Design of a Cardboard Bridge
Introduction: The Cardboard Bridge Design Challenge
Every year as a part of the CIV102 curriculum, the first year engineering science students participate in a Cardboard Bridge Design Competition. We are divided into design teams consisting of three people and are given the challenge of designing a beam bridge out of matboard and contact cement. We are limited to using one matboard with dimensions 1016 mm x 813 mm x 1.27 mm and two tubes of contact cement provided by the teaching team (a few of the constraints set out by the design brief). The Cardboard Bridge Design and the Design Process Our team focused on design for optimizing the second moment of area of our cross section, design for reducing shear stress, and design for withstanding compression at midspan. We met several times to decide on the best design for our cross section. Some of our low fidelity prototypes consisted of little sketches of different cross sections with calculations of their second moment of area. One of our team members developed a MATLAB program that could calculate the values of stresses our bridge could withstand and the total load it could bear based on the cross sections we drew by hand. We used this program as one of the tools to choose which cross section would be the most sufficient and meet the Dfx's we had decided on as a team. As a checking process we did the calculations by hand as well to make sure that the values we received from MATLAB were accurate. During this phase of the design, we had to constantly go back to the design brief and make compromises towards our designs to meet the given constraints. This was an iterative process, where we would design a cross section, determine how well it would perform based on our MATLAB program, and refine it based on the given constraints. We optimized the second moment of area by increasing the height of our cross section because that greatly affects the value of the second moment of area. By increasing the thickness of the top flange of our cross section, we increased the amount of compressive stress our bridge could withstand at midspan. Finally we accounted for shear stress by placing our diaphragms where the shear force was highest (at the ends of the bridge). My Contribution to the Cardboard Bridge Design
My initial design for the cross section was used as the starting point for our iterations of the cross section afterwards (see figure). We later adjusted this design to save on material by reducing the width of the cross section from 110 mm to 95 mm. However, the overall shape of my design was the cross section used for our prototype bridge. This prototype was able to hold a uniform load equal to my weight (see figure). We chose to load the prototyped bridge until it failed to see how we might improve the cross section for our final design. I also wrote the report which is in the attached documents section. |
Reflections On Designing a Cardboard Bridge
This design project taught me a lot about approaching the task from various angles. As mentioned earlier, we started by brainstorming by sketching various cross sections, some that we learned about in class and the others that would theoretically work based on calculations. Then a team member developed a program that could take our sketches and determine how well they suited our Dfx's. We also looked at designs that had worked in past bridge contests to see how the problem has been solved before. I also learned the importance of prototyping. It helped us to figure out our construction process for when we would build our real bridge and the limitations of our current design. By failing our prototype bridge, we learned how to prevent the same kind of failing in our actual bridge and by altering the cross section and the placement of the diaphragms, we increased the load our bridge could carry. Attached Documents
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