Oak Grove Bridge
Covered Timber Town Lattice Truss
National Register of Historic Places status:
Elderberry Avenue over the Middle River, 7.5 miles southwest of Winterset, Section 14, T75N-R29W (Lincoln Township)
Like most of Iowa's counties, Madison began its bridge building on a modest scale, with a 40-foot timber structure built in 1854-55. In the 1850s and 1860s the county board of supervisors ordered numerous short-span timber pile and kingpost structures. Though inexpensive to erect, most of these spans tended to be structurally suspect and required frequent maintenance to prevent their collapse. Moreover, they were restricted to short-span crossings. Beginning in 1868, Madison County contracted with local builders H.P. Jones, G.K. Foster and Eli Cox to build a series of timber trusses, covered with wood sheathing, to replace the earlier structures. Although the county began building all-iron spans in 1872, the supervisors continued to hire the local builders to construct timber trusses well into the 1880s. The two last covered bridges built in Madison County were the Roseman and Cedar Bridges over the Middle and North Rivers, respectively. Both approved by the board in 1877, they were build in succession in 1883 by Jones and Foster. Located southwest of Winterset, the Roseman Bridge (also called the Oak Grover Bridge or, inexplicably the Haunter Bridge) cost $2,930. It was comprised, like most of the county's long timber spans, of an all-wood Town lattice truss, over which was superimposed a queenpost frame. Supported by iron cylinder piers, the superstructure was covered with plank walls and a bow roof. The Roseman Bridge was completed in 1883. It carried traffic for almost a hundred years before it was bypassed by another bridge nearby in 1981. Today it stands in place as the centerpiece for a small park.
Patented by Ithiel Town in 1820, the Town lattice truss was used extensively in America for early all-timber bridges. Its closely spaced diagonal members formed a web of considerable strength and rigidity, and the structurally indeterminate configuration lent itself to empirical design by craftsmen lacking formal engineering training. H.P. Jones, who was probably responsible for the design of the Roseman Bridge, strengthened the truss further by superimposing upon it a queenpost truss, thus creating a structural redundancy that bordered on overkill. The result is a bridge that has stood in place while almost all of the other timber trusses in Iowa have since been demolished. The Roseman Bridge is thus distinguished, as are all of Madison County's covered bridges, as a relatively well-preserved remnant from the state's formative years in bridge construction [adapted from Fraser 1992].