A4 - Final Report

 Background        

            The goal of this assignment was to design and construct the best possible bridge that was both strong and cost efficient.  At the end of the 10 weeks a competition would be held to determine which bridge had the best weight to cost ratio (efficiency). A key factor in the design process was strength, basically, how to manipulate the Knex to be as strong as possible.  This was determined individually by each team. Since the cost of each Knex piece was set at the beginning it was important to decide on a way to approach the task. As a team we decided that best way to go about the design process was to make the strongest bridge, and then make changes to lower the cost. Overall, the main goal was to build the lowest cost bridge that held the most weight.

Design process

      During the project, and in the early stages of designing, the main goals of the project remained relatively the same. Since it was more difficult to monitor cost we decided to focus mainly on this aspect because it was the ratio that would win the competition not the amount of weight held by the bridge.

During this term, we used different tools to help us make better decisions and learn about the bridge design and construction process. The West Point Bridge Designer Program (WPBD) mainly helped with the understanding how different bridges reacts to loads. It also displayed where the bridge would potentially fail when we tested it by showing the tension and compression forces. Truss Analysis was a good tool to use to show how different angles create different forces and showed that the larger the angle the greater the force. Lastly making individual bridges allowed for each team member to experiment individually and come up with new ideas.

            Our final design was motivated by being different in both technical and aesthetic aspects. The bridge featured a bottom cross-section for reason of adding strength. The elevation view included that of an incomplete cross section, where an additional member runs correspondingly in a primary Howe truss. Unlike the Howe design, both sides mirrored the other. The incomplete cross-section was incorporated for the sole reason of cutting costs while still providing weight displacement along the bridge. Nevertheless, complete cross-sections were added to the bridge ends to prevent compressed failure. Middle horizontal beams were also added for strength purposes.

       The only way that our final design was modified was during construction. We realized that the ‘x’ shaped connection that we had in the middle bottom connection was going to conflict with the testing apparatus so that needed to be removed. Before the competition, we predicted our load at failure to be around 20 pounds. In the end, our bridge failed at 9.8 pounds.

Final Bridge

Final Cost $342,000

Top View

Close up

Side View

Testing Results
            The 36” bridge failed at only 9.8 lbs., under less weight than predicted. This early failure can be attributed to many mistakes. These mistakes are made clear in the video of our bridge being tested (which can be found either on the Drexel Bridge Design Blog dashboard or on the Bridge testing page). After watching this video, it is clear that the X cross-sections should not have been removed. These beams were removed to lower cost which ultimately caused its early failure. The bridge, as seen in the video, failed more toward the center. This shows that the bridge was not able to disperse the weight of the sand outward. Basically the bridge was structurally weak at the center most likely due to exclusion of the X cross-section. Another error that attributed to the failure of the bridge was the fashion in which the sand was placed into the bucket. The sand should have been placed slowly and evenly into the bucket so that it did not sway and wobble. I believed this put uneven pressure on the bridge which also could have attributed to its early failure.

Conclusion

The final bridge design did not function as predicted. Not only did it not hold as much as predicted but it also broke in an unexpected spot.  The 36” bridge failed under less weight than predicted. The prediction weight was 10lbs while, during the experiment the bridge failed under just 9.8 lbs. This is less weight than our initial bridge held.   The final bridge design should have been our initial and vise a versa because our first design held more weight than our final.  The ratio of actual cost to pound held for our final bridge was higher than our initial bridge, since the final bridge cost was the same as the initial but the initial bridge was a foot shorter.

Future work
            Our final bridge design had some good ideas. If these ideas were morphed with our first bridge design the bridge would have been able to support more weight. The aspects included would be an under truss, as well as X cross-sections, which were both utilized in the first bridge design. The middle support beams in the second bridge would also be utilized. The under truss distributed the weight of the sand outward allowing the bridge to hold more weight.  The X cross-sections on the sides of the bridge would make the center more structurally sound and would keep the bridge from twisting. The middle support beams would also help disperse the weight and make the center stronger. While combining these aspects would increase the overall price, the bridge able to hold more weight.