Elevated Bridge B/C

[dragonfly]

I know several of the big guns are getting ready for nationals; we got second in our state but our team wasn't anywhere near to moving on to nationals. So it looks like our bridge building career may be over. I'm ready to get started on towers. Has anyone built towers before? How do you connect the sides? What is you want to use three sides instead of four? Bass for the big compressions pieces, laminated balsa? What about beam shapes? L, V, T shapes. If you laminate where the two sides meet, then how do you miter those joints along the long side of the wood. I'm probably way ahead of myself here since they haven't even announced the specs yet, but I want to get started over the summer. Don't I say that every year?

Until nationals has passed my focus is solely on bridges, however throughout my middle school years I built towers. Side connection varies between teams, but unlike bridges almost definitely requires a jig for connection of sides. I haven't built science olympiad towers in a while (though I did build a toothpick one earlier in the year for a physics competition), but after the two years of designing and building in high school I imagine my approach at building towers here will be very different.

You're asking about laminating where the two sides meet... I'm not exactly sure what sides you are referring to. Are you confusing connection with lamination? or simply talking about laminating the initial two separate sides?

[T-B]

Until nationals has passed my focus is solely on bridges, however throughout my middle school years I built towers. Side connection varies between teams, but unlike bridges almost definitely requires a jig for connection of sides. I haven't built science olympiad towers in a while (though I did build a toothpick one earlier in the year for a physics competition), but after the two years of designing and building in high school I imagine my approach at building towers here will be very different.

You're asking about laminating where the two sides meet... I'm not exactly sure what sides you are referring to. Are you confusing connection with lamination? or simply talking about laminating the initial two separate sides?

Good luck to you at nationals. Thanks for the tip on building a jig to put it together. You're right, I was thinking that you joined the sides by creating some sort of L-beam or V-beam and I was thinking of that as a form of lamination. But I guess that isn't right. I clearly have some work to do.

Again, good luck at nationals!

[Balsa Man]

What I said is that, for pieces of equivalent weight, a smaller cross-section piece will have more tensile strength than a larger one. That is, a 20cm, 1 gram piece of 1/16 by 1/16 will be stronger in tension than a 20 cm, 1 gram piece of 1/8 by 1/8. This does, of course, mean that the piece is denser, as you suggested. ........ Hopefully that makes more sense!

Ah, now I understand what you're saying- I, too didn't understand what you first posted. You're absolutely correct.

For example- using some numbers from the tensile strength vs density table we've developed. If you take a 1/8th square piece at a density of 6 lbs/cu ft, tensile strength is about 10.05 kg. If you take a piece of 1/16th (1/4 of the cross section), at a density of 24 lbs/cu ft (4 times the density- which means the same weight per unit length as the 1/8th piece), the tensile strength is about 15.74 kg. I say "about", because there is always the factor of grain structure and inherent variability of wood at work; two pieces of the same density & cross section will not always have the same strength; variability can run as much as +/- 20%, though most of the time its more like +/- 10%.

Here's a set of numbers for tensile strength of 1/16th square. With this, you can build up a table at what ever cross sectional dimensions you want:

Ten Str Den
kg lbs/cu ft
0.48 2.5
0.82 3
1.16 3.5
1.49 4
1.83 4.5
2.17 5
2.51 5.5
2.85 6
3.19 6.5
3.53 7
3.87 7.5
4.21 8
4.55 8.5
4.89 9
5.23 9.5
5.57 10
5.91 10.5
6.25 11
6.59 11.5
6.93 12
7.27 12.5
7.61 13
7.95 13.5
8.28 14
8.62 14.5
8.96 15
9.30 15.5
9.64 16
9.98 16.5
10.32 17
10.66 17.5
11.00 18
11.34 18.5
11.68 19
12.02 19.5
12.36 20
12.70 20.5
13.04 21
13.38 21.5
13.72 22
14.06 22.5
14.40 23
14.73 23.5
15.07 24
Its this relationship of tensile strength vs density that took us to using high density 1/64th paired strips for tension; higher tensile efficiency, combined with larger glue area......

[Balsa Man]

I know several of the big guns are getting ready for nationals; we got second in our state but our team wasn't anywhere near to moving on to nationals. So it looks like our bridge building career may be over. I'm ready to get started on towers. Has anyone built towers before? How do you connect the sides? What is you want to use three sides instead of four? Bass for the big compressions pieces, laminated balsa? What about beam shapes? L, V, T shapes. If you laminate where the two sides meet, then how do you miter those joints along the long side of the wood. I'm probably way ahead of myself here since they haven't even announced the specs yet, but I want to get started over the summer. Don't I say that every year?

