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

 

Calculating Triangle Angles and Trigonometric Values

To analyze a truss there is often a good deal of trigonometry involved to calculate the components of forces.  To make things simpler (I hope), I’ve created a simple excel spreadsheet that will allow you to calculate angles, sines, cosines, and tangents if you know the length of the legs of any triangle.  This should ease the calculation burden somewhat.  You might well wish to use this sheet as a starting point to make your truss calculations even readily modifiable.

Here’s the link to the spreadsheet.

Download the spreadsheet to Excel to see the diagram that explains the notation.

Week-6 Bridge Results Recording

Before you leave the lab in Week-6 we’d like you to record the results of your bridge test along with the characteristics of your bridge.  We’ve set up the linked Google Docs Form to submit it.

We’ll show the results in class or summarize them in the blog.

Note that this form is very much like the one you’ll be asked to complete for week-9 to PREDICT the results for your final bridge.

Late submissions to Bb Vista

Over the past few weeks Dr. Mitchell, Deepak, and I have noticed a large portion of our inboxes dedicated to students either forgetting to submit their course assignments on Bb Vista, submitting the wrong links, or something related. We have decided that we will no longer allow for corrections to submissions or submissions of assignments, blog posts, and etc past the due date times as set forth on Bb Vista.

This will go into affect officially next week. To help avoid problems in your assignment submissions, take note of the following:

1. Stay Up to Date:There is something due every week in Bb Vista for our course. The Bb Vista assignments don't show up however until a week before they are due. You should be checking the course Bb Vista site every week to stay updated on everything.
   
2. Check your Hyperlinks: Make sure you check your hyperlinks before you submit them. A lot of you end up submitting links to the blog post editor or to the blog post preview. These will not work for anyone except for you. Try following the hyperlinks in a different browser, or even following them after signing out of your Google account.
   
3. Resubmissions Before Due Date: If you submit early and notice a mistake, you can still take your assignment, modify, and resubmit. This will only work however if it is done before the assignment due date. I've set it up in Bb Vista so that every single one of your assignments is due on a Wednesday at 9AM. The only variable is the week in which each assignment is due.

If you have trouble submitting or if Bb Vista is acting strangely, feel free to email anyone of us before the due date and we will gladly assist.

Bridge over CSX Tracks–Photos

You can see the photos I took this morning when the bridge was put in place over the CSX tracks.

Picasaweb Photos

Truss Bridge Installation Wednesday Night and Thursday AM May 2nd and 3rd

Here’s a news release from the Schuylkill River Parks Alliance that may be of considerable interest to this class.  It’s tonight and tomorrow morning!

The delivery of the truss portion of the bicycle-pedestrian bridge that will span the CSX tracks between Locust and Spruce streets is supposed to be delivered overnight Wednesday, May 2nd into the early morning of Thursday, May 3rd. The Philadelphia Streets Department will use the Swift Reach  telephone system to notify residents along the route. The contents of the message is below in italics.

Rockport, the contractor, expects that it will take approximately one hour to assemble the truss Thursday morning, then it is going to set the bridge into place between 8:30 AM and 9:00 AM.

The telephone Swift Reach message from the Streets Department:
Philadelphia Departments of Streets and Parks & Recreation is pleased to announce the delivery of the new Truss Bridge that will soon connect the Schuylkill River Park between Spruce and Pine Streets to the Schuylkill River Trail. Truss bridge is scheduled to arrive at the project site on late Wednesday night into early Thursday morning. Bridge will be escorted to the project site by Philadelphia Truck Enforcement unit and transported on two (2) oversized hauling trucks. In order to safely make the move, temporary parking restrictions will be posted along the trucking route, and will be effective from Wednesday, May 2nd at 10pm through Thursday, May 3rd at 7am. No parking signs will be posted at the intersections of Washington Avenue and 22nd Streets and 22nd and Locust Streets, as well as along Locust Street between 22nd and 25th Streets.

Truss Bill of Materials Typo Correction

Question:

I think there is an error in the excel spreadsheet that calculates the bridge cost. The member length after 1.25 should be 2.125 instead of 1.125 correct?

Response:

Indeed there is a typo in the Truss Bill of Materials.  Part #2 should be 2.125” in length rather than the 1.125” indicated in the description.

I corrected it on the server at about 7:15 on 5/1/2012

The cost is still correct.