Term Review

Experiencing the design of large bridges was a very interesting process to follow for the Spring Term; especially for student engineers with few experience in industrial related creative-release-opportunities such as the project for past module.  While designing large bridges, the entire class learned about angles, pressure, and the possible structural benefits as well as possible disadvantages.  I personally became very involved in how the pressure from the superior force pressed by motion and weight was distributed differently through out the parts of the respective bridge structures and the different designated joints and pieces, bridge sections, and their evolution through out their improvement upon changes chosen by the group members.  Hopefully it all serves me more towards the future and its upcoming engineering opportunities.

- Ana M. Franco

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. 

Term Review

This term has been full of new experiences and has taught me a lot.  I had never before realized what it was that real engineers do in the real world.  I can also honestly say that the goals posted for this term were met and I have learned something about each of them.  This was a team project so teamwork was an integral part of being successful and getting along as a group.  Planning and documenting were needed when we had to design our own bridges, plan what worked best, and then document our final results with each test.  Modeling was a huge part of this quarter.  We modeled on computer programs and using physical pieces to model our ideas.  I learned new computer programs that I had never even before heard of and it has been years since I have played with K’nex.  Finally, we used analytical methods to determine the weaknesses of our bridge designs and to help us improve our designs.  

I really liked how this class was run.   However, it was not connected to the Engineering 103 lectures that we had to attend every week.  Instead of changing the course to fit the lecture and layout of every other class, I think the engineering lectures should be more generalized.  Maybe they could talk about companies in the area or what employers are looking for or something else besides the syllabus for the rest of the Engineering 103 labs.  I felt that this class should not be changed.  We didn’t need to write a design proposal or do half of what the other labs did, but I felt that it was an appropriate level of work for this class.  I also liked how we were responsible for our own blog posts each week.  I have been in groups where people don’t always pull their own weight and it is nice that my work is reflected in my grade, and no one else is riding off of the work I am doing.  It is a big change from Engineering 101 and 102 and I really welcomed the chance to do the work for me and only me.  I also really appreciated the fact that we did not have to do presentations during week 10.  I felt that since everyone was doing the same thing, it would have been really redundant.  Also, without having to prepare for a presentation, we could focus more on our designs and documenting our results.  The only thing I think I would change if doing this module next year is to have some more to do during class time.  A lot of the time my group would be done early in class and we wouldn’t know what else to do until we were dismissed.  Also, at some points we needed more K’nex pieces.  Otherwise, I would absolutely advise any incoming Engineering 103 student to take this class. 

Last week in lab we finally got to test our final design of our bridge.  We were really excited because we made a lot of changes to it and really thought it was going to do well.  However, we weren’t all that surprised when it failed relatively early.  We knew our bridge was going to fail from twisting, and it did so after only adding 14.7 pounds of sand.  This is a low number compared to most other groups, but we know that we did the best we could and we tried our hardest.  However, I am happy that there was at least one group who had a worse cost to load ratio; it makes me feel a little bit better about our efforts.

Term Review

Last week in class, our group tested our 3' bridge design. Before doing so, we'd submitted a survey about the bridge to our professor who put the info provided by the survey into an Excel sheet. Using the Excel sheet, the entire class was able to compare the cost and strength of each bridge, side-by-side. As a class, we decided to test the bridges in order of highest to lowest predicted cost-to-strength ratio. Which group went first? None other than ours. Even worse than the 17 pounds we'd predicted our bridge would hold, it only supported 14.7 pounds in the final test. To make things worse, this meant our cost-to-strength ratio was even  greater than we'd estimated.

Now that the final competition is over, I can look back at all the steps we'd taken throughout the course and honestly say that I learned something worthwhile about every single one of the topics identified in the goals for the course. Working with a group required teamwork. Planning, documenting, computer modeling and physical modeling were all part of the design process which our group followed throughout the course. We used some computer software I'd never used before, and we used some computer software that I've already been familiar with for years. Lastly, we used static and forensic analytical methods to learn more about the weaknesses of our bridge designs.

I think the only thing that was not beneficial to me was the fact that this Engineering 103 lab was disconnected from the Engineering 103 lecture section of the overall course. If the bridge lab could be formatted to follow the general pattern of the assignments for all the other labs, I think I would feel like the Engineering 103 lecture section of the course was more important to me, since it is something I am required to attend. Basically, I just felt that attending the lecture session wasn't very important because I was told to ignore what they told everyone about assignments being due and ways to earn extra credit. On the other hand, I feel that having individual blog posts benefitted me alot. I think this is something that every lab should have. Although teamwork is important--especially as engineers--we are not paid to do the work for our classes, which means some students simply do not care as much as other students. Some students know that other students will do the work for them so that their own grade is not negatively affected by the actions of a lazy student. I've experienced this in Engineering 101 and 102, and the students who did not do all of their work did not have anything held against them. In the real world, they could potential be fired from their job, but in class, they won't even receive an F because someone else will end up doing the group assignment this student was responsible for. Instead, individual assignments reflect the work of each student, even if the assignments show similar results because of two or three students being in a group together.

