COMP2 - Final Results

See the following link for the comp 2 results: https://docs.google.com/spreadsheet/ccc?key=0Aq9AvRYJR07tdFpoRExuUlpGcVZMa2VONm8zZjdxemc

Group highlighted in Red had the lowest cost-to-strength ratio.

Group highlighted in orange had the smallest deviation between predicted and actual failure.

Comp-2 Corrections

Question 1

I believe there is an error in the COMP2 description where it says "When you have completed your initial WPBD design you'll submit the results to an online spreadsheet including....". I believe it should say Knex instead of WPBD, correct?

Response

You are correct.  I have corrected the page to reflect that.

 

Question 2

… only one person per group needs to turn in the COMP2 right? Or do all the members of the group have to complete/submit it?

Response

Yes.  Only one person per group needs to submit Comp-2

Refining Bridge Design

I've had two good questions about refining your bridge designs that may be of general interest.

Q1 - Why does an "X" type arrangement fail when using the Hopkins "Bridge Designer".

Response:

Probably the best answer is that you have created a "redundant" structure with an "X".  You can test that by removing one leg of the X and probably it will work as in the example above.  If you add the member from the lower left to the center of square so you have a complete "X" Bridge Designer won't calculate it because you have an unnecessary or "redundant" member.

Q2 - Additional Analysis

Throughout the project, I have been replicating the Knex bridge on WPBD as an extra load test. But thinking about what you said today after lab, I realize that's not the best way to test our truss. What at-home methods do you suggest would be best to see if the truss is stable?

Response:

Unfortunately you're running up against the limits of the tools you will understand in your first year.  There are indeed many tools for additional analysis and testing, but to use them effectively you need the knowledge that you'll gain in your more advanced courses.  WPBD and Bridge Designer have to make simplifying assumptions that your real bridges violate.  Let's hope this is your incentive to continue learning.

Units for Results

At 10:47pm the day before the assignment was due I was asked the following question.

“Is it ok if we converted pounds to newtons and feet to meters to do the calculations?”

Response

You may work internally in any units you wish, but results should always be shown in the units of the country in which you work unless told otherwise.  If I were working in almost anyplace other than the US then newtons and meters would be the appropriate units.  In our backward (in terms of units) country we still use feet, inches and pound-force and expect to see them in the results.

A3 - Part-5 Clarification

Question:

Did you say that only one group member needed to complete task 5 for Assignment 3? It asks us to use Bridge Designer to model our Knex Truss, and right now only one member of our group has the bridge with them.

Response:

You only have to perform the "Bridge Designer" analysis once in each group.  Each member, however, is to include the results of the analysis (part #1) and perform part #2 of Task 5 individually, though you're welcome to coordinate with your team members.

An Interesting Discussion

I had an exchange with a student who was unhappy about our policy of being timely with the submissions.  It led, I’m happy to say, to a more fruitful discussion about the differences between the WPBD software and the Knex experience.  Below is a portion of that exchange in chronological order with my latest response added via this blog

JEM

I did read your blog entry and appreciated your thoughtfulness about the difference between a Knex and a “real” bridge. I’d add to what you wrote that there’s a significant difference between WPBD and Knex in that WPBD is 2-D only (no torsional effects considered). In that system the gusset plates are perfect connectors as well – which is hardly the case with the Knex

Student

I would like to to thank you for taking the time to read and provide feedback regarding my worthless blog entry. While I did write that the differences are endless, (thankfully in this situation did not waste my time in further elaborating on these due to the fact that it is now resulting in a zero) I did like the specific portions you added and for the most part agree with them. I agree very much with your statement about the gusset plates in WPBD; they can be connected at any desired angle and are also never a point of failure, making them "perfect" connectors. While you are correct in saying that WPBD does not consider torsional effects, I would have to say the same goes for the K'Nex as well. The only reason that bridges in class were experiencing this twisting effect was due to the fact that the applied force was attached to the uppermost portion of the bridge. In WPBD the loads (dynamic in this case, unlike the static type used on the K'Nex bridges) are being applied at the same elevation as the bridges anchor points, not allowing for much torsion on the bridge. In the lab the bridges were tested by having the load applied from a point higher than that of the anchor points, causing the tall flimsy bridges to twist and fail. The use of cross members is another large portion of this, but for the sake of time and possible material for this weeks blog post I will end my thoughts here.

JEM – New Response

In any real bridge the loadings are always uneven.  The torsional effects are thus ones that must be considered and were important in your Knex designs, leading to failures in several cases.  In WPBD the single truck crossing a two-lane bridge would have asymmetrically loaded the bridge and thus torsional effects would have mattered, but were ignored by the software.

Week-6 - Bridge Testing - Lessons Learned

Your Results - You can see the spreadsheet here

Lessons About the Rules 

• The SPAN is 24" - NOT the length of your bridge
  
  • Your bridge must be longer than 24" to allow the supports at the end
  • The next bridge will require a SPAN of 36"
• The Width of your bridge matters
•  For your last bridge the width must be at least 3.5"
• Vehicles should be able to drive across the bridge
• For your last bridge there must be a "tube" at least 3" wide x 2" high - continuous

•  You can only contact the supports at one level at each end
• For the next bridge we will span between the tables, a plywood surface.
• There will not be the "V" to take advantage of from the sawhorses

• No one will be penalized this time for not meeting the requirements, except it must span 24" - there was no grade for this testing.  There will be for week-9

Lessons about Bridge Building - Your Observations

•  Most failures occurred at gusset plates
   
• There were unexpected failure modes - e.g. twisting 
• Long pieces passing through gusset pieces helped some groups