The Dearborn River High Bridge is thought to be the last example of a pin-connected half-deck truss bridge in the United States. It spans the upper drainage of the Dearborn River as it winds through a narrow canyon. Still in use on a graveled county road, the bridge originally carried horses and wagons, and early automotive traffic. Unlike most steel trusses, where the traveler either passed between and under the truss members or where the trusses were located underneath the deck, the Dearborn River High Bridge is a rare example of a deck attached midway on the trusses. In the 1890s and 1900s, structures of this type were occasionally built at canyon crossings.
In December 1896, the Lewis and Clark County Commissioners advertised for the construction of a bridge across the Dearborn River on the road between Augusta and the Montana Central Railway Station at Wolf Creek, 30 miles to the south. The road was charted through the foothills east of Montana’s dramatic Rocky Mountain Front, high, rugged terrain cut by rivers cascading from the Continental Divide. The county commissioners awarded the contract for the bridge to the King Bridge Company of Cleveland, Ohio, for its low bid of $9,989, and in July 1897, the firm completed the project.
The Dearborn River High Bridge consists of four spans: the main truss span and three plate girder approach spans. The main span is 160 feet long and 16 feet wide. The bridge ends originally rested on rubblestone abutments dug into the canyon walls. The bridge piers are concrete encased in riveted steel jackets. The lower chords are eyebars; the upper chords are continuous steel plates riveted atop two channel sections with lacing bars riveted to the lower flanges. The mid-lateral chords are angle sections with batten plates. Verticals members are laced channel sections and the diagonals are eyebars, and eyebars with turnbuckles. The steel I-beam floor beams support timber stringers that brace the timber deck. The materials used for the bridge were typical of steel truss bridge construction in Montana before 1915, when the Montana State Highway Commission moved to standardized bridge designs.
While the style of the bridge is unusual, the method by which it was contracted and constructed was typical in late-nineteenth- and early-twentieth-century Montana. The Dearborn River High Bridge represents a period in Montana when the counties utilized a system known as “pooling” to improve the state’s transportation infrastructure through private bridge contractors. Though the counties ordered hundreds of steel truss bridges, it was the responsibility of the contracted companies to determine what type of bridge was most appropriate at a given site. Once the county agreed with the bridge selection, the contractor ordered the structure to the correct specifications from one of several steel bridge fabrication companies located in the Midwest and the East Coast. The fabricator manufactured the bridge and assembled it in the factory, with pin connections facilitating the process. The bridge was then dissembled, shipped to Montana, and reassembled on-site by the contractor. Prior to creation of the Montana State Highway Commission, bridge construction companies operated in a very limited market in Montana. Other states with the same problem solved it by developing unwritten agreements among the contractors by which they obtained contracts in specific counties, guaranteeing them business, while keeping those who were not party to the agreements out of the market. Between 1892 and 1901, the King Bridge Company obtained all the county bridge contracts in Lewis and Clark County. The Dearborn River High Bridge was one of at least six bridges King Bridge built in the county during that period.
In 2003, recognizing its rarity, the Montana Department of Transportation rehabilitated the Dearborn River High Bridge and listed it the National Register of Historic Places. The original abutments were strengthened and the rubblestone replaced with concrete scored to simulate the original stonework. The truss members were straightened and tightened with new pins installed where necessary. The stringers were replaced, piers repaired, and a new deck installed. New crash resistant aluminum guardrails to simulate the original structures were installed and the bridge was repainted in its original Venetian red color.
Axline, Jon. Conveniences Sorely Needed: Montana’s Historic Highway Bridges, 1860-1956. Helena: Montana Historical Society, 2005.
Murphy, Kevin. “Dearborn River High Bridge, Spanning Dearborn River, Augusta, Lewis and Clark County, MT.” HAER No. MT-23, Historic American Engineering Record, 1984. Prints and Photographs Division, Library of Congress, Washington, D.C.
Quivik, Frederick L. Historic Bridges of Montana. Washington: National Park Service, 1982.