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First, HistoricBridges.org is EXTREMELY dubious of the National and Historic Bridge Inventory given construction date of 1876. 1876 is the patent date for a Pratt truss by Wrought Iron Bridge Company, and many bridges built by the company had plaques listing the patent date and not the construction date. Many bridges by this company are thus improperly dated in inventories. The only reason HistoricBridges.org is even listing the 1876 construction date is because there was no patent date on this bridge. The plaque on the bridge just lists the company, no construction or patent plaque. If the date is wrong, the bridge could date to as late as the early 1880s.
What is not in dispute is that this bridge's design details represent the Wrought Iron Bridge Company's earliest design for a Pratt through truss.
Although overlooked by the Historic Bridge Inventory, there is strong evidence that this bridge was relocated from an urban location to this rural location, potentially in the 1942 rehabilitation date given by the National Bridge Inventory. Firstly, the bridge sits on concrete abutments, of a sort that would not have been built in even the 1880s, let alone the 1870s. Second, the bridge's 18 foot roadway width is unusually wide for any rural pin-connected truss bridge, let alone one from the 1870s or 1880s. Next, the ends of the floor beams are rough and show evidence that they were cut off. This suggests that the bridge originally had cantilevered sidewalks, further evidence of a different and more urban location originally. The design of the floor beams originally would have extended the built-up fishbelly design out under the sidewalks, like this bridge.
Following field visit to this bridge, communication with David Simmons of the Ohio Historical Society and Tom Barrett of ODOT, it was learned that this bridge was originally located at South Main Street in Mt. Gilead just north of the fairgrounds, and had two cantilevered sidewalks. Not only is it nice to have the mystery of this bridge solved, this bridge is a great example of how much you can learn just by careful observation during a field visit. The only thing that was not possible to learn from the field visit was where the bridge came from.
Information and Findings From Ohio's Historic Bridge InventorySetting/Context The bridge carries a 1 lane rural road over a stream in a rural area of active farms. Physical Description The 1 span, 90'-long, pin-connected Pratt pony truss bridge supported on concrete abutments is a superb and complete example of an early WIBC thru truss, complete with built-up, shaped floorbeams, use of the bulbous T section for the built up verticals, and boxy connection between end posts and upper chord. The loop connection at the end-panel floorbeam hanger has been modified but is an in kind repair. Integrity Rust on bottom flange of floorbeams. Summary of Significance The bridge is one of the two oldest WIBC thru truss bridges in the state, and it remains in remarkably complete condition making it historically and technologically significant. It is also the product of on important in-state fabricator whose work is recognized nationally. Why the bridge was chosen as non-select in the previous survey is not known. Pratt trusses were undoubtedly the most popular truss design of the last quarter of the 19th century and continued to be built into the 20th century. The design, which initially was a combination of wood compression and iron tension members, was patented in 1844 by Thomas & Caleb Pratt. The great advantage of the Pratt over other designs was the relative ease of calculating the distribution of stresses. More significantly, it translated well into an all-metal design in lengths of less than 200'. Prior to about 1890, a variety of panel point connections (including bolts, cast-iron pieces, and pins), end panel floorbeam connections, and lower chord designs were in widespread use. Many of the connection details were proprietary and associated with individual builders or companies, and thus earlier examples are generally taken to be technologically significant in showing the evolution of the design. Post-1890 Pratt trusses show a progression toward less variation in their details such that by 1895 the design was quite formulaic with few significant differences between the designs of various builders. This marked the end of the pin-connected Pratt's technological evolution and, in fact, it was soon eclipsed in the highway bridge market by more rigid, rivet-connected truss designs, particularly the Warren design, but also the Pratt design as well. The transition to riveted field connections, which happened even earlier with railroads than highways, was in no small part due to concerns about stress reversals at the pins under heavier loads and improvements in pneumatic field riveting equipment in the early 1900s. In Ohio, there are 185 Pratt trusses dating from ca. 1874 to 1945 with at least 60 dating prior to 1900 (Phase 1A, 2008). The technologically significant unaltered examples of pin-connected Pratt trusses for the most part date prior to 1894. Post-1895 examples are less technologically significant. Justification The bridge is one of over 150 extant pin-connected truss bridges dating from 1874 for pony trusses and 1876 for thru trusses. Twenty six predate 1888 and represent the era of experimentation that evolved into standardized designs by about 1888. This example ranks as one of the earliest and one of the finest early WIBC bridges in the state and nation because of its age and completeness. Bridge Considered Historic By Survey: Yes |
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