Elevated Bridge B/C

[sewforlife]

Our coaches are just insane. that's why. even now that the official events table has come out, we're stuck testing bridges with three load points. ugh.

[blue cobra]

Question: Where do people get wood as tiny as 1/32 and 1/64? I'm partial to specializedbalsa.com, but the smallest they offer is 1/32 by 1/8. Does anyone know of a different source from which to get those really tiny pieces?

Have you checked all hobby shops? My local Hobby Town sells balsa sheets down to 1/32 and bass sticks and sheets down to 1/32. Nothing at 1/64, however.

[JimY]

nejaminb, there are several reasons to include an inverted triangle under the load point. First, it is called for in the structural design. That is, if you don't have a series of interconnected triangles, you can no longer calculate the stresses on the pieces. Without it, you have a trapezoid in the middle. Second, when you calculate the stresses, they turn out to be zero on the legs of the V. Does this mean that you don't need them? NO! Again, they are needed to calculate the stresses in the first place. The zero stress just means that you can use small pieces. Third, you never get perfect load distribution on your load point, no matter what. Therefore, in reality, one leg of the V will always be slightly in compression and the other slightly in tension. This will help to even out the uneven load from one side of the truss to the other. Fourth, with zero calculated load on the legs of the V, they don't put a downward load on the horizontal tension member where the V is attached. That member is only pulled on from its ends.

You should probably spend some time on the following website to see some of this for yourself.

http://www.jhu.edu/~virtlab/bridge/truss.htm

The structural design part is so basic to building a great bridge. Unfortunately, I think that most students don't understand it and therefore dismiss it as not needed. Yes, you can build miniature bridges without knowing anything about structural design. They will, however, be less efficient than what they could be if they are properly designed.

[nejanimb]

JimY,

Hm. Thanks for the explanation. I did try putting something like that into that bridge designer, and did see that the stress there is zero. Although I still don't understand why that is, I'm not going to disagree with the program. Given that, I do understand your reasons for having it.

Our design from this year never worked in those programs. It isn't a standard triangles-truss design, so the program doesn't accept it. Personally, I think that it's possible to be successful without doing any math or stress calculation. I think having an understanding of how forces work and how to utilize compression and tension in a design is important, but I think that sticking to a standard truss leaves too many restrictions, and doesn't allow for some of the best techniques for scioly structures. For example, you mentioned the trapezoid. While apparently that stress calculator (And perhaps all, I don't really know how it works) doesn't approve of a trapezoid existing in the truss, I think the trapezoid is a critical facet of this type of a scioly bridge when you have to accommodate a load that doesn't exist at a single point.

This year, at least in C division, I know that none of the top 4 bridges would've stood up to that stress-calculator, but all (obviously) were very successful designs. All 4 (I say 4 only because I actually saw all of the top 4 - I don't know about the rest of the top bridges) involved a trapezoid to support the loading block, and all 4 had bracing that I am sure their builders couldn't figure out individual stress values for. While I may be one of those people who dismisses it because I don't fully understand it, I don't think that the only way to be successful in this event is to be a structural engineer. If you have a decent concept of what's going on with the structure and can figure out how to make it work in wood, and are willing to put forth a lot of time testing and trying different things, then I think that can work too.

[rjm]

Designing by blind trial and error is weak, time consuming, and uninstructive. I'd recommend doing as Jim suggests, testing your work against the jhu applet or a similar static analysis program and making adjustments.

Something that the jhu program does NOT do, is to show you the deflection of the structural members under a load, and the changes in both the geometry of the truss and the stresses in each member as the lengths and angles between nodes change. That's a limitation of any static analysis. You could probably find a finite element analysis program for that, but the variability of the strength and elasticity of wood reduce the value of such analysis. That's where the trial and error method shines: you start with a sound design from statics, learn how to read the wood, learn how to build with good technique, and observe how the trial structures behave under load. You end up with a good, intuitive understanding of structure.

For what it's worth, the jhu program and most other static truss analysis programs, including manual calculations, assume that the nodes are perfectly hinged, that is, the joints would freely pivot unless the truss is triangulated. Without the triangles, the truss should fold up like a cardboard box. The analysis relies on that concept and assumes that ALL forces in the truss members are aligned with the length of the member, no matter what angle thay are at in the truss. When you leave out part of the triangulation, as with trapezoid shapes, the analysis method breaks down. In the real world of sticks and glue, it means that the joints have to resist bending and pivoting and that the members will be subject to bending stresses also, which will almost always mean the members have to be thicker and heavier. Additionally, even though the analysis indicates that the "inverted V" in the center of the truss carries no load, it will carry load as the truss deflects and the geometry changes. You can usually see the struts buckle as loading progresses.

BTW, when you set up a truss on the jhu grid, be sure to define nodes everywhere that lines cross, and be sure that your members stop and start at each node without passing through.

Bob Monetza
Grand Haven, MI

[dragonfly]

Designing by blind trial and error is weak, time consuming, and uninstructive. I'd recommend doing as Jim suggests, testing your work against the jhu applet or a similar static analysis program and making adjustments.

Hm....I have to say, those are three very strong adjectives that I, for the most part, disagree with. While initially trial and error may be a bit time consuming, 'blind' is not the correct way of describing how we build. Simply building a ton of bridges won't get you anywhere, but building a lot of bridges to test intriguing ideas is the only true way to achieve the answer you're looking for. I believe that Nejanimb was trying to say that while programs, of any sort for that matter, are indeed useful in some simpler cases, if you understand the general idea of how the forces would work in a design whose loads are distributed more complexly, that the best way to test your theory is to take a couple hours and get it over with That way, you will end up with a straight up answer by the remains (or lack thereof) of your bridge!

