Otter Tail County Bridge, County Road 8 SAP 056-608-028

In cooperation with Minnesota State Aid, Region 4, Otter Tail County, we replaced an existing bridge with a 62-foot 2-span bridge over the Otter Tail River on Otter Tail CSAH 8. The bridge was built in 1959 and classified as structurally deficient, with a sufficiency rating of 58.4. The existing bridge consisted of a continuous cast-in-place concrete deck supported by timber substructures. The bridge roadway width was 32.5 feet, with a total deck width of 34.3 feet and a 2-inch bituminous pavement deck-wearing surface. The roadway approach profiles on each end were relatively flat and had reasonable sight distance.

Interstate Engineering analyzed the existing structure and provided five alternative bridge replacement structures with USACE HEC-RAS hydraulic modeling. The alternative bridge structures primarily consisted of two or three spans with precast box beams or continuous concrete configurations. Hydraulic modeling for this project was more challenging than typical rural bridges. Little Pine Lake is located upstream of the existing bridge site, and the dam outlet structure is only 25 feet upstream. The outlet structure controlled the stream water surface profiles. Interstate Engineering, working with the Northwest Region of the DNR, provided alternatives that met the DNR’s criteria. Otter Tail County ultimately selected to proceed with the final design of a 76-foot, 2-span continuous concrete bridge having a 36-foot bridge opening. Interstate Engineering completed the bridge design per MnDOT criteria. It was constructed in 2021.