General Discussion

[thsom]

Good to know and that helped but that's not what I meant. My fault let me clarify: what I meant was that they loaded it quickly (as you understood) but the reason I thought that it could help there score was not because they got a little more sand to fall in after it snapped because of the speed, however that the tower held more because it wasn't subject to holding the wait for a long time so they poured in as much as it would stand to hold whereas if they poured slowly the tower would have to withstand and hold the sand for a longer amount of time and it would snap sooner.

[foreverphysics]

Good to know and that helped but that's not what I meant. My fault let me clarify: what I meant was that they loaded it quickly (as you understood) but the reason I thought that it could help there score was not because they got a little more sand to fall in after it snapped because of the speed, however that the tower held more because it wasn't subject to holding the wait for a long time so they poured in as much as it would stand to hold whereas if they poured slowly the tower would have to withstand and hold the sand for a longer amount of time and it would snap sooner.

There are no rules against it. You load however you like.

[guest382512345]

So I'm guessing the best way to load the sand is just as fast as possible but controlled so there's no jerking or uneven piles of sand? And for the automated system, you can't really avoid having one large mound of sand in the bucket, so do you just want to go slow in the beginning and then just gun it at the end?

[foreverphysics]

So I'm guessing the best way to load the sand is just as fast as possible but controlled so there's no jerking or uneven piles of sand? And for the automated system, you can't really avoid having one large mound of sand in the bucket, so do you just want to go slow in the beginning and then just gun it at the end?

From my experience, it's best to start off slows and pour in spirals. Slowly speed up. Don't just jump into superspeed pouring. At the end, you do want to go fast, but work up to that speed. That seems to work the best.

[Littleboy]

You have to let it grdually increase. If say you dropped a brick on it then it would break even though it is capable of holding more. You can't just release it suddenly. Also think about this if you are holding a heavy weight, it is easier to hold in the beginning then after a long time. The same is true for your tower. The longer it has to hold, the more likely it will break at higher loads.

[lllazar]

What would be the cheapest way to construct a testing apparatus? I'd like to as closely as possible mimic the testing procedure at the national competition, any ideas?

[thsom]

are ladders/horizontal pieces necessary or recommended for the very bottom edge of the base and very top edge of the chimney? Because I have seen designs without them. so it would look like a tower side for the base: top horizontal piece, any number of x's or z's, and then NO horizontal piece and the bottom. for chimney sided: bottom horizontal piece, all z's or x's and horizontal pieces or not, but no horizontal piece at the very top.

[sj]

I have a couple of questions...

We are usually very good at building towers (9th at Nats last year), but this year we seem to have entered a slump. I suppose it goes to show how anyone can do really well at this event or not so well. Our towers are getting efficiences around the high 50's, low 60's. They usually mass between 9-12g depending on the iteration, and are all 70 cm tall. They usually hold around 10kg. We test our wood for stiffness and density and we know the wood we are using with the spacing we are using should be able to hold all the mass. However we are finding that the top is breaking early. Does this suggest that the top is swaying/bending and needs to be reinforced with more diagonals?

Right now our tops have horizontals 1/16 x 1/16 every 3 cm and diagonals for every 3 of those so like:
____
|\__|
|_\_|
|__\|
|__/|
|_/_|
|/__|
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|_\_|
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|__/|
|_/_|
|/__|

Excuse the ASCII art.
The diagonal goes over the middle 2 braces without connecting (no glue).

Any tips as to wether there should be more diagonal braces or not? Maybe more closer to the top?

We built plenty of good towers last year but copying the design and adding extra height does not seem to work as the top snaps prematurely. Thanks for your help!

[Balsa Man]

An interesting problem, SJ. There was some discussion early in the season on the issues that come along with a significant increase in height (and that you seem to be running into). Some thoughts, in no particular order...

When you say the top is breaking, and then ask if it’s swaying/bending, it sounds like you’re not testing with a safety tower. Using one will allow you to see exactly where initial failure is happening. That, more than anything else, will tell you what’s happening, and what you need to do to fix it. If you set the slack (the distance the load block can fall) really close- in the 1/16th to 1/10th of an inch range, you may be able to see the bending before something breaks.

