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This is a two-span Pratt through truss. It is unusual because the two spans are not identical. Although both spans have the same number of panels (six), the western span appears to be slightly shorter in span length. In any case what is certain is that the composition of the two trusses is not identical. With the eastern span, the three center vertical members are all composed of back-to-bach channel with v-lacing on each side. In contrast, the three center verticals on the western span are not all the same: the centermost vertical differs in design and is composed of paired angle with v-lacing. There may be other more subtle differences in other member dimensions, but this was not investigated during the site visit.
Constructed in 1882, this is an early surviving example of a Pratt truss by the King Bridge Company. It has an unusual detail with its pins in that the outside ends of the pin appear to have fixed heads (meaning the pins are like giant bolts, rather than just simple threaded rods with nuts at each end). The fixed heads have square heads, a contrast to the hex nuts usually seen on pins, and indeed seen on the interior ends of the pins on this bridge. This design of pin can be seen in another early King through truss, the Hendricks Ford Bridge.
As preservationists, the first and foremost goal should always be to try to preserve a truss bridge in a way that its trusses continue to function as originally intended to carry live load. That said, a compromise that is sometimes made (and may in some cases be the best solution to meet the needs of a crossing while not losing the historic truss) is to add load-bearing stringers to the crossing, which carry the live load of traffic. This means the trusses become decorative, no longer carrying the loads of traffic, and only serve to support themselves. This compromise solution is good for situations where heavy, unposted traffic is needed at the crossing, and/or where rehab of the truss to carry live load might require substantial alterations to the truss causing loss of integrity in terms of original design or materials.
All that said, there is a major risk that comes with this compromise. Once the stringers are added, there is no longer an incentive for the owner to maintain the truss because the condition of the truss is no longert important to keep the bridge open to traffic. The consequence of this is twofold. The truss might not be as well-maintained. Worse, when the truss does deteriorate, it might be allowed to deteriorate to a far worse condition than a load-bearing truss would ever be allowed to, so if and when anyone decides to repair the truss a lot of work will be needed. This is exactly what has happened here. It appears when the stringers were added the floorbeams were not repainted (the argument probably being that nobody sees them and they would no longer be load-bearing). As such they are in terrible condition today (far worse than the trusses which were repainted blue). The u-bolt hangers (one of the most important and critical elements of a load-bearing truss) have even completely broken in some places. What is really sad is it looks like the county may have planned for this neglect. Under the load-bearing stringers there are u-shaped rods that enclose the floorbeams. They almost look like they were put there to "catch" the floorbeams, when the u-bolts fail, dropping the floorbeam.
The effort to render the trusses decorative back in 1996 was done in the interest of preserving this bridge. This was a commendable effort, and it makes sense to repair the floorbeams and hangers now so that the efforts to preserve this truss in 1996 are not ultimately in vain.
Please enjoy the comedy that is the findings of the Ohio Historic Bridge Inventory below. These are consultants who were paid a lot of money to make the below statement. HistoricBridges.org, (without payment) in five minutes of visiting bridge proved that these consultants either failed to visit this bridge or did so with their eyes closed. It claims that the bridge is a product of the Wrought Iron Bridge Company. It is not. The design details do not match those of the company. The square heads on the pins are a detail of an early King Pratt truss, and have never been seen on a product of Wrought Iron Bridge Company. The inventory claims there is no plaque. There is in fact a partial plaque on the bridge. Although partially broken it is in the shape of the King Bridge Company tombstone plaque design (never used by Wrought Iron Bridge Company). There are enough letters on what is left on the plaque to make it physically impossible that it says Wrought Iron Bridge Company. So how on earth did consultants paid lots of taxpayer dollars manage to get away with a claim that there is no plaque on this bridge and that it was built by the Wrought Iron Bridge Company?!
Information and Findings From Ohio's Historic Bridge InventorySetting/Context The bridge carries a 2 lane road over a stream in a sparsely developed, rural setting. Physical Description The 2 span, 165'-long, pin-connected Pratt thru truss bridge was rehabilitated in 1996 relieving the trusses of live loads by the insertion of a steel stringer superstructure. It is now functioning as a steel stringer, and that's how it is correctly reported in BMS. The trusses are traditionally composed of built-up compression members and eyebar or rod tension members. It has lattice portals. Integrity Steel stringer superstructure inserted, 1996. End-panel floorbeam hangers changed. Flooring system altered. Railings and plaques lost. Summary of Significance "Although correctly a stringer bridge because a new superstructure was placed in 1996, the stringers support an 1882 pin-connected Pratt thru truss bridge. There is no plaque, but the bridge is attributed to the Wrought Iron Bridge Company based on the comparison with other documented examples. More complete examples better represent the significance of the bridge type/design and the company's work. 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, although eventually superseded in popularity by Warren trusses. The design, which initially was a combination of wood compression and iron tension members, was patented in 1844 by Thomas & Caleb Pratt. Ohio has three covered bridges that use this combination configuration, but they are all modern reconstructions based on the Pratt patent. 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'. Significant surviving examples of all-metal Pratt trusses mostly date to the last quarter of the 19th century, and they are found with thru, pony, and the less common bedstead configuration. Prior to about 1890, a variety of panel point connections were in widespread use (including bolts, cast-iron pieces, and pins), but engineering opinion was coalescing around pins as the most efficient and constructible. 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. Later post-1890 Pratt trusses show a progression toward less variation in their details such that by 1900 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 waning and eclipsed in the highway bridge market by more rigid, rivet-connected truss designs, particularly the Warren but also riveted Pratts. The transition to riveted 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, Pratt truss highway bridges, whether pinned or riveted, were almost always built under the auspices of counties and local units of government; the Pratt was not a standard design of the state highway department. 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 1900 and have documented or attributed builders and dates of construction and/or significant connection or member details. Later post-1900 examples are less technologically significant. Significant unaltered examples of riveted-connected Pratt trusses date from ca. 1900 to 1915." Bridge Considered Historic By Survey: No |
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