Final bridge post
The design that we (Bobby and Spencer) decided to construct was one of the designs which we had created through the West Point Bridge Design program. We chose this design for the balsa wood model for many reasons. Primarily, we chose this design because its performance in the virtual load test was highly promising for its small stature. Secondly, it was agreed that this design would be very cost effective in that it would not need much material to build in the balsa wood scale. Along with its material efficiency, it would be light weight. We also agreed that this design was pretty cool looking. From the start of our building of the bridge, it was obvious that it was not going to weight much, especially when compared to the 120 gram weight limit. In the long-term, our goal was to build a light weight but strong bridge. With the final product weighing in at 76.5 grams, it certainly wasn’t heavy. Let’s rewind for a moment, back to day one of the balsa wood bridge build. Immediately we encountered some difficulty. When we attempted to scale the bridge design from the program into the 16 inch long Balsa Bridge, the scaling wasn’t exactly easy to do. After spending a while with the Pythagorean Theorem and a calculator, we drew up scaled designs for our bridge. Another problem that we faced in the design process was that the underside of the bridge could only go out 14 inches, one inch on each side form the outside edge, because of the stand that the bridge would sit on. This was a major problem because a great deal of the strength of the design came from the underside support truss. Again, after a while of fooling around with a calculator, our problems were solved. If you check earlier posts on this page, you will find my video (it is tumblr’s fault that the orientation of the video is messed up) describing the early stages of the bridge build process. When we were building the base of the bridge and the road bed, we knew that the exact middle of the 16 inch bridge was going to be the location of the most stress, not because that is where the weight was placed, but because the middle of the bridge is the spot on the bridge that is farthest from the two sides which held the bridge up. This is due to torque, the application of force at a certain distance. Because of this, the torque in the middle, the most stress will be at this middle point. With the exception of the one inch on the underside of the bridge, the design of the bridge did not change during the build process. We had one idea and kept with it until the end which, in my opinion, helped our group visualize the final product every step of the way. When the day came to ‘test’ our final product, it was in our minds that the downfall of this bridge will be the lack of wood that was used to build it. However, being light weight had the advantage of a potentially higher weight to load capacity ratio. As we began the load test, the bridge held strong and did not give up. I was surprised with the amount of weight that it held. Weighing in at 76.5 grams, it held a load of 413 N. Interestingly enough, the first place that the bridge broke was that the block, which the metal bar was resting on, fell through the road bed, making a nice square slot in the middle of the road. Aside from our bridge collapsing in a wave of both awe and sadness, the bridge did not fail our expectations. After everything was over, it was concluded that we should have used a little more material to strengthen the bridge, but I think it was good that we kept it light weight. As far as the bridge itself, it broke in a few locations. The roadbed broke exactly where I thought it was going to break, in the middle. For the upper part of the bridge, only a few beams actually broke, which hopefully says a lot about the distribution of the forces on the bridge. It is interesting to note, the joints, where I placed the plates on both sides, did not break. Every single plated joint held together, which also says a lot about the structural importance of using plates to create a stronger joint. On the lower part of the bridge, one side of the lower truss broke, however, this may have just been due to the bridge falling from the high stand it was on. Be sure to catch my other (most of) my blog videos on YouTube http://m.youtube.com/user/Landbeforephysics The few early videos are on my blog page below (tilt your head to watch them) in which I describe the early days of the bridge build.
