Bridges

Identify Problem

Design and build a small scale truss bridge for it to be tested under load for strength and efficiency in a class competition.

Constraints:

Made out of 3/8" balsa wood strips
12" long, 3.5" wide (max), 6" tall (max)
Only hot glue or wood glue
No excessive glue (only what's needed to form joints)
Max of 12 balsa strips

Brainstorm

Research

Truss bridges are made up of many triangles, as the joints created on each vertex are the most stable out of any shape. I researched many different types of bridges for their efficiency at a small scale and found multiple promising bridges but the best one I found is the Pensylvania Petit Truss. I chose this design for its high strength at a small scale, even though it was more heavy.

Sketch

Design

Once I finished the research portion of the project, I began designing the bridge in AutoCAD. the design was mostly the same as my original sketch, but I felt it needed more support underneath, so I added an undertruss. I also simplified the top of the bridge because it isn't load-bearing and could use less material.

Build

To build the bridge, I used 12 12x3/8" balsa wood strips and hot glue. I really enjoyed this portion, though it took much longer than expected and I wasn't able to build the undertruss. I built both side before fastening them to the plank using more hot glue. At the end, I added "sneaky beams," which are just beams on the bottom of the bridge in place of an undertruss in order to add more structure.

Test & Evaluate

In the testing portion, I put the bridge on a machine that applied a great amount of force to the center of the bridge deck, the weakest part of the bridge deck. I measured the dead load (the weight of the bridge), the live load (the total weight held), and the efficiency (live load/dead load). The efficiency is the most important part, as opposed to the live load, as it's the best indicator of how well the bridge performs based on the weight.

Results

Though the bridge broke, it wasn't a fault of the strength of the truss, but the joints of the truss to the deck. I'm attributing this to having used hot glue instead of wood glue because I glued one of the joints with wood glue and it was one of the only that stuck.

Bridge Video.mp4

Stats

Dead Load: 0.93 oz

Live Load: 42 lbs

Efficiency: 722.58:1

Class Rankings (out of 16)

Live Load: 13

Efficiency: 3

Redesign

If I were to redo this project, I would simplify the top to reduce weight and use the unused wood strips to connect the truss to the deck with wood glue instead of hot glue. The full length strip not only makes it easier to glue, but also adds more surface area of wood glued to the deck, meaning it would be a lot less prone to disconnecting.

Conclusion

This project taught me a lot about not only the forces and physics of a bridge, but that I tend to spend a lot of time on the smaller details, such as how the glued joints look, instead of getting the project done first. In future projects, I will work on getting the project to a functional stage before hyperfixating on the minute features.

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