General Discussion

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General Discussion

Post by dragonfly » September 7th, 2011, 5:09 pm

Hi everybody. I know that I found it frustrating last year seeing a lot of repetitive forum topics created about the same things or personal issues.....Here I hope that everyone can talk about most anything and 16 different forums mustn't have to be read to get a good gist of how things are going or to ask/answer questions. As such, I took the liberty in making this topic. Hope you all agree.

Personally, the rules are already stressing me greatly and already have me thinking about new designs. Maybe it's early for most everyone....but I'd love to hear how people are starting off their building seasons!
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Re: General Discussion

Post by hpfananu » September 7th, 2011, 6:26 pm

Great idea! At the end of last year, everything became so closely related that nothing really belonged in a single forum.
Personally, I think this year the rules aren't TOO bad (seeing as the base height is the same, just an added 20 cm) but it will take a lot of testing. I wonder if anybody will even attempt less than 70 cm. The (DRAFT) scoring looks pretty heavy set on 70cm probably winning, so I don't see anybody really doing less than that. (Unless they get to an inhumane weight...)
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Re: General Discussion

Post by chalker » September 7th, 2011, 7:17 pm

hpfananu wrote:Great idea! At the end of last year, everything became so closely related that nothing really belonged in a single forum.
Personally, I think this year the rules aren't TOO bad (seeing as the base height is the same, just an added 20 cm) but it will take a lot of testing. I wonder if anybody will even attempt less than 70 cm. The (DRAFT) scoring looks pretty heavy set on 70cm probably winning, so I don't see anybody really doing less than that. (Unless they get to an inhumane weight...)

Actually, one of the committee members did some very extensive and sophisticated calculations to try to balance the scoring formula such that the added weight mostly offsets the added height. We're hoping to see a variety of designs (i.e. some short and lightweight, some tall and heavier).

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Re: General Discussion

Post by SLM » September 7th, 2011, 9:29 pm

chalker wrote: Actually, one of the committee members did some very extensive and sophisticated calculations to try to balance the scoring formula such that the added weight mostly offsets the added height. We're hoping to see a variety of designs (i.e. some short and lightweight, some tall and heavier).
If the tower is designed properly, constructed perfectly and loaded carefully (to avoid swinging of the chain, although this might prove to be challenging), any additional height past 40 cm does not increase member forces or the chance of failure due to buckling. Therefore, theoretically, the score for a 70 cm tower would be higher than that of a 40 cm tower, assuming the same design is used for both towers.

Here is an example demonstrating the point. Our 50-cm tower from last year weighs about 7 g. The tower has a rectangular chimney and base where each weighs about 3.5 g. Given that the chimney is 35-cm long, then it weighs about 0.1 g per cm (3.5g/35 cm = 0.1 g/cm). So, if we duplicate the design for a new tower that is 40-cm in height, its chimney would weigh about 2.5 g (25 cm x 0.1) and the entire tower would weigh about 2.5 (chimney) + 3.5 (base) = 6 g.

If we extend the height of the chimney from 25 cm to the maximum height of 55 cm, then the total weight would increase by about 3 g; the entire tower would weigh about 9 grams.

Using the scoring equation for the Nationals, the shorter tower would get (15/6)(40-25) = 37.5, and the taller tower would get (15/9)(70-25) = 75.
Here, I am assuming that both towers hold the entire load since, theoretically, this is a valid assumption.

Here is the equation relating the change in height to the score for the above example.

Score = (15)(40 + d - 25) / (6 + 0.1 d), or,
Score = 15(d + 15)/ (6 + 0.1 d) where d is the additional height past 40 cm.

The above equation indicates that for valid d values (between 0 and 30), Score increases with increase in height (under the above-mentioned assumptions).

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Re: General Discussion

Post by jander14indoor » September 8th, 2011, 2:31 pm

I think your analysis may be missing something. If you can increase the height without changing the basic design of your chimney portion, it is overdesigned and COULD be lighter!

An "ideal" tower should have every member just short of failing due to compressive failure AND buckling at the max load. If you just made it taller without changing any other features (increased cross section, added bracing, etc.) It should now buckle before the max load.

So I don't think you can assume a linear increase in chimney weight with height. That only works if it is overdesigned for buckling at the shortest length. Thus overweight.

Something to consider at least.

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Re: General Discussion

Post by SLM » September 8th, 2011, 5:11 pm

jander14indoor wrote:I think your analysis may be missing something. If you can increase the height without changing the basic design of your chimney portion, it is overdesigned and COULD be lighter!

An "ideal" tower should have every member just short of failing due to compressive failure AND buckling at the max load. If you just made it taller without changing any other features (increased cross section, added bracing, etc.) It should now buckle before the max load.

So I don't think you can assume a linear increase in chimney weight with height. That only works if it is overdesigned for buckling at the shortest length. Thus overweight.

Something to consider at least.

Jeff Anderson
Livonia, MI
Good point. This is a valid point if the overall buckling of the tower (chimney) has to be considered as a mode of failure.

