1. Background
- In ENGR bridge design, students received the opportunity to explore and practice the design process of large projects; which could be bridges, buildings, highways, etc. The goal of the course was to have student understand and experience the process of working in team, planning, documenting, modeling and analyzing. Through these goals the students receives a better grasp of how engineering uptakes in the field. The most efficient method of any project is to create prototypes or smaller scale of the structure and conduct analysis from the testing results to make improvements to the project. Therefore, the students were assigned several checkpoints throughout each week that needed to be satisfied to proceed. These checkpoints introduced the students to various such as West Point Bridge Designer, Bridge Designer, and K'nex pieces and allowed them to experiment their prototype of the bridge design. The ultimate task for the course was to work in a team to inquire the best solution for the problem given a set of constraints. Constraints such as building and testing a serviceable K'nex bridge that spans a certain length, width and height. During the whole process, students were able to expand their knowledge on the various physical properties and concepts related to the designing process of truss bridges.
2. Design Process
- Our primary objective was to construct a simple truss bridge that satisfied all the design constraints and had a best cost to weight ratio. First we utilized West Point Bridge Designer to design a bridge, and it made all of the calculations for us. Therefore, the designing aspect was easy, we just had to make sure to, counteract the tension and compression on the bridge chords. Then the task was to make a bridge using K'nex pieces, which took a hands on approach. This time the bridge will be evaluated based on the price of the bridge and the weight it could withstand. As the course progressed, our group was introduced to various tools and more knowledgeable about the designing process. Our initial bridge that spanned 24'' constructed using the blue membranes was only able to withhold five pounds. During our Truss analysis we discovered that by increasing the number of membranes and decreasing the amount of connectors will increase the tensile pull-out force of K'nex rod from a K'nex connector. Next, we learned about the "Method of Joints" (MOJ), system that calculates the exact tension on our bridge. Therefore, for our final design we chose to replace the longer membrane which will result in less connectors and still satisfy the new set of constraints. The predicted load of failure was 26 pounds. This was calculated based upon the testing results conducted by truss analysis conducted each week.
3. Description of Final Bridge
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| The Bill of Materials |
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| Elevation View |
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| Inside View |
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| Plan View |
4. Testing Results
a) Load at Failure
- The Bridge was able to upheld a 31.8 pound load.
b) Describe the failure mode of the Bridge
Our Bridge exceed past our expectations and held more load than it did during the first testing period. We initially believed that the middle section of our bridge was extremely strong and our fail towards the end points of the bridge. However, it actually failed due tension more towards the center of the bridge, in between the center and right end point.
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| The failure mode of the bridge |
5. Conclusion
- In conclusion our bridge exceeded our expectations and held a excellent amount of 31.8 pounds when we only guessed 26 pounds. The final bridge design that consisted of 265 K'nex pieces resulted in a cost to weight ratio of around $12,437 per pound. Unfortunately, we lacked in our analysis and predicted the method of failure wrong for the final bridge. The mode of failure was the separation of the 180 gusset and the yellow membrane. The bridge failed gracefully without pieces exploding everywhere. This demonstrates the strength of the bridge design. Overall, it was a great learning experience and also gave us a good opportunity to perceive the engineering aspects on designing process. It was a good insight for our future engineering careers.
6. Future Work
- For our future bridge, we would primarily attempt to bring the total cost down by using less numbers of pieces. Also try to use less of the red beams, which costs 2,000 dollars each. Overall, we believe that our final design has a effective structure and contains all of our best concepts. Therefore, we would not change much of the bridge, besides manipulating the pieces to reduce the total cost.
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