What it shows
Long river bridges in India, like the ones at Bhagalpur and Rishikesh, are cable-based structures. A model with two towers, a main cable and hangers shows how deck loads travel through the hangers to the cable and then to the anchors. The project measures cable sag and tension and checks them against the parabola theory.
The science
When a load is spread uniformly along the deck, the cable forms a parabola. The horizontal tension is H = w L² / (8 d), where w is the load per unit length, L the span and d the sag. The cable tension is greatest at the towers and is shared between the vertical support and the backstays. If the load is concentrated, the cable shape changes and the deck can deform, which is why stiffening is added.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two wooden towers 40 cm tall
- A strong cotton or nylon cord as the main cable
- Thinner thread for hangers, 12 of them
- A deck made of a balsa or ice-cream stick strip 80 cm long
- Two clamps or heavy bases as anchors
- Two spring balances (kitchen type, up to 1 kg) at the anchors
- Small weights, a ruler and a pointer
Safety
For this project: Do not overload the cord and keep faces away from the cable when loading to failure. An adult must supervise, and use eye protection.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
11 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Choose the span of 80 cm, the sag of 10 cm, and calculate the expected horizontal tension for a given load per metre.
Fix the two towers upright in bases and brace them with backstay cords at the back.
Hang the main cable over the towers and anchor the ends through the spring balances.
Attach the 12 hangers at equal spacing and calculate their lengths using the parabola formula, so the deck is level.
Tie the deck to the hangers and check that it is horizontal.
Add weights evenly along the deck in steps of 100 g and measure the sag at the middle and the readings on both spring balances.
Compare the measured tension with the theoretical H and note the differences.
Apply a concentrated load at the centre and then at quarter span and measure deck deflection.
Add a stiffening truss made of sticks along the deck and repeat the tests to see how the deflection reduces.
Plot the load against sag and the load against anchor tension.
Write the report on the structural behaviour, calculations, results and comparison with a girder bridge.
What you should see
Under uniform loading the cable follows a smooth parabola and measured anchor tensions are close to the formula. Concentrated loads cause local dips that the stiffening truss clearly reduces.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is a parabola formed?
Because the load per horizontal metre is constant, the cable shape that balances it is a parabola.
2What do the anchors do?
They hold the ends of the cable and carry the horizontal pull of the cable into the ground.
3Why does a suspension bridge move in wind?
The deck is light and flexible, so it needs stiffening and aerodynamic shape to avoid oscillation.
4How does a cable-stayed bridge differ?
The deck is held by cables that go straight to the towers instead of hanging from a main cable.
Take it further
Hang a small motor with an off-centre weight under the deck to shake it and find the resonance frequency with a phone accelerometer.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present
Science projects
Tools, Materials and Safety for Model Making
Everything you need to build neat, strong and safe science models