This impressive bridge was built to carry the railroad over the Mill Creek and its associated valley. The bridge is 60 feet above the bottom of the valley. The valleys carved by the Mill Creek and nearby Black River in this area constitute the only notable terrain relief in the entire thumb area, which is otherwise flat as a pancake. Thus, bridge and setting are both unusual in this region.
An exact construction date is unknown for this bridge, although local media reports it was built in the late 1800s. A c. 1890 date seems appropriate for this structure until further research reveals a more specific date. The bridge was built by the Flint and Pere Marquette Railroad, which constructed a line from Yale to Port Huron sometime soon after 1889. The bridge was last owned by CSX prior to abandonment and subsequent conversion into rail-trail. The remains of a builder plaque was found on the bridge. The remains were enough to identify that the Detroit Bridge and Iron Company built the bridge, but the date portion was missing. The company used the same style plaque over many years. In 1904, the bridge was strengthened by the addition of a third truss line in between the two original trusses. Similarly, a third girder was added in between the originals on the approach spans.
The main span of this bridge is a deck truss with a polygonal bottom chord, which is described in HAER documentation as a "fishbelly" deck truss. The fishbelly design was generally employed when a replacement bridge that could carry more weight was to be constructed on top of existing substructure while also maintaining the existing track elevation. In order to make the truss stronger, the first thought would be to increase the depth of the truss. However because the tracks could not be raised and altering the piers might be costly, the only other apparent way to increase the strength of the truss without making the truss deeper at the ends would be to drastically increase the size of the members, which was expensive. However, a design was developed to make the bottom chord of the truss polygonal... so that the shallower depth that would fit the piers and tracks could be employed at the ends, while a deeper truss at the center would increase the strength of the truss. All that interesting history said, it is unclear whether in fact that is why the Mill Creek Railroad Bridge displays this fishbelly design. Here, the substructure appears to be the same age as the superstructure. However the ability of the fishbelly truss ability to function with a shorter substructure may have reduced costs for the railroad by requiring a shorter substructure to support the main span. This may explain the unusual design in this instance.
The 1904 addition to this bridge was documented briefly in the Report of the Pere Marquette Railway Company For the Six Months Ending June 30th, 1904. The brief mention stated "Avoca, Bridge P-71.0, steel trestle 640 ft. long. Material received and charged out for strengthening this structure to permit using heavier power. The work of erection was not commenced until after June 30th." The third truss line was cleverly built in between the two original truss lines. New bents were constructed to support this. The third truss line lacks the fishbelly design, instead featuring a constantly horizontal bottom chord. Otherwise, the third truss line is stylistically similar to the original trusses, aside from being slightly more massive in design.
This bridge features an extensive approach system of deck plate girder spans. The supports for the bridge are a complex system of steel bents resting on stone footings. Abutments are also stone. Some connections on the bridge are riveted and others are pin connected. The primary connection type is pin connections. The bridge's main span stands on steel bents which rest on stone footings. Built up members are the primary beam type and include v-lacing, and lattice, adding to the geometric beauty of the structure.
While comprehensive national inventories of railroad bridges do not exist in the way that the National Bridge Inventory does for highway bridges, very few examples of fishbelly deck truss bridges have been identified nationwide. There likely are other examples, however this truss design should still be considered exceedingly rare. It is even more rare in the context of Michigan which has very few railroad deck trusses. The third truss added to the bridge was added so long ago that the alteration is itself historic. The bridge is historically and technologically significant as an unusual and complex railroad truss bridge in Michigan.
This bridge is used as part of a pedestrian trail, and is no longer used by the railroad. Although the deck has been converted to a walkway, the actual bridge structure remains rusted and has not been painted or repaired at all since the railroad abandoned it. There also is no easily accessible viewing area for trail users to marvel at this extremely rare bridge's unique superstructure and substructure. To really see the details of this bridge a visitor would have to be along the banks of Mill Creek, far below.
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