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Bridge Challenge Logbook

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Bridge Challenge Logbook

A creek crossing needs a lightweight model bridge. Use the evidence and calculations below to plan a fair test, explain the forces at work, and recommend an improvement. Unless a question says otherwise, treat the bridge as stationary during each load test.

Read the test brief

The creek is 30 cm wide. Each model must bridge a 30 cm gap and use no more than 80 g of supplied materials. A test load is added at the centre in equal steps until the bridge fails or bends too far. The team records the greatest load held. For this investigation, compare two designs made from the same material and tested using the same gap, load position and loading steps.

1.A team changes the bridge design and measures the greatest load held. Name the independent variable, the dependent variable and two controlled variables from the brief.

2.Which procedure best makes the comparison fair?

Use the same gap, material, load position and load increments for both designs; change only the design.
Change the material and design together, but keep the bridge the same colour.
Use a different span for each design so each can show its best result.
Add load in large steps for one bridge and small steps for the other.

Forces and structures

Forces can change an object's shape or motion. In a bridge under load, compression pushes or squashes, tension pulls or stretches, shear makes parts slide past one another, and torsion twists. A truss uses connected members arranged in triangles to share loads.

3.Match each force with the best description.

Compression
Tension
Shear
Torsion

B. A twisting action
D. A pushing or squashing force
C. A pulling or stretching force
A. Opposing forces make parts slide past one another

4.A load is placed at the centre of a bridge deck. Explain how a triangular truss could help transfer that load towards the supports. Include the idea of members sharing forces.

5.Two bridges were tested with the same method. Bridge A had a mass of 40 g and held 12 kg. Bridge B had a mass of 60 g and held 15 kg. Calculate each bridge's structural efficiency using: maximum load (kg) ÷ bridge mass (g). Show your working and give the unit.

6.Which bridge held the greater maximum load? Which was more structurally efficient? Use the figures to support both conclusions.

7.During a test, one diagonal member becomes longer and thinner as the load increases. Which force is most likely acting on it? Give one possible way the design could reduce this failure risk.

8.A team tests each design only once. Explain why this limits confidence in the results, then suggest one improvement to the testing method.

9.Sketch a truss bridge spanning a 30 cm gap. Label the deck, supports and at least two members. Add arrows showing where a central load acts and how forces could travel towards the supports. Then write one sentence explaining a design choice that could improve strength without exceeding the 80 g material limit.

Draw and label your proposed bridge here.