US Bridges

SR 20 over Thunder Arm Bay

Fair

Whatcom County, Washington · 13.8 E Skagit CO · Structure no. 0006585B0000000

Built
1961
64 years old
Condition
Fair
inspected June 2024
Type
Stringer/Multi-beam or Girder
Concrete
Length
210 ft
7 main spans
Daily traffic
1,322
2023 count
Owner
State
State Highway Agency

About this bridge

SR 20 over Thunder Arm Bay is a concrete stringer/multi-beam or girder carrying SR 20 over Thunder Arm Bay in Whatcom County, Washington. It was built in 1961, so it is 64 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.

It carries about 1,322 vehicles a day, roughly 8% of them trucks, projected to reach 1,927 by 2039. The structure is 210 ft (64 m) long and 31 ft wide. If it were closed, the detour would be 98.8 mi (159 km).

Location

Open in Google Maps
CarriesSR 20
CrossesThunder Arm Bay
Location13.8 E Skagit CO
RouteState Highway 20
CountyWhatcom County
StateWashington
Coordinates48.69094, -121.09592
Functional classRural Minor 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
Deck7 GoodSome minor problems.
Superstructure6 SatisfactoryStructural elements show some minor deterioration.
Substructure7 GoodSome minor problems.
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionJune 2024
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation6: Equal to present minimum criteria
Deck geometry4: Meets minimum tolerable limits to be left in place as is
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment6: Equal to present minimum criteria
Operating rating76.9 US tons
Inventory rating46 US tons

Structure

Structure typeStringer/Multi-beam or Girder
MaterialConcrete
DeckOther
Main spans7
Design loadHS 20
Structure length210 ft (64 m)
Longest span30 ft (9.1 m)
Deck width31 ft (9.3 m)
Roadway width26 ft (7.9 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic1,322 (2023)
Projected traffic1,927 by 2039
Trucks8%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed98.8 mi (159 km)
TollOn a free road
National Highway SystemNo

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$3.8 million (2022 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was SR 20 over Thunder Arm Bay built?

It was built in 1961, making it 64 years old in 2025.

What condition is SR 20 over Thunder Arm 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 June 2024.

How much traffic does SR 20 over Thunder Arm Bay carry?

About 1,322 vehicles per day (2023 count), of which roughly 8% are trucks.

How long is SR 20 over Thunder Arm Bay?

The structure is 210 ft (64 m) long with a longest span of 30 ft (9.1 m) and a deck 31 ft wide.

Who owns SR 20 over Thunder Arm Bay?

It is owned by the State Highway Agency.