What is the Difference between Bridge and Box Culvert?

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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).  

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	Bridge	Box Culvert
Definition	A 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.
Parts	Major 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 Materials	Reinforced concrete and steel.	Concrete, steel, plastic, aluminum, and high-density polyethylene.
Spans	A 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.
Height	Bridges 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.
Loads	Bridges can withstand heavy and speeding vehicles.	Culverts are not designed to withstand speeding vehicles.
Construction methods	The 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 Cost	The 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 type	Bridge 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 mechanism	Abutments 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 shape	Bridges 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 traffic	Bridges allow traffic movement only over its decks.	Culvert adapts to roadways both under and above its deck.
Transportation	Bridges 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.
Others	The bridge structure does not have floors.	It has a roof, sides, and roofs.

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