Civil Engineering Unit

The Build Phase

Before building my bridge, I needed to design the structure on paper. My main design plan was to incorporate as many triangles as possible, as they are known as the strongest shape.

With a design to execute, I moved on to building the two sides of the bridge. This was a very tedious process as the bridge strength depends heavily on how carefully each member is cut and glued together. To keep the bridge from sliding around while the glue dried we used small pins as a brace.

Once I had one complete side, I began work on the other duplicate side. This second side was much easier to put together since I had not only a drawing but a 3D version to look at as I was building.

Finally, with two sides built I moved on to connecting the two sides together. This part was the most difficult because we were not allowed to use overlapping members. Also, it was very tough to cut all of the connecting pieces to the exact same length. However in the end I was able to create a solid connection between the two separate pieces.

Testing

Once my bridge had been a sufficient amount of time to dry, it was time to put it to the test. As mentioned before, the goal was to maximize the strength to weight ratio. Keeping this in mind, I had kept my design relatively simple to minimize its weight, and it payed off as my bridge came in a 12.21 grams, a little below average for the class.

To be able to calculate the failure load exactly, we set up an apparatus that involved a bucket of water hung below the bridge, allowing us to add water until failure. This system is preferable to adding incremental weights because it allows for more precision.

Overall, my bridge was very successful. The failure load of 10,134 grams was one of the highest in the class, and the strength to weight ratio of 830 was even better.

Seeing many other bridges tested before mine, I predicted that the bottom members would be first to fail(I had six members holding the weight), but the result was slightly different. Instead, the whole bottom section ripped off. The video of this can be found below at the bottom of the page.

Conclusions

In the end, the learning goal of understanding forces on a bridge was met fully. Along with that, two informal lessons I learned were:

1. When producing a member to fit in-between a gap of two members, always make the initial cut at a longer length than the gap. This way the piece can be trimmed accordingly, but is not completely wasted because it is too short and doesn't connect.
2. When working in small scale, craftsmanship is just as important as design. A good design can set you up for success, but care and time must be taken to execute at a high level.

If I were to start over, I would consider using members of shorter lengths, because based on testing of many different bridges this seemed to help with strength. This makes sense because in compression, shorter members have a higher failure load.

I thought this was a good project, but we were not given a sufficient amount of class time to build our bridges with precision. It required quite a bit of tedious work, and it seemed a majority of the class had to work on the bridges at home, causing use of different materials that in turn caused some complications in comparing our bridges. What should be changed for next time as well would be to allow students to overlap members on the bottom of the bridge. Although metal bridges don't overlap because welding is used, wooden bridges need this overlap or else the strength being tested is the strength of the glue.