. The approach span superstructures were removed from the bridge, but because of their extremely high level of historic significance, the main bowstring spans were not demolished and were left in place as an abandoned ruin. The bowstring trusses remain with excellent historic integrity and in an easily restorable structural condition. With appropriate funding and public interest, the spans could either be restored in place or in a new location for non-motorized use.
Bowstring truss bridges are sometimes called bowstring arch bridges because they have similarities to both structure types. Beginning with Squire Whipple's Whipple Arch Bridges, such as the Ehrmentraut Farm Bridge, the bowstring truss bridge is the bridge type that began a transition away from wood and stone and began to make metal a common bridge building material. It also began a period of experimentation until a good bridge form was developed, leading to a gradual standardization of bridge design. During this period, numerous bridge companies all experimented with metal, trying to design the best bridge. Each company had their own distinctive bowstring design, including unique and creative design details. These designs were often patented. Most bowstring truss bridges were built in the 1870s. Also during this time, cast iron was still used in addition to wrought iron for the construction of bridges, so many bowstrings built during this period include details such as connection assemblies that are made of cast iron. By the 1880s, bridge companies decided that the pin-connected Pratt truss was a better structure type, and construction of bowstring bridges sharply dropped after 1880. Because of the period in which Iowa was first being settled, a much larger number of bowstring truss bridges were built in the state than in other states. As a result, even today, Iowa has more historic bowstring truss bridges than any other state, although the number of bridges statewide is under 20, a very small number. However, a number of states do not have even a single historic bowstring truss within their borders. As such, while bowstring truss bridges are very few in number in Iowa, they are extremely rare on a national scale. It is imperative that each surviving bowstring in the county be preserved to protect this key period in bridge building history.
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.
View Excerpts From Other HAER Documentation Describing the King Bridge Company's HistoryEach time HAER documents a bridge built by King Bridge Company, they often create a historical narrative for the history of King Bridge Company. HistoricBridges.org has searched through these narratives and offers here a convenient PDF version of two of the longer and more informative narratives.
View The First King Bridge Company Bowstring Patent (PDF)Zenas King first patented a bowstring design in 1866.
View The Second King Bridge Company Bowstring Patent (PDF)Zenas King first patented a bowstring design in 1866. This second patents is closer to the construction date of the Green Mill Ford Bridge, and thus reflects more closely the design seen in the Green Mill Ford Bridge.
Information and Findings From Iowa's Historic Bridge InventoryDiscussion of Bridge The Green Mill Ford Bridge represents two-thirds of a bridge built in 1872 as a bridge crossing the Cedar River in the town of Waverly. In 1870 the Waverly City Council petitioned the Bremer County Board of Supervisors, stating, "The bridge across the Cedar River in Waverly is nearly worn out and the needs of the people of the whole county demand the completion of a strong construction of bridge." The supervisors proposed a special mill levy on the county property tax; at a special election that year the citizens of the county passed the initiative. In spring 1871, John R. Price and Brothers used stone from the Anamosa quarries to build the bridge's substructure for a county contract of $5,000. The board then hired William Crickett, an agent for the King Iron Bridge and Manufacturing Company, to build a wrought iron bridge of three 125-foot spans and an overall bridge width of 18 feet, for a total contract cost of $11,000. The Waverly Bridge served in its original location until 1898 when it was replaced by a new girder bridge erected by the Toledo Bridge Company. Three years later, in May 1902, the old bowstring trusses were dismantled. One of its three spans was moved to a site across the Cedar River in Franklin Township; the remaining two spans were placed over the Cedar River in Jefferson Township. Know locally as the Green Mill Ford Bridge, these latter spans still stand in this location today. The bowstring arch-truss was the iron span of choice for Iowa counties in the late 1860s and 1870s. Marketed extensively throughout the Midwest by such industry giants as the King Iron Bridge and Manufacturing Company and the Wrought Iron 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 corresponded with the initial development of Iowa's road system; as a result, perhaps thousands of these prototypical iron spans were erected throughout the state. The bowstring had some rather severe structural flaws, however, relating primarily to lateral stability of the arches, and it was largely superseded by the pin-connected truss in the early 1880s. Despite this, some bowstrings were still erected in Iowa in the 1880s, although the number dwindled precipitously by decade's end. Through subsequent attrition, almost all of Iowa's bowstrings have since been replaced and demolished. Although it no longer carries traffic, the Green Mill Ford Bridge is historically and technologically significant as of the last remaining examples in the state of what was once a mainstay structural type [adapted from Fraser 1990]. Bridge Considered Historic By Survey: Yes |
This bridge is tagged with the following special condition(s): Unorganized Photos
This historic bridge has been demolished. This map is shown for reference purposes only.
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