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Bridges & Forces

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Catherine Dawes

on 19 March 2014

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Transcript of Bridges & Forces

What makes a bridge strong enough to withstand force?

Bridges and Forces
When building a bridge, engineers have to build it strong enough to resist force.
It needs to be...
Strong enough to resist the stress of weight
Strong enough to withstand weather/storms

The 2 main stresses or forces that affect a bridge are ...

What are these forces?
Tension Compression
Tension is the force when something is pulled tight, like a rope in a game of tug o'war.
Compression is when something is being pushed or squashed like a spring.
Both of these forces can be applied vertically or horizontally.
What force do the red arrows depict?
What force do the blue arrows depict?
Here is a diagram of a suspension bridge
You now have 15 minutes to complete the hand out sheet on compression & tension stresses on bridges - Remember to use the hint diagram at the top of the sheet if you need a reminder.
If a bridge is not built correctly it can have devastating consequences.

The Tacoma Narrows Bridge was opened in Pennsylvania, U.S.A in 1940 but, due to engineering issues, it lasted just 4 months.

Take a look...
Strengthening Bridge
Bridges can be strengthened with something called 'Trusses'.

In the first diagram we see a beam bridge. All of the stress on this bridge is divided between just 6 arrows.

In the second diagram, the compression &tension forces are being spread across 9 arrows. Just by adding a triangular shape to the bench bridge, the stress has been spread out.

In the last diagram, the stress is shared even further by adding more trusses. There are now 12 arrows depicting how the stresses are spread.

The arrows in this diagram represent the effect of Compression & Tension force.
Here are some examples of trusses. Notice how they are made up of geometric shapes.
In pairs, you have 15 minutes to make & strengthen 2 geometric shapes with trusses. You will have only the 3 following items to make your shapes with...
Drinking straws
There is a page of truss examples in the back of your hand-out booklet to help you.
In engineering, a truss is a system of triangular structures that help support load. The method of adding triangles for strength is called triangulation.
What did we discover?
Remember to come prepared for your trip next week.
Wearing sensible footwear and a waterproof jacket.
Have a packed lunch (preferably in a plastic bag that you can throw away after you've eaten it).
Bridges don't have to just be ugly and functional.

Look at some of the new exciting designs of bridge architecture that aren't just functional, their aesthetics & engineering bring visitors from around the world to see them...
Choose 2 from the list below and write a paragraph about each, explaining why you chose it.
Thomas Heatherwick's Rolling Bridge
Gateshead Millennium Bridge
Aiola Island Bridge
Langkawi Sky Bridge
Banpo Fountain Bridge
Octavio Frais de Oliveira Bridge

1. What is compression?
2. What type of shape is commonly seen in trusses?
3. What is tension?
4. What is the purpose of trusses?
5. Name the 4 main types of bridges we have talked about over the past 2 lessons?
6. What 3 things make a bridge withstand force?
7. What is the name given to the method of adding triangles for strength?
I need 1 volunteer to sit in the hot seat and everyone else should write 1-7 on a piece of paper.
The volunteer will be asked questions.If the volunteer answers correctly, you put a , if you think it's wrong, put a .

Which of the 2 bridges I have made
is stronger?


You have 5 minutes to complete the Triangulation task on the back page of your hand-out booklet.
Learning Objectives...
• To be able to identify what forces act on a bridge structure and how these forces affect the structure by completing the work booklet.
• To demonstrate an understanding of the purpose of trusses in bridge strengthening by completing the triangulation task and drinking straw task.
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