Jared Frank; Aneesh Lodhavia; Alex Shortt
Intro & Specs
Our assignment was to build a bridge 20 cm tall, and 60 cm wide. The two bases touching the ground have to be no wider than 5 cm, and there needs to be an open space under the bridge 10 cm tall and 50 cm wide. [Figure 6] The bridge cannot weigh more than 454 grams (about one pound) and needs to carry at least forty times its weight and the center of the top before breaking.
Our first idea going into the build process was to make as many triangular shapes as possible. According to common knowledge, the strongest shape in the natural world is an equilateral triangle, so it makes sense to incorporate as many triangles into our build sketch as possible. We ended up using equilateral triangles as our side, supporting the bridge roadway at the top [Figure 5]. Regarding the base, we simply tried to layer Popsicle sticks enough to pass the 20 cm height expectation [Figure 2]. We reinforced the base with vertical X-shapes, connecting the sticks together, thus dispersing the weight equally to each stick of the base. We also connected the left and right sides of the base with a single stick, preventing the diagonals of the rectangle created by the base to shift in length [Figure 5]. When putting it all together, we took a unique approach to connecting the side to each base. We stuck the end Popsicle sticks into the top half of the bases, and laid the bottom of the ends of the side on sticks resting on the top half of the base. To prevent the side from detaching from the base, we put anchor sticks on top of the other side of the stick mentioned above [Figure 3, 4]. At the end of the build, we had a choice to either glue the roadway to the side supports or leave the two sections of the bridge unglued. We chose to leave the two unglued because we believed that the initial space between would allow the side supports to carry less weight.
Weight of Bridge: 360 grams (0.8 lbs.)
Weight held by Bridge: 75 lbs.
Weight Ratio: 94.9%
Video of Weight Test: https://www.youtube.com/watch?v=r5gEwAo49VM&feature=youtu.be
Our goal was to hold 100 times the weight of the bridge; we fell short of the goal, holding around 95 times the weight. Using the weight test, we discovered that the bridge broke in the middle, closer to the right; the side supports were mainly responsible for the damage. Looking back at our design, we realized that our supports were under-reinforced. We could have either added extra sticks throughout the depth of the bridge or used a cross-x design, rather than an equilateral triangle design. Either way, any assistance to the side supports would have saved us an extra ten or so pounds, giving us the reach we needed to achieve the 100 ratio goal. Although we are not able to know when the base would give, it is safe to say the base was the strongest part of the design. The base did not give way before the side supports, and looked stable during the crash of the bridge. We could have used the extra 90 or so grams (the weight limit of the bridge was one pound, or 454 grams) to solely add extra reinforcement to the side supports. Also, we could have added extra reinforcement under the roadway [Figure 1] by adding sticks perpendicular to the roadway, especially in the middle.
Jared: Bases; Connecting the pieces together; Blog; Video
Aneesh: Side Support; Blog Edits
Alex: Roadway; Blog Edits; Weight Tester
Pictures & Figures