The Suspension Bridge

How the suspension bridge works

As the name implies the roadway is suspended or hung from two long cables that are placed over two tall towers. The roadway is made from a hollow beam to make it light in weight.
The huge cables are made up of hundreds of smaller steel cables, each of which is only 2-3mm in diameter. Even a 3mm cable is capable of supporting very heavy weights; half of a ton, so having hundreds of them bound together produces a cable that has incredible strength. The cables are secured to the bank at each end of the bridge.
The huge cables are made to span distances from 600m to over 2000 meters.
This makes the Suspension bridge ideal for spanning very wide rivers, gorges or valleys.
The engineer designs the bridge is stiff enough to safely allow traffic to travel from one end to the other, even in very extreme weather.
The engineer needs to ensure that all of the forces of tension, compression, twisting and bending are in equilibrium, or balance.
With any bridge project the choice of materials and form usually comes down to cost, and suspension bridges tend to be the most expensive to build, because of the technology and time needed to make them.
The high towers enable the main cables to be draped over long distances. Most of the weight or load of the bridge is transferred by the cables to the anchorage systems. These are imbedded in either solid rock or huge concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load to earth and to prevent the cables from breaking free.
The Akashi Kaikyō Bridge is the world's longest suspension bridge. The Humber Bridge has the worlds longest central span.

The Humber Bridge

Here are some facts about the Humber Bridge.

  • The Humber Bridge is a single-span suspension bridge.
  • It spans the Humber(the estuary formed by the River Ouse and the River Trent).
  • One side of the bridge is in Barton-upon-Humber, the other is in Hessle.
  • It has a total length of just over 2200 metres, and a width of about 28 metres.
  • The bridge was opened in 1981 and it cost nearly £100 million to build.
  • More than 100,000 care cross the bridge every week.
  • Until 1997, the Humber Bridge was the longest single-span suspension bridge in the world.
  • Construction of the bridge began in 1972.
  • The bridge’s towers are more than 150 metres tall.
  • The bridge is designed to bend more than 3 metres in heavy winds.
  • More than 400,000 tonnes of concrete was used in the bridge’s construction, and the bridge’s deck weighs more than 16 tonnes.
  • Each of the main cables is more than 60cm thick.
  • The Humber Bridge has a larger span than the Golden Gate Bridge in the United States.
  • The bridge was designed by Freeman Fox and Partners.

 

"Galloping Girtie" : A very famous bridge fail.

You have learned that an engineer must be able to have control over the forces that act upon a bridge, otherwise the bridge will fail.
The Story of the Tacoma Narrows Bridge is the most important development in Bridge Engineering.
The failing of this bridge helped engineers, scientists, mathematicians and technologists to ensure bridges can stand up to the massive forces that are created by the wind.
A terrified little dog, called Tubby, was the only victim of this catastrophic bridge failure.

RESEARCH CHALLENGE:

What can you find out about the Tacoma Bridge that failed, and the one that replaced it?

DRAWING CHALLENGE:- PRODUCE A DETAILED DRAWING OF A SUSPENSION BRIDGE

Ensure you add lots of useful information about Suspension Bridges for example the roadway, anchorage points, the Tower, cross section of the cable, compression forces and tension forces.
Stretch and Challenge yourself.
Why not produce a drawing of the Humber Bridge, and add as many interesting facts as you can find out.

PRACTICAL CHALLENGE:- FEEL THE FORCES IN A SUSPENSION BRIDGE !!

Look at the images below.

Using Bendable straws, Normal straws, String and tape you should be able to hold the string and feel the load applied to the bridge

You become the most important part of the suspension bridge:- The Anchorage Point.

Write about and record what you find out.

 

PRACTICAL CHALLENGE:- MAKE A MODEL OF A SUSPENSION BRIDGE

In this challenge you will be given access to card, wooden blocks, string and dowel.

You will also be given access to glue guns, pillar drill, cordless drill, scissors and modelling tools and equipment.

Make and test a model of a suspension bridge that spans a 300mm gap.

You may work individually, with a partner or in a team.

Handy Hint: the roadway must be suspended from the cables, and the cables need to be securely anchored at either ends.

Research Exercise: Famous Suspension Bridges around the world

Carry out research into famous Suspension 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 : Akashi Kaikyō Bridge, Humber Bridge, Yavuz Sultan Selim Bridge, Golden Gate Bridge, Forth Road Bridge, Severn Bridge, Tacoma Narrows Bridge (1950), Stord Bridge.