Ah, towers. Its been 5 years. My older son was a senior in 2005, the last year.
Obviously, many of the same basic engineering principles we've been discussing apply, but there are some significantly different challenges.
As with all the "wooden structure" building events (bridge, elevated bridge, boomilever, tower), the specs will get harder each year they continue a particular structure. Tower, last time around, first increased in height, then a .....constriction was imposed, where at a certain distance above the base, it had to "pinch down" to pass through a circle of a specified diameter. The opening the base had to span was large enougn that you couldn't run straight legsfrom the base to the top and pass through the constriction; it meant either curved legs, or a..... "two-piece" approach, where the portion up to the constriction was a.....broader pyramid, and above it was a narrower upper section.

A decent jig will, indeed, be important; very important. If the structure is not symmetrical, load on one leg will be heavier, and.....down it comes. There are a number of ways to approach it. Building sides on a flat jig, and joining them is indeed one way to go. If each side is built with.....leg elements on each side, when you join up the sides, you do get the strength advantages of lamination. Three legs vs four legs- its tradeoffs, just like in bridge that you'll have to look at. If you use 3, they'll each have to carry more load than a 4-leg configuration would; heavier, but one less leg.

[lllazar]

I know several of the big guns are getting ready for nationals; we got second in our state but our team wasn't anywhere near to moving on to nationals. So it looks like our bridge building career may be over. I'm ready to get started on towers. Has anyone built towers before? How do you connect the sides? What is you want to use three sides instead of four? Bass for the big compressions pieces, laminated balsa? What about beam shapes? L, V, T shapes. If you laminate where the two sides meet, then how do you miter those joints along the long side of the wood. I'm probably way ahead of myself here since they haven't even announced the specs yet, but I want to get started over the summer. Don't I say that every year?

Ah, towers. Its been 5 years. My older son was a senior in 2005, the last year.
Obviously, many of the same basic engineering principles we've been discussing apply, but there are some significantly different challenges.
As with all the "wooden structure" building events (bridge, elevated bridge, boomilever, tower), the specs will get harder each year they continue a particular structure. Tower, last time around, first increased in height, then a .....constriction was imposed, where at a certain distance above the base, it had to "pinch down" to pass through a circle of a specified diameter. The opening the base had to span was large enougn that you couldn't run straight legsfrom the base to the top and pass through the constriction; it meant either curved legs, or a..... "two-piece" approach, where the portion up to the constriction was a.....broader pyramid, and above it was a narrower upper section.

A decent jig will, indeed, be important; very important. If the structure is not symmetrical, load on one leg will be heavier, and.....down it comes. There are a number of ways to approach it. Building sides on a flat jig, and joining them is indeed one way to go. If each side is built with.....leg elements on each side, when you join up the sides, you do get the strength advantages of lamination. Three legs vs four legs- its tradeoffs, just like in bridge that you'll have to look at. If you use 3, they'll each have to carry more load than a 4-leg configuration would; heavier, but one less leg.

I don't know anything about tower efficiencies, wat would u say is a tower efficiency equivalent to, in terms of competitiveness, a bridge eff of 1000?

[StampingKid]

Up until my last bridge, I had a hard time getting past 1050. I finally hit 1500+ at state. Last year, i hit 1700+at state. In 2008, my tower efficiency at state was 2600+. My tower weighed less than 5.4 gr.and held over 14 kg. My best bridge this year was 9.0 gr. having picked up .5-.6 gr due to rain at state competition and held 13.9. Last year my bridge weighed 8.6 gr. and held the whole weight.

If I recall it was a 50 cm tower with the top 20 cm having to fit through a 7-8 cm circle.I think the national towers were hitting 3000.

[Balsa Man]

If I recall it was a 50 cm tower with the top 20 cm having to fit through a 7-8 cm circle.I think the national towers were hitting 3000.

That sounds about right (for 2005). My son went to Nats that year. Mission-P gobbled up all his time. So, the night before we left for U of I, he took his 2004 State tower, and slapped on a 10 cm extension to the top- taking it from 40 to 50 cm; it held ; efficiency was right at 2000, which was good enough for mid-pack; 30th.

