Garretson Outlet Bridge
Map
Description
This long-span pony truss carries an unpaved county road across the Garreston Outlet Ditch north of Whiting. Comprised of a riveted Warren truss with polygonal upper chords, the structure was fabricated and erected for the county by the Iowa Bridge Company of Des Moines in 1913. IBCo held the county's annual bridge contract that year, as it had for 1912, building several small- and medium-scale bridges over the newly dredged system of channelized streams and drainage ditches that crisscrossed the county. Supported by a timber pile bent substructure, the bridge still functions in place in unaltered condition.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].
National Register information
- Status
- Posted to the National Register of Historic Places on March 12, 1999
- Reference number
- 99000313
- Architectural style
- Other architectural type; Warren pony truss
- Area of significance
- Engineering
- Level of significance
- State
- Evaluation criteria
- C - Design/Construction
- Property type
- Structure
- Historic function
- Road-related
- Current function
- Road-related
- Period of significance
- 1900-1924
- Significant year
- 1913
Sources
- Dave King
- National Register database - Reference number 99000313 (Search for documents)
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