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What is the Difference between Bridge and Box Culvert?

What are the differences between Bridge and Culvert Structures

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It may appear that bridge and culvert structures are similar as they both serve the vital need of transportation. However, they are very different in aspects like structural design, construction, components, load-carrying capacity, costs, etc.

Culvert is defined as a tunnel-like structure that facilitates the movement of water from one side to another and supports vehicular, human, and animal loads. On the other side, bridges are linear structures built over large bodies of water or physical obstruction and allow the passage of heavy and speeding vehicles. Bridge structure requires detailed and accurate design in comparison to culvert, and it also needs considerably higher budget, time, and human resources for construction.

Both bridges and culverts follow "American Association of State Highway and Transportation Officials-Load and Resistance Factor Design specifications" (AASHTO LRFD). Still, culverts are not considered bridges because their spans do not exceed 6 m, as per the Federal Highway Administration classifications (FHWA).  

Figure-1: Components of a Bridge
Figure-3: Components of Culvert Structure

What is the Difference Between Bridge and Box Culvert?

Table-1 presents the differences between bridge and culvert in terms of definitions, components, design, load-carrying capacity, construction, etc.

Table-1: Difference between Bridge and Culvert Structures

Items BridgeBox Culvert
DefinitionA bridge is a large structure constructed over sizable water or physical obstruction that allows the passage of vehicles and people.A box culvert is a tunnel-like structure built under roadways or railways to provide cross drainage from one side to the other.
PartsMajor parts of bridges are superstructure, decks, and substructures. (see Figure-1 and Figure-2).The main components of a box culvert are single or multiple concrete boxes, pipes, a top deck slab, and supporting parts. (see Figure-3).
Construction MaterialsReinforced concrete and steel.Concrete, steel, plastic, aluminum, and high-density polyethylene.
SpansA minor bridge spans about 60 m, whereas a large bridge spans up to 120 m.Commonly, the length of a culvert does not exceed 6 m.  
HeightBridges are built more than 6 m above obstruction or waterbody.Culverts are constructed at a height of less than 6 m above the physical obstruction.
LoadsBridges can withstand heavy and speeding vehicles.Culverts are not designed to withstand speeding vehicles.
Construction methodsThe Cast-in-situ method and sometimes precast technique can be considered for a certain bridge component. However, it requires a great budget and a massive labor force.Both cast-in-situ and precast concrete can be used to construct culverts without difficulties. The latter construction method requires less time but is expensive. However, it does not require a high budget and more manpower.  
Design and CostThe bridge requires detailed and accurate analysis and design; hence it requires a lot of time to design and construct, and consequently, a high budget is needed.  Culverts do not require such a high budget as bridges.
Foundation typeBridge requires a stable deep foundation to transfer the load safely to the soil underneath.Culverts do not need a deep foundation to support their load. Spread footing is sufficient.
Load transfer mechanismAbutments and piers support and transfer the bridge's load to the foundation.Culverts are buried in soil that support a great part of the culvert loads.
Structural shapeBridges are commonly straight and linear.Culverts are enclosed structures that can be rectangular, semi-circular, elliptical, or pear-shaped.  
Construction conditions  A bridge structure is constructed over a sizable water body with variable flow.Culvert structure is built to transfer water through tunnels or channels under roadways.
Modes of trafficBridges allow traffic movement only over its decks.Culvert adapts to roadways both under and above its deck.
TransportationBridges offer an easier route of transportation that reduces distances and saves time.Culverts permit natural water flow under a roadway and impede flood, erosion, and waterlogging.
OthersThe bridge structure does not have floors.It has a roof, sides, and roofs.
Figure-2: Components of Superstructure of a Bridge
Figure-4: Culvert Structure


What is a culvert structure?

Culvert is defined as a tunnel-like structure that facilitates the movement of water from one side to another and supports vehicular, human, and animal loads. 

Is culvert structure considered a bridge?

No, culvert structure is not the same as a bridge because its span does not surpass 6m, per FHWA classifications.

What are the different types of culverts?

1. Pipe culvert (Single or Multiple)
2. Box culvert (Single or Multiple)
3. Arch culvert
4. Bridge culvert
For more details on culvert types, please click here.

What is the difference between minor and major bridges?

A minor bridge spans about 60 m, whereas a large bridge spans up to 120 m.

What are the components of bridge structure?

1. Abutments
2. Piers
3. Wing walls
4. Beam and girders
5. Bearing
6. Parapet wall and handrail
7. Decks
8. Arch and cables (may not be present in all bridge structures)

What are the components of culvert structure?

1. Single or multiple concrete boxes or pipes
2. Topdeck slab
3. Supporting sides
4. Floors

Read More

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