US Bridges

Forest Road over Swan River

Fair

Lake County, Montana · Porcupine Creek Road · Structure no. 011001000000109

Built
1956
69 years old
Condition
Fair
inspected August 2023
Type
Stringer/Multi-beam or Girder
Steel
Length
231 ft
3 main spans
Daily traffic
20
1992 count
Owner
Federal
U.S. Forest Service

About this bridge

Forest Road over Swan River is a steel stringer/multi-beam or girder carrying Forest Road over Swan River in Lake County, Montana. It was built in 1956, so it is 69 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 20 vehicles a day, projected to reach 30 by 2045. The structure is 231 ft (70.4 m) long and 15 ft wide. If it were closed, the detour would be 98.2 mi (158 km).

Location

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CarriesForest Road
CrossesSwan River
LocationPorcupine Creek Road
RouteFederal Lands Road 10229
CountyLake County
StateMontana
Coordinates47.88666, -113.86047
Functional classRural Local

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.
Channel and channel protection7 GoodSome minor problems.
Last inspectionAugust 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 geometry6: Equal to present minimum criteria
Waterway adequacy9: Superior to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating58.5 US tons
Inventory rating45 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialSteel
DeckWood or timber
Main spans3
Design loadHS 25 or greater
Structure length231 ft (70.4 m)
Longest span91 ft (27.7 m)
Deck width15 ft (4.7 m)
Roadway width14 ft (4.3 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic20 (1992)
Projected traffic30 by 2045
Trucks—
Lanes on structure1
Traffic directionOne-lane bridge for two-way traffic
CarriesHighway
CrossesWaterway
Detour if closed98.2 mi (158 km)
TollOn a free road
National Highway SystemNo

Ownership and planned work

OwnerU.S. Forest Service
Maintained byU.S. Forest Service
Historic statusHistorical significance not determinable at this time
Work proposedNone recorded
Estimated improvement cost—
NavigationNo navigation control on waterway

Frequently asked questions

When was Forest Road over Swan River built?

It was built in 1956, making it 69 years old in 2025.

What condition is Forest Road over Swan 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 August 2023.

How much traffic does Forest Road over Swan River carry?

About 20 vehicles per day (1992 count).

How long is Forest Road over Swan River?

The structure is 231 ft (70.4 m) long with a longest span of 91 ft (27.7 m) and a deck 15 ft wide.

Who owns Forest Road over Swan River?

It is owned by the U.S. Forest Service.