Week 10

In the past week, we tested our bridge in class using the same method we used to test the 24" span bridge.  A bucket was hung from the bridge and sand was added until the bridge failed.  We predicted that our bridge would hold 35 pounds before it broke under pressure.  We were only 1.6 pounds off.  The actual weight it was able to support was 36.6 pounds.  We were pleased with the results, our improvements were successful but there were still more aspects that could be improved to lower the cost to weight ratio. Our major accomplishment was testing the bridge and seeing results that were more successful than our 24" span bridge.  What we focused on changing worked for our design.  There were no major issues that we encountered.  This week we are going to review the results of the survey that Professor Mitchell has conducted.  It will be interesting to see my peers reactions to the course.  I am expecting to see a positive reaction from most of the class.

This course was extremely beneficial to me as an engineering student.  I learned a sufficient amount in each area of the course goals.  I learned a great deal about teamwork throughout the entire engr 10X sequence.  Teamwork is something that we are going to use for the rest of our careers being in the engineering field.  It is extremely important to develop good teamwork skills early on.  Teamwork gives us the chance to see problems and solutions from different point of views.  Our minds all work differently and we come up with different ways to accomplish the same task.  This course did a very good job integrating planning, documenting, and the design processes.  We were forced to do this each week on our blogs.  It proved that this it is extremely important and beneficial to document the progress and steps in a process.  There are things that we did in the first few weeks of class that I would have forgotten about if they were not documented in our blog.  We mainly documented information after we did tests.  We could have improved our documentation method by taking more pictures and notes while were in the middle of the design and building process.  The design process was learned by working hands on.  We worked with computer software to make computer models of bridges to analyze.  Then we worked on physical modeling, testing, and forensic and static analysis.  Each of these stages is crucial to the design process and this course enforced that.  I learned something from each part of this course.  There was not a part that I felt was a waste of time and not beneficial in some way.  The least beneficial component for me was the method of joints.  It was interesting to learn and compute but I did not feel that it was really used in the course to help with analysis and prediction.  It would have been helpful to apply it more to make accurate predictions about the loads our bridge will hold.  The most beneficial part of this course was the split between individual work and teamwork and learning about the design process.  The split between the work was important and beneficial because it forced us to collaborate as a team but we each got the experience of competing each part of the work.  The design process is best taught by experience.  This was a main component of the class, I learned a lot about the entire process overall and it helped me to understand more clearly how actual engineering projects are carried out.  The only suggestion that I have to improve this course is to spend more time using analysis to improve designs and predict the outcomes.  Overall, I really enjoyed this course and benefited tremendously from it.

A4-Group 14

Background

Figure 1

The bridge pictured above was created by three freshman engineering students in order to gain hands on experience with truss design.  Using their knowledge of K’Nex pieces, West Point Bridge Designer software, and the method of joints truss analysis, students were asked to create a bridge to span 36 inches from one flat table top to another using only one support on each end.  The overall goal was to produce a bridge that was able to hold the most weight for the lowest cost.

Design Process 

The initial goal for this bridge was based off of the previous bridge which was designed to span only 24 inches.  Although it covered a shorter distance, the design used for the 24 inch span proved to be fairly sturdy and held 21.2 pounds before the point of failure.  Because the bridge for this assignment was to span 36 inches, a goal of 25 pounds was set based on two factors: the first being that the original bridge had definite room for improvement in terms of support and weight distribution and the second being that the second bridge would span further which is the only reason why the goal was not much higher.

The group determined that by adding more support to the plan, or top view, of the bridge that it would be less likely to succumb to torsion forces as did the first bridge.  After reinforcing the top and bottom connections, work was done to more efficiently distribute the weight.  The initial idea was to create a smaller “secondary” truss portion which was placed on top of the main portion of the truss.  After performing three load tests, the group decided that the secondary truss component was most effective on the under part of the bridge.  The three load tests consisted of testing the original design with the secondary upper truss, then again with no secondary truss, and finally with the secondary under truss.  The upper truss was the least successful in comparison to the other two as it had the highest cost to weight ratio.  It was only able to support 21.0 pounds, which is not only less weight than the initial 24 inch design could hold, but it came at a much higher price.  The bridge which utilized the upper truss had a cost of $490,500.  Similarly the bridge with the under truss also cost much more, but was able to support 33.6 pounds when first tested.  

After testing, the effect of adding more connections to a gusset plate in increasing the force needed for each member to pull out of the gusset plate was taken into account and minor changes were made.  These changes include the addition of the 1.125” white members added to the outermost gusset plates as these were the failure points during testing.  After the three load tests, the prediction of 25 pounds was reevaluated and raised to 35 pounds when the addition of the white pieces was considered.

