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The goal of our project was to design and build a bridge that could support the required load of 10lb while using the least amount of material possible.
Special thanks to my teammates, Anthony, Liam, and Brandon for their collaboration on this project.
First Design: Popsicle Stick truss
We decided to use Artlicious craft sticks and carpenter’s wood glue. The craft sticks weigh less than K’NEX pieces, reducing overall weight.
For the truss, we chose a 60° Warren truss. Since forces are greatest at the center of the bridge, we planned to reinforce that section with an extra layer of sticks. However, we avoided reinforcing the entire span to prevent excess weight.
We also explored adjusting the bridge width to reduce buckling risk while maintaining adequate support for the load string. Mechanical intuition suggested that a width of about 4 inches would suffice. The center point would have three layers of 9-inch craft sticks, with 5.5-inch sticks on each side to meet the required span.
We finalized the plan and began construction of our prototype. The middle sections were reinforced to counteract buckling and tension deformation.
Triple supports were used in the center.
Double supports on adjacent sections, with 2-inch stick overlap.
Single supports elsewhere, with 1-inch overlap.
To maintain the full popsicle stick length and avoid excessive cutting, we slightly altered our truss design. We built the four truss pieces longer than originally planned (60 inches each) and instead of a perfectly symmetrical 60° Warren truss, our triangle angles were adjusted to about 70°, 70°, and 40°.
Front View
Isometric View
Calculations
We calculated forces on diagonal members and maximum compression/tension forces for triple, double, and single support members.
After further investigation the team concluded that the popsicle stick bridge was structurally unstable and unable to withstand the required force due to warping along the members and alack of symmetry due to workmanship.
Second Design: K’NEX truss
The K’NEX design will be a 45-45-90 degree Warren Truss (with verticals) spanning 60 inches in length with interlocking members in the middle to prevent structural mishaps because of the compression on the top. Along the top and bottom we insert small green pegs to reduce the tension forces in the bottom members.
After the K’NEX truss successfully passed the 1-minute load test, we removed the top green pegs, since those members were in compression and the pegs added unnecessary weight, while keeping the bottom ones. This reduced the mass from 693 g to 667 g without compromising the bridge’s strength.
After removing the pegs the truss was still able to withstand the test.
Calculations
Compression Strength of a member:
Tensile Strength of a member:
Due to time constraints, the team found out that the pull out force for the K’NEX without packing is 11.8lb, and with packing is 33.0lb courtesy of Tom, Julien, Hao, Aliyana, and Tiffany.
Force Analysis
The strength of each member is bigger than the force that will be applied on that member.
Final Design: Popsicle Stick Redesign
We replicated the 45-45-90 Warren truss design using popsicle sticks for a lighter and stronger structure. Diagonals at the ends were removed to reduce weight, while top and bottom members were double-layered for additional stiffness.
Calculations
Compression Strength of a member:
Tensile Strength of a member:
Force Analysis
Each member’s strength exceeds the force applied to it. One factor we initially overlooked was the strength of the glue; however, wood glue is typically stronger than the wood itself, allowing the bridge to support the required 10-lb load for the full minute.
Presentation
On presentation day, we determined that our bridge had a mass of 269.9 g, supported a maximum load of 21,000 g, and achieved a load ratio of 77.8. The failure occurred at the center on a single-support member, which was expected since that section experienced the highest forces and a single support can withstand far less load than a double support. Additionally, one of the compression members failed, which is reasonable because members typically have a lower capacity in compression than in tension.
Photo of the bridge after the test. The red circle indicates the failing point.
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