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.

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

 

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.

Comp-1 = A1 Survey – Please complete this survey of your design results

Updated 4/17/2012 4:05PM to make it clear this is the Comp-1 assignment.

In the week-3 lab we’ll be reviewing the results of your A1 designs and then each team will develop an improved design in class.
To allow us to compare the designs and perhaps draw some inferences we’re asking you to complete a Google Docs survey that documents some aspects of bridge that you’ve designed, as well as the URL of your A1 blog post.
You’ll receive a very similar email asking you to do the same thing.  This post is just a readily accessible place to go to the survey.

Link to Survey - This is the Comp-1 survey

You’ll be able to see the results in spreadsheet form here.

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Bring your Bridge Design with you

As the survey notes, you’ll want to modify your bridge design in class so please bring your file with you:

• On a flash drive
• On a web site such as Google Docs or DropBox
• Email it to yourself