Many structures are built around frameworks made of straight, rigid pieces of wood or metal, called members, connected together. Forces generated in the members support the structure.

In this activity, drinking straws are used as members, to make a self-supporting dome.

1. You’ll need 35 long straws at 13cm and 30 short straws at 11cm. Pick a colour for your long straws and cut them to the correct length. Use the other colour for the short straws and cut those to the correct length too.
2. Follow the steps in the diagram, and stick the straws together to start building your dome.

#### green = sticky tape

3. At first, your dome should look like this.
4. Continue following the diagram and stick your straws together.
5. Continue following the diagram until the top of the dome is secure.
6. Your finished dome should look like this!
7. See if you can use the straws to make some different 3D shapes.

• What different shapes can you identify in the structure of your dome?
• What were the challenges you faced while working as a team to build your dome?
• Why doesn’t the structure fall down?

### Investigate…

• Can you make your dome bigger, or scale it down?
• What other materials could you use to improve your finished product?
• Can you make a dome using shapes different to the one in the pictures?

### Did you know?

Domes made of triangles are called geodesic domes – the largest, in Fukuoka, Japan, has a diameter of 216 metres.

### What’s the science and maths?

Some of the straws in this activity are in compression: they are being squashed, and as a result they produce a force that pushes back. It is those forces that support the weight of the dome. The other straws are in tension: they are being stretched, and as a result they produce a force that pulls back. These tension forces prevent the structure from splaying apart and collapsing.

It is this combination of tension and compression forces that makes a triangle a strong shape, and the interconnected triangles in this activity make the dome a stable, self-supporting structure.

### Science and maths in your world

Architects and engineers always think carefully about the combination of compression and tension forces when designing a new self-supporting structure, whether it’s a simple garden chair or a huge complicated bridge or building. You can often work out which parts of a structure are generating which kind of force.