For the final project of my Solid Mechanics class, we were tasked with designing a wooden footbridge and constructing a 1:16 scale model to test its strength. The bridge needed to be designed to meet certain specifications:
- Full-scale design spans a distance of 6.4 meters with a width of 1.22 meters
- Scaled-down model holds at least 1 kN of downward force applied in the center
- Weight is minimized to decrease cost of materials
- Visual design is creative and aesthetically pleasing
The Design Process
My team designed an arched K-truss bridge in order to maximize strength and minimize material usage without compromising on aesthetics. After doing some preliminary sketches and stress calculations on paper, we turned to SOLIDWORKS to create and evaluate a fully detailed design.
I was the primary person responsible for designing the vertical railings with the trusses, and other teammates designed the curved horizontal beam and supports. We chose to pre-bend the beam in the opposite direction from the expected load so that the plywood would not need to deflect as far relative to its original shape, thus decreasing the internal stress. I enjoyed the challenge of translating my vision for the bridge into a CAD drawing – although it was challenging at times, I learned quite a lot.
Once the preliminary design was finished, we conducted a Finite Element Analysis (FEA) to simulate the effect of a 1 kN load being applied from an Instron machine. Based on the results, we iteratively modified the design to include additional supports underneath the beam, fillets to add material and reduce stress in the corners, and braces across the railings to decrease twisting.
Along the way, we also built prototypes of the bridge to test and inform future modifications. This involved laser cutting the plywood pieces and gluing them together.
As a final touch, we named our bridge after our team’s new favorite singer: Phoebe Bridgers.
Testing and Results
On testing day, our bridge significantly outperformed the required specifications. Upholding the minimum required load of 1 kN was no problem for our bridge; the beam deflected less than a millimeter. It was able to hold over 4.7 kN before the railings broke – despite being a scaled-down model, it could still hold the weight of approximately 6 people!
The moment of failure is shown in the video clip below. The railings failed first due to torsion, but the beam was able to bend much further before breaking.
In addition to being quite strong, our bridge also weighed the least out of all the bridges made by the class! We were proud to deliver a result that maximized the efficient use of materials without compromising on strength or visual design.
