370th Street Bridge
© Iowa Department of Transportation
BH Photo #270770
Located southwest of Henderson in northwestern Mills County, this rigid-connected pony truss carries a paved county road (once a part of a state highway) over the Nishnabotna River. The bridge is comprised of two skewed trusses, each configured as a Warren pony with polygonal upper chords. The trusses are supported by concrete abutments and piers, and they are approached by steel stringer approach spans on both ends. The Nishnabotna River Bridge was designed by engineers for the Iowa State Highway Commission in the summer of 1929. The bridge's skewed orientation and relatively long span length prompted ISHC to produce a special design for the trusses, rather than rely on its standard plans. On August 20, 1929, the state highway commission contracted with the McCormack Construction Company of Lohrville, Iowa, to construct the bridge for $30,900. Using steel rolled by Inland, McCormack completed the bridge the following year. It has since functioned in place, without substantial alteration.
Although at least one Iowa bridge company employed polygonal-chorded Warren trusses on a limited basis in the early 1910s, this inherently long-span structural type never found much favor among the counties. As a result, relatively few such trusses were built in the state between 1910 and 1913--the year that the state highway commission began issuing standard plans for bridges. ISHC's standard pony trusses ranged in span length between 35 and 100 feet, all featuring straight-chorded Warren configurations. The sloped upper chords of the Nishnabotna River Bridge are an anomaly, apparently an attempt by ISHC to develop a more materially conservant structural type for its long-span ponies. It is not known whether the Nishnabotna River Bridge was the first example of this truss type, or whether others were ever built, but this bridge is today is distinguished as the only example of its kind by ISHC remaining in the state. It is thus technologically noteworthy as a well-preserved example of an uncommon structural type [adapted from Fraser 1990].
- Pony truss bridge over Nishnabotna River on 370th Street
- Mills County, Iowa
- Open to traffic
- Built ca. 1940
- - Inland Steel Co. of East Chicago, Indiana (Steel mMnufacturer)
- Iowa State Highway Commission of Ames, Iowa (Designer)
- McCormack Construction Co. of Lohrville, Iowa
- Riveted warren pony truss
Length of largest span: 100.1 ft.
Total length: 291.0 ft.
Deck width: 19.7 ft.
Posted to the National Register of Historic Places on May 15, 1998
- Approximate latitude, longitude
- +41.08846, -95.48022 (decimal degrees)
41°05'18" N, 95°28'49" W (degrees°minutes'seconds")
- Approximate UTM coordinates
- 15/291680/4551541 (zone/easting/northing)
- Quadrangle map:
- Land survey
- T. 73 N., R. 40 W., Sec. 29
- Inventory numbers
- IA 246630 (Iowa bridge number)
NRHP 98000496 (National Register of Historic Places reference number)
BH 14034 (Bridgehunter.com ID)
- Inspection (as of 03/2015)
- Deck condition rating: Serious (3 out of 9)
Superstructure condition rating: Serious (3 out of 9)
Substructure condition rating: Critical (2 out of 9)
Appraisal: Structurally deficient
Sufficiency rating: 26.2 (out of 100)
- Average daily traffic (as of 2012)
- November 20, 2013: New photo from Luke Harden
- March 12, 2012: Updated by Luke Harden: Added category "Iowa State Highway Commission"
- July 30, 2011: New photo from Luke Harden
- July 29, 2011: Updated by Luke Harden: Added description and photograph, updated bridge type