*** OVERVIEW ****

A suspension bridge made from rope and wood that children can walk across!

Possible activities:
1. Children enjoy just walking over the bridge!
2. Have one child stand on the bridge and trace the forces down to the ground. Unclip the roadway to see what holds it up. Tension.
3. Put one kid on and see how much the bridge is bent. What if we add more children?
4. Why do we want the cable to sag? Use a seperate rope with a weight in the middle. Need to pull very hard to hold up the weight with the cable horizontal, but less hard if you let the rope sag.

Other things to talk about:
1. Real life suspension bridges
2. Why build suspension bridges?
3. How to build suspension bridges - sinking piers into water, digging to bedrock, anchoring cables, etc.
3. Material science - tension and compression and cracking.

Tips for demonstrating: Don't be fazed by the crowds - keep control and take time to ask questions and demonstrate the science. If children are beginning to run over the bridge or get overexcited, limit the number of children on the bridge to one at a time.

*** BASIC PROCEDURE AND EXPLANATION ***

Get children to walk over the bridge, then ask them questions. On a busy day you could stand blocking the entrance to the bridge until a group have listened to the science, then let them walk over.

What holds up the roadway? You can unclip it to show that it hangs from the sagging cables. suspension means hanging.

How does it work? Have one child stand in the middle of the bridge. He's pushing down on the roadway because he's heavy. What holds up the roadway? It's hanging from the sagging rope. Pushing and pulling forces - the rope is being pulled. Trace tension in rope over the piers and down into the ground. Get them to touch the various cables feel the different tensions. Ropes are anchored to the ground by the stakes so they have something to pull on. What would happen if we took out the stakes? Which way do the wooden piers get pushed (down into the ground).

Why do we have the rope sagging and not taught horizontal? Use a seperate rope with a weight in the middle. One child holds each end. Ask them to raise the weight into the air by pulling on the ends of the cable. Need to pull very hard to get the rope horizontal. Why? Because they're pulling sideways but they're trying to pull the weight upwards. So most of their pulling is wasted. Much easier if the rope is sagging.

Because the rope is sagging, we need these big wooden piers to hold it up. We don't want the whole roadway to sag though - cars would have to drive up to the top of the pier and down and up and down again - very silly! We want a flat roadway. So we hang the flat roadway from the sagging rope.

What about the roadway bending? Put one kid on and see how much the roadway is bent. Will the roadway be more or less bent if you add more kids all along the bridge? So the roadway doesn't bend if you spread out the load. Real suspension bridges don't bend as cars go across them! This is because the deck is usually either a beam or a truss, so can spread out the loads like when people stand all along our bendy roadway. If a train goes over the bridge then the engine is a very heavy load in one place so it will bend, so we don't tend to use suspension bridges for railways.

*** OTHER THINGS TO TALK ABOUT ***

Real life suspension bridges. Severn bridge. Clifton bridge in Bristol. Menai and Conwy suspension bridges in Wales. Humber bridge. Union bridge (over River Tweed). Brooklyn bridge in new york. Golden Gate bridge at the entrance to San Francisco Bay.

Why build a suspension bridge rather than another type of bridge? Imagine building an arch bridge. Would need lots of bricks, which are heavy. Rope is very light and cheap, so easier to make longer bridges. Arch bridge would need lots of arches and more piers on the river bed, suspension bridge only needs two. Can be built high to allow ships to sail underneath.

Building them in real life. Steel cables not rope. Need to sig down through earth to bedrock to start building piers. May need to dig underwater using a pressurised diving chamber called a caisson - this was first done when building the Brooklyn bridge in New York. Anchor cables by dropping a heavy lump of rock on them. May have two bits of rock with interlocking teeth and cable between.

Talk about tension and compression. Rope is good under tension, brick cracks under tension. Making steel rope - wind strands of iron into a small length, wind many of them together. Testing for strength!

*** SCIENCE BACKGROUND FOR DEMONSTRATORS ***

'Why' suspension bridges are a good idea is to do with the stability of tension - no need to waste strength and weight resisting buckeling, so can make lighter and hence longer spans. Resistance to Buckeling is mostly a geometeric effect. We really need a good demo to communicate this. This is in contrast to the strength of materials in tension and compression - most do better in compression as cracks are such a big problem in tension.

'How' they work is a matter of vector addition of forces. When explaining on tour, i used a knot with three strands of rope comming from it. That was enough for a very basic feel for what is happening, but I would really like something with springs in it to demo the actual vector addition, explain resolving into components and so on (i have an idea using some peg-board and newton-meters to do this, but making the geometry of the vector addition clear will be a real challenge).

Pedestrian suspension bridges often place the deck under tension to resist bending.

Suspension bridges aren't used for trains because they can apply such concentrated force, requiring a more concentrated deck so that a suspension bridge is no longer an efficient solution.