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Iroquois 600 Bridge

TR-565 Bridge

Iroquois 600 Bridge

Primary Photographer(s): Nathan Holth and Rick McOmber

Bridge Documented: May 8, 2010

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Key Facts

Facility Carried / Feature Intersected
Iroquois 600 (TR-565) Over Spring Creek
Rural: Iroquois County, Illinois: United States
Structure Type
Concrete Through Girder, Fixed
Construction Date and Builder / Engineer
1915 By Builder/Contractor: Unknown

Technical Facts

Rehabilitation Date
Not Available or Not Applicable
Main Span Length
61 Feet (18.7 Meters)
Structure Length
64 Feet (19.5 Meters)
Roadway Width
16.4 Feet (5 Meters)
1 Main Span(s)
NBI Number

Historic Significance Rating (HSR)
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Bridge Documentation

View Archived National Bridge Inventory Report - Has Additional Details and Evaluation

This bridge is an excellent example of a standard plan concrete through girder in Illinois. The bridge is unaltered and retains near-perfect historic integrity, and also remains in good structural condition despite never having been repaired or rehabilitated. This particular example is noteworthy for its span of over 60 feet in length, which is quite long for a concrete through girder. For comparison, in the state of Michigan, where a relatively large number of straight chord through girders survive, the largest known span is 50 feet. In Ohio and Indiana, the few concrete girder spans that reach 60 feet are generally curved chord through girders, not straight chord through girders. It appears that 60 feet is also among the longest spans in Illinois as well. Girders longer than this tended to quickly become inefficient and costly in terms of materials. The long length of this Iroquois County girder was likely made easier by the narrow 16.4 foot deck. Narrower decks require less massive girders to support the load. This bridge is also significant for its very early 1915 construction date. It was around this time when concrete girders began to be built with frequency. As such, this bridge should be considered historically and technologically significant.

Concrete through girders are a nationally rare structure type today. Their popularity only lasted for about 20 years, mainly between 1915 and 1935, mainly because they were inefficient for deck widths much wider than 22 feet. Michigan and Illinois both appear to have the largest numbers of surviving concrete through girders in the country. This does not mean either state has a large actual number however. As such, surviving examples that have integrity should be considered to have historic value. The bridges tend to be heavily built and very durable. Those examples that are located on quiet rural roads such as this one make good candidates for preservation.


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