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Ontonagon Railroad Bridge

Ontonagon Railroad Bridge

Primary Photographer(s): Nathan Holth

Bridge Documented: October 1, 2012

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Key Facts

Facility Carried / Feature Intersected
Railroad (Escanaba and Lake Superior Railroad) Over Ontonagon River
Ontonagon: Ontonagon County, Michigan: United States
Structure Type
Metal 6 Panel Pin-Connected Pratt Full-Slope Pony Truss, Fixed and Approach Spans: Metal Deck Girder, Fixed
Construction Date and Builder / Engineer
1899 By Builder/Contractor: Lassig Bridge and Iron Works of Chicago, Illinois

Technical Facts

Rehabilitation Date
Not Available or Not Applicable
Main Span Length
100 Feet (30.5 Meters)
Structure Length
470 Feet (143.3 Meters)
Roadway Width
15 Feet (4.57 Meters)
2 Main Span(s)
NBI Number
Not Applicable

Historic Significance Rating (HSR)
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Bridge Documentation

This bridge was originally built for the Chicago, Milwaukee, and St. Paul Railroad. Pony truss bridges are uncommon on railroads, with through truss bridges and plate girders being far more common. As such, this bridge stands out as unusual, and even more so because it has two pony truss spans. With an 1899 construction date and pin connected trusses, the trusses are older examples among Michigan railroad truss bridges, and the pin connections speak to an earlier generation of railroad bridge design.

West of the pony truss spans are five steel deck plate girder approach spans totaling 130 feet in length. The two eastern spans are the largest of these, and the easternmost span is unusual because the girders come up to the track level, rather than completely hiding under the deck like a normal deck plate girder (and the other deck girder spans on this bridge). This easternmost span is also not symmetrical, with the eastern end's bottom chord sloping upward at the end. East of the central pony truss spans is a series of timber stringer approach spans totaling 150 feet in length. At the eastern end, the railroad splits to the north and south in a Y formation. The bridge appears to retain good historic integrity with no major alterations noted. The pony truss spans are outstanding examples of railroad pony truss bridge construction.

The reality that railroad pony truss bridges are far less common than highway pony truss bridges is quite striking. A potential reason for their limited use on railroads is detailed in The Principal Professional Papers of Dr. J. A. L. Waddell, Edited by John Lyle Harrington, dated 1905. The exact text from a discussion of railway bridges follows:

"The pony truss for railway bridges has happily fallen into oblivion. Its top chord was rarely well stayed and was often subjected to severe and unknown stresses, while the economy involved in its use was very small."

It would appear that the specific variety of stresses that a train puts on a railroad bridge were too great for the top chord of a pony truss. In contrast, a through truss with its extensive overhead bracing system must have provided the stability against these stresses.



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