Be sure to view the page for the Walnut Street Bridge, a nearby cast iron truss built by the same company as this bridge. The Walnut Street Bridge page provides a more comprehensive overview of the significance, design, and builders of this bridge. Further, the HAER documentation for the Old Mill Road Bridge is extensive and provides a good history and description of the bridge. The narrative for this bridge is intended to describe the Old Mill Road Bridge by contrasting it to the Walnut Street Bridge.
Like the Walnut Street Bridge, the Old Mill Road Bridge is one of the smallest and oldest group of remaining metal bridges in the United States... those with cast iron compression members. Along with the Walnut Street Bridge, the Old Mill Road Bridge is one of the only known bridges in the country to have floorbeams that are cast iron. Although the Old Mill Road Bridge is not a through truss like Walnut Street Bridge, and is not quite as old, it does have one key element that keeps this bridge up among the most unique and historically/technologically significant bridges in the country. This is the fact that the bridge is a continuous pony truss. Continuous pony trusses are exceedingly rare, as are pre-1900 continuous truss bridges of any kind. This bridge is both, and further is a cast iron truss. It may be the only such bridge remaining in existence. It is worth noting however that some engineers (including the narration in Historic American Engineering Record) question whether the bridge really functions as a continuous truss bridge. The author in the HAER documentation comments that the bridge's members are not particularly suited to the forces of a continuous truss bridge, and further the manner in which some members were adjusted would effect where the forces were distributed. The author suggests that the bridge might be thought of as a simply spanning truss bridge with a shared end post at the pier. If this is the case, the bridge may be even more unusual and unique. Either way, the detail at the pier is a key feature of this bridge.
The Old Mill Road Bridge differs from the Walnut Street Bridge in terms of member design. While the Walnut Street Bridge has circular-shaped members, the Old Mill Road Bridge has octagonal shaped members. Further, the cast iron top chord connection boxes on the Old Mill Road Bridge have a beautiful decorative flower shaped design on their faces. Also, unlike the Walnut Street Bridge, the Old Mill Road Bridge features a more traditional bottom chord consisting of eyebars connected by a pin.
The Old Mill Road Bridge retains its cast iron floorbeams, however modern floorbeams were added below the original floorbeams. The pier at the center of the bridge is apparently the original stone pier encased in concrete.
The bridge has been closed to vehicular traffic for some time, although it remains open to pedestrian use. HistoricBridges.org recommends a comprehensive in-kind restoration for the bridge for continued non-motorized use in its current location. In-kind restoration means that as much original material must be preserved as possible, and anything replaced on the bridge must be an exact replica. The removal of modern additions to the bridge may also be part of such a project.
Information and Findings From Pennsylvania's Historic Bridge Inventory
Discussion of Bridge
The 2 span, 104'-long through truss bridge appears to be a technologically significant example of its type/design. The bridge has an octagonal top chord with cast fittings for top and bottom chord connections. Bottom chord is very light eyebars, and diagonal are rods. Concrete pier may have been added later. Stone abutments, wingwalls and approach parapets. Floorbeams are double rolled sections. The bridge has been found to be a contributing resource to the Ehrhart's Mill Complex Historic District.
Discussion of Surrounding Area
The bridge is located in a parklike setting with adjacent residences. The one lane bridge is closed to all traffic. The Ehrhart's Mill Complex Historic District includes the bridge, which is rated as contributing to the district.
Bridge Considered Historic By Survey: Yes
This bridge is tagged with the following special condition(s): Cast Iron
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