1) The objectives of this course are to learn about the construction of bridges, analyze the forces in a bridge, become exposed to West Point Bridge Designer (WPBD) and to work as a team to construct a bridge using Knex. There are three people in a group that work together to reach the objectives. The bridge competition is won by whichever team has the lowest cost to weight-held ratio. In the beginning of the quarter, we set up a blog website that held all of our weekly blog posts and assignments. Throughout the term, we gained knowledge about bridges through the West Point Bridge Designer program, playing around with Knex pieces and doing thorough truss calculations to understand the forces. We learned that it was easier to stick with a cheaper bridge that held a lot of weight for its cost, rather than having an expensive bridge that could hold a greater amount of weight. We also figured out that triangles should be formed within the pattern of trusses in the bridge, which created the most stable bridge.
2) Our first objective was to make a bridge that would support a lot of weight while maintaining affordability. We started off with a simple and standard truss design, thinking that since they are used in the real world, they should be well rounded designs. We wanted to make a bridge that would hold a lot of weight using many pieces, so we added many webs into the bridge to make it stronger. We did not think too much about price and continued to make the bridge stronger by adding more pieces to the weak points of the bridge.
Our design goals changed most after we were told that the bridge must have an opening in the middle 3 inches wide and 2 inches tall. The bridge that we had already designed did not meet this requirement, so we decided to design a completely new bridge. We still kept the simple standard truss design and we built off of this. At this point, we decided to move from a strong bridge with many pieces, to a cheap bridge with a minimum number of pieces. We decided to do this because the objective was to make a bridge with the best cost/weight ratio, and we thought that this would be a good idea. Instead of increasing the weight we could hold, we decreased the cost of the bridge.
The individual designs using WPBD allowed us to find that a standard truss design may be one of the best designs around. This was one of the reasons we decided base our bridge design off of a standard truss bridge. The truss analysis helped us because we were able to locate the weak points in our design and reinforce these points so that the bridge was more difficult to break. We were also able to get rid of the pieces that did little or no work so it was cheaper. The actual individual designs using the Knex pieces helped because we could actually calculate and see which design was bad or good. Using this information, we chose the best bridge design and based our final bridge off of this.
We chose our final design by using trial and error. The group came up with two different designs at the end, so we tested both of them and we saw that the bridge one of us had designed was significantly better in both durability and cost than the other design, so we decided that the final bridge would be built off of that bridge.
After testing our bridge for the first time, we realized that it would twist and break when a lot of weight was put on it so we decided to add trusses on the top plane of the bridge in order to keep it from twisting when load was added to it. We also made the bridge as cheap as possible by replacing pieces with cheaper pieces. By doing so, we were able to keep the price down.
In the first test of our final bridge, our design was able to support a weight of 14.4 pounds. Using this as a reference, we predicted that our final bridge was going to hold around 13 pounds, but the actual bridge ended up supporting a maximum weight of 15.2 pounds which was better than what we expected.
3) The final bridge design was a simple and cheap over truss bridge, with a web on top. Our design was aimed toward a cheap bridge with a small amount of pieces rather than a strong sturdy bridge with many pieces. In order to make the bridge like this, we only have a small web on the top plane of the bridge. We also thought about how to use the 7” pieces. Instead of connecting two short pieces with a gusset, we made the 7” piece go through the hole of Knex pieces so that there were less gusset plates. We designed it in this manner because we wanted to make the bridge have less gussets since the bridge commonly breaks at the joints.
Elevation
Plan
Bill of Materials
The total cost of the bridge was $211,500.
4) The final bridge ended up holding 15.2 lbs. This was about 2 lbs more than we expected. Our bridge collapsed by having a few trusses pop out of their gusset plates, around the center of the bridge. There was a small amount of twisting before it collapsed and we all noticed that the joints were experiencing a lot of stress before it broke by viewing the bent trusses.
5) We predicted it would behave like our first Knex bridge, which went through an extreme amount of twisting before it finally collapsed. However, the final Knex bridge only twisted a little bit before we saw some of the joints were starting to pop out. We think the reasoning behind this is because we swapped out 5 inch trusses along the base for 7 inch trusses. Since there are fewer pieces being used, the bridge has fewer gusset plates to bend at. With that being said, more force is being dissipated along one big truss as opposed to have it divided among many, fewer trusses. This caused our bridge to give a more “sudden” collapse rather than gradually bending at the joints.
6) If we were able to modify our bridge further, we would exchange the 7 inch trusses along the base for shorter trusses and more gusset plates. This would give our bridge the ability to bend rather than putting a lot of stress on fewer, large trusses. We were also thinking about adding another level/deck to increase the height of the bridge. This would move a lot of the force away the center of the bridge and distribute it along the sides.
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