Difference between culvert and bridge

A very common confusion between two vital structures is present among students and engineers as well that is What is the difference between culvert and bridge.
Culvert and a bridge both span a clear space and looks similar to each other, but there is a clear difference between their purpose as well as their design.
In this post we will be trying to understand that difference and after reading this post you will be able to easily distinguish between them just by looking;

What is the use of a bridge? 

First of all don’t confuse the word “Bridge” with that of the card game played mostly and is very popular among teens. 🙂

Bridge is a structure that spans a physical natural hurdle or obstruction like river or a valley. A bridge carries a passage that can be a roadway or railway across an obstruction that can be a river or a railroad or even another road as well”

A Bridge spanning a river crossing a roadway
Famous Sydney Harbour Bridge spanning a river crossing a roadway

Thus bridge allows easy movement of vehicles or people across a river or a valley without difficulty, and in this way it reduces the distance and time required to cross such a distance.

Culvert in Brief

culvert is a structure that allows the easy passage of water through a physical obstruction that can be a hill or a roadway or a passage or a walkway”.

Culvert providing access for the water to cross the roadway
Culvert providing access for the water to cross the roadway

Culvert is typically embedded by a soil or surface. Culvert can be made up of a reinforced concrete or even steel as well.

Difference between culvert and bridge

Now if you go through their introduction you will be at ease in differentiating the culvert and bridge, firstly and importantly culvert in civil engineering is made for passage of water where as bridge is designed to allow the easy passage of traffic or community.
From the image we can make in our mind of bridge and culvert is; bridge is a bigger structure as compared with a culvert, Bridges mostly don’t have a floor; by floor we mean in bridges the two piers or abutments are not joined at bottom by a surface where as in culverts they have a proper floor and are circular, rectangular or elliptical.
Now if from just a look you are not able to decide whether it is a culvert or a bridge the deciding factor is its size, If the size of the structure is more than 20 feet it is a bridge and if less than it would be a culvert. 

Difference between culvert and bridge
Difference between culvert and bridge


Difference Between Box Culvert and Slab Culvert

I have been recently asked by one of my friend about difference between a slab culvert and box culvert; So I thought it would be worth sharing with you as well.
Both box culvert and slab culvert and two types of culverts. Designers are often confused while deciding which to choose. So this comparison will help you in decision while keeping in view the project and area requirements.
A box culvert in closed on all sides by side walls, top deck and floor slab while in slab culverts only the top deck slab is present that is either rested on the side walls or directly on the ground without having the floor slab. Due to this feature, the velocity of flowing water may vary in box culvert which is not the case in slab culverts.

Difference between slab culvert and box culvert
Difference between slab culvert and box culvert


Deciding Bridge or Culvert for a Project?

Now you have learned what is the difference between culvert and a bridge; Now I’ll discuss about the factors helpful for you while deciding between both of them.
For most highway crossings, the choice is between a culvert and a bridge. Storm drain analysis is sometimes required before a decision is made between a culvert bridges.

In the coming paragraphs I will discuss the deciding factors for culvert vs a bridge.

Comparing culverts to bridges the designer must determine which type of structure is best for a particular location, and then decide how to analyze the crossing. For example, in many respects a large box culvert begins to resemble a small single-span bridge with vertical wall abutments, and so:

Which one is preferred hydraulically, aesthetically and economically?

How should the structure be analyzed (as a bridge with free surface flow using gradually varied, open channel flow concepts or as a culvert with headwater based on traditional culvert analysis)?
Culverts are used:

  • Where bridges are not hydraulically required
  • Where debris and ice potential are tolerable
  • Where more economical than a bridge (including guardrail and safety concerns)

Bridge are used:

  • Where culverts are impractical
  • Where more economical than a culvert
  • To satisfy land-use and access requirements
  • To mitigate environmental concerns not satisfied by a culvert
  • To avoid floodway encroachments
  • To accommodate ice and large debris.

You may want to know about the different types of bridges that are commonly used.

Traditionally, economic considerations were of primary importance in deciding between the use of a bridge or a culvert at stream crossings where either will satisfy hydraulic and structural requirements.
The initial cost of a culvert is usually less than a bridge since the use of increased headwater at a culvert installation normally permits the use of a smaller opening, compared to a bridge which is normally designed with freeboard at the design discharge.
However, this advantages must be balanced against possible floor damages associated with increased headwater, especially at higher discharges.
Safety and aesthetic considerations are also involved in the choice of a bridge or culvert. Safety considerations for culverts include the use of guardrails or safety grates. It is important to recognize that culverts exceeding a 20 feet span width are considered bridges. Bridge decks often constrict shoulder and median widths and are subject to icing which can present traffic safety problems. A bridge may be considered more aesthetically pleasing in traversing a scenic valley or a canyon.

Saad Iqbal
Hi there, I am Saad Iqbal from Pakistan. I am an enthusiastic blogger, passionate content creator, construction geek, and a creative graphic designer. Connect with me at my social channels.