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Bridge Materials Strength Investigation

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Bridge Materials Strength Investigation

A creek crossing needs a small test bridge. Work with an adult and compare sample materials. Use the same span, starting position and load size each time. Add loads one at a time, stop if a sample slips or becomes unsafe, and record the greatest load held. Never use sharp or heavy objects.

Plan a fair test

Compare equal-sized strips of cardboard, plastic, wood and metal. Keep the gap between supports and the way you add loads the same. Change only the material. Write a prediction before testing. If you cannot safely test a material, use the sample results below.

1.Which material do you predict will hold the greatest load before bending a lot? Explain one reason for your prediction.

Record the greatest load held and what you observed. Leave a dash if a material was not tested.

Material

Greatest load held (g)

Observation: bend, sag or break?

Cardboard

Plastic

Wood

Metal

2.In this fair test, what is the one factor you change, and name one factor you should keep the same.

Sample results

A class tested equal-sized strips over the same gap, adding 50 g loads. These are their results; your own results may differ.

Cardboard — 150 g

Plastic — 250 g

Wood — 300 g

Metal — 400 g

3.Using the sample results, which material held the greatest load, and which held the least?

4.How much more load did the metal strip hold than the cardboard strip in the sample results? Show the subtraction.

5.The cardboard strip sagged as loads were added but did not snap straight away. What does sagging tell you about how the material responds to a load?

6.Based only on the sample results, which conclusion is best supported?

Metal held the greatest load in this test.
Metal is always the best material for every real bridge.
Cardboard cannot be used for any purpose.
All four materials would hold exactly the same load in another test.

7.Why would it be too strong to conclude that metal is always the best choice for every real bridge from this test alone?

8.Suggest one change that would make the results more reliable, and say how it would help.

9.Sketch a small bridge that uses two different materials. Label which material you would use for the supports and which for the bridge deck. Explain why you chose each.