Historic Bridges

Boone River Bridge

Wright county

Bridge information

Years constructed: 1912
Bridge type: Rigid-Connected Warren Pony Truss
National Register of Historic Places status: Listed
Length: 128 feet
Width: 15.2 feet
Spans: 1
FHWA: 359190
Wright County Engineer
aclemons@co.wright.ia.us / 515-532-3597
Location: Buchanan Avenue over the Boone River, Section 20, T93N-R26W (Boone Township)


During the first twenty years of the twentieth century, the Iowa Bridge Company was responsible for building most of Wright County's steel bridges. Prior to 1909 the county's board of supervisors awarded contracts for either individual or multiple bridge projects, but in subsequent years the board often issued annual contracts to the Des Moines-based firm. One of the bridges built by IBCo in 1912 was this long-span Warren truss, erected over the Boone River north of Goldfield, in Boone Township. Steel for the superstructure was fabricated at the Cambria Mills in Pittsburgh; the truss itself was fabricated at IBCo's Des Moines shops. The bridge continues to carry vehicular traffic in its original location, and possesses a high degree of physical integrity.

After about 1910, rigid-connected Warren pony trusses began to supersede earlier pinned Pratt configurations for use on short- to intermediate-span highway bridges. Although these bridges displayed variations in their web configurations--some were "pure" Warren configurations without verticals, others had verticals at all or alternate panel points--virtually all of these early Warren trusses featured straight upper chords. A few rigid-connected Warren ponies were fabricated with polygonal upper chords, however. These were comparable to the basic Warren in their load-carrying capacity, but required somewhat less steel in the vertical and diagonal members of the outer panels. As a more materially efficient design, the polygonal-chorded Warren represented a technological refinement over its straight-chorded counterpart. Unlike long-span through trusses, in which the savings of material in a polygonal top-chord could be substantial, the relatively modest savings in material in these medium-span pony trusses was more than offset by greater inventory, fabrication and erection costs. As a result, the polygonal-chorded Warren truss never became a mainstay structural type. Those few polygonal-chorded Warrens identified in Iowa have been traced to the Iowa Bridge Company of Des Moines. Soon after the company began fabricating this bridge type, however, the Iowa State Highway Commission issued standardized bridge designs for use on the state's roads. The commission's Warren truss designs, widely adopted bye the counties under the directive of the state legislature in 1913, featured straight upper chords. The polygonal Warren configuration was thus rendered obsolete in Iowa soon after it was introduced. The Boone River Bridge is distinguished as a well-preserved example of this uncommon Warren truss subtype [adapted from Fraser 1991].


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