US Bridges

56th Ave So over Duwamish River

Fair

King County, Washington · 0.1E Interurban Ave · Structure no. 081662000000000

Built
1985
40 years old
Condition
Fair
inspected April 2023
Type
Box Beam or Girders - Multiple
Concrete Continuous
Length
244 ft
2 main spans
Daily traffic
2,900
2023 count
Owner
City
City or Municipal Highway Agency

About this bridge

56th Ave So over Duwamish River is a concrete continuous box beam or girders - multiple carrying 56th Ave So over Duwamish River in King County, Washington. It was built in 1985, so it is 40 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 2,900 vehicles a day, roughly 30% of them trucks, projected to reach 3,200 by 2043. The structure is 244 ft (74.4 m) long and 35 ft wide. If it were closed, the detour would be 98.8 mi (159 km).

Location

Open in Google Maps
Carries56th Ave So
CrossesDuwamish River
Location0.1E Interurban Ave
RouteCity Street 1427
CountyKing County
StateWashington
Coordinates47.4825, -122.26744
Functional classUrban Local

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.
Superstructure7 GoodSome minor problems.
Substructure6 SatisfactoryStructural elements show some minor deterioration.
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionApril 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; action required to protect exposed foundations
Structural evaluation6: Equal to present minimum criteria
Deck geometry4: Meets minimum tolerable limits to be left in place as is
Waterway adequacy6: Equal to present minimum criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating52.9 US tons
Inventory rating31.7 US tons

Structure

Structure typeBox Beam or Girders - Multiple
MaterialConcrete Continuous
DeckConcrete, cast-in-place
Main spans2
Design loadHS 15
Structure length244 ft (74.4 m)
Longest span149 ft (45.4 m)
Deck width35 ft (10.6 m)
Roadway width28 ft (8.5 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic2,900 (2023)
Projected traffic3,200 by 2043
Trucks30%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway and pedestrian
CrossesWaterway
Detour if closed98.8 mi (159 km)
TollOn a free road
National Highway SystemNo

Ownership and planned work

OwnerCity or Municipal Highway Agency
Maintained byCity or Municipal Highway Agency
Historic statusHistorical significance not determinable at this time
Work proposedOther structural work, including hydraulic replacements
Estimated improvement cost$3.5 million (2022 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was 56th Ave So over Duwamish River built?

It was built in 1985, making it 40 years old in 2025.

What condition is 56th Ave So over Duwamish 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 April 2023.

How much traffic does 56th Ave So over Duwamish River carry?

About 2,900 vehicles per day (2023 count), of which roughly 30% are trucks.

How long is 56th Ave So over Duwamish River?

The structure is 244 ft (74.4 m) long with a longest span of 149 ft (45.4 m) and a deck 35 ft wide.

Who owns 56th Ave So over Duwamish River?

It is owned by the City or Municipal Highway Agency.