TRY THIS! Build a Bridge

TRY THIS!

Design a bridge out of a limited number of toothpicks and marshmallows that is strong enough to support at least 10 pennies in a small cup.  Your bridge must also meet the following requirements:

  1. It must span a gap of at least 10 cm in length
  2. No part of the bridge can touch the table or floor between the boxes or books.
  3. It must be at least the same width as the cup you’re using for the pennies

What You'll Need

  • 100 toothpicks
  • 50 mini marshmallows
  • Ruler
  • At least two flat and stable objects of equal size to prop up each end of your bridge (cereal or photo boxes are great for this)
  • At least 10 pennies
  • Small cup to hold the pennies (we recommend a small paper cup since it will add the least amount of weight)

If you don’t have the exact materials on hand – look around to see what other materials you can use instead.  Check the bottom of the page for suggested substitute materials.

Be Safe!

Always check with a parent or guardian before you start a project and make sure it is okay to use materials you find around the house.

 

 

Bridge design

There are many different kinds of bridges. You have probably crossed several of them. Each type of bridge offers specific advantages, and comes with different limitations.

The Suspension Bridge

The Golden Gate Bridge

The Golden Gate Bridge, which spans the one-mile strait between San Fransisco Bay and the Pacific Ocean, is suspension bridge. Its deck (the load-bearing portion that supports the roadway) is held up by vertical suspenders that attach to large suspension cables.

Suspension bridges are capable of spanning great distances and can be built high above ground or water level. They are also very flexible and can withstand some movement during high winds or earthquakes. However, they require very strong foundations and cannot carry as heavy loads as other bridge types because the deck has to be relatively light.

The Arch Bridge

Richmond Bridge, Richmond, Tasmania

Arch bridges like The Richmond Bridge, in Richmond, Australia, above, are some of the oldest operational bridges in the world. Arch bridges work by transferring part of weight of the bridge and its load horizontally into abutments – places where the bridge rests or make contact with the ground.

Arch bridges are incredibly sturdy - and even get stronger over time if they are well maintained. They can be built with many different kinds of materials and can withstand much heavier loads than some other bridge types. A single arch can only span a limited distance, so longer bridges require multiple arches.  This makes arch bridges more time-consuming, and often much more expensive to build.

The Truss Bridge

Metal truss bridge

This railroad bridge in Croatia is an example of a truss bridge. This particular design is a variation of the Baltimore Truss, which is common in railroad bridges.

Deck truss bridge in Lockport, New York

This bridge in Lockport, New York, is a deck-truss bridge.  In this design, the deck rests on top of the truss rather than inside it.

Forth Bridge

The Forth Bridge in Scotland is a beautiful example of a cantilever truss bridge.  Cantilevers are only supported on one side and, in large spans, are often built in pairs. This design can be built outward from set points rather than from end to end.

A truss is a structure of connected (usually straight) pieces that acts as a one piece. Trusses can dramatically increase the overall strength of a structure without adding a lot of weight or material.  They can also can be built in one place, then transported to somewhere else for assembly.  Truss bridges are often made from steel or other metal - but can just as easily be constructed out of wood or other material.

There are many more variations on the truss bridge.  Once you understand how trusses work, you'll start to see them all around you: on bridges, in buildings, supporting towers, even in the cranes that are used to build other structures.

Getting Started

As an engineer, you’ll want to consider how long your bridge needs to be to connect two places (it’s “span”), how heavy of a load it needs to hold (in our case: pennies), and meeting budgetary restrictions (limited materials) when designing and constructing your bridge. Pro tip for your design: when working with toothpicks and marshmallows, triangular shapes will provide the best bridge strength and stability.

  1. Think about a design for your bridge that will optimize strength. Sketch out your ideas.
  2. Construct your bridge using only the amounts of materials described above. This is your material “budget.”
  3. Once you’ve designed and built your bridge, prop it on top of two boxes or textbooks of the same height, leaving a gap of 10 cm in between (see the introduction video for an idea of what this setup should look like).
  4. Set the small cup on top of the center of your bridge. Gradually and gently add the pennies one at a time to the cup until you’ve added all 10 pennies. If your bridge stays intact with 10 pennies, try adding more. How many pennies can your bridge support before pieces start coming apart? How many before it collapses?

Try this next!

If you enjoyed this activity – take it further with more design challenges!

  1. Earthquake test: How might you simulate an earthquake? (Some ideas: try setting your bridge on top of a tray of jell-o and shaking the tray or try pounding on or shaking the table that your bridge is on directly). Does your bridge stay intact? What happens if you increase the force of the earthquake? If your bridge falls apart, how can you modify its design to maximize earthquake resistance?
  2. Using the same materials (or amounts of each material), design a tower that’s at least 20 cm high. How many pennies can your tower support? Can it withstand a simulated earthquake?
  3. What if you could use pencils, straws, or popsicle sticks in addition to toothpicks? How might you decide the best material for strength? For earthquake resistance?
  4. What if the weight on your bridge was distributed evenly? Although rare, this can occasionally happen during heavy traffic.  Try using multiple small cups and distributing them evenly along the top of your bridge. Then add the same amount of pennies to each cup.  Does that change how many total pennies your bridge can hold?  How could you strengthen the bridge so that it can hold a heavier weight distribution?

Thought challenge: What materials would you use if you needed to design a bridge to last more than 100 years?

Share your design!

Ask your parents to share photos/videos/drawings of your creations with @EngineerGirlNAE on social media and use #EGirlTryThis in the posts!

Submit images and stories for publication on EngineerGirl.

Learn About Civil Engineering

Substitute materials

The goal of this activity is to get creative and practice engineering design! If you don’t have the exact materials on hand – check around to see if there are other materials that you can use instead. A few alternative building supplies you might consider:

In place of toothpicks: try pencils, straws, or popsicle sticks

In place of marshmallows: try Play-Doh, tape, craft glue, homemade modeling clay.

If you use glue or clay, you’ll need to give it plenty of time to dry before testing the strength or earthquake resistance of your bridge. Also, toothpick bridges made with glue are much stronger than those made with marshmallows, so try testing the strength by placing large textbooks or cans of soup on top instead of pennies.

In place of pennies: try other small coins, bottle caps, individually wrapped pieces of candy...

For propping up your bridge: try textbooks or shoe boxes, photo boxes, tissue boxes, or cereal boxes of the same size with something inside of them to keep them from sliding


Photo credits:
Image of The Golden Gate Bridge by Stephen Kapinos, found on freeimages.com
Image of The Richmond Bridge by JJ Harrison, found on Wikipedia
Image of Railway bridge over the Mura river in Croatia by Silverije, found on Wikipedia

Image of Deck truss bridge in New York by Leonard G, found on Wikipedia
Image of Forth Bridge by Andrew Shiva found on Wikipedia, shared via CC BY-SA 4.0

TRY AGAIN!

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