US Bridges

Main Boulder RD over Sep I 90

Fair

Sweet Grass County, Montana · Big Timber · Structure no. S00298000+08841

Built
1980
45 years old
Condition
Fair
inspected March 2024
Type
Stringer/Multi-beam or Girder
Steel Continuous
Length
395 ft
2 main spans
Daily traffic
958
2023 count
Owner
State
State Highway Agency

About this bridge

Main Boulder RD over Sep I 90 is a steel continuous stringer/multi-beam or girder carrying Main Boulder RD over Sep I 90 in Sweet Grass County, Montana. It was built in 1980, so it is 45 years old, younger 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 958 vehicles a day, roughly 3% of them trucks, projected to reach 1,006 by 2043. The structure is 395 ft (120.4 m) long and 34 ft wide.

Location

Open in Google Maps
CarriesMain Boulder RD
CrossesSep I 90
LocationBig Timber
RouteState Highway 298
CountySweet Grass County
StateMontana
Coordinates45.82634, -109.9486
Functional classRural Major Collector

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.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Last inspectionMarch 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourBridge not over waterway
Structural evaluation5: Somewhat better than minimum adequacy to tolerate being left in place as is
Deck geometry6: Equal to present minimum criteria
Approach roadway alignment6: Equal to present minimum criteria
Operating rating42.9 US tons
Inventory rating33.1 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialSteel Continuous
DeckConcrete, cast-in-place
Main spans2 (+2 approach)
Design loadHS 20
Structure length395 ft (120.4 m)
Longest span140 ft (42.7 m)
Deck width34 ft (10.5 m)
Roadway width31 ft (9.6 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic958 (2023)
Projected traffic1,006 by 2043
Trucks3%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesHighway
Detour if closed—
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 proposedNone recorded
Estimated improvement cost—
NavigationNot applicable (no waterway)

Frequently asked questions

When was Main Boulder RD over Sep I 90 built?

It was built in 1980, making it 45 years old in 2025.

What condition is Main Boulder RD over Sep I 90 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 March 2024.

How much traffic does Main Boulder RD over Sep I 90 carry?

About 958 vehicles per day (2023 count), of which roughly 3% are trucks.

How long is Main Boulder RD over Sep I 90?

The structure is 395 ft (120.4 m) long with a longest span of 140 ft (42.7 m) and a deck 34 ft wide.

Who owns Main Boulder RD over Sep I 90?

It is owned by the State Highway Agency.