Elevated Bridge B/C

[nejanimb]

We all work together. I'm not sure exactly what you mean, but in PA, every school only gets one team, and our Olympiad team is fully a cooperative unit. For bridges, deciding which one is going to go to competition is really easy - highest score wins. Am I misinterpreting the question?

[cypressfalls Robert]

...highest score wins...

Who gets the medal (if recieved)?

[hbk.showstopper]

...highest score wins...

Who gets the medal (if recieved)?

I thought they each got a medal (not trophy) but like a medal that goes around you're neck......or do you mean like besides the people on the team itself????

[cypressfalls Robert]

besides the poeople on the team....the people that help

[hbk.showstopper]

oh I see.......so what's the answer nejanimb???

[Greg Doe]

nejanimb
You wanted to know if anyone had built successful bridges with slopped sides. I coached two different B division schools that built similar bridges. The regional bridges both weighed 15 grams, and held the 15 kg. Their efforts earned them first and second, but it was necessary to go to the second tie breaker to determine the winner. Those bridges sides slopped inward 4 degrees on each side. For the state we redesigned the bridge down to 10.7 grams. We only tested them to an efficiency of 1000 (11 kg.) because they didn't have time to build another bridge. Those bridges sides slopped 2 degrees, and took first and second at state. Both bridges failed the same way. An examination showed that the omission of a 10th gram reinforcement that we had used on the regionals bridges caused the failure. Those bridges again took first and second, and the winning bridge had an efficiency in the 1400's. We agreeded that if we had advance to the third design we were going to try a 1 degree slope.
In the beginning they tried parrallel sides, but had problems with rollovers.
Greg Doe
Smyrna, TN

[rocketchicka]

I don't have a partner for bridge. I just build it myself. I think that it wouldn't be good to have 2 people building the same bridge because 1. The bridge won't be even on both sides because different people cut differently, use different amount of glues, etc. 2. You would have to meet to finish building a bridge, which in my opinion is a waste of time and 3. While you build one bridge your partner could be building another bridge that you don't have time to build.
That's why I prefer to work alone. Then for competition, my coach just throws someone on the event with me to fill up the empty space.

I agree with you here. my partner and I tried to do it together and it ended up being lopsided. now I work on it and she just gives a helping hand if I need it. it works better that way.

[croman74]

I don't have a partner for bridge. I just build it myself. I think that it wouldn't be good to have 2 people building the same bridge because 1. The bridge won't be even on both sides because different people cut differently, use different amount of glues, etc. 2. You would have to meet to finish building a bridge, which in my opinion is a waste of time and 3. While you build one bridge your partner could be building another bridge that you don't have time to build.
That's why I prefer to work alone. Then for competition, my coach just throws someone on the event with me to fill up the empty space.

I agree with you here. my partner and I tried to do it together and it ended up being lopsided. now I work on it and she just gives a helping hand if I need it. it works better that way.

Why not have your partner work on a different design?

[Balsa Man]

I appreciate your thoughtful post, nejimb- I think the best way to continue the discussion is some paragraph by paragraph thoughts back. Looking over this after I wrote it, its …..long, and covers a lot of things. But, for some, it will provide interesting insights, and things to discuss further, so here goes……

“I have to disagree with Balsa Man on this one. Although a well built jig obviously cannot hurt, I think it'd be misleading to say that it is the only way to build bridges competitive at the 1200+ ranges. I think that, with bridges, jigs don't make sense in a lot of ways. With Towers, jigs would help a lot, as the whole structure had an entirely different dimension - usually 4 symmetric sides. With bridges though, I know the strategy that a lot of people take (myself included) is to build just two symmetric sides of the bridge, and then connect them in an entirely different way. Because of this, I think that those two sides can be built fairly easily on a 2d surface, such as building on top of a schematic or drawn template. Where the jig becomes more useful would probably be with connecting the two sides - and even still this can be done accurately without a jig.”

Disagreement respectfully noted, and truly appreciated. Its this kind of dialog we all learn from, and get new ideas to try from, and develop our own knowledge and opinions from.

I’m not saying the only way to build a bridge in the 1200+ efficiency range is with a very precise jig. I am saying, and I do believe, that a pretty high level of precision and symmetry IS necessary.

I think I could fabricate to the level of precision needed with ……simpler jigging than I am suggesting, but I’ve been building things out of balsa (and other stuff) since I was a kid, many more years than I’d care to admit. I believe one of the missions of a coach is to find ways that allow someone who is learning, developing the feel, skills, etc, to get to the necessary precision. A carefully built jig will do that- for bridges, towers, and booms. Do I think this is the only way? Not at all. I simply present it/discuss it as something that has worked. I first got involved in 2000, with my older son; now with my younger son. Been through bridges 3 cycles, booms two cycles, and towers; multiple State medals in all 3; one time to Nats, top1/3. We have learned many things, together, every year. Out of that a way of approaching; a “system” has emerged – just as it has, I’m sure for you

Let me clarify that I was talking about 2 different jigs for bridge- a 2-D one for fabricating the sides, and a 3-Done for putting the sides together into a complete structure It is necessary that a) the sides be flat, symmetrical, and pretty darn close dimensionally, b) the pieces in the sides be straight, and c) the sides be…..symmetrically aligned. These two jigs will do that. I agree it is easier to get the sides fabricated accurately with…..minimal jigging, than getting the sides put together into a bridge.

