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This bridge is a King Bridge Company bowstring. Historicbridges.org completed an extensive photo-documentation of the McDowell Bridge, another King Bridge Company bowstring in Iowa. Visit the McDowell Bridge page to learn more about King bowstrings and see a more extensive photo-documentation of the many noteworthy details on a King bowstring truss.
This bridge is a privately owned bridge, since the road it once served at this location was officially abandoned. However, the bridge appears to have been very well-maintained and it has been painted with an interesting two-color scheme and the bridge appears to retain excellent historic integrity with few alterations.
The Lower Road Bridge is rare, not only as a bowstring truss bridge, but in addition as a multiple span bowstring, since the bridge contains two spans, one of those a small pony span and the other a through span.
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. Later, two more patents were issued. These later two patents are closer to the construction date of the McDowell Bridge, and thus reflect more closely the design seen in the Lower Road Bridge. This is the first patent.
View The Second King Bridge Company Bowstring Patent (PDF)Zenas King first patented a bowstring design in 1866. Later, two more patents were issued. These later two patents are closer to the construction date of the Lower Road Bridge, and thus reflect more closely the design seen in the Lower Road Bridge. This is the second patent.
Information and Findings From Iowa's Historic Bridge InventoryDiscussion of Bridge
The bowstring arch-truss was the iron span of choice for Iowa counties in the late 1860s and 1870s. Marketed exclusively throughout the Midwest 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 configurations 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, and 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 the decade's end. Through subsequent attrition, almost all of Iowa's bowstrings have since been replaced and demolished. Because of its excellent state of preservation, the Lower Road Bridge is an outstanding early transportation-related resource [adapted from Hybben, Roise, and Fraser 1992] Bridge Considered Historic By Survey: Yes |
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