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LKN wrote:BalsaMan or SLM,

I understand that in the chimney, the horizontal bracing are under compression and the diagonal bracing is under tension....My partner firmly believes he has a chimney design that has horizontal and diagonal bracing under tension...What do you think?

Please see the attached pdf file for my explanation on this issue. Let me know if further clarification is needed.

I'm not sure if I am allowed to ask this, but how wide does the chimney have to be? Does it have to be a certain size or does it have to be able to hold a 5cm block?

iTzDiamondFirexD wrote:I'm not sure if I am allowed to ask this, but how wide does the chimney have to be? Does it have to be a certain size or does it have to be able to hold a 5cm block?

Must be able to hold 5x5 block and fit within 8cm tube above a certain height that depends on the division

iTzDiamondFirexD wrote:I'm not sure if I am allowed to ask this, but how wide does the chimney have to be? Does it have to be a certain size or does it have to be able to hold a 5cm block?

You question leads me to believe you don't have a copy of the rules. You really should get one and review them closely since there are a lot of other specs your tower needs to meet.

Since balsa is naturally bent, how would you make sure it's straight when building the chimney? I have several rulers, but only one is close to being close to perfectly straight. The way I've done it was just lining it up and taping it down, but so far it's been extremely time consuming and sometimes inaccurate. Also i've been having trouble connecting the base and chimney sections of the tower so that they're glued straight. Currently, I'm building the base and chimney separately and just eyeballing when it looks decently perpendicular.

guest382512345 wrote:Since balsa is naturally bent, how would you make sure it's straight when building the chimney? I have several rulers, but only one is close to being close to perfectly straight. The way I've done it was just lining it up and taping it down, but so far it's been extremely time consuming and sometimes inaccurate. Also i've been having trouble connecting the base and chimney sections of the tower so that they're glued straight. Currently, I'm building the base and chimney separately and just eyeballing when it looks decently perpendicular.

I'd recommend you read the accumulated information (including pictures in the Gallery) on making and using jigs. That's how you get all the pieces straight, and symmetrically aligned.

SLM wrote:

LKN wrote:BalsaMan or SLM,

I understand that in the chimney, the horizontal bracing are under compression and the diagonal bracing is under tension....My partner firmly believes he has a chimney design that has horizontal and diagonal bracing under tension...What do you think?

Please see the attached pdf file for my explanation on this issue. Let me know if further clarification is needed.

A very good and interesting analysis, as always. SLM.
An important clarification/understanding on "horizontal bracing under compression and diagonal bracing under tension." I've used these descriptions in describing the ladder & Xs we use; there is an important aspect that didn't come across. As SLM points out, until a leg starts to buckle, there is esentially no force on any of the bracing (unless you're pre-loading in your construction). It's when buckling starts that the bracing sees force. The ladders see compression when a leg starts to bow inward- towars/ento the end of the ladder; the Xs see tension when a leg starts to bow outward.

Pre-loading/ building in intentional distortion is a very valid and useful technique. I described a form of it in bridges year before last. By inducing failure in the direction you want, and then bracing against it , you are controlling things- real engineering. Your partner is very perceptive, and right-on, that if you put a little bit of outward bowing in, you've set the failure mode as outward buckling, and proper tension bracing will keep that from happening You mentioned 0.5cm- I assume that was mm. In an Xs and ladders configuration, with an induced outward bowing, both the ladders and Xs would see tension. Butt-jointed, low density ladders would be very weak/ineffective carrying tension- hi-density strip Xs are very strong/effective. More importantly, the ladders would be redunant. Food for thought and experimentation......

Has anybody thought about a parabolic design?

thsom wrote:Has anybody thought about a parabolic design?

very difficult to do with the 8cm tube restraint

My thinking of this, which might not be the popular opinion, is that since for years there is a basic design for towers, its not b/c its the only thing that anyone could think of- its because it works. There may be a few more efficient ways to make them, but not very many if national winners and attendees or state champions use the basic outline you see so often.

It comes down mostly to who can build it better more than who has a better design. The higher you go up in these competitions the more you see the same things over and over, with the exception of some points that one isn't always better than the other when made correctly

Just my view on this. Density is the idea to focus on once you have an outline that you see working consistently


Speaking of which, I know that no ones going to tell me or others their specific densities for parts (and rightfully so) but as I'm new to the specified densities game, what is the range of densities one can order for balsa? And at what number of this range does it go from useful in this event to unnecessarily dense and heavy even for the 3 or 4 main structural beams?

mrsteven wrote:

thsom wrote:...what is the range of densities one can order for balsa? And at what number of this range does it go from useful in this event to unnecessarily dense and heavy even for the 3 or 4 main structural beams?

From last year (division C): The tower has a height of 50 cm and weighs 6.92 g.
Chimney (main compression members): Balsa density around 190 kg/m^3. Each member weighs about 0.37 g.
Legs: Composite design (different parts having different densities): Each legs weighs about 0.5 g.
Bracings: around 160 kg/m^3.

Here is a picture of the tower: