Activity: Bridge Building

Nikita and the bridge

On SCOPE's Bridges and Tunnels episode, you saw Nikita build some different bridges and test them to breaking point. Here's how she did it:

Watch this clip.

What you need

• A ruler
• 7 drinking straws
• Sticky tape
• 2 metres of dental floss
• Pair of scissors
• 4 large paperclips
• A small plastic or paper cup
• Some coins

What to do

Stage 1

1. First, cut two 3 centimetre-long pieces of straw, then tape two long straws on either side of the short pieces and tape their tops together too.
2. You've just made two towers for your bridge. Secure them to the edges of two tables or chairs of the same height.
3. Position the towers about 15 cm apart, then place another straw between the towers so its ends rest on the short pieces.
4. Find the strength limit of the bridge by attaching a cup with a paper clip to the middle of the bridge and loading coins into it.

Stage 2

1. Tie the middle of your piece of dental floss cable around a new straw and place the straw between the two towers, a bit like last time.
2. Then, pass each end of cable over a tower and down the other side.
3. Anchor the bridge by tying each cable end to a paper clip, before securing them firmly to the table.
4. Then test the strength again.

What's happening?

The first bridge constructed is a simple beam bridge with a straw deck. But once you add the dental floss it becomes a cable-stayed bridge and they have much more strength over distance.

Your new cable-stayed bridge should be able to support many more coins than the original beam bridge. Why? Well, in your cable-stayed bridge the downward forces on the deck are transferred up the cables to the towers, and then through the towers to the anchor points on the ground.

Cable-stayed bridges are the best sort to use to span distances between 500 m and 1 km.