US Bridges

NFSR 347 over Grayback Gulch

Good

Boise County, Idaho · 2.5 MI SW of Idaho City · Structure no. 040203000032691

Built
2018
7 years old
Condition
Good
inspected September 2023
Type
Stringer/Multi-beam or Girder
Steel
Length
90 ft
1 main span
Daily traffic
50
Owner
Federal
U.S. Forest Service

About this bridge

NFSR 347 over Grayback Gulch is a steel stringer/multi-beam or girder carrying NFSR 347 over Grayback Gulch in Boise County, Idaho. It was built in 2018, so it is 7 years old, younger than the Idaho average of 45.3 years.

The most recent inspection reported to the Federal Highway Administration rates it in good condition with a lowest component rating of 7 out of 9. All primary components (deck, superstructure, substructure, or culvert) are rated 7 or higher. No significant problems were noted.

It carries about 50 vehicles a day, projected to reach 75 by 2045. The structure is 90 ft (27.4 m) long and 16 ft wide. If it were closed, the detour would be 620.7 mi (999 km).

Location

Open in Google Maps
CarriesNFSR 347
CrossesGrayback Gulch
Location2.5 MI SW of Idaho City
RouteFederal Lands Road 347
CountyBoise County
StateIdaho
Coordinates48.80728, -115.86854
Functional classRural Local

Condition and inspection

Good

All primary components (deck, superstructure, substructure, or culvert) are rated 7 or higher. No significant problems were noted.

ComponentRatingMeaning
Deck8 Very GoodNo problems noted.
Superstructure8 Very GoodNo problems noted.
Substructure7 GoodSome minor problems.
Channel and channel protection8 Very GoodNo problems noted.
Last inspectionSeptember 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour within limits of footing or piles
Structural evaluation7: Better than present minimum criteria
Deck geometry6: Equal to present minimum criteria
Waterway adequacy9: Superior to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating50.4 US tons
Inventory rating38.6 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialSteel
DeckCorrugated steel
Main spans1
Design loadHL 93
Structure length90 ft (27.4 m)
Longest span88 ft (26.7 m)
Deck width16 ft (4.9 m)
Roadway width14 ft (4.3 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic50 (—)
Projected traffic75 by 2045
Trucks—
Lanes on structure1
Traffic directionOne-lane bridge for two-way traffic
CarriesHighway
CrossesWaterway
Detour if closed620.7 mi (999 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 NFSR 347 over Grayback Gulch built?

It was built in 2018, making it 7 years old in 2025.

What condition is NFSR 347 over Grayback Gulch in?

The National Bridge Inventory rates it in good condition. All primary components (deck, superstructure, substructure, or culvert) are rated 7 or higher. No significant problems were noted. The last reported inspection was in September 2023.

How much traffic does NFSR 347 over Grayback Gulch carry?

About 50 vehicles per day (latest count).

How long is NFSR 347 over Grayback Gulch?

The structure is 90 ft (27.4 m) long with a longest span of 88 ft (26.7 m) and a deck 16 ft wide.

Who owns NFSR 347 over Grayback Gulch?

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