Leonardo da Vinci Bridge

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, you will build a self-supporting stick bridge to explore structural engineering concepts like shear forces, bending moments, load distribution, and yield points. Watch the video above to see how it is done.

Popsicle stick bridge materials & ingredients

For this activity:

• Wooden Popsicle sticks or craft sticks (at least 15 to 20 sticks)
• Heavy books or weights (to test load capacity)

How to build a self-supporting bridge: Step by step instructions

Step 1: Interlock the Popsicle sticks

1. Begin weaving the Popsicle sticks together into a self-supporting lattice pattern, placing transverse beam sticks beneath angled longitudinal sticks.
2. Continue adding interlocking sticks to extend the length and arch of your bridge structure.
3. Example of the stick layout in included below:

Image

Step 2: Test the unweighted structure

1. Observe the assembled bridge before applying any weight. Notice that without an external load, the design feels slightly loose and can slip back and forth due to the light weight of the Popsicle sticks.

Step 3: Apply load and test the yield point

1. Place a light weight onto the central beam of the bridge. Notice how the structure instantly becomes rigid as the beams interlock tightly.
2. Gradually add heavier weights or books one by one to observe how the bridge handles increasing downward force.
3. Continue adding weight until the structure passes its yield point and deforms or collapses. For extra data, measure the weight of each weight before placing it on the bridge. Then you will have an accurate record of the heaviest weight the bridge could sustain before collapsing.

Popsicle stick bridge: The science explained

This engineering demonstration illustrates structural mechanics, friction, and material limits:

• Reciprocal frame design: The bridge holds itself together without glue, nails, or tape. The geometry relies on interlocking beams where each stick supports and is supported by its neighbours.
• Shear forces and friction: When weight is applied to the top of the bridge, shear forces bend the wooden beams slightly. This bending action forces the connection points to press tightly against each other, increasing friction and making the structure more rigid.
• Load distribution: External weight placed on the central arch is spread throughout the connected framework down to the support base.
• Yield point and structural failure: Bridge stability does not increase indefinitely with weight. Once the load exceeds the elastic limit of the wood, the bridge reaches its yield point, causing the beams to deform permanently and collapse.

Science fair projects & taking it further: Experimenting with variables

• Span length: Build bridges with different numbers of interlocking sections. How does increasing the span length affect the maximum load the bridge can support?
• Surface friction: Test building the bridge on a smooth table versus a textured mat to see how base friction affects stability under load.
• Material comparison: Attempt building the same self-supporting bridge design using wooden pencils or chopsticks to compare material stiffness and friction.

Enjoyed this experiment? Explore more hands-on activities on the Science Lab ANU YouTube channel.