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What is a Truss Bridge? - Designs & Definition

Instructor: Amanda Robb
In this experiment, you'll be studying the effect of structural arrangement of supports in truss bridges. Through this experiment, you'll learn about different variations of truss bridges and how they change the amount of weight a bridge can support.

Introduction

Research Question: How do structural variations of truss bridges affect how much weight they can hold?
Age: Middle school and up
Safety concerns: None
Time: At least 3 days
Independent variable: Type of structure
Dependent variable: Amount of weight held
Control variables: Number of popsicle sticks, length of bridge

For thousands of years humans have been trying to span rivers, valleys, chasms, and even parts of the ocean. Bridges are used to accomplish this task and range from the incredibly simple beam bridge, made of only a flat deck, to complex cable bridges. One successful type of bridge is called a truss bridge. Truss bridges use triangles as supports above or below the deck to increase the amount of weight the bridge can hold.

The Liberty Bridge in Budapest is an example of a truss bridge
Truss bridge

The triangles can be arranged in many ways, and today we'll be testing how different arrangements affect the weight the bridge can handle. Before you start, look at some of the examples here. Which ones do you want to test? Which type do you think will be the strongest? There are many more variations you could try as well!

Variation of truss bridge structure
truss bridges

Materials

  • 200 popsicle sticks per bridge
  • 1 bottle craft glue
  • Scale
  • Objects to test your bridge with (coins, small books, bricks, scientific weights)
  • Data table:

Bridge type Weight held



Methods

1. In this experiment, you can build one bridge at a time, or work on both simultaneously. Whichever you choose, you'll start by building the deck, or the part that people and cars move across. Make your deck at least 2 feet long. You can glue the popsicle sticks vertically, horizontally or even stack them. It's up to you! Let the deck dry overnight when you're done.

2. Now it's time to assemble the supports and attach them to the deck. It's usually easier to start at one side of the bridge. You can glue the supports directly to the deck, or assemble the scaffold and then glue the whole thing together at once. You might need to hold the sticks together while they dry. Remember to include a triangle pattern for your bridge; that's the main characteristic of truss bridges. Now, let your scaffolding dry overnight.

3. If you need to add any more scaffolding or supports, do this now. Otherwise, move on to constructing your other bridges to test by repeating steps 1-3.

4. Once all the bridges are dry, it's time to test them. Weigh any objects you'll be placing on the bridges, such as books or bricks. Now, start stacking the objects on the bridge slowly. When the bridge collapses, that's the maximum weight it can hold. Record this in your data table.

Troubleshooting

If you're having trouble getting your glue to dry, you can use small binder clips to hold the pieces in place while they dry. You could also get a friend to help you hold the pieces as you construct your bridge. The warren bridge is the easiest to build, so if you're bridge isn't working out, try that variation first.

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