Paper Bridge Design Challenge

Background Information for Teachers

In this project, your students will build bridges out of single sheets of paper, that will potentially hold dozens of pennies. You might wonder: how can a single piece of paper hold up dozens of pennies when it cannot even support its own weight (Figure 1)?

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 1. A single piece of paper between two stacks of books falls down under its own weight.

The answer lies in folding the paper to change its cross section (the shape you see when you look at the paper edge-on). Think about bending a ruler. A ruler has a (roughly) rectangular cross section that is very thin in one direction and thicker in the other direction. It is very easy to bend the ruler in the thin dimension, but much more difficult to bend it in the thick dimension (Figure 2).

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 2. A ruler is easy to bend one way but nearly impossible to bend the other way.

The exact same concept applies to paper. A flat sheet of paper has a very thin cross section. That makes it very easy to bend, so it cannot even support much weight, let alone carry a load (additional weight, like people and cars on a real bridge). However, you can fold or roll the paper into a variety of cross-sectional shapes (Figure 3), all of which are much harder to bend. These shapes can easily carry a load in addition to their own weight.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 3. Paper bridges with different cross sections.

Which shapes work the best? That is for your students to find out in this project!

Additional Background Links

• Cross Sections, Math is Fun
• All About Bridges: Importance and Types, Easy Science for Kids

Prep Work (5 minutes)

• Set up an area where you can do a demonstration for the whole class. Use a ruler to set up two stacks of books 10 inches (about 25 centimeters) apart. Put the paper on top of the books, with one edge on each stack. The paper should fall down under its own weight. If it does not, move the books slightly farther apart. Remove the piece of paper and save it for the class demonstration.

Engage (5 minutes)

1. Show this video to your students and discuss it as a class. Make sure your students know that all the people on the bridge were safely rescued and that this exactly why designing strong bridges is important!
   

   https://www.youtube.com/watch?v=knaNXcKS-qM

   What did you notice in the video?

   As more and more people walked onto the bridge, it started to sag, until it collapsed and fell in the water. Luckily, the water was very shallow, and the people were rescued!

   Why do you think the bridge collapsed?

   The bridge collapsed because there was too much weight from all the people walking on it.

2. Briefly discuss the importance of bridges as a class. You can tailor this discussion to your geographic region (for example, if you live in a town with a bridge that most students will be familiar with).

   Why do we need bridges?

   We need bridges so people and vehicles can cross bodies of water, canyons, other roads, etc.

   Why is it important for bridges to be strong and sturdy?

   Bridges need to be strong so they do not fall down due to the weight of all the people and vehicles on them. Depending on where they are built, bridges might need to withstand other things, like strong winds or even earthquakes.

3. Explain that today we will make "bridges" out of paper, and the bridges will need to support "people" (pennies). Show the students your two stacks of books and piece of paper.

   What do you think will happen if I try to use this flat piece of paper as a bridge? Will it be able to hold any pennies?

4. Ask a few students for their predictions, then place the paper across the books to see what happens. If you are using stiffer paper that can support its own weight, add a few pennies to the middle to make it fall.

   Why did the paper fall?

   The paper fell because it was too flexible and bent too easily.

5. Explain that now the students will work in pairs to build a bridge using one sheet of paper. Their goal is to design a bridge that can hold as many pennies as possible. They will get multiple sheets of paper so they can test more than one bridge design.

   What can you do to the paper to make it support more weight?

   You can ask some students for their ideas, but now they will do an experiment to find out! It is OK to share their ideas with the whole class, this is not a competition.

Explore (30 minutes)

Have each group follow these directions:

1. Use a ruler to set up two stacks of books 10 inches (about 25 centimeters) apart. You might need to help your students do this if they have not learned how to use a ruler yet. Do not move the books once you have them set up, it is important for their distance to remain constant throughout the experiment.
2. Figure out how to make a bridge with a single piece of paper. The bridge should be able to sit with one end on each stack of books, without sagging down and touching the table. Important: don't give away the solution! Let your students explore and figure out that they need to fold (or roll) the paper. If some groups don't get it, encourage them to look to other groups for help.
3. Once every group understands that they need to fold or roll the paper to make a bridge, challenge them to design a bridge that can hold as many pennies as possible. Follow these rules when testing:
   1. The bridge must be able to hold pennies without the pennies falling off. So, for example, a circular bridge will probably not work, because the pennies will slide off the top.
   2. You can use up to two pieces of tape on your paper to prevent it from unrolling/unfolding. You cannot tape the paper to the books.
   3. Start out by placing one penny at one end of the bridge. One at a time, keep adding pennies single-file along the length of the bridge (Figure 4). This simulates more and more people walking out onto the bridge.
   4. If your pennies reach the far side of the bridge, go back to the other side and start stacking the new pennies on top of the old ones.
   5. Keep adding pennies until the bridge collapses (it falls down and touches the table), or the pennies start to slide/fall off the bridge. If the bridge is sagging, but not yet touching the table, and all the pennies are still on the bridge, you can keep adding pennies.
   6. On the student worksheet, make a drawing of the bridge's cross-sectional shape (the shape you see when you look at the bridge from the end), and record how many pennies it could hold.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 4. Adding pennies to a bridge until it collapses.

4. After your first bridge has collapsed, think about how you could improve your design. Is there a way you could make your bridge stronger, or try a different design? Use a fresh piece of paper and repeat step 3.

   Did your new bridge hold more pennies than the first one? If so, why do you think it was stronger? If not, what do you think went wrong?

5. After each group has completed two bridge designs, discuss your results so far as a class.

   Which bridges held the most pennies? The least? What shape were they?

   Are there other factors that affect the strength, like how careful you are when building the bridge, or how firmly you crease the paper when folding?

   Based on what you learned from the other groups, how would you change your bridge design to make it stronger?

6. Using what they learned from the rest of the class, each group should repeat steps 3–4 and build and test at least two more bridges (more if time allows). Try to make each new bridge stronger than the last one. Remember to use a new piece of paper for each bridge.

Reflect (5 minutes)

1. Make a data table on the board to collect results for each group's strongest design. Draw a cross-section of the bridge design in the "Bridge Shape" column, and record the number of pennies it held.

2. Discuss the results as a class:

   Which bridge designs were the strongest? Do the strongest designs have anything in common?

   Which designs were the weakest? Do those have anything in common?

   How do you think a bridge's shape affects its strength? Why does a flat piece of paper not make a good bridge?

3. Discuss the design process students went through. This serves as a good transition point to the next lesson plan, where they will try making bridges from different materials.

   How different were your first design and your last design? What changes did you decide to make and why?

   What other changes would you make if you had more time?

   What if you could use materials other than paper? What materials would you want to use to build your bridge?

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Civil Engineers

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Career Profile

Civil engineers design huge structures like skyscrapers, stadiums, and bridges. It is their job to make sure these structures can safely support the weight of all the people on or inside them. This is a very important job, since people can be badly hurt when buildings or bridges collapse. A civil engineer probably helped design the school you are sitting in right now! Read more

Lesson Plan Variations

• What happens if you make the bridges out of different materials like cardstock, aluminum foil, or wax paper? See this lesson plan for instructions for this version of the activity.
• What happens if you change the distance between the stacks of books? Try moving the books closer together and repeating the experiment with some of the same bridge designs. Or, move the books farther apart, and try building bridges made from two pieces of paper taped together end-to-end.
• How repeatable are your bridge designs? Perform multiple trials with the same design (using a new piece of paper for each one). Can each one hold the same number of pennies?