Final Bridge Blog (Lorenzo G and Karan B)
The original design we chose was the “Warren Bridge” design, which simply has angled cross members to form equilateral triangles. We chose this design for a couple of reasons. First of all, it held the most weight for the cheapest price in the USMA stimulation, compared to our other arched design. Second, we thought ahead and figured that to manually build an arched bridge out of balsa wood would create a greater challenge. However, we learned a couple of things during the time period of going from the USMA Computer Program to actually building the bridge. For example, when making the scale, we made everything precise and expected everything to work our perfectly, but that didn’t happen at all. It worked for the first couple of pieces, but eventually, pieces became too short, too long, or uneven, so we had to cut and glue pieces based off our wood bridge, not the scale we had prepared beforehand. Because of this, our final design differed from our original design.
As stated before, our original design was a simple Warren Bridge. All of our top trusses were equilateral triangles, no reinforced base, and no deck trusses. Once we completed our basic design, we still had a lot of materials left, and our bridge’s weight wasn’t even close to 120 grams, so we made it stronger, which led to our final design differing from our original design. First, we reinforced our roadbed by adding on two extra layers. Then, we added deck trusses. On the far left and right hand side, we had trusses that were right triangles, and between these right triangles were tiny trusses that consisted of more equilateral triangles.
After about the fifth or sixth brick, our bridge was weakened. One of the cross bars snapped out of place, causing the top horizontal member to bend out. This is where our bridge failed. Only three or four bricks were able to be placed in the bucket before that section of the bridge completely gave out, causing the whole thing to collapse. We could have done a lot of things differently to have prevented this. For example, we made a mistake on one side of the deck truss by gluing a part that should not have been glued. The crazy glue dried it so fast, that if we were to take this part off, our whole bridge would’ve been put at risk. This unwanted part that was glued on made it very hard to make bottom trusses on this side, so we only had bottom trusses on one side. The side on which this error was made was the same side that collapsed on our bridge. Second, our right triangle trusses on each end did not fit “snug” with the sixteen inch span. We feel that this could’ve made our bridge stronger if it were to have been “snug”.
Although the bridge building was a lot of fun, there was a lot of important physics involved like force, pressure, and torque. In order for a structure, like our bridge, to work, it must be in equilibrium, which goes back to the Newton’s Laws that we learned about. For an object, like our bridge, to be in equilibrium, the forces on a particular point or points must be balanced in all directions, and the sum of the torques must be equal to zero. Pressure also is very important. If you recall, Pressure is a newton per meter squared, or P=F/A. As you can see, Pressure and Area are inversely proportional. When building something like a bridge, or any other structure, if one increases the area on a certain point, the pressure is decreased at a certain point, therefore making that area stronger.