Key Concepts
Geometry, trusses
A triangle and square both made of popsicle sticks and held together with small binder clips

Introduction

Have you ever driven across a bridge or seen a building that is under construction and noticed the large metal support beams? What about all the wooden beams in a house that is under construction? Did you notice how sometimes the beams formed different geometric shapes, like triangles or squares? In this project you will be a structural engineer and make your own “support” shapes out of popsicle sticks. What shape will be the strongest?

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

When civil engineers build large structures like bridges and buildings, they have to take into account how forces will act on the building materials. Forces, like the weight of cars driving across a bridge, can push (compression) or pull (tension) on things, causing them to break if they are not designed properly. Engineers also have to design things to handle torque, or “twisting.” You exert a torque on a screwdriver when you twist the handle with your hand, or on a ruler when you hold it on both ends and bend it.

Bridges and buildings usually have their frames built as a “truss,” or series of beams that are connected at their ends. The engineer’s goal is to design a truss that will not bend or break under load. In this project you will make trusses by connecting popsicle sticks end-to-end with binder clips. Can you guess what simple geometric shape will resist bending the most? Get ready to find out!

Materials

  • Popsicle sticks (at least 7)
  • Small binder clips (at least 7)

Procedure

  1. Clip two popsicle sticks together end-to-end using a binder clip.
  2. Hold one end of each popsicle stick in one end. Gently try to twist them back and forth, rotating about the joint where they are connected by the binder clip (note: do this by sliding the flat surfaces of the popsicle sticks against each other, do not try to “break” them by bending them). How easy is it to rotate the popsicle sticks about the joint?
  3. Now, make a square out of popsicle sticks that are connected by binder clips at the four corners.
  4. Grip two adjacent popsicle sticks with your fingers, and gently try to rotate them relative to the joint that connects them. How easy is it to rotate the popsicle sticks? What happens to the shape of the entire square?
  5. Grip two popsicle sticks that are opposite each other, and gently try to slide them back and forth parallel to each other. How easy is it to slide the popsicle sticks back and forth? What happens to the shape of the entire square?
  6. Now, make a triangle out of popsicle sticks that are connected by binder clips at the three corners.
  7. Grip two adjacent popsicle sticks with your fingers, and try to rotate them just like you did with the square. What happens? Can you rotate the popsicle sticks, or does the triangle maintain its shape?

Extra: if you have more popsicle sticks and binder clips, try making a larger truss structure out of multiple connected squares and/or triangles. What happens when you push, pull, or twist different parts of the truss? Does one shape (triangle or square) tend to “hold its shape” better, while the other one slides around?

Extra: what happens if you add a diagonal across the square, dividing it into two triangles (secure two popsicle sticks together with multiple binder clips to make a single, longer stick that cannot rotate)? Can you still easily rotate the square?

Extra: if you have building toys available like K’nex, Tinker Toys or LEGO Technic pieces (note that this will not work with regular LEGO bricks, you need beams with holes that can be connected by pegs), try using them to make trusses instead of popsicle sticks. Do you get the same results?

Observations and Results

You should have found that it was very easy to rotate the popsicle sticks in your square truss. When you rotate two adjacent popsicle sticks, or slide two sticks that are opposite each other, the sticks all stay connected, but the entire square deforms to form a parallelogram.

However, when you try to rotate the popsicle sticks in the triangle truss, they do not move. The triangle design is “stronger” – the popsicle sticks are arranged such that when you push or pull on them, none of them can rotate.

This happens because of something engineers call “degrees of freedom.” The square truss has one degree of freedom, which means it can move in one direction (in this case, a rotational direction – it can rotate from a square to a parallelogram). The only way to prevent the square from rotating at all is to pinch down very, very hard at the joints (imagine using much stronger binder clips). You can probably imagine why it would not be good to only use squares when building structures – all of the joints would have to be super strong! The triangle, however, has zero degrees of freedom – all of its popsicle sticks are fixed in place and cannot rotate. This means you can build a truss structure out of triangles that does not rotate or deform, even though the joints are only lightly held together by binder clips.

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Credits

Ben Finio, PhD, Science Buddies

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Key Concepts
Geometry, trusses
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