ingridscience

Stands alone as a 1.5hr lesson, with a good amount of time for free exploration, ending with looking at photos of local bridges and understanding the Forces in them.

Tell students they will be making different kinds of bridge structures 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 or intermediates, 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 as well as the bridge supports.
A girder bridge (a strengthened beam bridge) is made by folding up the sides of one or both sheets of paper - make sure the students fold the long sides and the folds are across the gap between the supports.
A truss bridge (a strengthened beam bridge) is made by creasing one sheet of paper into an accordion length wise, then laying the second sheet of paper over the accordion - make sure the folds are across the gap.

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.
If a pot starts to get filled, stack on a second cup, so that the force is still in the same spot of the bridge deck (and not spread out).

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 and arch bridge data seems to be quite clean), but the highest point can be looked at, or the rough average of the points. the variability in truss bridge strength may be because students make differently-sized folds in their paper.

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 paper bends.
The folds of the girder bridge direct 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 than complete triangles).
(More details on bridge construction and forces at http://science.howstuffworks.com/engineering/civil/bridge.htm)

Look at bridge photos (local if possible):
Beam bridge: a log bridge, or plank over a river (see https://afriprov.org/wp-content/uploads/2020/05/aug-1-05.jpg)
Girder bridges: Oak Street, Knight street.
Truss bridges: Granville Street, Burrard Street, Second Narrows Ironworkers Memorial
Arch bridges: Second Narrows
Other bridge shapes not covered with this activity:
Suspension bridge: Lions gate
Cable-stayed bridge: Alex Fraser (longest in the world when built), Port Mann

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.

Discuss the elements that are used to construct a real bridge:
Structural elements are long beams, made of steel or wood. They are rigid and transmit forces along them, to distribute the force of the load throughout the structure.
Fasteners attach the structural elements together: bolts, straps to crimp structural elements together. Welding, gluing and cement are also used.

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?
Students can design their own bridge structures.

More info on bridge websites:
http://www.historyofbridges.com
http://www.pbs.org/wgbh/buildingbig/bridge/index.html
https://www.explainthatstuff.com/bridges.html
http://www.highestbridges.com