A4: Breslin, Duane, and Ngov

BACKGROUND

The objective of this project was to design a bridge created from Knex pieces while considering the required constraints and parameters. The bridge needed to span a specific length while aiming to be most cost efficient. The efficiency was determined by dividing the total cost of the bridge by the total weight held. The first task in the bridge design process was to use computer software to get an understanding of to design a bridge. After information was gained, Knex pieces were used to create a bridge spanning two feet. Testing was done on the bridge to discover its strength. Following the testing, observation was done and calculations were taken. These results aided in the ability to lengthen the bridge to a span of three feet.

DESIGN PROCESS

Prior to the designing process, teams were established to set goals and collaborate during a ten week period to design the most cost efficient three foot bridge made from Knex pieces. Weekly progress was recorded on a team blog which showed a steady development of team work and design improvements. Many resources were utilized throughout the design process. The resources West Point Bridge Design, Knex pieces, Truss Analysis, and Bridge Designer were considered while creating the bridge throughout the entire ten week period.

The first asset used to aid the bridge design process, which spanned three feet, was a computer program called West Point Bridge Design. This program assisted students in understanding basic truss bridge designs, compression and tension forces, and the importance of cost. General truss designs of an arch, box, and under hang truss bridges were explored to allow groups to determine the most efficient design. A virtual simulation calculated the compression and tension forces of each member of the bridge. The greater the value of the force meant the member was weaker. This allowed for modifications to the design for improvement of the overall efficiency of the bridge design. In addition, different materials for members and gussets in West Point Bridge Design allowed students to consider the importance of cost. The strength of a material gave potential to increase the efficiency of the bridge while also raising the cost. Force and material were vital in the creation of a truss bridge throughout the Knex building process. This is because the ratio of the cost to weight determined the efficiency of the bridge. During the entire process, the cost of the design was extravagant. Consequently, the design was altered numerous times to decrease the cost while attempting to maintain the strength.

The next type of source that assisted in the development of the best bridge was Knex. Knex allowed a physical representation of how a bridge was built and failed. The construction with Knex pieces showed real life limitations. For example, the limited amount of members and gussets represented how manufactures do not have every size available for construction, while numerous fail factors can be physically seen. West Point Bridge Designer did not show geometric contortion like Knex. The bridge development through the use of Knex exposed vertical and horizontal failures while allowing failures to occur at multiple points on the bridge. This resulted in necessary changes to improve a similar example of a real bridge experiencing outside forces such as wind, earthquakes, and traffic. In addition, Knex allowed accessible and simple changes because every piece was readily available for modifications if necessary improvements were needed.

The following resource utilized was the concept of truss analysis. Calculations on the members of the bridge were calculated and the results were similar to the forces given from West Point Bridge Design. This reinforced the importance of improving the design at the weak members on the bridge to increase the efficiency. In addition, experiments with Knex pieces during the load testing allowed physical observations of the weakest members and contortion. The use of Bridge Designer also showed the weakest members of a basic bridge design. The disadvantage with the use Bridge Designer was the inability to input any design. The Bridge Designer program required triangular connections at every node. Nodes are similar to gussets in which they are the connections joints for members. The program did have an advantage where the general concept of any truss design could be applied and simply scaled. If the height was scaled by a factor of two, the forces of the members would decrease by roughly a factor of two.

In the final bridge created, every resource used was applied in designing the best possible bridge. The final bridge can be seen in Figure 1. The overall design was created after the observations of many bridge failures occurred during the load testing. Designs that did not include gussets with grooves appeared to withstand the most amount of weight. This resulted in using the least amount of grooved gussets; refer to Figure 2 for two groove gussets together forming a connection joint and Figure 3 to see a one groove gusset. The connection at the grooves would not fail as a result of weight but horizontal force. In addition, the cost was considered and determined that the use of the regular gussets had more of an advantage over the grooved gussets. Many sockets of the gussets held the maximum number of members possible to increase the pull out load and the load capacity. In addition, the height was increased to allow the forces to evenly distribute. The goal of the bridge was to create a design that would fail as a consequence of weight, not contortion or vertical and horizontal force. Throughout the improvements of creating the final design, the weakest members were constantly changed to decrease the force applied. After testing and considering prior resources, the predicted failure load was estimated to be at thirty-five pounds.

Figure 1: Final Design of Bridge. 

Figure 2: Two Groove Gussets Together.

Figure 3: One Groove Gusset.

FINAL DESIGN

            The numerous design attempts, research, analysis, and testings group one created a final design seen in Figure 1. The plan and elevation views can be seen in Figures 1 and Figure 4 respectively. After discussion and consideration of research, analysis, and testings, group one extended the height to eight and a half inches this is because it allowed more even distribution among all the members. In addition, the overall design from the previous bridge was kept because the strength and efficiency was effective. Also, the connection joints were altered because it was discovered that the gussets with grooves, as seen in Figure 2, was weaker then regular gussets without grooves. Another influential aspect was the design of the bottom of the structure. The cords on the bottom were pulled through the holes of the gussets to allow for more flexibility in the bridge; as a result, this allowed the bridge to withstand greater compression and tension forces. This cost and materials list can be seen below in an Excel spreadsheet in Figure 5. 

Figure 4: Plan View of Bridge.

Figure 5: Excel Spreadsheet of the Materials List.

The total number of Knex pieces used was 212 and cost $375, 500.

FINAL RESULTS
            The final bridge was tested by setting an apparatus on top of the bridge. This can be seen in Figure 6. Connected at the bottom of the loading apparatus was an empty bucket. The apparatus was placed as close to the center of the bridge to allow an even displacement of the weight throughout the members. The bucket was slowly filled with sand until the bridge failed. The weight the bridge supported until the failure was then placed on a scale to be found. In the final testing, the bridge spanned a length of three feet. This bridge was able to withstand a downward force of 32.2 pounds and failed as a result from weight. The weakest area of the design was at the reaction points of the structure.  Similar to the first design, the second bridge also had a graceful failure where the deflection of the bridge was visible. A bridge cannot be improved if it failed as a result of weight unless an entire design is created.

 Figure 6: Apparatus on a Bridge spanning three feet.

CONCLUSION
            The course showed how the process of designing a bridge can be applied to Knex pieces. After testing the bridge, group one was able to conclude that the bridge behaved as expected.  In terms of the final bridge, the failure was thought to be as a result of weight. The results confirmed this prediction as the sand was slowly poured into the bucket. The overall geometry of the bridge did not contort but failed as a result of the downward force of weight.  The failure point at a reaction point can be seen in Figure 7. The group knew after computer and computational analysis that the end points were the weakest parts of the bridge. The predicted downward force the bridge was expected to withstand 35 pounds.  After testing, it held 32.2 pounds giving the group roughly an eight percent error in their prediction. Different aspects of strategies in teamwork, calculations, computer testing, physical modeling, and physical testing were explored. The course allowed further development in each skill. 

Figure 7: Failure Point of Bridge After Load Testing.

FUTURE
            As a result of the testing, modifications could be made to improve the design of the bridges.  Members should be added to the side of the bridge in order to prevent the bridge from contorting.  In addition, more reinforcements towards the end of the reaction points would allow more even distributions in every member; consequently, the efficiency of the bridge would improve.