US Bridges

I 15 NB over Sun River

Fair

Cascade County, Montana · Great Falls · Structure no. I00015279+09761

Built
1966
59 years old
Condition
Fair
inspected January 2025
Type
Stringer/Multi-beam or Girder
Prestressed Concrete
Length
485 ft
5 main spans
Daily traffic
5,254
2023 count
Owner
State
State Highway Agency

About this bridge

I 15 NB over Sun River is a prestressed concrete stringer/multi-beam or girder carrying I 15 NB over Sun River in Cascade County, Montana. It was built in 1966, so it is 59 years old, older than the Montana average of 47 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 5,254 vehicles a day, roughly 9% of them trucks, projected to reach 5,516 by 2042. The structure is 485 ft (147.8 m) long and 32 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

Open in Google Maps
CarriesI 15 NB
CrossesSun River
LocationGreat Falls
RouteInterstate 15
CountyCascade County
StateMontana
Coordinates47.49955, -111.34288
Functional classUrban Principal Arterial - Interstate

Condition and inspection

Fair

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

ComponentRatingMeaning
Deck8 Very GoodNo problems noted.
Superstructure8 Very GoodNo problems noted.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionJanuary 2025
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 geometry3: Basically intolerable, requiring high priority of corrective action
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment7: Better than present minimum criteria
Operating rating58 US tons
Inventory rating45 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans5
Design loadHS 20
Structure length485 ft (147.8 m)
Longest span97 ft (29.6 m)
Deck width32 ft (9.8 m)
Roadway width28 ft (8.5 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic5,254 (2023)
Projected traffic5,516 by 2042
Trucks9%
Lanes on structure2
Traffic directionOne-way traffic
CarriesHighway
CrossesWaterway
Detour if closed1.2 mi (2 km)
TollOn a free road
National Highway SystemYes
Strategic Highway NetworkInterstate STRAHNET route

Ownership and planned work

OwnerState Highway Agency
Maintained byState Highway Agency
Historic statusNot eligible for the National Register of Historic Places
Work proposedReplacement because of substandard load-carrying capacity or roadway geometry
Estimated improvement cost$1.4 million
NavigationNo navigation control on waterway

Frequently asked questions

When was I 15 NB over Sun River built?

It was built in 1966, making it 59 years old in 2025.

What condition is I 15 NB over Sun 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 January 2025.

How much traffic does I 15 NB over Sun River carry?

About 5,254 vehicles per day (2023 count), of which roughly 9% are trucks.

How long is I 15 NB over Sun River?

The structure is 485 ft (147.8 m) long with a longest span of 97 ft (29.6 m) and a deck 32 ft wide.

Who owns I 15 NB over Sun River?

It is owned by the State Highway Agency.