US Bridges

S 320th ST over I-5

Fair

King County, Washington · 1.8 N JCT SR 18 · Structure no. 0006262A0000000

Built
1960
65 years old
Condition
Fair
inspected October 2024
Type
Stringer/Multi-beam or Girder
Prestressed Concrete
Length
332 ft
4 main spans
Daily traffic
49,879
2021 count
Owner
State
State Highway Agency

About this bridge

S 320th ST over I-5 is a prestressed concrete stringer/multi-beam or girder carrying S 320th ST over I-5 in King County, Washington. It was built in 1960, so it is 65 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 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 49,879 vehicles a day, roughly 6% of them trucks, projected to reach 70,429 by 2041. The structure is 332 ft (101.2 m) long and 76 ft wide. If it were closed, the detour would be 3.7 mi (6 km).

Location

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CarriesS 320th ST
CrossesI-5
Location1.8 N JCT SR 18
RouteCity Street 5
CountyKing County
StateWashington
Coordinates47.31514, -122.29819
Functional classUrban Other Principal 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
Deck6 SatisfactoryStructural elements show some minor deterioration.
Superstructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Substructure7 GoodSome minor problems.
Last inspectionOctober 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 alignment8: Equal to present desirable criteria
Operating rating63.9 US tons
Inventory rating37 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans4
Design loadHS 20
Structure length332 ft (101.2 m)
Longest span100 ft (30.5 m)
Deck width76 ft (23.2 m)
Roadway width73 ft (22.3 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic49,879 (2021)
Projected traffic70,429 by 2041
Trucks6%
Lanes on structure5
Traffic directionTwo-way traffic
CarriesHighway
CrossesHighway
Detour if closed3.7 mi (6 km)
TollOn a free road
National Highway SystemYes

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$12.9 million (2022 estimate)
NavigationNot applicable (no waterway)

Frequently asked questions

When was S 320th ST over I-5 built?

It was built in 1960, making it 65 years old in 2025.

What condition is S 320th ST over I-5 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 October 2024.

How much traffic does S 320th ST over I-5 carry?

About 49,879 vehicles per day (2021 count), of which roughly 6% are trucks.

How long is S 320th ST over I-5?

The structure is 332 ft (101.2 m) long with a longest span of 100 ft (30.5 m) and a deck 76 ft wide.

Who owns S 320th ST over I-5?

It is owned by the State Highway Agency.