The Cantilever Bridge

The image opposite is a very famous photograph taken around 1900.

It shows Sir Benjamin Baker illustrating the structural principles of the cantilever bridge.

Benjamin baker is the man in the middle.

The drawing you can see in the picture is of the most famous Cantilever Bridge in the world; the Forth Bridge, which crosses the Forth in Scotland.

 When you understand this image, you will understand the science behind the amazing Cantilever Bridge.


The Forth Bridge is a cantilever railway bridge across the Firth of Forth in the east of Scotland, 9 miles west of Edinburgh City Centre. It is considered as a symbol of Scotland, and is a World Heritage Site. 
  • Construction started: 1882
  • Total length: 2,529 m
  • Height: 110 m
  • Architects: Benjamin Baker, Sir John Fowler,
  • Location: United Kingdom, Edinburgh, Fife
  • Length 8,094 feet (2,467.05 m),
  • The double track is 150 feet (45.72 m) above the water level at high tide.
Watch the video opposite to find out more.

Understanding levers

To understand the cantilever bridge, you need to understand how levers work.

Levers are mechanisms, they seem simple, but to the Engineer they are an invaluable application of Maths and Science.

Engineers who build Cantilever Bridges understand how important it is to balance forces.

Forces that are balanced are said to be in 'EQUILIBRIUM'.

When the forces in a bridge are balanced, or in equilibrium, the bridge is Stable, Rigid and Steady. It will not fall down.

Bridges fall down when the Engineer cannot control the forces acting on the bridge.


What is a Lever?

A lever is a rigid bar or beam that rotates around a fixed point called a fulcrum or pivot.

By putting effort on the opposite end of the load it moves up, the farther away you put the effort the easier it is to move the load.

 Levers have been used for thousands of years

 Some examples of levers are scissors, hammers, tweezers, seesaws, and crowbars.

 Levers make work easier by increasing distance and decreasing force.

What do all of the examples shown have in common? What makes each example different?

The three classifications of Lever.

As we move closer to understanding how the Cantilever bridge works, we need to understand the three different types of Lever.

All levers have a ''Pivot point'', All levers have a ''Applied Effort'', All levers have a ''Applied Load''.

They are known as the ''Class 1 lever'', ''Class 2 Lever'' and ''Class 3 Lever''.

Practical Challenge: Experiment with Levers and make a "Levers Teaching Aid"

You will be given wooden pivots blocks, wooden beams and weights to experiment with.
Work on your own or with a partner to make each of the 3 Classifications of Lever.
Challenge yourself to make a "Interactive" card teaching aid.
The levers should be able to pivot when moved, and the card "Load" and "Effort " sliders should be easily moved along each of the levers to change position and distance from the Pivot Point. This teaching aid would make an excellent contribution to your team presentation for assessment criteria 1.7.

What Class of Lever is the Cantilevered Bridge?

The Cantilever Bridge in its simplest form can be seen in the image opposite. The image is of a stone constructed bridge in Tibet.

Two stone levers can be seen "Pivoted" or fixed firm to each bank.

The two stone levers have a stone beam placed between the two.

The beam is not heavy enough to rotate the levers around the Pivots.

All of the forces are balanced, and are in "equilibrium", so the bridge does not fail.

Using your Teaching Aid, can you identify the Class of lever being used?

What is a Cantilever? How does the Cantilever Bridge work?

A Cantilever is any beam or lever that is fixed or pivoted at one point.
Take a look at this video link to understand more.
The simplest form of a Cantilever is a beam that has been embedded into a wall.
If you apply a weight to the end it may bend if the weight is too big.
All of the usual rules apply, the top of the beam will be in Tension, and the lower part of the beam will be in compression.
All of the forces are transferred through the beam, and down the wall.
The cantilever will fail when the load is too big.
Look at the images on the slider opposite that show how the Cantilever Bridge is developed by using levers that are fixed at one end.

Practical Challenge:- Make a model of a Cantilever Bridge

For this challenge you are going to experiment and make a model of a cantilever bridge.
You will be supplied with wooden batons, Card, Wooden beams, wire, string, nails.
You will also have access to an Engineers try square, Glue Gun, double sided tape,  paper/card punch, pillar drill and cordless drill.
You can build your own model or work with a partner.
The slider opposite shows a series of steps that you could take.
Your teacher will show you a completed example of the model bridge, and will demonstrate the steps to make it.


Research Exercise: Famous Cantilever Bridges around the world

Carry out research into famous Cantilever Bridges around the world.
Some famous ones are shown in the images opposite, can you identify them?
Find out the location of the bridge.
Find out when the bridge was built.
What does the bridge carry? Pedestrians, Trains, Cars, waterway?
What material is the bridge made from?
What other interesting facts can you find out?
How will you present your information?
Handy hint : Forth Bridge, Quebec Bridge, Minato Bridge, Commodore Barry Bridge, Tokyo Gate Bridge, Astoria–Megler Bridge, Oakland Bay Bridge.