Bridge Description

The design of the bridge consisted of a main truss and a smaller under truss.  From the results of prior tests, the under truss proved to be the most supportive design.  Figure 2.1 is an elevation drawing of the 36" span bridge.  Figure 2.2 is a plan drawing of the same bridge.  The three different colors represent the three levels.  The red plan view in figure 2.3 is the top level plan view.  This is the view if you removed the bottom two sections.  Figure 2.4, the green plan view is the middle section plan.  The blue plan view is the bottom plan drawing.  They are pictured together in figure 2.2 and separately in figures 2.3, 2.4, and 2.5 to show a more simple view.  The total cost was calculated using the bill of materials in figure 3. The total cost for this bridge is $494,500.  Figure 4 shows a photograph of the bridge built out of K'nex.  

Figure 2.1 - Elevation

Figure 2.2 - Plan

Figure 2.3 - Top Level Plan

Figure 2.4 - Middle Level Plan

Figure 2.5 - Bottom Level Plan

Figure 3 - Bill of Materials

Figure 4

Testing Results

This bridge held a total of 36.6 pounds during the final test.  This was slightly better than the previous prediction of 35 pounds.  Since the design focused on distributing the weight throughout the bridge, the bridge failed at several points as the stress increased. The most noticeable points of failure were the beams at the very ends of the bridge. The tension forces on the bridge caused those two diagonal beams at the ends to bend and snap off the gusset plate. There were also numerous other pieces that slipped off the gusset plates when the bridge failed. The bridge failed in a graceful and spread out manner, which was the intent of the design

Conclusion

After testing the bridge during Week 9, it was predicted that the bridge would be able to support a load of about 35 pounds and the failure would occur at one of the outermost gusset plates.  This prediction proved to be very close to reality as the bridge ended up supporting 36.6 pounds and failing gracefully at the third gusset plate from the end.  The failure most likely did not occur at the predicted gusset plate due to the increased number of pieces in each outermost gusset plate as well as the redistribution of weight.

Future Work

In designing a future bridge, more emphasis would be put in increasing the pull out force needed to pull members from their respective gusset plates.  The current bridge design functioned well in terms of weight distribution, but further testing could have proven beneficial and have allowed for further modification.  Using WPBD and the method of joints, efforts could be made to more efficiently distribute the loads.

Week 10-Cameron

After rereading the course goals, I feel that great strides were made throughout this course by myself as well as my group mates.   Teamwork is something that not just this course, but the ENGR 10X sequence as a whole, has brought to the forefront of our education here as Drexel engineering students.  Working as part of a team is an integral part of any engineering project as other group members may have a different vision than your own.  That difference of opinions does not in any way prove either one wrong, but it allows each opinion to be reworked to include the best elements of both.  This is how engineers collaborate. 

I also think that the planning stages rely heavily on the teamwork aspect as each group member had their own ideas for each of the projects.  This variety allowed for multiple initial plans, which then forced us to analyze each one specifically to decide which one was the "best."  Just as any type of design, our designs went through many different stages before reaching a point of finalization.  One thing that is mentioned in the course goals is documentation which I think we could have improved upon a little bit.  We seemed to focus too much on the after aspect of documentation, but should have been taking photos and recording our progress all along.  One thing that helped pick up our slack was the blog site that we created as we could use it to refer back to what we had done in the previous week and compare it to the point that we were currently at. 

In terms of the remaining course goals, I think things were learned in each area.  Before completing this course I had no idea what West Point Bridge Designer software was, but after using the program I definitely can see myself using it in the future either for fun or for actual modelling. 

In my opinion the only aspect of this course which I did not find beneficial was learning the method of joints.  Although it is an extremely important method of truss analysis, it is basic physics which I have been learning since my junior year of high school.  I do appreciate learning the method of joints, but I think that less time could be spent on it in this course and the same effect would be achieved.

The most beneficial aspect of this course was the hands on design process from start to finish.  I enjoyed designing a truss using computer aided design, implementing that design using K'nex, and finally testing a small-scale version of the design to determine its strengths and weaknesses, literally.  I find it hard to learn new things out of a textbook, which is why the hands on approach that this course took was very beneficial for me.  I honestly can not think of ways to improve this course, as it was one of the most rewarding courses I have taken during my freshman year.

Last week in class, we tested our designs using the same method as we did for the 24 inch span bridge.  Based on tests that we had conducted during week 8, we predicted that our bridge would be able hold 35 pounds which was only 1.6 pounds away from reality.  After the final test was completed our bridge held 36.6 pounds.  Overall we were happy with the results; our cost to weight ratio could have been lower, but considering the improvement we made over the previous design, we were satisfied with the results.

This week we look forward to going over the results of the survey that Dr. Mitchell had sent out earlier in the week.  It will be interesting to see everyone's feedback on the course.