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If your child is an engineer at heart or just loves to push things to their limits, this experiment is for them.
This experiment involves using popsicle sticks to build a sturdy Pratt truss bridge that can hold several pounds of weight. The unique design of the truss allows for weight to be distributed in such a way that it can hold a significant amount of weight.
Ready to put your engineer hat on? Let’s get to building!
How to make the Building Bridges engineering experiment
Supplies you will need
For this experiment, you will need the following:
- Popsicle sticks
(we used 60) - Hot glue gun
- Optional: markers or paint
Here is a kid’s book about the types of bridges 
out there and how they bring people together to accompany this experiment!
Before you start
We’re using hot glue and popsicle sticks that have the potential to splinter, so please watch your child around the materials.
Instructions
Here is how to do this experiment with your child:
Step 1: Plan out your design
We’re going to be making a Pratt truss bridge in today’s experiment, so the design will reflect that style of bridge.
The Pratt truss is made by a simple web arrangement of diagonal beams (or sticks in our case) in tension and vertical beams in compression.
If you want to use a different truss bridge design, go ahead and plan it out now. Be sure to plan out how many popsicle sticks you will need too.
Step 2 (optional): Paint your popsicle sticks
This part is optional but can make your bridge a lot more fun!
You can use markers or some project paint to color the popsicle sticks before you begin assembling the bridge.
Step 3: Create the top and bottom pieces of the side supports
First, we are going to create the two sides of the truss bridge. The Pratt truss bridge has a specific design to it, which you can see in the drawing I made of it.
Start by placing three popsicle sticks side by side, touching at their endpoints. You are going to glue two popsicle sticks to join those, so add hot glue near and around the joining endpoints and place a popsicle stick on top. Repeat that on the other joining endpoints. That’s the bottom!
We’re going to do something similar for the top, except it will be two popsicle sticks joining at their endpoints and one stick gluing them together.
Step 4: Connect the top and bottom with vertical pieces
When building a truss bridge that will carry people or vehicles across, getting the angles just right is very important. In our case, since we won’t be holding quite as much weight, it’s not as crucial.
In my bridge, I have 5 vertical beams and 6 angled supports.
First, place the bottom piece and top piece on the table, the length of a popsicle stick apart, and try to line up the midpoint of each as well as you can.
I started by placing the 5 vertical beams equidistant to join the top and bottom pieces, starting with the middle beam. I then added the top end beams and finally, placed the last 2 vertical beams equidistant from the middle and end beams.
Step 5: Add the angled supports
Now, we’re going to add the angled supports that make it the Pratt-style truss bridge!
All of the angled supports are going to angle in toward the center bottom.
Start with the middle vertical beam. To the left of the vertical beam, add an angled support with the bottom piece touching the bottom of the vertical beam. Repeat this on the right side of the middle vertical beam, having the bottom piece touch the bottom of the middle beam.
Repeat with the next set of angled supports, with the bottom of the popsicle stick touching the bottom of the innermost vertical beam of that set.
Finally, add an angled support to the outside on each end. In this case, the angle will be the opposite of those inside, since you’re forced to angle it a certain way because the bottom beam is longer than the top beam.
Step 6: Repeat Steps 3-5
We need to create one more side for the bridge, so repeat Steps 3-5 to build an identical side piece.
Step 7: Connect the sides
For this step, I added supports along the top and bottom of the structure to connect the two sides we made in Step 3-6.
I added about 5 sticks to the base of the bridge by applying hot glue to the tip of each of the sticks and placing one of my sides at a 90-degree angle on the glue. Then, I added glue to the other end of the bottom sticks and placed the other side piece at a 90-degree angle. This will give you the base and two sides of the bridge all connected.
Next, I made a slightly more intricate top to the bridge so it was more secure.
I added 5 vertical beams equidistant from one another (about where the vertical beams are in the side supports).
Then, I added some angled supports just like I did in Step 5.
Step 8: Make cross supports for the ends of the bridge
Finally, I made a simple X-shaped cross support for the two ends of the bridge.
I joined two popsicle sticks to ensure they were long enough and made an “X” shape with them at the ends of the bridge with some hot glue.
Step 9: Test it out!
All that’s left now is to test out the bridge to see if it can hold weight!
I made 2 of these bridges. The first wasn’t quite as sturdy and held about 30 pounds.
This second one, the one I showed you in this experiment, held more. I wanted to test out with some dumbbells first, so I added 15 pounds, then another 15 pounds, then a 10-pound dumbbell, and finally another 10-pound dumbbell (that’s all the dumbbells I had). That’s 50 pounds!
Then, we thought it’d be fun to have our oldest child stand on it. Oh boy.
We placed a cutting board on top of the bridge so that she could stand on it, then we had her stand on it. She’s 45 pounds, so it didn’t even test the limits of the bridge.
If I had more weight, we would have tried it out, but for the sake of showing the sturdiness of this bridge, just know that it can hold more than 50 pounds. And it’s just made from popsicle sticks!
The engineering behind the Building Bridges experiment
This experiment teaches:
- How truss bridges work
- Tension and compression
- Creativity
How it works
Trusses are used for transferring loads from where you don’t want them to where you do. They’re used in a broad range of structures, mainly where there is a requirement to hold a lot of weight for very long spans, such as inairport terminals, skyscrapers, sports stadium roofs, cranes, auditoriums, and of course, bridges.
A truss bridge takes the weight of whatever is passing over it and transfers it to the piers. What makes a truss different is the efficient way it carries the load using geometry in its favor.
How the Pratt truss bridge work
In very basic terms, the Pratt truss is a series of triangles that help distribute the load across the structure. The diagonals of the triangles all slope toward the center, except for the two diagonals on the outsides.
The beams of the truss work together, either under tension or compression, to hold significant amounts of weight.
Tension and compression
The Pratt truss bridge works through tension (stretching something) and compression (squeezing something together).
There are a lot of calculations that go into figuring out if each piece of the truss is under compression or tension.
In the case of our Pratt truss bridge with a downward force, which would be the force produced by humans or vehicles going across it, the compression is found in the top beam and vertical beams, and tension is found in the bottom beam and diagonals (except the outermost diagonals, which are under compression).
If you’re really into calculations, here’s a good article breaking down the forces in each of the beams with all of the calculations to go along with it.
Creativity
If you choose to paint or draw on your popsicle sticks before building the bridge, it’s a good lesson on creativity!
Not only will your child be able to paint the sticks freely, but they will get to see what the bridge looks like when put together.
Bonus points if they choose which painted stick goes where on the bridge!
More experiments about making structures to try out with your child
- Straw-tastic structure: Make a structure out of straws and see what it can hold!
Melissa
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