My analysis above assumes that only local buckling controls the design; only the individual compression members (the ones between the bracings) could buckle. But, the chimney as a whole would not buckle regardless of its height. Based on this assumption, if a consistent bracing pattern is used to prevent local buckling, then a 70-cm tower has the same (local) buckling strength as a 40-cm tower.

So, the question is "Would a 70-cm tower tend to buckle as a whole under the maximum load?" We will not know definitively until a few towers are built and tested. But, using Euler's Buckling Equation and simple experimentation, it is easy to conclude (as I did) that overall buckling would not be a governing factor in the design of the tower. My analysis indicates that for overall buckling to become critical, the tower has to be taller than 100 cm. I can provide the details of my analysis if you are interested.
Last edited by SLM on September 8th, 2011, 8:21 pm, edited 4 times in total.

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Re: General Discussion

Post by iYOA » September 8th, 2011, 5:53 pm

hmm so what are this year's rules?
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Re: General Discussion

Post by Guest » September 8th, 2011, 6:56 pm

There are different ways to look at this. Our Div. C Nationals tower last year weighed 5 grams and would score 75 under these rules. It would also be legal per the new rules parameters. (Too bad it exploded on the test stand at 15.01 Kg...) A 9 gram 70 cm tower would also score 75 and lose the tie breaker. Our Nationals tower would have scored 135 at Regionals compared to 108 for your 9 gram example.

The rules were structured to make it easier to do well with a shorter tower at Regional level competition, but the rules strongly favor taller towers at State and National rules. In your example, the taller tower still wins at Regional, but the scores are much closer. Presumably, some competitors at Regional tournaments, who may not be as proficient or as capable of building stable, geometrically perfect towers as others, will have a way to compete effectively. This doesn't dumb down the event, it changes the strategy.

Strategic choices were not really a factor in previous tower rules. These rules aren't so different, but they allow competitors to make choices and emphasize their strengths as designers and builders. I suspect we'll see towers built toward the extremes of the height ranges: very short and light, or very tall and somewhat heavier. I am anxious to see where this goes as the season plays out.

I fully agree that global buckling of the towers shouldn't be a factor. Local buckling and dynamic loading effects will almost certainly crop up. The rules include a provision that competitors must be allowed to steady the loading bucket by some effective means, and with very tall towers, any swing or rotation of the bucket will be brutal. I also want to emphasize that any faults in the geometry of the tower or the craftsmanship of the builder will be more pronounced with taller towers. On paper, the towers are stable; in the real world of balsa and glue, maybe not. It's part of the trade-off in going after the height score. Also, as builders try to reduce the weight of tall towers, they will make compromises on wood density, cross section sizes, spacing of the nodes on the bracing, etc., all of which have the potential to reduce strength and stability.

The example calculations in the rules were rewritten in late edits; the original examples highlighted the relative differences between Regional, State, and National scores for realistically scaled different towers. It doesn't matter, competitors will generate real data soon enough.

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Re: General Discussion

Post by Fossil Freak 25 » September 8th, 2011, 7:18 pm

Ok. so the new scoring system makes no sense to me :(
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Re: General Discussion

Post by chalker » September 8th, 2011, 7:56 pm

Guest wrote:There are different ways to look at this. Our Div. C Nationals tower last year weighed 5 grams and would score 75 under these rules. It would also be legal per the new rules parameters. (Too bad it exploded on the test stand at 15.01 Kg...) A 9 gram 70 cm tower would also score 75 and lose the tie breaker. Our Nationals tower would have scored 135 at Regionals compared to 108 for your 9 gram example.

The rules were structured to make it easier to do well with a shorter tower at Regional level competition, but the rules strongly favor taller towers at State and National rules. In your example, the taller tower still wins at Regional, but the scores are much closer. Presumably, some competitors at Regional tournaments, who may not be as proficient or as capable of building stable, geometrically perfect towers as others, will have a way to compete effectively. This doesn't dumb down the event, it changes the strategy.

Strategic choices were not really a factor in previous tower rules. These rules aren't so different, but they allow competitors to make choices and emphasize their strengths as designers and builders. I suspect we'll see towers built toward the extremes of the height ranges: very short and light, or very tall and somewhat heavier. I am anxious to see where this goes as the season plays out.

I fully agree that global buckling of the towers shouldn't be a factor. Local buckling and dynamic loading effects will almost certainly crop up. The rules include a provision that competitors must be allowed to steady the loading bucket by some effective means, and with very tall towers, any swing or rotation of the bucket will be brutal. I also want to emphasize that any faults in the geometry of the tower or the craftsmanship of the builder will be more pronounced with taller towers. On paper, the towers are stable; in the real world of balsa and glue, maybe not. It's part of the trade-off in going after the height score. Also, as builders try to reduce the weight of tall towers, they will make compromises on wood density, cross section sizes, spacing of the nodes on the bracing, etc., all of which have the potential to reduce strength and stability.

The example calculations in the rules were rewritten in late edits; the original examples highlighted the relative differences between Regional, State, and National scores for realistically scaled different towers. It doesn't matter, competitors will generate real data soon enough.

Bob Monetza
Grand Haven, MI
FYI, Bob was the person I was referring to that did some extensive calculations. I'm glad he chimed in here and he said much more eloquently what I was trying to convey at the start of this thread.

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