Jackson Road Bridge

Information and Findings From New Jersey's Historic Bridge Inventory

 Summary

The skewed rivet-connected 4-panel Warren pony truss bridge is a complete example of its type, but its design is typical of the period. It is composed of channels and plates for the chords, and of angles joined by battens for the diagonals. The Warren is the most common 20thcentury truss type for highway bridges. The newest of the 10 Warren pony truss spans in the county, it is an extremely well preserved example of the historically significant bridge type, and it is thus notable.

Setting/Context

The single-lane bridge spans the river and connects Branch Road on the west bank with a private lane (Jackson Road). The surrounding area is hilly and rural with pastures, wooded lots, and scattered 19th- and 20th-century residences.

Physical Description

The skewed 62'-long riveted Warren pony truss bridge is supported on a concrete substructure. The span is extremely well preserved. The upper chords and inclined end posts are built up box members with channels with cover plate and lacing. The diagonals are toe-down angles with battens, and the lower chords are toe-up angles with battens. Asymmetrical gusset plates are placed on both sides of the lower panel points, and they too are stiffened with a batten plate. The floor beams and stringers are rolled I sections. Lattice railings are still in place on the inside face of the trusses. As complete as the trusses is the rural setting of the bridge, which services and unimproved road.

Historical and Technological Significance

The Warren pony truss bridge on Jackson Road was built in 1927, and it is the latest of the surviving examples of metal truss bridges in the county. While exhibiting no innovative or patented details, the span is technologically and historically significant as a well preserved survivor of the last years of metal truss bridge erection in the area (criterion C). It was apparently designed by County Engineer Oscar Smith, Jr., who succeeded longtime County Engineer Joshua Doughty, and it was built at a time when most county-designed spans were encased stringers. There is no indication that this bridge was moved to Jackson Road from another location. Prior to the widespread acceptance of rolled steel stringer and reinforced concrete arch bridges in the 1910s, the Warren pony truss was the most common early-20th century highway bridge for crossings of less than 100'. The Warren truss bridge type was patented in 1848 by two British engineers, James Warren and Willoughby Monzani. It differed from other trusses in that it did not have a vertical (compression) member and that alternate diagonals slope in opposite directions. Because some of those diagonals are compression members, the design was not well suited for pinned connections. With the perfection of the portable pneumatic equipment in the late-19th century, field riveting was possible, and the simple but rigid Warren truss came to the fore during the 1890s. During the early 20th century, steel Warren trusses appeared in rapidly growing numbers, and were used for both highway and railroad spans. However, by the end of the 1920s. few, if any, metal pony truss highway bridges were being built in New Jersey.

Boundary Description and Justification

The bridge is evaluated as individually distinguished, although its rural setting does offer fine integrity of setting. The significant boundary is limited to the span itself, both the superstructure and substructure.

Sources

Somerset County Engineer: Bridge File: E141. Musti, J. Somerset County New Jersey 1688-1930. Camden, N.J., 1930.

Bridge Considered Historic By Survey: Yes