[nejanimb]

Really impossible to speculate about Tower rules or how scores will be... scores are almost entirely dependent on the rules. From B towers a few years ago, I remember that probably about a 2500 was the same level as a 1000 is for current EBs, maybe a little better. Back in the day (late 90s), the winning towers at nationals were in the 9000s though, and 3000 would have put you in like 40th place. Just different specs.

What we can start to predict is scores for nationals this year. I predict that only one, mayybe two teams break 2000 for C division (though it wouldn't surprise me if no one does), and that it takes a 1600+ to medal in Div C. I don't think we'll see anything like the 2400+ we saw in C last year. I don't know B division as well... what do people think? I'd bet we see Witchita Collegiate back up there for a medal, and Troy as well. I'll be curious to see what happens with Centerville - they were awesome last year, but they were 11th at Ohio states (I assume they had some kind of disaster...), and it'll be interesting to see what Solon brings. I know FM won their States with 1400+ a month or so ago, so we'll see if they improved, and Grand Haven was over 1500... or any of those other 50 teams!

Is everyone underway building their bridges for nationals?

[JimY]

If anyone purchased the 2010 CD for the technology events and went through the Elevated Bridge powerpoint, there were a number of photos of C division entries from the 2009 national competition while on the testing stand in the various setup phases. The photos were all of teams that finished the event in the top half, and includes the 2nd place bridge as well as aia's 7th place bridge (though from a distance). Many of these bridges were what I call A frame designs, which feature a near triangle resting on either two sets of supported vertical legs or two 4-legged towers. So, of the C teams that did this design last season, all they had to do to compete this season was to increase the height of their towers or legs, while the triangle part could stay essentially the same.

Doing nothing to the triangle part and just increasing the height of the bottom section by 5 cm for this season will likely add less than 0.5 grams to the best builds. Assuming that some schools decide to improve their triangle section, I think the top C scores will be in the 2100-2300 range, and maybe a bit higher. So, just short of last year.

As for B division, I recently posted a 2000 and a 2300 of my own, which obviously doesn't count. I also built two bridges early in the season that each went to about 2200. Again, not counting. None of these four went to full load, so my recent work in the event was to get our national entry either closer to or all the way to full load while only adding a relatively tiny amount of mass. So, the incremental efficiency of just beefing up the cross members between the two trusses and doing nothing else (see my post somewhere around page 70) could be HUGE if it allows the build to break at 2 kg higher load while adding 0.1 grams. The efficiency of this added mass would be 2000/0.1 or 20000! The result is that it allows a bridge's efficiency to increase by several hundred points as long as it was failing well before 15 kg in the first place. So, I predict about the same efficiency for the B winner this year. It could be a bit higher than last year as well.

By the way, we're scheduled to test at 1:50 PM on the 22nd. When is your team testing? I'd like to meet as many of you as I can and say hello, but I'll only be at the testing location from about noon to 3 PM. Anyway, please give team names and scheduled testing times.

[packer-backer91]

Really impossible to speculate about Tower rules or how scores will be... scores are almost entirely dependent on the rules. From B towers a few years ago, I remember that probably about a 2500 was the same level as a 1000 is for current EBs, maybe a little better. Back in the day (late 90s), the winning towers at nationals were in the 9000s though, and 3000 would have put you in like 40th place. Just different specs.

What we can start to predict is scores for nationals this year. I predict that only one, mayybe two teams break 2000 for C division (though it wouldn't surprise me if no one does), and that it takes a 1600+ to medal in Div C. I don't think we'll see anything like the 2400+ we saw in C last year. I don't know B division as well... what do people think? I'd bet we see Witchita Collegiate back up there for a medal, and Troy as well. I'll be curious to see what happens with Centerville - they were awesome last year, but they were 11th at Ohio states (I assume they had some kind of disaster...), and it'll be interesting to see what Solon brings. I know FM won their States with 1400+ a month or so ago, so we'll see if they improved, and Grand Haven was over 1500... or any of those other 50 teams!

Is everyone underway building their bridges for nationals?

That was the actual scores that year [I think it was 1997, I found the results in the book in my teacher’s back room with all of the ancient science Olympiad events]. I didn't have the rules for that year but that must have been some each requirements. I thought that they had some weird formula that they used to get the scores that year to get scores that high.

I don’t think my states compaction was score reflect the potential of some of the bridges. I know that my schools bridge broke about 100 before it should have, Troy [MI]'s bridge broke prematurely, and I don’t know if any other school had mess ups besides Troy and my school but the added humidity hurt the scores as well [slightly higher mass means lower scores].