# Structural Engineering : Straw Bridges (Membership)

**Objective**

In this activity, students will learn why triangles are considered a stable shape and how they are used in the structural design of a Truss Bridge. Students will further explore these ideas by designing and building a Truss bridge of their own out of straws. At the end of this activity, students will have a chance to put their bridges to the test.

**Materials**

- blank paper
- markers/pencils
- straws
- tape
- random items to create load at the top of our bridge (empty water bottles, metal scraps, etc.)
- box, tray, or any type of container that can hold our load at top of the straw bridge

**Essential Keywords**

- Load
- Weight
- Force

**Attention**

Why are bridges built? To help people move from one place to another easier.

Bridges are built in various shapes and sizes but the bridge we will focus on is the Truss bridge. A truss bridge is a structure created using a series of triangles.

Let's take a look at this video:

SciShow Kids: What Makes Bridges So Strong?

**Learners Guidance**

So what shape is used to create a truss bridge? Triangles!

Why triangles?

Due to the shape of the triangle, if you add a load or forces to the corners, the load will be evenly distributed throughout the rest of the triangle. It does this the best compared to other shapes.

What is a load? Load is weight or force.

###### img reference

For example, if we build a square and push on it, the square can be easily turned into a parallelogram because of the way the corners are connected. A square is not the best shape to build a sturdy truss bridge.

###### img reference

We will be building a truss bridge of our own. We will place our bridge in a gap and then add some load. However, before we can start building, we must first design a truss bridge. Knowing what a truss bridge looks like and using the examples below, go ahead and draw out your own (Activity #1).

**Explain & Show how to build a triangle out of straws:**

- grab 2 straw
- pinch one end of the straw until there are indentations
- fold that same end of the straw in half
- tuck the folded end of the 1st straw into the 2nd straw.
- Push it in so it does not slip out
- Repeat that same pinching process to connect both ends of the straw.
- Shape the long straw into any triangle shape you want

**Start building each section 1 at a time. TIP: wrap the tape tightly especially when connecting sections together.**

- Start with the 2 wall sections. Create 3 triangles for each side wall, thats a total of 6 triangles
- Arrange the triangles in an alternating pattern
- Tape the triangles together.
- Next create the bottom piece.
- For the bottom section, we need to create 4 triangles. Preferably 90degree triangles which can be hard for most students to understand so they will need help shaping these triangles.
- Tape 2 triangles at a time to form a squared shape. The bottom piece requires 2 squares therefore 4 triangles.
- For the top section, make 2 triangles which will be taped in a square. The top only requires 1 square
- Tape each section together. Start with connecting each wall 1 at a time to the bottom section.
- Then, tape the top section to the walls

**Explore/Activity**

**Activity #1:**

(10 - 15 mins)

Let students draw out a side view of a truss bridge to help them better understand how the triangles connect to one another.

**Activity #2:**

Once students have completed their bridges, let them test their own bridge. Create a gap with chairs or tables and place the bridge in between. Set a tray or small box over the top of the bridge. Add objects to the tray until the bridge seems to not be able to hold anymore.

**Share**

Let students share how they decided to make their triangles and their bridge.

Did their bridge have any weak points?

Did they have any troubles while building?

**Closure & Essential Questions**

What is load?

What shape do truss bridges use?

Why do we use triangles instead of squares? Triangles are stable shapes

**Evaluate**

Give students feedback on their work.