US Bridges

US 521 over Thorntree Swamp

Fair

Williamsburg County, South Carolina · 5.3 MI S of Kingstree · Structure no. 000000000000587

Built
1931
94 years old, rebuilt 1965
Condition
Fair
inspected June 2023
Type
Culvert
Concrete
Length
34 ft
3 main spans
Daily traffic
2,200
2023 count
Owner
State
State Highway Agency

About this bridge

US 521 over Thorntree Swamp is a concrete culvert carrying US 521 over Thorntree Swamp in Williamsburg County, South Carolina. It was built in 1931 and reconstructed in 1965, so it is 94 years old, older than the South Carolina average of 48 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 2,200 vehicles a day, roughly 22% of them trucks, projected to reach 3,828 by 2043. The structure is 34 ft (10.4 m) long and 55 ft wide. If it were closed, the detour would be 3.1 mi (5 km).

Location

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CarriesUS 521
CrossesThorntree Swamp
Location5.3 MI S of Kingstree
RouteU.S. Route 521
CountyWilliamsburg County
StateSouth Carolina
Coordinates33.5877, -79.83714
Functional classRural Principal Arterial - Other

Condition and inspection

Fair

The lowest-rated primary component is rated 5 or 6. Minor deterioration is present but the structure is sound.

ComponentRatingMeaning
Channel and channel protection6 SatisfactoryStructural elements show some minor deterioration.
Culvert6 SatisfactoryStructural elements show some minor deterioration.
Last inspectionJune 2023
Inspection intervalevery 24 months
Operating statusOpen, no restriction
ScourFoundations stable; scour above top of footing
Structural evaluation6: Equal to present minimum criteria
Deck geometry3: Basically intolerable, requiring high priority of corrective action
Waterway adequacy8: Equal to present desirable criteria
Approach roadway alignment8: Equal to present desirable criteria
Operating rating47.5 US tons
Inventory rating28.2 US tons

Structure

Structure typeCulvert
MaterialConcrete
DeckNot applicable
Main spans3
Design loadH 20
Structure length34 ft (10.4 m)
Longest span10 ft (3 m)
Deck width55 ft (16.8 m)
Roadway width28 ft (8.4 m)
Vertical clearance over deckUnrestricted

Traffic and service

Average daily traffic2,200 (2023)
Projected traffic3,828 by 2043
Trucks22%
Lanes on structure2
Traffic directionTwo-way traffic
CarriesHighway
CrossesWaterway
Detour if closed3.1 mi (5 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 proposedReplacement because of substandard load-carrying capacity or roadway geometry
Estimated improvement cost$255,000 (2024 estimate)
NavigationNo navigation control on waterway

Frequently asked questions

When was US 521 over Thorntree Swamp built?

It was built in 1931, making it 94 years old in 2025. It was reconstructed in 1965.

What condition is US 521 over Thorntree Swamp 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 2023.

How much traffic does US 521 over Thorntree Swamp carry?

About 2,200 vehicles per day (2023 count), of which roughly 22% are trucks.

How long is US 521 over Thorntree Swamp?

The structure is 34 ft (10.4 m) long with a longest span of 10 ft (3 m) and a deck 55 ft wide.

Who owns US 521 over Thorntree Swamp?

It is owned by the State Highway Agency.