Popsicle Stick Trusses: What Shape Is Strongest?

A structural science project by Science Buddies

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What makes a structure superstrong? Would you believe it can come down to simple shapes? Learn how with this fun engineering activity! 

Key conceptsGeometryTrusses

Introduction

Background

Bridges and buildings usually have their frames built as a "truss," or a series of beams that are connected at their ends. The engineer's goal is to design a truss that will slightly flex but not bend or break—even with strong forces acting on it. 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 seven)
• Small binder clips (at least seven)

Procedure

• Clip two popsicle sticks together end to end using a binder clip.
• Hold one end of each popsicle stick in each hand. Gently try to twist them back and forth, rotating about the joint where they are connected by the binder clip. (Do this by sliding the flat surfaces of the popsicle sticks against one another, do not try to "break" them by bending them.)
• Now make a square out of popsicle sticks that are connected by binder clips at the four corners.
• Grip two adjacent popsicle sticks with your fingers and gently try to rotate them relative to the joint that connects them.
• Grip two popsicle sticks that are opposite one another and gently try to slide them back and forth parallel to one another.
• Now make a triangle out of popsicle sticks that are connected by binder clips at the three corners.
• Grip two adjacent popsicle sticks with your fingers and try to rotate them just like you did with the square.
• Extra: If you have more popsicle sticks and binder clips, try making a larger truss structure out of multiple connected squares and/or triangles.
• Extra: Try adding 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).
• Extra: If you have building toys available, such as K'nex, Tinkertoys or LEGO Technic pieces, try using them to make trusses of different geometric shapes. (Note that this will not work with regular LEGO bricks, you need beams with holes that can be connected by pegs.)

Observations and results

When you try to rotate the popsicle sticks in the triangle-shaped truss, however, they do not move. The triangle design is "stronger"—the popsicle sticks are arranged in such a way in this shape 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 now see why it would not be good to only use squares when building structures—all of the joints would have to be superstrong! 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.

More to exploreThe Effect of Bridge Design on Weight-Bearing Capacity, from Science BuddiesAll about Bridges: Importance and Types, from Easy Science for KidsSphere-Based Science: Build Your Own Geodesic Dome, from Science Activities for All Ages!, from Science Buddies

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