Bridge building & forces
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Learning Objectives
- 1Identify the main forces that act on a bridge
- 2Compare beam, arch, and suspension bridge designs
- 3Explain why triangles make truss structures strong
- 4Plan a fair test for the load capacity of a model bridge
Mini Lesson
Bridges are structures engineers design to carry loads across a gap. Every bridge has to handle two main forces: compression (a squeezing push) and tension (a stretching pull). Different bridge shapes spread these forces in different ways. In this project lesson you will build a small model bridge, test how much weight it can hold, and look at how shape affects strength.
The Two Main Forces
- Compression — a force that squeezes or pushes a material together. The top of a beam under load is in compression.
- Tension — a force that stretches or pulls a material apart. The cables of a suspension bridge are in tension.
- Load — the weight pushing down on the bridge: cars, people, the bridge itself.
- A strong bridge directs every load safely down through its supports into the ground.
Three Bridge Designs
- Beam bridge — a flat deck on two piers. Simple and short. The top of the deck is in compression; the bottom is in tension.
- Arch bridge — a curved arch carries the deck. The arch turns the load into compression and transfers it down to the abutments at each end.
- Suspension bridge — a deck hung from cables that pass over tall towers. The cables are in tension; the towers are in compression.
Why Triangles?
A square frame can squash sideways into a diamond shape — but a triangle cannot deform without changing the length of one of its sides. That is why bridge supports use trusses: networks of triangles. When a load presses on a truss, the force spreads out along every member, so no single piece has to take it all.
- Triangles distribute load into compression and tension along their members.
- A truss made of triangles is much stronger than a frame of squares of the same weight.
- Engineers stack triangles to build long, light, strong spans.
Testing a Model Bridge
- Span the bridge between two desks of equal height.
- Add weight gradually — small amounts at a time — at the center of the deck.
- Watch for sagging (the deck bowing down) and buckling (a member bending sideways or folding).
- Record the load at the moment of failure. Compare designs using the same procedure each time.
Tip: a fair bridge test uses the same span, the same materials, and the same way of adding weight. Only the shape of the bridge changes between trials.
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