The different designs of bridges have to take into account the load that they have to carry. One of the fundamental things students should understand is that the design of a bridge is usually built around certain shapes that are known to have great strength. Materials also influence the finished design. Over the years bridge building went from wood to iron and steel. Now cement and aluminum are also used. One of the best ways to demonstrate this is to do the following demonstration:
Demonstration: Shape Matters
You will need about eight thick books all about the same size. Take two of the books and put them on a table about 6 inches apart from each other. Place a piece of paper between the two books and begin to place paperclips in the middle of the paper. See how many clips the simple paper bridge can hold. With the next two books set them up 6 inches apart but this time curve the paper between the two books like an arch. Begin to add paper clips at the middle of the curve.7 How many clips can it hold?
Now we want to combine shapes. Put up two more books. Put a paper in between curved and then lay another paper on top of the books. Add paper clips. Does it hold more than the other two bridges?
Finally, take a piece of paper and fold it like an accordion, and put it on the 4th pair of books. Take another paper, fold it into an accordion pleat, and then glue a piece of paper to the top and bottom without squishing the folds of the middle paper. Which of these last three bridges will be strongest. Keep adding a load until one falls. Which is strongest? How could you improve on the strongest to make it even better?
Perhaps some of the greatest bridge builders were the Romans. They ruled throughout Europe and over five hundred years. They are the first great arch builders. The arch is considered to be both beautiful and one of the strongest shapes in construction. The Romans used the stones that were around them to construct these arches. Before the arch beam bridges over water were impossible without piers out in the water. This often disrupted ship traffic. The advent of the arch allowed builders to span greater distances without the need of piers and so larger ships were able to go underneath.
The arch works because the curve carries the weight outward from the top down to the abutments at the end. All sides are being compressed and since stone works well when compressed the arches have superior strength. The most important part of the arch is the keystone or top piece. The two sides of the arch are weak until the keystone is added. Then the arch can support itself.
Challenge: building an arch
Students will need to collect a number of stones about the same size and they can use clay or clay dough or the recipe for mortar given below. The Romans knew that the arch could not stand until the keystone was put in so they supported the bridge with a frame called centering. For this activity students can use a cylindrical shaped container like an oatmeal box or a block that is the correct shape.
To make homemade concrete you will need:
Spatula or wide craft sticks
Double boiler (you can use a smaller pan in a larger pan)
Pan or kettle to boil water
1 cup of sand
handful of pebbles or gravel
The recipe is as follows:
1. Boil water in the double boiler and kettle.
2. Take the small pan and mix sand and cornstarch
3. When both pans of water are boiling add ½ cup of the boiling water in the pan to the mixture in the top of the double boiler. Put the pan in the top of the double boiler. Stir the mixture, as it gets thick. If it seems to thick add a little more hot water. Let the mortar cool till you can touch it.
4. Now you can try to make something like the arch
5. If there is any left add pebbles and turn the mixture into concrete.
6. You can place whatever objects you make on a cookie sheet and set the oven to 275°. Let them stay there until they dry.8
Which arch is stronger?
In this demonstration students will test the strength of two different arch bridges. One will have vertical supports like some steel arch bridges. You will need the following materials:
Arches: 2 poster board strips 2" X 14"
Decks: 2 poster board strips 2" X 11"
Piers: 4 poster board strips 2"X 4"
2 heavy books or bricks to use as abutments
1. First tape the center of one deck strip to the center of one arch strip. Tape a pier between the ends of the deck and arch at both sides. Be sure not to bend any of the strips.
2. Make another bridge like the first.
3. Add supports to this bridge by taping between the deck and the arch.
4. Put the books on the table about 11inches apart. Put the bridges in between them, one behind the other. Now test their strength by adding similar coins to each bridge one at a time. Which bridge can hold the heaviest load before sagging? Are the arches retaining their shapes?9