The Straw Bridge: Choosing a Structure Under a Material Limit
The JSS2 class at Model Junior Secondary School is entering the inter-school bridge-building competition. Each team gets exactly 30 drinking straws, 2 metres of cellotape and one pair of scissors, and must build a bridge that spans a 40 cm gap between two desks and carries a 2 kg mass at the centre for 30 seconds. Team A plans to lay all 30 straws side by side and tape them into one flat deck. Team B plans to tape the straws into triangles, using a deck as the top of a truss with a triangular side on each side. Team C wants to cut some straws into short pieces to make joints but is worried about wasting materials. The teams have one hour of lesson time to build and test. Last year's entry collapsed at the joints, not in the middle. The teacher will allow only one final test per team.
1. Which design is most likely to carry the 2 kg load, and why? Use ideas about how forces travel through a structure.
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Team B's truss. A straw is fairly strong when pushed along its length (compression) or pulled along its length (tension), but it bends easily when a force pushes across it. Team A's flat deck has no depth, so the 2 kg load bends the middle, the top surface stretches, the bottom squashes, and the tape peels off - the shape changes and the bridge sags.
A triangle cannot change shape unless one of its sides changes length, so it is rigid. In a triangular truss the top members are in compression, the bottom members in tension, and the diagonals carry the load out to the desks at each end. The truss gives the bridge depth, which spreads the load instead of concentrating it at one point.
The judgement is that the material is not the problem - the shape is. Thirty straws used as a flat sheet are weaker than thirty straws arranged as triangles.
2. Describe how the teams should test their models fairly before the final test.
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Build a test rig that never changes: same 40 cm gap, same two desks, same loading point (centre), same support at each end.
- Load in steps - for example 200 g at a time - waiting 10 seconds after each step and watching the bridge.
- Measure the sag at the centre with a ruler after each step and record it in a table.
- Note where it fails first: a taped joint, a straw in the middle, or the supports. Last year the joints failed, so the joints are what the test must watch.
- Change only one thing between trials - the design - not the span, the load or the loading point.
- Keep a spare straw or two aside; do not use the final test as the practice run, since only one final test is allowed.
A fair test is one where any difference in result can be blamed on the design, not on the testing.
3. Given that last year's joints failed, suggest how Team C should prepare the joints without wasting straws.
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Team C's instinct to cut straws is right, but it must cut for a purpose.
- Make short sleeves. Cut 2 cm pieces, slit them lengthwise, open them out and wrap them round a joint over the two straws meeting there. The sleeve spreads the force over a wider area instead of one taped point.
- Flatten and overlap. Flatten the ends of two straws with the scissors and tape them side by side in a lap joint, so tape grips flat surfaces rather than round ones.
- Tape along, not round. Run tape lengthwise along the straw for strength; tape wrapped round a straw adds weight and very little grip, and wastes the 2 m limit.
- Dry-fit first. Assemble the whole frame with the tape still on the roll, check the triangles, then tape once. Re-doing joints is where straws and tape are lost.
- Triangle every joint. A joint that sits inside a triangle is held by the other members; a joint on a flat run carries the load alone.
The theme: spend the scarce material where the force is, which is at the joints.