The Coburg Railroad Bridge spans the McKenzie River adjacent to the Armitage State Park alongside Coburg Road between Eugene and Coburg, Oregon. The bridge was fabricated in 1887 (company unknown) for the Oregon Railway and Navigation Company. The OR&N originally erected the bridge over the John Day River in north central Oregon. By 1907 the bridge had become obsolete at the John Day location and was acquired that year by the Southern Pacific Railroad Company. The Southern Pacific during this period, 1906-1907, was replacing most of its early wooden bridges with iron structures. The relocation of what was then the John Day Bridge to Coburg was carried out by the American Bridge Company. The iron Coburg Bridge replaced an earlier wooden structure of a significant span, built in 1891 at the same location over the McKenzie.
The overall form of the Coburg Bridge is that of a long trapezoid. The bridge, constructed of rolled iron members, is a double-intersection Pratt through truss structure. It is riveted and pin-connected throughout. The bridge has a single clear span of 405 feet. The large trapezoidal truss consists of 16 panels each 25 feet in length. The overall breadth measures 25 feet and its height measures 44 feet from the top of the rails to the top of the superstructure. The railroad clearances measure 16 feet 8 inches horizontally and 18 feet 7 inches vertically. The entire structure rests on concrete supports; at the eastern bank are two iron-clad concrete piers and at the western bank the bridge is supported by one large rectangular concrete abutment.
The vertical frames of the bridge are braced by a diamond lattice pattern that is riveted in place. The end frames are also braced in this manner and are capped with decorative railings and date plates. All major horizontal, vertical and diagonal members are pin connected.
The bridge has been painted the standard railroad black in past years. It has not seen a fresh coat of paint in many years however and the paint has long since deteriorated on most surfaces and only remains in areas of the bridge protected from the weather. The overall appearance is rusty, but since the bridge is constructed of wrought iron, the rust has stabilized, and the structure is in sound condition otherwise.
The current status of the line which the bridge serves is inactive, and abandonment status is being considered by the Southern Pacific. With the present inactivity of the line there has arisen a community effort to develop the line, including the bridge, into a community recreation corridor for walking, jogging, and bicycling.
Statement of Significance
The Coburg Railroad Bridge achieves significance as a rare survival of truss bridge construction. The particular type of construction, the double-intersection Pratt through truss, is noteworthy as it was used in the first all-iron truss bridge ever constructed. That first bridge was erected over the Mississippi River [sic] at Glasgow, Missouri in 1879. The bridge pioneered the use of iron and later steel in truss bridge design and construction. The pin-connected truss designs, of which there were several, quickly began to replace the earlier composition, lattice and girder structures, in. the 1870s. The new designs were to enjoy popularity over the following seventy years. The success of the design was due to at least three significant features: 1) the design reduced material and the number of members to a minimum necessary to carry the load. 2) manufacture and erection were greatly facilitated through the standardization of parts and assembly. 3) the life of the structure was increased while maintenance costs were reduced as a result of the use of iron. These three factors strongly reflect the ideals of industrialization during the Progressive Era of American History. The Coburg Railroad Bridge is one of the few remaining bridges of that era in the Pacific Northwest.
In addition to the construction type, additional details of particular note on the structure include diamond lattice bracing between all vertical frames, decorative head railings and date plates atop both entry frames.
The moving of the bridge in 1907 from the John Day River to the McKenzie River is a significant event. Apparently it was common practice amongst the frugal railroad companies of the time to move bridges rather than erect new ones continually. 1906-1907 saw the replacement of most of the wooden bridges and trestles on the Oregon lines with iron structures; several were moved from other locations both from inside the state and from other states as far away as Utah and Colorado. The covered wooden span that predated the present iron was one of the longest such structures ever built. It measured 380 feet overall and its main span was 260 feet.
The wooden structure built in 1891 was part of a spur line of the Springfield branch of the Southern Pacific which was intended in the early 90's for increased timber and agricultural production in the area. The replacement of the wooden structure in 1907 with the existing iron bridge became necessary to handle increased traffic and heavier loads.