This is the CHaOS demonstrator website, mainly intended for our student volunteers. Click here if you were looking for our main front page.

Arch Bridge

Public summary: 

Can you build a bridge strong enough to walk on?

Building an arch bridge, and then walking over it.
Useful information
Kit List: 

Bridge Base (flat board and two triangular pieces, they attach together with wing nuts)
5 Blocks (all the same)
2 wooden supports (scaffolding to assist the building process)

Packing Away: 

Take the base apart by undoing the wing nuts that hold the triangular pieces to the base. Store them by reattaching to the triangular pieces.

Frequency of use: 

*** OVERVIEW ****

This is an arch bridge built of 5 identical blocks, which the children can then walk over.

Possible activities:
1. Build bridge.
2. Walk over it.

Other things to talk about:
Forces on blocks.

Tips for demonstrating:
The blocks are quite heavy for small children, so you may need to lift them for them/get parents to help. Don't let them jump on the bridge/too many on at once - jumping off sideways may cause the bridge to fall to the side, too many children on at once may push each other off.


To build the arch bridge, get the children to place the wooden supports between the ends of the bridge (say there like the scaffolding used around the edge of buildings or under bridges), then get them to add the blocks, starting at the ends. If they have trouble working out which way round to put the blocks try showing them the trapezium shaped side of the block. Ask them whether the longer or shorter side needs to go on top to make the arch shape.

Check that the blocks are all lined up straight and that the tops of the blocks all meet. Ask them what will happen when you take the wood away - will the bridge stay up? Why?

Take the wood away - the bridge stays up! Talk about how the shape of the brick means that they are being squashed in from the sides ('in compression') and they can't fall down. Ask if they think the bridge is strong enough for them to walk over (subtly check that the bridge is sturdy first) - take care that they don't fall when they do this.

When the child is standing on the middle of the bridge, ask them what is stopping them from going straight down. Depending on the child's age, could talk about the force of their weight, and explain how it is pushed out sideways and that it is because the ends of the bridge cannot move away from one another that they don't fall down.

Explain that this is how real arch bridges work - the mortar 'gluing' the bricks together isn't very important, all the strength is due to the bricks being squashed together horizontally. Bricks are strong in compression, but no good in tension.

It can be useful to ask the children how the other blocks would have to move to allow one of the blocks to fall, to get them to see the arch as a lot of wedges jammed in between the piers. Very wide stone arches have a tendency to force apart their piers and collapse.

Risk Assessment
Date risk assesment last checked: 
Fri, 03/01/2020
Risk assesment checked by:
Date risk assesment double checked: 
Mon, 20/01/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 
Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Blocks May fall on feet and injure people. 4 1 4 Blocks are designed to be fairly light and covered with rubber. Demonstrators may need to help younger children carry the blocks.
Call a first aider in the case of an injury.
2 1 2
Blocks Blocks may become smooth over time and slip against each other (bridge collapse) 1 2 2 Blocks should be checked for smoothness at the beginning of the event. Demonstrator should remain vigilant for blocks slipping during the event.
Demonstrator must stop experiment if they suspect that the blocks are getting smooth.
Call first aider in the event of injury.
1 2 2
Blocks May trap fingers between blocks during construction. 2 3 6 Demonstrators may wish to help small humans when placing the blocks. Mention that it is better to hold them across their sides rather than ends.
Call a first aider in the event of an emergency.
1 2 2
Built bridge Children/demonstrator falling off the bridge. 3 4 12 Make sure that the bridge is erected somewhere the floor is flat, with no sharp corners to fall on. Don't let kids jump up and down on the bridge (both because the blocks can slip and the child can fall off). Children should only go on the bridge one at a time. Don't let children crawl under the bridge. Demonstrator to check the bridge is stable before allowing children to stand on it.
Call first aider in the event of an accident.
1 3 3
Bridge Bridge falling over sideways (falling on children, bridge falling with children on it) 1 4 4 Demonstrators should test the bridge by walking over it before any children do. Children should not be allowed to jump off the bridge sideways, as this may cause the bridge to fall.
Call first aider in event of injury.
1 3 3
Bridge scaffolding Being hit by carried bridge scaffolding 2 3 6 Small or overexcited children should be supervised when moving bridge scaffolding. Demonstrator should volunteer to do it themselves.
Call first aider in event of injury.
1 3 3
This experiment is sometimes run outside during CBS!, see separate risk assessment.
Publicity photo: 
Experiment photos: