Activity

Bridge shapes and forces

Summary
Build a beam, arch, girder and truss bridge and compare how much weight they can carry. Discuss their relative strengths in terms of force distribution.
Science content
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Materials

For each pair of students:

  • 10 pieces of paper (quarter sheet of letter paper)
  • 100 pennies/washers or 50 glass counters
  • small pots to contain counters e.g. disposable ketchup pot, plastic shot glass
  • bridge supports: tubs of sand or stack of books, about 5cm high. They must stay the same distance apart for all experiments - the sand tubs can be taped together
Procedure

Tell students they will be making different kinds of bridges and testing their strength. Bridge supports are tubs of sand or stacks of books. All the bridges are made with the same amount of material - two sheets of paper (quarter of a letter-sized sheet), so that the different shapes can be compared for strength.
For Ks, show them how to make only beam and girder. For older primaries show them how to make beam, girder and truss. For intermediate students show them how to make beam, arch, girder and truss.

A beam bridge is simply two sheets of paper on top of each other, spanning the bridge supports.
An arch bridge is made by curving one sheet of paper between the two bridge supports, then laying the second sheet of paper over the arch so that it rests on the top of the arch and the bridge supports.
A girder bridge is made by folding up the sides of one or both sheets of paper.
A truss bridge is made by creasing one sheet of paper into an accordion length wise, then laying the second sheet of paper over the accordion.

For each bridge type, test its strength by adding a small pot to the centre of the bridge, then adding counters until the bridge collapses.

Tell students after testing these bridges, they can also make up their own bridge shapes, but to compare their strength to the others, they must also only be made from two sheets of paper.

Students come together to record their results on a class graph. The class results will show the data pattern clearly, even if individual results are not so striking.
It is expected that the beam bridge will be the weakest and the girder or truss bridge the strongest (though depending on the number of creases in the truss this can make a huge difference to its strength). There is a fair amount of variability in the data (though beam bridge data seems to be quite clean), but the highest point can be looked at, or the rough average of the points.

Discuss the forces in each bridge:
When the load is added to the beam bridge there is force pushing down in one area of the flat surface of the paper, soon bending it. When the load is added to the arch bridge the force is spread out by the arch to the sides, so that less force is experienced in more places. Hence more load can be added before the card bends.
The folds of the girder bridge directs force sideways and distributes it through the bridge. The truss bridge has triangles underneath, which are strong shapes that do not easily distort and spread the forces out so the bridge can take even more load (though these triangles are not strapped at the top, so are weaker that complete triangles).
(More details on bridge construction and forces at http://science.howstuffworks.com/engineering/civil/bridge.htm)

Look at local bridge photos:
Beam bridge: a log bridge, or plank over a river
Girder bridges: Oak Street, Knight street. Cantilever (built protruding from sides to middle), Cambie Street, No.2 road bridge
Truss bridges: Granville Street, Burrard Street, Second Narrows Ironworkers Memorial
Arch bridges: Second Narrows
Other bridge shapes not covered with this activity:
Suspension bridges: Lions gate
Cable-stayed bridges: Alex Fraser (longest in the world when built)

Depending on how wide the river that the bridge crosses, the materials that the banks are made from, and whether piers can be built in the river, and the cost of the materials, different bridge styles are chosen for each location. For each location the engineers determined the best bridge type. In general, a simple beam bridge is not used for longer bridges.

Ideas to continue experimenting and graphing:
If you build a beam bridge from several stacked pieces of paper, how many pieces of paper makes it as strong as an arch bridge/truss bridge made from 2 pieces? How much more material does the beam bridge need than the other kinds of bridges to be as strong?

Notes

Need a better weight holder than a cup, as they get top heavy when filled with coins. Need a long box the width of the paper with some height to it, so that all the load is added across the centre of the bridge (for fair comparison).

Grades taught
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7
Teaching site
Champlain Heights Annex
Gordon Elementary
Sexsmith Elementary
Tyee Elementary