US Bridges

I-5 over Stillaguamish River

FairPosted for load

Snohomish County, Washington · 0.7 N JCT SR 530 · Structure no. 0001652A0000000

Built
1933
92 years old
Condition
Fair
inspected March 2024
Type
Truss - Thru
Steel
Length
859 ft
3 main spans
Daily traffic
45,626
2023 count
Owner
State
State Highway Agency

About this bridge

I-5 over Stillaguamish River is a steel truss - thru carrying I-5 over Stillaguamish River in Snohomish County, Washington. It was built in 1933, so it is 92 years old, older than the Washington average of 49.8 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. Its operating status is: posted for load restriction.

It carries about 45,626 vehicles a day, roughly 8% of them trucks, projected to reach 65,795 by 2039. The structure is 859 ft (261.8 m) long and 49 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

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CarriesI-5
CrossesStillaguamish River
Location0.7 N JCT SR 530
RouteInterstate 5
CountySnohomish County
StateWashington
Coordinates48.1974, -122.21048
Functional classRural 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
Deck7 GoodSome minor problems.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionMarch 2024
Inspection intervalevery 24 months
Operating statusPosted for load restriction
ScourScour critical; foundations unstable for calculated scour conditions
Structural evaluation5: Somewhat better than minimum adequacy to tolerate being left in place as is
Deck geometry4: Meets minimum tolerable limits to be left in place as is
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating41 US tons
Inventory rating25 US tons

Structure

Structure typeTruss - Thru
MaterialSteel
DeckConcrete, cast-in-place
Main spans3 (+5 approach)
Design loadH 15
Structure length859 ft (261.8 m)
Longest span200 ft (61 m)
Deck width49 ft (15 m)
Roadway width48 ft (14.6 m)
Vertical clearance over deck17 ft (5 m)

Traffic and service

Average daily traffic45,626 (2023)
Projected traffic65,795 by 2039
Trucks8%
Lanes on structure3
Traffic directionOne-way traffic
CarriesHighway
CrossesHighway and waterway
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 proposedRehabilitation because of general structure deterioration or inadequate strength
Estimated improvement cost$24.5 million (2022 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was I-5 over Stillaguamish River built?

It was built in 1933, making it 92 years old in 2025.

What condition is I-5 over Stillaguamish 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 March 2024.

How much traffic does I-5 over Stillaguamish River carry?

About 45,626 vehicles per day (2023 count), of which roughly 8% are trucks.

How long is I-5 over Stillaguamish River?

The structure is 859 ft (261.8 m) long with a longest span of 200 ft (61 m) and a deck 49 ft wide.

Who owns I-5 over Stillaguamish River?

It is owned by the State Highway Agency.