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US65 Winnebago River Bridge


North Approach To Winnebago River Bridge On Southbound U.S. Highway 65 Looking South

Photo by Bruce A. Harms, LBA, Summer 1996, for HAER

View photos at Library of Congress

BH Photo #122212

Street View 


"Design No. 426, Cerro Gordo County, is unique in being the longest monolithic concrete girder in the state," the state highway commission reported in its typical deadpan manner in 1926. "It is 122' long over all but makes use of the cantilever principle, the distance between piers being 70'." Built in 1926, this is actually the second bridge at the Winnebago River crossing just north of Mason City, replacing an earlier 120-foot truss. When ISHC engineers designed the concrete bridge, they employed the original stone masonry abutments, covered with a new concrete sheathing. The replacement structure consisted of three concrete spans--a 70-foot center span cantilevered from shorter anchor spans on both ends--all supported by a concrete substructure. The drawings for the bridge were completed in the spring of 1926; competitive bids for this bridge and four others were solicited by the county in early June. On June 15th, the county received eleven proposals and divided the bridges between two Mason City contractors, C.A. Holvik and William Henkel. Henkel was awarded the construction contract for the cantilever deck girder, a 50-foot pony truss and a steel stringer structure for $21,920. The Concrete Engineering Company was hired to encase the stone abutments. Apparently completed later that year, the Winnebago River Bridge has functioned in place since. The sidewalk that cantilevers from one side may or may not be original, but the bridge superstructure and substructure remain in unaltered condition. The choice to use a rather esoteric structural type at this crossing was for the Highway Commission a logical extension of its design policies. ISHC had first developed the Standard Series H design for simply supported concrete girder bridges among its first standardized bridge plans in 1914. Three years later the Highway Commission designed its first cantilevered deck girder for a three-span structure over the Boyer River at Woodbine. In its 1917 annual report, ISHC deemed the cantilevered deck girder design "particularly well adapted for use on deep drainage ditches and streams subject to widening." A cantilevered span could carry more weight than a simply supported girder of the same length. Alternately, a cantilevered girder could span a greater distance, while carrying the same weight as a shorter, simply-supported girder. Moreover, the arched profile of ISHC's cantilevered girders was considered more architecturally accomplished than the straight spandrel of simply supported girders. Finally, although ISHC engineers espoused the advantages of standardized construction, they apparently could not resist the opportunity to experiment occasionally. Built on the substructure of an earlier bridge, the Winnebago River structure allowed ISHC the opportunity to demonstrate the superiority and flexibility of concrete for bridge construction. With a 70-foot center span, the Winnebago River Bridge marks the culmination of this type of construction for ISHC. It is thus the most technologically significant of the state's numerous concrete girder bridges [adapted from Hybben, Roise and Fraser 1992].


Concrete deck girder bridge over Winnebago River on southbound US 65
Cerro Gordo County, Iowa
Open to traffic
Built 1926
- Concrete Engineering Co (Abutment Encasers)
- Henkel Construction Co. of Mason City, Iowa
- Iowa State Highway Commission of Ames, Iowa
Concrete cantilever tee beam.

"The Winnebago Bridge, the longest monolithic concrete girder in Iowa, is the most technologically significant of the state's numerous concrete cantilevered girder bridges designed by the Iowa State Highway Commission. It also marks the culmination of this type of construction by ISHC."

Historic American Engineering Record, Summer 1995

Length of largest span: 69.9 ft.
Total length: 124.0 ft.
Deck width: 24.0 ft.
Posted to the National Register of Historic Places on June 25, 1998
Approximate latitude, longitude
+43.19279, -93.20961   (decimal degrees)
43°11'34" N, 93°12'35" W   (degrees°minutes'seconds")
Approximate UTM coordinates
15/482968/4782244 (zone/easting/northing)
Quadrangle map:
Mason City
Inventory numbers
NRHP 98000812 (National Register of Historic Places reference number)
IA 19010 (Iowa bridge number)
BH 13109 (Bridgehunter.com ID)

Update Log 

  • November 22, 2013: Updated by Luke Harden: Added builders
  • July 30, 2010: New Street View added by Jason Smith
  • August 22, 2008: New photos from J.R. Manning


  • HAER IA-78 - Winnebago River Bridge
  • J.R. Manning - thekitchenguy [at] sbcglobal [dot] net
  • Jason Smith - flensburg [dot] bridgehunter [dot] av [at] googlemail [dot] com
  • Luke


US65 Winnebago River Bridge
Posted September 27, 2020, by Bruce Myers (bruce [at] brucemyers [dot] com)

This bridge was replaced in 2009.

See: https://www.news.iowadot.gov/newsandinfo/2009/05/us-65-traff...

See: http://bridgereports.com/1107455

Google street view also confirms replacement.