US Bridges

US Highway 12 over Little Missouri River

Fair

Slope County, North Dakota · At Marmarth · Structure no. 0012006555

Built
1950
75 years old, rebuilt 1987
Condition
Fair
inspected July 2022
Type
Stringer/Multi-beam or Girder
Steel Continuous
Length
414 ft
5 main spans
Daily traffic
580
2019 count
Owner
State
State Highway Agency

About this bridge

US Highway 12 over Little Missouri River is a steel continuous stringer/multi-beam or girder carrying US Highway 12 over Little Missouri River in Slope County, North Dakota. It was built in 1950 and reconstructed in 1987, so it is 75 years old, older than the North Dakota average of 48.2 years.

The most recent inspection reported to the Federal Highway Administration rates it in fair condition with a lowest component rating of 6 out of 9. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

It carries about 580 vehicles a day, roughly 24% of them trucks, projected to reach 1,118 by 2039. The structure is 414 ft (126.2 m) long and 45 ft wide. If it were closed, the detour would be 123.7 mi (199 km).

Location

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CarriesUS Highway 12
CrossesLittle Missouri River
LocationAt Marmarth
RouteU.S. Route 12
CountySlope County
StateNorth Dakota
Coordinates46.29785, -103.91658
Functional classRural Minor Arterial

Condition and inspection

Fair

The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

ComponentRatingMeaning
Deck6 SatisfactoryStructural elements show some minor deterioration.
Superstructure7 GoodSome minor problems.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
Channel and channel protection7 GoodSome minor problems.
Last inspectionJuly 2022
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour within limits of footing or piles
Structural evaluation6: Equal to present minimum criteria
Deck geometry9: Superior to present desirable criteria
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment4: Meets minimum tolerable limits to be left in place as is
Operating rating69.6 US tons
Inventory rating41.6 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialSteel Continuous
DeckConcrete, cast-in-place
Main spans5
Design loadH 15
Structure length414 ft (126.2 m)
Longest span90 ft (27.4 m)
Deck width45 ft (13.7 m)
Roadway width42 ft (12.8 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic580 (2019)
Projected traffic1,118 by 2039
Trucks24%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed123.7 mi (199 km)
TollOn a free road
National Highway SystemNo

Ownership and planned work

OwnerState Highway Agency
Maintained byState Highway Agency
Historic statusNot eligible for the National Register of Historic Places
Work proposedRehabilitation because of general structure deterioration or inadequate strength
Estimated improvement cost$696,000
NavigationNo navigation control on waterway

Frequently asked questions

When was US Highway 12 over Little Missouri River built?

It was built in 1950, making it 75 years old in 2025. It was reconstructed in 1987.

What condition is US Highway 12 over Little Missouri River in?

The National Bridge Inventory rates it in fair condition. The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound. The last reported inspection was in July 2022.

How much traffic does US Highway 12 over Little Missouri River carry?

About 580 vehicles per day (2019 count), of which roughly 24% are trucks.

How long is US Highway 12 over Little Missouri River?

The structure is 414 ft (126.2 m) long with a longest span of 90 ft (27.4 m) and a deck 45 ft wide.

Who owns US Highway 12 over Little Missouri River?

It is owned by the State Highway Agency.