- Melissa Wetzel

Bridge Process

Now that the term is almost over, I can take the time to reflect on what I have learned.  Before I had taken this class, I didn’t even know what a truss was, let alone that it was a type of bridge.  I was new to this whole topic and was forced to learn a lot in order to succeed in this class.  I thought it was really interesting that bridge designing was such an intense process.  It really has a lot of little details that need to be perfected in order to have a successful bridge.  I had no idea that so much happened behind the scenes during the planning stages of a bridge.  It really is a complicated process.  You need to consider what type of materials to use, how big/long the bridge will be, what type of gusset plates to use, the angles of the trusses, the thickness of each member.  The list goes on and on.  I am just so amazed that all of this goes on.  It has really opened my eyes up to what engineers have to do when designing a real life project. 

Last week in class my group tested our three foot design.  Unfortunately, we weren’t doing very well.  We changed our design a few times and added a lot of gusset plates and more members.  We knew we were raising the cost of our bridge, but it didn’t hold a lot of weight at all.  Our bridge kept failing due to the twisting of the members at the joints.  We tried putting in cross sections in the middle of the bridge to reduce this twisting, but we didn’t get to test it again so we weren’t sure how it was going to work. We hope that this has made our bridge stronger, especially since it is now more expensive.  Next week we hope our prediction of a low weight is wrong.  

Bridge Process

During class last week, my group and I tested a three-foot bridge we'd designed over the course of the previous week. This bridge held only 14.2 pounds and its point of failure was caused by weak joints for the cross beams. Under 14.2 pounds, the bridge twisted and its members popped apart. To fix this, we replaced the weak connections with stronger ones. The original connections consisted of a Knex chord laying in the gusset plate joint. These connections were weak because the chords could slide freely through the plates, although being held against them. The new connections were created with two grooved 360-degree gusset plates joining the chords' ends like two puzzle pieces interlocking. This type of connection does not allow for movement and helps minimize the bridge's horizontal displacement. Next week we will test out three-foot bridge and compare our results to the rest of the groups' results to see which group has the best overall bridge.


Now that my group and I have almost completed the bridge design process for the term, I am realizing how much I have learned, specifically about bridge design. In class yesterday, Dr. Mitchell put three rough bridge analyses on the board. All were the same width and comprised of three triangles in the same order, however each was a different height. According to the analyses, a bridge whose height is the taller, or whose triangles are the least obtuse, will have the least amount of compression and tension on its members under a given load. Another important key to designing bridges that I've learned is that joints tend to be the weak spots. Before taking this class, I always thought that chords were the "weakest links" in bridges, but it is truly the "links" which are weaker. In testing the Knex, I have read information about which joints can handle the greatest stress force before failing, but we have never analyzed the force it would take to snap a chord in half. This is because of weak joints.

A3 - WETZEL

Truss Overview:

Free Body Diagram & Calculations:

Replication of Analysis in Bridge Designer:

In order to make the results of the hand analysis correspond to the online Bridge Designer program, some simple scaling was used. While using Bridge Designer, I mentally set each grid square to be 3"x3". Because my bridge constraints set the bridge length to be 36", that meant 18 squares on the grid would correspond to the length of my hand-drawn bridge. The height, needing to be 10", I scaled at about 3.3 squares on the Bridge Designer grid. This enabled me to use Bridge Designer to see if my own calculations were right about this bridge by creating angles similar to the ones I used. As long as the triangles composing my bridges corresponded, the tension and compression forces would also correspond. I was happy to see that the results calculated by Bridge Designer were very closely related to the results I'd calculated myself.


Knex Bridge Designer Analysis:

Just as I've done for the hand and computer analyses of the small bridge, I've now designed the Knex bridge using the Bridge Designer program. In order to scale the draft's calculations to the actual bridge, I will need to measure the actual bridge and set a value for each square on the grid in the program. After we test the bridge and see the weight it fails under, I will be able to scale the real load with the one I've used in the designer to determine the stress on the Knex bridge when it is holding a specific load.

Given the testing information about Knex joints, I might use this analysis as a guideline for the Knex bridge. I can say that, theoretically--and based on the computations of the Bridge Designer--the bridge will be able to hold x amount of weight when there are y joints. But if I optimize the strength of the joints by having more chords meet in a single joint, the bridge should be able to hold more than the program suggests. The testing information about Knex shows that a joint is stronger when more member meet at it. At the same time, the Bridge Designer's unique formula "members + 3 = 2*nodes" leads users to do the same thing: have multiple members meet at a single node. By following this pattern, my group and I should be able to increase the strength of our bridge enough for the cost to strength ratio to benefit from the modifications.