A4- O'CALLAGHAN, WETZEL, FRANCO

Introduction

Engineering 103 was comprised of exploring and experiencing the design and modeling process for building bridges.  This process was followed by a sequence of designing, building, and testing through the use of a variety of tools such as the computer software West Point Bridge Designer 2012, model materials such as K’nex tools, and innovative in-class testing of the finished versions of the various bridge designs.  The objective was to understand how the bridge would, in theory, behave when the real-world bridge project was completed, as well as the effectiveness of each group's bridge and the groups' abilities to work in teams efficiently.

Each group entered their design into the class competition.  The different models were tested in comparison to each other which resulted in determining a model that satisfied the best cost to weight ratio.  As the experimental process followed an in-depth analysis, many steps were executed by each team.  The teams went through several phases which included planning, documenting, computer modeling, static analysis and finally, physical modeling.  Each failure, as well as each success, was observed and analyzed in order to gain more knowledge of the module through blog documentation.

The Design Process

Throughout the course, the students learned what happens behind the scenes when a bridge is being designed.  When the class was first introduced to the topic of bridge designing, group seven had the obvious goals of designing and building the most efficient bridge possible.  This was determined by who had the lowest cost to weight ratio, or the bridge that could hold the most weight relative to its cost.  The instructor had many ideas as to how to help the students understand how to complete this goal.   

Before the class could start designing their own bridges with K’nex pieces, they were instructed to learn about bridge designs through their own research and to use a bridge designing application, West Point Bridge Designer 2012.  This program let the students design their own truss bridges from a lateral point of view.  The students had the option of using different types of materials as well as different lengths and thicknesses of these materials.  After the design was complete, the program then took the design and turned it into a 3D animation, demonstrating a truck driving over the bridge.  If the truck made it all the way across the bridge, the bridge was successful.  However, if the truck failed to clear the bridge, it meant that the bridge design failed and at least one member of the bridge had too much compression or tension acting on or against it.  The program also showed how much tension and compression was on each member and calculated the cost of the bridge based on the member size, length, slenderness, cross section, and material type.  West Point Bridge Designer 2012 really helped the students to learn which designs were most efficient and which designs dispersed the weight of the truck most evenly.  After playing and experimenting with WPBD, the group decided that a deck truss would work best.  This is when the truss sits atop the bridge.  This was picked because it was most common out of the group’s designs and the group felt it would be easiest to build out of K’nex pieces. 

The instructor also required the students to complete an exercise where they had to figure out the tension and compression on each member of their bridge.  This gave the group an estimate of how their bridge would fail and what changes they should make, if any.  Group seven decided that they should add more support pieces to their bridge and try to add some middle cross section pieces, rather than just connecting the two sides of the bridge together on the top and bottom.  At this point the bridge was composed of nine cubes lying side by side.  In the three middle cubes, there was an X shape along the two vertically standing edges of the box with a gusset plate at the middle of the X.  A K’nex piece was used to connect the X’s front to back to keep the bridge from twisting. This can be seen in Figure 1.

Figure 1. Bridge before Modification

This was the one problem that was noticed with the bridge design – the bridge would twist in opposite directions when weight was placed on top of it.  This was taken into careful consideration when the group redesigned their bridge.  Up to this point, this was the final design chosen for the three foot long bridge, but could only hold a maximum of thirteen pounds.  Only after testing the bridge which resulted in this outcome did the design change once more.  The bridge as designed was not very efficient and the group hoped to increase the amount of weight their bridge was able to support.   After the bridge was modified one more time, the group predicted that the bridge would be able to support seventeen pounds. 

The Final Design

Figure 2. Drawing of Final Bridge Design

After designing, testing, and analyzing the three-foot bridge in numerous ways, the design was ready to be finalized. The final design for the bridge consisted of nine squares, making up the length of the bridge.  The plan for the final design bridge can be seen above in Figure 2. Each square had an X through it. In other words, each square was composed of four right triangles, their right angles at the center of the square. To give the bridge width, enabling for vehicles to travel across, 3.375” chords were extended to and from the corners of each square, respectively, between the two side trusses of the bridge.

 

Figure 3. Final Bridge Design

Having learned that the weak joints were those composed of a chord clipped into the gusset plate and lying perpendicular to the gusset plate, all but eight of these joints were replaced. The old joints allowed sliding of the chords which were responsible for preventing twisting of the bridge, causing the bridge to twist under light loads. The new, stronger joints were composed of two interlocked three-hundred-sixty degree grooved gusset plates with chords snapped securely into their sockets for better support against twisting of the bridge. By analyzing the way the bridge twisted as the weight of its load was increased, the best set of joints to leave unchanged was determined. Because the price of each new joint was quadruple the price of each old joint, eight old joints were carefully chosen to remain intact, saving $24,000 from being added to the cost of the bridge.  The final design is pictured above in Figure 3.

Overall, the bridge was priced at $409,500 and was comprised of two-hundred-sixty-five parts, as calculated on the spreadsheet in Figure 4. During the competition, the bridge collapsed under the weight of 14.7 pounds after undergoing too much tension, as seen in the images below, Figures 5 and 6.

 

Figure 4. Bill of Truss Materials

Figures 5 and 6. Breaking Point

Final Results

The bridge was designed as a deck truss made up of equal size cubes in a line.  When weight was placed in the bucket, the bridge had a tendency to wiggle back and forth.  In other words, the top level of the bridge would move back and forth while the bottom stayed still.  This resulted in more and more twisting as the bucket of sand got heavier.  After the bridge reached its maximum load weight, 14.7 pounds, the bridge twisted so much that a member from the top level snapped out of the gusset plate that it was connected to.

The group tried to prevent this twisting from occurring.  After adding in the X shapes all along the bridge, the front and back pieces of the bridge were able to be connected in three layers top to bottom instead of just two.  The group thought this would support more weight because the bridge wouldn’t be able to twist as far if more pieces were holding it together.  However, this really did not help at all since the bridge repeatedly broke in the same place and only held half a pound more.  This was insignificant compared to the cost of all the members we had to add to create the X shape design.

Conclusion

          As engineers of all fields, the class gained valuable experience by undergoing the bridge design process. One did not need to be a major in civil engineering to learn that there is much testing, analyzing, and teamwork required for the final product of an engineering design process to be successful. Although the bridge designed by group seven was not successful compared to other bridges in the competition, it was successful in putting the group through a small-scale engineering project that required them to understand all of the major—and some of the minor—steps in any engineering design process. The strategies, plans, elevations, calculations, computer testing, physical modeling, and physical testing are all aspects of engineering projects that the group members will grow to be very familiar with, even as mechanical and electrical engineers. 

Future Work

Based on the final product of this project, one of the changes that could be applied in the future would be to change the way the K'nex grooved gusset plates are joined to each other.  During the design process, the group learned that grooved gusset plates tend to slip apart under tension.  Therefore, whatever pieces join the chords of the bridge together need to be very strong in order to successfully sustain the amount of weight and force distribution on the bridge.  Additionally more care should be given to the variety, magnitude, and location of the possible angles of the structures that form the bridge.  There might be new undiscovered advantages if there were more angles available for use in the bridge structure, but K’nex pieces limited which joint angles could be used.