US Bridges

SR 520 over Peat Bog

Fair

King County, Washington · 0.2 E JCT I-405 · Structure no. 0014039G0000000

Built
1993
32 years old
Condition
Fair
inspected October 2024
Type
Stringer/Multi-beam or Girder
Prestressed Concrete Continuous
Length
194 ft
3 main spans
Daily traffic
32,900
2023 count
Owner
State
State Highway Agency

About this bridge

SR 520 over Peat Bog is a prestressed concrete continuous stringer/multi-beam or girder carrying SR 520 over Peat Bog in King County, Washington. It was built in 1993, so it is 32 years old, younger 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.

It carries about 32,900 vehicles a day, roughly 5% of them trucks, projected to reach 74,202 by 2039. The structure is 194 ft (59.1 m) long and 22 ft wide. If it were closed, the detour would be 1.2 mi (2 km).

Location

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CarriesSR 520
CrossesPeat Bog
Location0.2 E JCT I-405
RouteInterstate 405
CountyKing County
StateWashington
Coordinates47.63183, -122.18386
Functional classUrban 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
Deck6 SatisfactoryStructural elements show some minor deterioration.
Superstructure7 GoodSome minor problems.
Substructure7 GoodSome minor problems.
Last inspectionOctober 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourBridge not over waterway
Structural evaluation7: Better than present minimum criteria
Deck geometry9: Superior to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating84 US tons
Inventory rating50 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialPrestressed Concrete Continuous
DeckConcrete, cast-in-place
Main spans3
Design loadHS 20 + Mod
Structure length194 ft (59.1 m)
Longest span66 ft (20.1 m)
Deck width22 ft (6.8 m)
Roadway width64 ft (19.4 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic32,900 (2023)
Projected traffic74,202 by 2039
Trucks5%
Lanes on structure3
Traffic directionOne-way traffic
CarriesHighway
CrossesOther
Detour if closed1.2 mi (2 km)
TollOn a free road
National Highway SystemYes

Ownership and planned work

OwnerState Highway Agency
Maintained byState Highway Agency
Historic statusHistorical significance not determinable at this time
Work proposedRehabilitation because of general structure deterioration or inadequate strength
Estimated improvement cost$6.1 million (2022 estimate)
NavigationNot applicable (no waterway)

Frequently asked questions

When was SR 520 over Peat Bog built?

It was built in 1993, making it 32 years old in 2025.

What condition is SR 520 over Peat Bog 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 SR 520 over Peat Bog carry?

About 32,900 vehicles per day (2023 count), of which roughly 5% are trucks.

How long is SR 520 over Peat Bog?

The structure is 194 ft (59.1 m) long with a longest span of 66 ft (20.1 m) and a deck 22 ft wide.

Who owns SR 520 over Peat Bog?

It is owned by the State Highway Agency.