US Bridges

S 91 over Boulder River

Fair

Sweet Grass County, Montana · Big Timber · Structure no. P00091002+06221

Built
1938
87 years old
Condition
Fair
inspected July 2023
Type
Stringer/Multi-beam or Girder
Steel Continuous
Length
286 ft
3 main spans
Daily traffic
2,133
2023 count
Owner
State
State Highway Agency

About this bridge

S 91 over Boulder River is a steel continuous stringer/multi-beam or girder carrying S 91 over Boulder River in Sweet Grass County, Montana. It was built in 1938, so it is 87 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 5 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 2,133 vehicles a day, roughly 5% of them trucks, projected to reach 2,240 by 2043. The structure is 286 ft (87.1 m) long and 32 ft wide. If it were closed, the detour would be 6.8 mi (11 km).

Location

Open in Google Maps
CarriesS 91
CrossesBoulder River
LocationBig Timber
RouteState Highway 91
CountySweet Grass County
StateMontana
Coordinates45.83479, -109.93581
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
Deck5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Channel and channel protection7 GoodSome minor problems.
Last inspectionJuly 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour within limits of footing or piles
Structural evaluation5: Somewhat better than minimum adequacy to tolerate being left in place as is
Deck geometry3: Basically intolerable, requiring high priority of corrective action
Waterway adequacy9: Superior to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating48 US tons
Inventory rating28 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialSteel Continuous
DeckConcrete, cast-in-place
Main spans3 (+2 approach)
Design loadH 15
Structure length286 ft (87.1 m)
Longest span90 ft (27.4 m)
Deck width32 ft (9.9 m)
Roadway width26 ft (7.9 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic2,133 (2023)
Projected traffic2,240 by 2043
Trucks5%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed6.8 mi (11 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$608,000
NavigationNo navigation control on waterway

Frequently asked questions

When was S 91 over Boulder River built?

It was built in 1938, making it 87 years old in 2025.

What condition is S 91 over Boulder 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 2023.

How much traffic does S 91 over Boulder River carry?

About 2,133 vehicles per day (2023 count), of which roughly 5% are trucks.

How long is S 91 over Boulder River?

The structure is 286 ft (87.1 m) long with a longest span of 90 ft (27.4 m) and a deck 32 ft wide.

Who owns S 91 over Boulder River?

It is owned by the State Highway Agency.