Failure in the top says it IS bending- the question is where and why. From what you say & show, I understand that last year you ran a 36cm chimney as shown and described, and this year, you’ve added six 3cm.....increments (two of the 9cm diagonally-braced increments). It also sounds like you’re running the “same wood” (to the extent that can be done) as what worked last year. Yes? Given your fine results at Nats last year, I’d guess that your leg size/density, and braced interval (effective column length between bracing) is pretty close to the limit. With “perfect” alignment (to each other and to the center-line), each leg’s carrying 3.75kg (at a 15kg load), so let’s say you’ve got a column strength of 3.8, maybe 3.85. You were able to achieve some level of precision in alignment over the 36cm length that kept the load distribution between legs pretty close to even- such that the most heavily loaded leg was not going over that 3.8/3.85. Failure in the top/chimney says one leg is getting a disproportionate part of the load- more load than in last year’s shorter tower. No build is going to be perfect; there is always going to be one leg carrying more than the others. Last year, that “extra increment” still left you within your column failure limit. Now you’re outside it. That says it has to be a precision issue, driven by the extra length.

The possibilities include:

A less than level testing surface. To whatever degree your test surface is less than perfectly level (and it won’t be perfectly level- and more importantly, you have to assume at competition it won’t be- and may be less level than your test setup, and build in some level of safety factor), one leg is going to get disproportionate loading. Let’s say at a 50cm height, the top of the “low-side” leg is a millimeter off from where it would be with a perfectly level base. At a 70cm height (a 50% increase), its going to be 1.5mm off.

The alignment precision over the longer length. For discussion purposes, let’s say last year you were able to build to have alignment within a millimeter from bottom to the top of the chimney. The additional height/chimney length increase is 50%. With the same degree of linear precision, top could now be out by 1.5mm. If you get top mis-alignment and test surface “slope” in the same direction, the total mis-alignment at the top is additive, meaning major disproportionate loading on one leg....
Weight variation of 3 grams (9-12), but all with failure “around 10kg” – with knowing your wood weight/density.....seems a bit strange. Unintentional extra glue could easily account for a gram, but a couple grams in denser (heavier/stronger) legs should mean a 12-gram one would be significantly stronger than a 9 gram one- +15, maybe +20%.

So, for the test surface level issue, there is nothing you can do but build in a safety factor. For the alignment precision issue, you might be able to help some by increasing precision of measurement, but there’s a limit, and beyond that, a safety factor is the only way to manage it.
Two ways to build in a safety factor- one is stiffer/stronger/heavier leg wood; the other is shorter effective column length. I’m not sure what your current effective column length actually is, given diagonal bracing at every 3rd ladder. Its 9cm or less; certainly the points where ladders and diagonals meet are “pinned” – located, effectively isolating the column section in between. What the effect of ladders only in between are, beyond me to assess/calculate.... I can tell you, if you run diagonals every 2 ladders, instead of every 3, you will significantly decrease the effective column length. Also, just tightening up the diagonal interval in the zone closer to the top would not be a good idea; a) failure will be at the weakest point- if you have longer effective column lengths anywhere in the chimney, that’s where buckling will happen, and b) I’ll bet if you test with a safety tower, you’ll find (in current configuration) failure close to the middle....

Last thing to consider- are you running any “lean-in”, or is the chimney straight-sided? Leaned in legs will give you a bit of a safety factor over straight up and down ones- a bit of latitude in the level of precision needed to keep the most loaded leg inside its column strength envelope.

Hope something here helps.

[Balsa Man]

are ladders/horizontal pieces necessary or recommended for the very bottom edge of the base and very top edge of the chimney? Because I have seen designs without them. so it would look like a tower side for the base: top horizontal piece, any number of x's or z's, and then NO horizontal piece and the bottom. for chimney sided: bottom horizontal piece, all z's or x's and horizontal pieces or not, but no horizontal piece at the very top.

Because they're at an angle, at the bottom of the base, the leg ends are pushing outward from the centerline- at full load `couple kilograms. Friction against the base holds them in position to some extent, but not much with a hard, smooth surface; certainly not enough to stop the outward movement. So, bracing between them needs to be tension bracing. At the top, even with a little bit of lean-in (a degree or two), the legs are essentially vertical; the load block induces neither inward or outward pressure; friction between the load block and leg top ends will likely hold them in place, which suggests they're not truely necessary. However, if you're running diagonals to the next ladder down, the start of bowing at that next ladder would pull on the top, and the friction between leg top and block might not be enough. A light ladder at the top would prevent that.