Photo from nps.gov
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, 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 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 by 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. This bridge in Monona County is a well-preserved example of the polygonal Warren pony truss. One of only a handful of such structures, it represents riveted truss construction in the years immediately before codification of bridge design by the state highway commission [adapted from Fraser 1992].