[smartkid222]

basicly, i've built tons of bridges, towers, etc., never used a program or calculated forces but was still good enough to medal. Obviously i see that this is not the correct method but it still works to an extent; i think that this is what they were trying to say. As a result of all this talk of always with stresses and that program keep coming up, this is one thing i hope to change this year. I always built stuff, but never really learned the physics/math behind it.

[Aia]

I don't see any faults with trial and error as long as you test new ideas methodically and carefully. While the ability to calculate forces on the bridge would be valuable, it's a very difficult proposition for most teams. I looked at several websites last year that would allow you to build virtual trusses, and most were fairly limited or only calculated forces for basic trusses. After I couldn't find any websites to help, I asked several physics teachers to help me calculate forces. They were unable to calculate the forces, and interpreted my current bridge in same way I did by looking at which pieces were in tension and compression, how the load transferred, etc. Simply put, calculating truss forces isn't feasible for most teams, especially if one considers the limitations of these truss programs and the countless variations in design this year. Unless you are a physics wizard or have a helpful contact, calculating these forces are exceptionally difficult.

I would also like to point out the largest virtue of trial and error. From personal experience, I know that every single tower, boomilever, and bridge I've built has taught me something new, whether it was knowledge about that particular design or an improvement in building technique. Even as I built bridges this year, I recalled knowledge from past towers or boomilevers that I tested, and was able to synthesize my research to build better structures.

[blue cobra]

I think programs such as the JHU one are most useful for general ideas. For example, If you were thinking of including a certain type of truss in your bridge, you could draw that and see a basic idea of which members would be in compression and which ones would be in tension, as well as how much relative to other peices. You could decide which members probably need more or less strength, and what kind. A lot of this can be instantly deduced by an experienced bridge-builder, but having the numbers would surely help. Being able to calculate the forces wold be fantastic, but, especially for students, learning how to do those calculations could be more difficult and time consuming than building numerous bridges. Plus, you get building experience and a definitive answer. Calculating stresses are certainly best from an engineering standpoint, but when you test your design in real wood in a few hours, people prefer to do that.

Anywho... lap joints are stronger than butt joints. I don't quite understand exactly why they are much stronger. It just seems a force on one of the pieces in a lap joint would pull the joint apart, while in a butt joint a force applied to a member on top would push against the other member and not pull the joint apart.

[nejanimb]

Truly "blind" trial and error would definitely be time-consuming, but I don't think uninstructive. On the contrary, like Dragonfly said, I think to put a design in wood and get it through its paces on the blocks is the only way to really see what's going on. I agree with Aia - we built close to 40 bridges this season as a team, and we learned something from every single one, all the way down to the bridges built in the week between states and nationals.

Since we have a number of people building bridges, what we did to start the season was to let everyone try whatever they wanted based on what they knew from past experience and their understanding of the physics involved. We'd all get together to watch tests, give critiques, and talk about what we could learn from the bridge and how it loaded. It took us a couple months, but eventually we hit upon a good design. Then, we spent the rest of the year perfecting it. We tried changing one variable at a time - one person would try using a different cross-section for the main compressions while another person would try changing the angle of the legs while another person would try tapering the sides together, etc. We'd regroup, watch the tests, talk about what we found, and then move on to a new round. One person would try .15g for the compressions while another tried .21. Rinse and repeat.

The point of this description: we tried consulting the truss-bulider program, but it couldn't actually teach us how to build a successful bridge. It might show some simple versions of sturdy triangle arrangements, and it can show relative loads... but how can you make that work in balsa wood? In the end, building 30+ bridges taught us a TON during the course of this past season - what works, what doesn't, and I know more for it. Not only do I know more about that each particular design, but I learned things about engineering in general and how structures and forces work. I was able to see in a very tactile, real way laid out in front of me in splintered balsa fragments what I should do next time and how we could improve. This was infinitely more real and useful to me than a numbers readout from a program that really doesn't teach me anything about how to make the next bridge better than the one we just broke. Without trial and error, how can you improve? Build one bridge, and then another, but learn nothing in between from the trial and error process? That seems insane to me. It's an absolutely essential part of the design process, to my way of thinking, and one that cannot be replaced by working with fancy spreadsheets or stress-analysis programs. For us high schoolers, this is critical to learning about how things work. It's building something, trying it out, and improving our design and methods that's really instructional - not just simply heeding the advice of a structural engineer we know or a statics program we couldn't emulate ourselves or a spreadsheet that we didn't write ourselves. To me, that would be far more "blind" than being methodical with trial and error, like we were.

Seems to work too: like I said before, none of the top bridges (the C bridges, at least) would have held up to that JHU app, but clearly they were successful nonetheless. Our bridge sits proudly atop my desk, and I'm happy when I look at it and see each individual piece of it, and I can remember the bridge that we built last season that taught me the lesson that resulted in it being exactly how it is now. I see "oh yeah, that gusset was added after it broke such a way" and "oh we moved that there after we almost hit minimum specs" and "we added that extra piece after it kept breaking at the top" and "we decided on .19g for that part since the ones heavier held everything and the ones lighter broke early" and so and and so forth. The final product was absolutely a result of a full year's hard work, with things learned at every step of the way through rigorous trial and error. While I may not fully understand static analysis, I don't think that means I know nothing about structural engineering, nor do I think that my method was weak. Above all though, I know it was as far as you get from uninstructive.

Aside from that rant, rjm, the clarification about how that statics program doesn't show deflection makes a lot of sense to me. I knew that was the case, but I didn't put it all together to come to that conclusion properly - thanks. That makes my misunderstanding of the inverted V and the rest of the issues much more clear - I get that all now. Thanks!