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.
I really enjoyed reading this post, and I must say that this student sounds like a great person to have in a group project like this. The points brought up are valid and I personally agree with their opinion on how torsion has an effect on the K’Nex bridge in comparison to WPBD. I think the student is saying that torsion only affects the K’Nex bridges because they are being tested in a much different manner than the West Point program. In my opinion, trying to make a comparison between these two bridges is pointless in the sense that the only thing similar about them is that they are truss designed bridges. One of them is a full scale, two lane passenger vehicle bridge designed and built using a free online computer program that tests the bridge by driving a truck across it (dynamic load), while the other bridge spans just two feet, is built and designed by hand using plastic K’Nex pieces, and is ultimately tested by seeing how much weight (dead load) can be dangled from the center of it. They are scaled differently, built differently with different materials, and are tested under two completely different circumstances. To then try and further compare either of these designs to that of a real bridge is just a waste of time; a real bridge is designed to be perfect in every aspect to ensure the safety and lives of anyone on or around it. A real bridge is designed for a specific purpose around codes, regulations, environments, load requirements, budgets, and readily available materials just to name a few parameters. Overall I think this student has done an outstanding job of thinking outside of the box and looking at this course from a broader perspective, and for that should be awarded the credit that he deserves for his blog post.
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