What I have seen – observing, and re-looking at a lot of video; booms, towers, and bridges, is the impact of asymmetry/imprecision. If you can see a bow, a twist, an unevenness (before testing), you will – except in some very special cases - see “premature” failure from it. Once a structure starts to distort (and I’m talking in competitively light structures), forces, particularly at joints and in compression members go up (from design levels) very rapidly, and a failure mode is reached quickly.

“Of course, these things aren't actual negatives of making a jig, just my input that I know that good construction can be done without one. The problem though is that using a jig (typically) allows for much less flexibility and opportunity for change in the design since it requires a major reworking of the jig itself. I think to suggest that teams should focus on getting a good jig built would be misleading - it's my opinion that keeping the door wide open to playing with even the most minute details of the design is more valuable, and that that will overall lead to greater success.”

Here we come to a fundamental difference in approach; from our respective different bodies of experience, we have learned, and concluded different things. Good (precise) construction can indeed be done with a minimal jig. It takes a very high level of skill to do it. Yes, if you come to the conclusion you need to make (or if you decide you want to make) a significant design change after “locking in” on a design (and by “design” I’m referring to the size and alignment of pieces; “configuration”, really), it can be a pain to re-do the jig.
So, what, in my mind, goes with the approach of building and using “good” jigs, is how you approach the design process; how you get to a configuration that warrants the time commitment that building good jigs involves. Here’s a summary of that process. Again, I’m not suggesting this is the only, or even the best way, but I can say it …..works.

Step 1- Analysis of options and selection of a configuration. The rules provide a set of constraints; a few points on a piece of paper; the base span, the block that has to be cleared, and the height. Peter looked at 8 different configurations, with a couple minor variants. Each configuration was first entered in the jhu Bridge Builder app to get forces- the load each member was carrying. Then, using information from previous years, each configuration was entered into a spreadsheet to estimate approximate weight. That information was from past data on what “weight” of wood carried what loads (and the size- as in cross-section). For members in compression, that data is a combination of weight of wood and exposed column length. When I say “weight”, I’m talking in terms of grams per centimeter- a very useful unit of measure. The spreadsheet has a line for each of the members; length, number, grams per centimeter. It included a glue factor (which is easy to get by weighing pieces, glueing a joint, and re-weighing). By putting in approximate grams per centimeter, you get an estimated structure weight. Sometimes (as was the case this year), this process will get you to one configuration that is clearly lighter than the alternatives. Sometimes, you may have more than one at similar estimated weight, and the question becomes which is easier to construct.

So, yes, this process leads to – and “locks you in”- to a configuration from which you build your jigs. It is that configuration, selected on a…..rational basis as the likely lightest way to get the job done that is really the starting point for the engineering design process. You use the term “keeping the door wide open to playing….” I would suggest that the place for this is in the comparison (on an estimated weight basis) of possible configurations. You then go on, “…playing with even the most minute details…” I’m not sure what you encompass in “minute details.” Given a basic layout/configuration, the “details” become a) the weight/configuration of each of the members (e.g. 1/8th square at 1.5 grams per 36” stick, or 1/8th at 0.9 gr/36” with “angle iron” lamination of 3/32 wide, 1/64th thick strips on adjoining sides), b) the joint details (e.g., gusset plates, butt vs lap, etc.)

Step 2- Component testing. This is where – in the way we approach it – testing comes into play; testing individual pieces, knowing what forces/loads they need to carry in the completed structure. Tension testing is pretty straight forward – set up the piece – and importantly the joint(s) that are under tension – where its hanging from one end, and you hang a container into which you can pour water onto the other end. Pour in water (1 liter = 1 kg) till it breaks. Compression testing is harder, but if you’re really “engineering” a structure, necessary. You need to be able to apply a measured load (again, we use a container for water) along the axis of a piece, with the piece vertical, and you need to be able to adjust the length of the piece you’re testing (by putting something under the bottom end). For pieces under compression, what you need to know is what load it will carry at the exposed length. The exposed length is the unsupported length – i.e., the length/distance between support/column bracing pieces. This sort of testing gets you to what is the lightest wood, whether it be a specific density (e.g. 1.2 gr/36”, or 0.8 gr/36” with angle iron lamination, etc) that will carry the “design load”). Given the nature of wood, you’ll want to apply some safety factor to your test results.

For the last two steps, I’m just going to briefly describe the process and givea few specifics from this year’s bridge. The basic process is applicable to all types of structures.

