US Bridges

SR 520 over Portage Bay

Fair

King County, Washington · 0.3 E JCT I-5 · Structure no. 0006819A0000000

Built
1963
62 years old
Condition
Fair
inspected March 2024
Type
Stringer/Multi-beam or Girder
Prestressed Concrete
Length
2,841 ft
29 main spans
Daily traffic
18,129
2023 count
Owner
State
State Highway Agency

About this bridge

SR 520 over Portage Bay is a prestressed concrete stringer/multi-beam or girder carrying SR 520 over Portage Bay in King County, Washington. It was built in 1963, so it is 62 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 18,129 vehicles a day, roughly 3% of them trucks, projected to reach 28,568 by 2043. The structure is 2,841 ft (865.9 m) long and 71 ft wide. If it were closed, the detour would be 3.7 mi (6 km).

Location

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CarriesSR 520
CrossesPortage Bay
Location0.3 E JCT I-5
RouteState Highway 520
CountyKing County
StateWashington
Coordinates47.64275, -122.31767
Functional classUrban Principal Arterial - Other Freeway or Expressway

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.
Superstructure7 GoodSome minor problems.
Substructure5 FairAll primary structural elements are sound but may have minor section loss, cracking, spalling or scour.
Channel and channel protection8 Very GoodNo problems noted.
Last inspectionMarch 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation3: Basically intolerable, requiring high priority of corrective action
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 rating18.2 US tons
Inventory rating13.9 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialPrestressed Concrete
DeckConcrete, cast-in-place
Main spans29
Design loadHS 20
Structure length2,841 ft (865.9 m)
Longest span100 ft (30.5 m)
Deck width71 ft (21.6 m)
Roadway width56 ft (17.1 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic18,129 (2023)
Projected traffic28,568 by 2043
Trucks3%
Lanes on structure4
Traffic directionOne-way traffic
CarriesHighway
CrossesHighway and waterway
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$136.3 million (2022 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was SR 520 over Portage Bay built?

It was built in 1963, making it 62 years old in 2025.

What condition is SR 520 over Portage Bay 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 SR 520 over Portage Bay carry?

About 18,129 vehicles per day (2023 count), of which roughly 3% are trucks.

How long is SR 520 over Portage Bay?

The structure is 2,841 ft (865.9 m) long with a longest span of 100 ft (30.5 m) and a deck 71 ft wide.

Who owns SR 520 over Portage Bay?

It is owned by the State Highway Agency.