Introduction
We were given the task of designing a bridge that could hold a weight of 75 pounds, however, the bridge itseof had to weigh less than 0.75 pounds. The bridge also had to be built economically, with the price determined by the following formula: $200*(weight in pounds) + $3*(numbers of joints). The bridge had to span a little over 2’-9”, and be composed of all natural material, excluding the joints. Each member had to have a cross-sectional area no greater than 0.1in2. our team was given twenty ¼ ¼ 24 pieces of basswodd and a one and half weeks to design and build the bridge.
Design Plan
To begin our design, the team ultimately decided on doing a truss based bridge. We were provided with 20 rods of balsa wood, which were no thicker than ¼ of an inch. With this, we began to mess around in the program provided to us, Visual Analysis 10.0. We had an initial idea of how the bridge was going to be shaped, but with the use of the program, we were able to add supporting beams that truly distributed the force to all members of the bridge. For us as a team, we all concluded that our goal was to have the wood snap, rather than the joints fail, and also to have the whole bridge simply bust, rather than just a single joint or piece of wood fail.
Material and Construction
With the provided 20 rods of wood and the wood glue, we began construction. Each of our first two days of work, the initial design of each truss was put together. When working, a flaw in our design was pointed out. We had only been fixing our members together by glue, instead of creating a way to interlock them, which would have made it much harder for the joints to break. So with this new information, each day glue was added to every joint to increase the strength, in hopes that they would not break. When the two sides were constructed, they were then joined together by cross-sectional members, yet only members on the same vertical plane as what they were connecting, were used. Once the truss was together, we began using the program to further strengthen and distribute the force about the members. With the bridge complete, we were ready for a day of testing in Estabrook 111.
Analysis and Results of Testing
Prior to test day, we used our model on Visual Analysis 10.0 to strengthen via gluing the joints that were going to experience the most forces as demonstrated by the visual analysis.
Our bridge held 186 pounds before failing. It failed on the bottom left side. As seen in the picture below, the bridge was buckling at the higher weights on only the left side—most likely caused by a not entirely uniform build.
Conclusions and Improvements
Overall, our bridge design was a success. Our goal was to reach 75 pounds and we reached 186 pounds, far succeeding expectations. We had a total of 28 joints and weighed 0.425 pounds. Using the cost formula, our total cost was $169.
To improve our bridge, we would design it so that the part of the bridge resting on the supports was reinforced. Additionally, we would add more crossing supports on the bottom connecting the two trusses to relieve stress. Finally, we would be more deliberate when building to assure that everything is symmetric.
The goal was to build a bridge that held 75 pounds. We far succeeded this, indicating that we could have used less material. While our bridge was a success, it could have been more efficient and cost effective if we used less materials and less joints.
Bill of materials
20 pieces of balsa wood: $17.91
Wood glue: $3.00
Poster board and miscellaneous items: $6.00
References