Elevated Bridge B/C

[JimY]

I haven't posted in quite a while now, and certainly haven't taken the time to read through the entire 70+ pages. However, I wanted to say just a couple things.

First, Balsa Man, you say that to get beyond 2000 in B division takes some magic mix of design, finding some special pieces of wood, and very precise building, and predict that the top one or two at nationals this year will score around 2500. Well, I agree with the design and 2500 part, but not the special pieces of wood and precise building part. Last season, we had 10 balsa and 88 bass pieces in the national entry, which weighed 5.96 grams, did not fail during loading, and took first place. This year, the design is very similar to last year (see the picture in the rules manual), but has 14 balsa pieces and 80 bass pieces. The bass was purchased at northeasternscalelumber.com, and the balsa at my local hobby store. Nothing special about either, except picking a density for the balsa and seeing whether or not it was high enough. By precise building if you mean perfect angles and great workmanship, then I disagree. If you mean great glue joints and just average workmanship, then I agree. A great glue joint, to me, has neither not enough nor too much glue (odorless CA, gap filling viscosity for us) and has no gap between the wood pieces where they contact each other. On our builds, the joints with higher stresses coming together at the node points are designed to have more glue surface area and use more glue, and vice versa. Considering that many of the 94 or 98 pieces in one of our builds has two ends, that's a lot of glue joints. I tell the students over and over that you don't want one of the higher stress joints to fail prematurely, causing the bridge to fail before it should. Rather, you want structural failure to occur somewhere away from a glue joint.

Now for my second point. Assuming that you can get structural failure to occur repeatedly, then how do you go about maxing out efficiency and pushing the 2500 efficiency mark? Probably the most notable thing is to figure out just how the bridges are failing. A week ago, I tested two prototypes that I built. The efficiency of the lighter one was about 2000 and the heavier one (by a whopping 0.11 grams) was just short of 2300. Both had the same failure mechanism, which was that the trusses pinched together at the center of the member between nodes 2 and 3 on our 9 node design. These members are the ones next to the members that the loading block rests on (see picture in rules manual). So, as each bridge was being loaded, you could watch how the cross piece and Xs on either side of the center cross piece for the members started bowing upward. At the point just before structural failure, the bowing increased dramatically. This happens very quickly and is therefore tough to catch for light builds. So, what's the remedy for this pinching/buckling? Use larger wood for the trusses? Find that magical piece of wood that doesn't break under the stress? These certainly are options, but not good ones in my book. I very much prefer to keep the trusses exactly the same (wood type and dimension) and use slightly larger cross pieces to connect the trusses, while using the same size Xs to connect the trusses. This worked for us last season leading up to nationals, and we will be doing the same this season. As far as the cross pieces, the website above sells three sizes that you can't get in most hobby stores (1/32" square, 0.040" square, and 3/64" square). Midwest Products also sells 0.0406" square pieces that are 11 inches long, with 15 per pack. Some hobby stores carry it and others don't. Of course, almost everyone carries 1/16" square. And by the way, I'm talking bass, NOT balsa. Getting back to the tests last week, they both used the same nominal size cross pieces and Xs. However, I noted as I was building them that the cross sectional area of one was a bit less than the other, as a new purchase was used for one and an old purchase was used for the other. The one with slightly smaller cross sectional area failed first. So, our national build, which the students are working on, will use the next size or two larger cross pieces between the two trusses in the center of those particular members, and the same size X pieces as my prototypes. So, I hope everyone is getting a clear picture of how to max out on your designs.

Good luck to all at nationals!

[cypressfalls Robert]

JimY are you talking about this bridge? 2000+

[nejanimb]

He is. He posted a further description of it on last year's thread.

[cypressfalls Robert]

...and the heavier one (by a whopping 0.11 grams) was just short of 2300.

wow really .11 I never went that precise on entire bridge weight, just to a .5 range....but I'm talking to a first place at nationals coach(<-- I think) so .....

[sean9keenan]

...and the heavier one (by a whopping 0.11 grams) was just short of 2300.

wow really .11 I never went that precise on entire bridge weight, just to a .5 range....but I'm talking to a first place at nationals coach(<-- I think) so .....

Most scales measure to .01 grams (with max loading to about 200g). Even my JYC scale from last year measured better then .5 grams (<-Shameless bragging about my JYC scale(<3 that scale))

Infact, in order to measure density, I know a lot of teams mass each piece of wood

[nejanimb]

That scale could measure the masses of our bridges pretty satisfactorily. =).

.5 grams is a HUGE margin of error, especially when your'e building bridges < 10 grams (that's plus or minus a full 5%...).

JimY, are you helping with C bridges at all this year, or just the TJ bridges for B division? From what I remember, the C division design last year was completely different from the B bridge (but also very successful, ~2100?). Is that design going to be coming to nationals?

[11madmic]

got back from states and the bridge broke a little soon and had an eff. of 1240 and got 4th place. It was sooo humid and i think most bridges weighed a little more than usual.

[nejanimb]

Congrats! That's very solid. How were the other bridges? Which are coming to nationals?

[11madmic]

Grand Haven like usual and West Ottawa and they both are very good with their bridges. I don't recall their placing though. I'll get back to ya on that.

[JimY]

JimY, are you helping with C bridges at all this year, or just the TJ bridges for B division? From what I remember, the C division design last year was completely different from the B bridge (but also very successful, ~2100?). Is that design going to be coming to nationals?

I coached both B teams in town this season, the same as last season, plus the high school team, where I'm one of the two paid coaches. We got the same results as last season at state with first and second in B and first in C. No, the C team won't be going to nationals, as we got 3rd overall. For the C group, I just gave the builders my truss program, told them how to use it, asked them to design every piece, checked the design to make sure it was not too far over or under-designed, and then told them to build what they had designed. I built no prototypes for them, so they did well with an average efficiency of 1900 at regionals and state. I can say that I did not care for what happened while loading the C bridge, but I had no time to delve into it. We had some issues with the B bridges as well, but since the C team's season is over, I've had a chance look into them further and come up with fixes.

Last season, the state C entry weighed something like 6.86 grams and went to full load without failure. It was a 9 node design, same as the B design. Same for both this season, with 9 nodes per truss. While they don't look the same due to the dimension differences, it's easy to tell that they're blood relatives.