Step 3 – Building. From early design analysis, we’d settled on 1/8 square stock for compression pieces, and 1/64th thick (1/8th and 3/16ths wide) strips for tension pieces. 1/8th balsa or bass will carry the design loads at the lengths we were looking at. With information from component testing, you can approach this and the next step from two directions. One is first using the lightest pieces that testing indicates will hold design load, and (likely) beefing pieces up in the refinement process. The other is first over-building – using pieces you know, or are pretty sure are stronger/heavier than you’ll need. This year, we took the second approach-the first bridge was done with basswood, except for the tension pieces. Why?- because the spreadsheet showed an estimated structure weight a little over 15gr – which we figured would be competitive for Regionals. It would provide sort of an insurance policy; a bridge that would place…..well. Then depending on the time available (and things always come up that cut into the time available for Science-O), later versions/modifications could be done to pull the weight down

Step 4 – Refining/improving. First cut- we knew from component testing that bass was serious overkill in some pieces. Building with bass confirmed fit- that the guide pieces produced bridge pieces the right length; all the pieces fit together; the sides come out the same; they went together to make a “true” structure; the right sequence for putting the pieces together became confirmed.

“That said... I've wished all year that we had a good jig to work with. Though I'm overall glad we've spent the time building and testing bridges instead and not having one has given us the opportunity to tweak the design (which we're still doing even this late in the season on a small scale, it definitely would be a more convenient way to build.”

There are many ways to skin a cat…. Again, its just personal perspective, based on what has worked, and how well its worked for us. Having good jigs, and focusing later work to “tweaking” materials was both time-effective and successful. Having a “good” (set of) jig(s) gave our guys “control” of THE major variable – precision/accuracy of construction – each side, each bridge is …..true- essentially the same. This allowed them to focus “tweaking” on materials and construction. The key is control and understanding of variables. If you only change one thing, you know what you’ve changed, and you know the results of that change. If you make multiple changes (e.g., changing both the configuration of one or more pieces, and changing the …..specs (density, gr/cm) of one or more pieces), you loose the ability to…..understand the effect of a given change. It’s the basic scientific method – dependent/independent variables, collecting unambiguous data; getting true understanding

One thing we have done as far as trying to keep the bridge as uniform as possible is to mass every individual piece. Previously, we would mass each full stick of balsa, and cut our members from pieces that had the same mass. However, we found that there was too much variation even within each stick, so we've been cutting every piece and making sure that they are actually the same mass before we put them in the bridge. It's a painstaking process, especially when our tolerance is 5 thousandths of a gram, but I think it does actually help.

You are, in my experience, absolutely right on here. It doesn’t just “help”, it is one of those critical factors for high-efficiency structures). As I’m sure you have seen in this process, there is significant variation in single sticks- even if you’re working with weight-graded stock (e.g. 1.0 gr/36”, 1.1gr/36”, etc.) Our data show 15-20% typically – for pieces in the 10cm range, and up to, and occasionally >25%. You need to know the actual density of pieces tested individually, and you need to use pieces that are the same weight/density as what you’ve tested, and you still need to apply a safety factor (we generally use 20%) to account for grain variation and the inherent non-homogeneous nature of wood. We have the luxury of a scale (at my office) that weighs to 1/10,000th of a gram. Being able to accurately know weights (gr/cm) of laminated construction is critical to using lamination (and comparing un-laminated and laminated options). You need to know wood weight, and laminated construction weight, which gives you glue weight. In testing compression pieces, you will see that different amounts of glue in laminations will give you significant differences in load carrying capacity, and weight. As with un-laminated pieces, for component testing results of laminated members to be reliably incorporated in construction – you need to be building with …..what you tested.

"On a different note but still addressing Balsa Man's post, I'm interested to see that you had more success using sides that were angled in. I've always been under the impression that doing so just makes the force that your main sides have to undergo even greater and it introduces a new force that the connecting pieces must take. Balsa Man, what made you decide to ultimately do this? And, to everyone else, have other people seen similarly positive results from this type of construction?"

There have been a couple other posts regarding this while I’ve been working on this reply; they’re right on. Its not that we “had more success” with sides angled in a bit; it was a basic design decision, early-on. Vertical sides give you no room for error, no latitude for a loading force that is not perfectly vertical (i.e. any bucket swing, or any difference in the height of the two sides, any error in sides being vertical, i.e. leaning slightly outward). Sides angled in a) do give you a margin for error/bucket swing, and b) give you …..inherent stability. Yes, looking at the legs, this does introduce “new”/additional forces, compared to vertical; the compression load goes up slightly (compared to vertical), but that additional load (when you work out the vectors) is a function of the cosine of the angle. When you look at a table of cosine values, you will see that in the first few degrees, it is a VERY small factor. It does mean the base of the legs will be……pushing out- that you have to put a piece between the ends of the legs on each side, that will be under very light tension loading (less than ½ kg).
I, too, would be very interested to see how many folk have......picked up on the value of this aspect of desogn.

Whew, long post. Lots of food for thought, and more discussion, and more learning, for all of us. That, in the end is what it is all about.

Len Joeris
Fort Collins, CO

[croman74]

That was like 15 posts in one!
I just finished building my bridge. It's weight is 16 grams. I'll test it tomorrow.