Show students a map of Vancouver and neighbouring cities, with the bridges that connect them. Show photos of local bridges, and name the type of bridge in each case. I used Oak Street bridge (and a log bridge) as examples of a beam bridge, Burrard Bridge as example of a truss bridges, Port Mann Bridge as an example of an arch bridge.
List of other lower mainland bridge types:
Girder (beam) 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: Port Mann bridge (Coquitlam to Surrey)
Suspension bridges: Lions gate
Cable-stayed bridges: Alex Fraser (longest in the world when built)
Show students how to construct each kind of bridge - beam, arch and truss - and test their relative strengths:
Bridge supports are tubs of sand or stacks of books. (See photos for the two kinds). All the bridges are made with the same amount of material - two sheets of paper.
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 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 pennies until the bridge starts to collapse.
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 truss bridge the strongest (though depending on the number of creases in the truss this can make a huge difference to its strength).
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, so that less force is experienced in more places. Hence more load can be added before the card bends.
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.
Look at the local bridge photos again. These bridges are of all different kinds: beam, truss and arch. Why are they not all truss bridges, as we found this to be the strongest in our experiment?There are many factors in building a bridge, including the length of the span, the material the banks are made from and where piers can be built, and the cost of the materials. For each location the engineers determined the best bridge type.
Keep experimenting and graphing:
1. Make and test a different kind of bridge: try a sheet of paper with the sides folded up along the length of the bridge - girder bridge).
2. Students can design and test their own bridge too.
3. 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?