Historic Bridges

Details

Located some five miles northwest of Bevington, this medium-span iron truss carries a gravel-surfaced county road over the North River. The Cunningham Bridge, as it is known locally, consists of a pinned Pratt pony truss, approached on both sides by timber stringer spans and supported by timber pile bents. The structure dates to 1886. That year the Madison County Board of Supervisors contracted with the George E. King Bridge Company of Des Moines to erect an 80-foot wrought iron truss at this Crawford County crossing and move the existing bridge to another site. Costing $820, the truss was shipped to the county and assembled by June. Since its completion, the Cunningham Bridge has functioned in place without substantial alteration.

Like virtually all of Iowa's counties, Madison County followed a definite progression in its bridge construction in the 19th century, in response to evolving transportation needs and technological development in the bridge industry. The first simple spans, built as the county was undergoing its initial settlement, were rudimentary timber structures: cheap and easy to build but lacking in durability and limited in span length. With greater revenues from increased settlement, the county could undertake more ambitious timber trusses in the 1850s and 1860s. Iron was used for tension members in the covered combination spans erected during the 1860s and 1870s. But even with their plank and shingle sheathing, these timber trusses lacked the strength and durability of all-iron trusses. As a result, timber trusses were superseded by wrought iron configurations (either bowstring or full trusses), which were readily available in the 1870s and 1880s through mass-production techniques. Although the county supervisors barely noticed the transition from iron to steel in the 1890s--and, indeed, continued to refer to new bridges as iron that were undoubtedly made of steel--this evolution marked a watershed that would continue into the 20th century for bridge fabricators and the rolling mills that supplied them. Of the many substantial wagon bridges built in the 19th century by Madison County, only a few remain intact. The Cunningham Bridge is one of these: a relatively well-preserved example of early all-iron truss construction [adapted from Fraser 1992].