Kern Bridge

Background

The Kern (Yaeger) Bridge is a single-span, wrought-iron, bowstring arch through truss bridge that rested on limestone abutments before its removal on February 6, 2020. The bridge is significant as the only example of a bowstring through truss bridge in Minnesota and for its exception span length.

This type of bridge was a popular bridge type from the 1850s to 1880s. Typically constructed of cast or wrought iron, these bridge types marked the transition from wood, stone, and timber building materials to metal. They peaked in popularity in the 1870s and could be found in states throughout the Midwest. 

By the 1880s, bridge builders shifted their attention to Pratt truss as the preferred crossing type and construction of bowstring arches declined. Today there are few bowstring arch bridges still standing in the Midwest or the United States. (Historic Bridge Foundation, "Seeking a New Home: Kern Bridge, America's Largest Bowstring Span", 2019). 

Key Facts

• Built: 1873 (one of Minnesota's oldest bridges) for a cost of $8,652 by John Mahowald and the Wrought Iron Bridge Company
• Type: Bowstring arch through truss (only one in Minnesota) 
• Length: 189ft. (longest existing bowstring span in the US) 
• Initial Crossing: Mankato/South Bend Ivywood Lane (Township Road 190) over the Le Sueur River
• Use: Initially used for horse and buggy traffic; closed to pedestrians and vehicles early 1990s
• Ownership: Transferred from Mankato & South Bend Township to MnDOT District 7 following the removal to be stored until a new owner can be found
• Listed on the National Register of Historic Places 

Removal and Storage

On February 6, 2020, the Kern Bridge was picked up from its location spanning the Le Sueur River and carefully placed along the river's banks to be disassembled and placed into storage.

Due to the design and age of the bridge, this project required intricate structural engineering analysis of the bridge's components strength characteristics as well as a carefully detailed pick and salvage plan in order to maintain the integrity of the bridge structure for disassembly and future re-use in an alternate location. Stabilization of the southeast bridge bearing during construction was also required because the limestone block abutments the bridge sat upon were failing. This stabilization helped to reduce the chance of failure prior to salvaging the bridge. 

Two cranes were used during the operation with one crane picking at each end of the bridge because of the bridge's length. A large crane was placed on a rock pad in the middle of the Le Sueur River to lift the west side; another slightly smaller crane sat on the east bank to lift the east side. The bridge was then lifted from its location and set down temporarily while the east crane re-positioned to set the bridge in its final location for disassembly. 

After removal, the structure was carefully marked and disassembled into large pieces for future refurbishing and re-assembly. 

The removal was completed by Carlton Companies, Inc. of Mankato, MN with a bid of $595,660. The project was funded with Federal (FHWA) aid (80%0 and State Aid Township Bridge funds (20%). The project was managed by Blue Earth County Public Works Director/County Engineer Ryan Thilges, P.E. & Assistant County Engineer Stefan Gantert, P.E. 

February 6, 2020 Bridge Removal Video