Week 9 term review

In our week 9 lab we tested our 3' bridge. It ended up holding 35 pounds before one of the bottom members seperated from the a gusset plate. We are content with this weight as it seems that we were pretty average with the rest of the class. In the next week we will all work to complete our A4 final report, which we will present in week 10. Overall I believe that our group did well in this class. Our 2' bridge held the most weight, and our 3' bridge did satisfactory. I feel that I have learned a lot about bridge design and the type of work that goes into designing a good real-life bridge. I have learned that the west point bridge designer program does a good job at designing a good bridge structure, but it lacks the ability to take into consideration the lateral forces that a real bridge will encounter. Knex were a choice for the materials for this project because they are similar to real bridge pieces but they lack the freedom to attach members at any requested angle. If I could add something to this course, it would be more opportunities to break bridge designs. I say this because it is the best way to learn what exactly is failing in the design, making it easier to improve the desing.

Week 9: Term Review

This past week the group tested the finalized 3' bridge in class. It held a total of 35.2lbs and costed $418,000.  The cost/load ratio turned out to be 11875 $/lb.  I believe this is probably an average ratio.  The group will be working on the A4 report to turn in for next class and it is coming along well.

I believe the group met the goals for this course and in the process learned a lot about bridge design and truss analysis.  Not only did the group learn about these things but they also learned about the importance of efficiency in engineering especially when dealing with the cost of a project.  I think the most beneficial part of this course was the experiment and calculations the group performed.  It really helped us get a hands on experience in designing and revising a plan for problem.  The least beneficial to me was the lack of things to do in the lab at times.  This was also a problem for the group because we would rather get out of the classroom so it was not necessarily due to the lack of work.  For future classes it would be important to make sure all designs make the specifications exactly as they are laid out.  It would also make things a lot more interesting to have the best groups make larger models of their bridges, like a 5' bridge, to see how the strength decreases for each bridge and how the cost/load ratios are affected.

A4- Fitzpatrick, Miller, Parker

1. Background 
   The course was set up in way that was conducive to learning, There were not many restraints put on the students and there were clear checkpoint tasks that were used to gauge our progress and pace the course. This was geared toward the civil engineering majors giving them a taste in bridge design and insight into some of the tasks they might face in the work force, in addition it provided a basic introduction to concepts that civil engineers are applying to designs on a daily basis. There are various types of bridges that are used around the world; most of which can be seen in the greater Philadelphia area however in this course we only focused on truss bridges Through various tasks throughout the term we were able to exceed the course goals. We were introduced to the design process in which there are many ways of solving a problem; we were given the task of finding the best solution for the problem given a set of constraints.
2. Design Process
   
   1. What goals you set at the beginning
   1. In the beginning of this project we wished to design a bridge that would adequately satisfy all of the necessary contraints and perform well enough to be considered a "safe" bridge  
   2. How they changed during the project
   1. Though we learned different key things about bridge design while constructing our bridges, our goals stayed the same. 
   3. The role of individual projects with WPBD, Truss Analysis, Individual Knex Bridge Designs 
   1. WPBD
   1. West Point Bridge Designer was used as our introduction and crash course in bridge design. Many aspects of this program are realistic while there are still parts of the program that are not so realistic; The blog post: Week 3: WPBD goes further in depth as to the strengths and weaknesses of WPBD. In working with WPBD one of the keys to success in lowering the cost of the bridge was getting the tension and compression forces in each member as close to 1 with out going over 1 as they could be. While we all had very similar designs the best design was Miller's. In the end the designs by Parker and Fitzpatrick were fairly close with all of the forces maximized. The goal of WPBD was to create a bridge with the lowest cost, when all of the bridges of the class were lined up our three bridges were the top three in the class.
   2. Individual Knex Bridge
   1. The individual knex bridge provided the stepping stones used to build up to the 36" span. For individual designs the constraints are as indicated in the link however the main concern for our designs was the 2' span. Each member of the group came up with a different design based upon the same general shape, this is one iteration: A2-Fitzpatrick . Each member of the group followed the same general rectangular frame with a different web design.
   3. Truss Analysis
   1. Truss analysis is used in order to determine the tension and compression forces acting upon the different members in the bridge. As can be seen in A3-Parker the truss analysis that we have completed closely relates to the truss analysis provided when using bridge designer for a simple truss bridge. When using truss analysis and bridge designer for a more complicated truss design such as our knex design the analysis was more complicated and it became more and more difficult to interpret the results as the complexity of the truss increased.
   4. How the final design was chosen 
   1. The final design was chosen based upon the results of our bridge from the individual bridge design . This design was a major part of our finished design which held the most in the class with 49.2 pounds. Moving up to the three foot bridge we did expect a decrease in the load that the bridge could handle due to a longer span without supports and the requirement for a 2" x 3" opening through the bridge which eliminated the cross bracing.
   5. How it was modified during construction - if at all 
   1. Modifications during construction include the addition of cross members on the top and the bottom of the bridge to help prevent the bridge from racking and to spread the load more evenly across the entire span.
   6. Predicted load at failure 
   1. 30 pounds, this prediction was based upon testing designs and modifying them along with the use of bridge designer software to show where the point of failure would most likely occur.

3. Description of Your Final Bridge

1. Plan View

1. Elevation View
2. Bill of Materials
3. 
4. Cost 


1. The cost of the bridge as calculated in the Bill of Materials above was $418,000.00
5. Photographs of the bridge

4. Testing Results

1. Load at failure
1. The bridge failed with a 35.2 pound load.
2. Describe the failure mode of the bridge 
1. The bridge failed at one of the lower gusset plates, the knex bars pulled out under a load of 35.2 pounds.
   

5. Conclusions About Your Bridge Design

1. Did it behave as predicted in terms of load and failure mode
1. Our bridge was able to hold more weight than we had expected, The design was expected to hold 30 pounds and during the testing our design held 35.2 pounds exceeding our expectations by 5.2 pounds. The mode of failure was separation of the gusset plate on the lower edge of where the load was applied. The location of the failure occurred in the location that we were expecting failure. It failed graciously, it was not an explosive failure it was a failure that would allow vehicular traffic to safely vacate the bridge.

6. Future Work

1. How would you modify your bridge if you were to design another version 
1.  If we were to modify the bridge to design another version with the same goal of lowest cost to weight ratio in mind then the bridge would take a completely new form with added internal cross supports to help spread the load.

 

Failure of the 24" Bridge Span

 

Failure of the 36" Bridge Span