Towers B/C

[DarthBuilder]

Yes! That’s it! Thank you

[bernard]

Several diagrams of the Test Base, Loading Block Assembly and Bucket Stabilization Sticks, ring gauge, and a sample tower setup have been posted to the national website.

https://www.soinc.org/towers-b
https://www.soinc.org/towers-c

[sciencepeeps]

You underestimate what happens outside of the rather elite sphere of Scioly.org. Though 1 kg is very heavy, I've certainly seen teams with tower masses upwards of 500 grams.

Point is, 1kg weight tower gets u at max 17 points.... which is (no offense) nothing.. and saw a hole through a solid block of wood and what you get is 1 kg.

Ya, if you can point me to a team that scored lower than 17 points at an competition (not practice or for fun) I will be amazed!!

Let me tell you a story.
At the 2017 IL state competition, all of my events were extremely early. I knew that competitive towers teams like Marie Murphy, North Shore Country Day, and Woodlawn were going to be testing around this time, so I headed over to the Towers room. We had already tested and wanted to see our competition. Some teams had already tested, and I was just waiting around. From the back of the room, I heard “CAN WE TEST OUR TOWER?” I turned my head aound, and there it was. A tower that was almost a meter tall that must have been made out of the biggest balsa you could find anywhere. It wasn’t a solid block of wood or anything, and the people had obviously tried to build a tower, which was the worst part. Some of the pieces were literally falling off. They went to weigh their tower at the scale, which went up to 1 kg. The evenst supervisor said, “why don’t you guys go up to the other scale and weigh your tower,” meaning the scale they used to weight the SAND. Then they started testing their tower, which despite its bad quality, would probably hold the entire weight. But it DIDN’T. It broke with only half of the sand in the bucket. At the most.
That’s the story of how we saw a tower get an efficiency of 7 or less.

[musicalwhang]

Hello Towers forum!
So last year, I built a similar base-chimney style tower that fits this years set of rules. I tested it at my Regionals (don't worry I did way better at States), and it weighed 23.2 g and held all the weight, putting the efficiency at about 650ish (under Div. B rules). I still have the tower and was wondering how I could improve on the design to cut down the weight. Back then, I had no idea what I was doing so I used random wood with no calculations. Can I use this similar design in Div. C?
https://drive.google.com/file/d/0B1BYt2 ... sp=sharing

Thanks,
Musical

[Raleway]

Hello Towers forum!
So last year, I built a similar base-chimney style tower that fits this years set of rules. I tested it at my Regionals (don't worry I did way better at States), and it weighed 23.2 g and held all the weight, putting the efficiency at about 650ish (under Div. B rules). I still have the tower and was wondering how I could improve on the design to cut down the weight. Back then, I had no idea what I was doing so I used random wood with no calculations. Can I use this similar design in Div. C?
https://drive.google.com/file/d/0B1BYt2 ... sp=sharing

Thanks,
Musical

Hullo!
There's (like we always say) amazing advice on this specific issue out in last year's forum- maybe we need that as an announcement Bernard *COUGH COUGH*

But to address your question, most teams use 1/8 inch square wood with either 1/16 bracing square or 1/16 by 1/32 or even the rare 1/16 by 1/64. Bracing wood handles the tension force so the wood does not need to be as thick (no compression). Most teams also do not use "ladders" especially in division C- they add unneeded weight most of the time (up to the builder though). Your building and gluing technique look slightly sloppy (don't take it to heart, just improve!) but the idea is there! Best of luck in your future endeavors!

[Balsa Man]

Hello Towers forum!
So last year, I built a similar base-chimney style tower that fits this years set of rules. I tested it at my Regionals (don't worry I did way better at States), and it weighed 23.2 g and held all the weight, putting the efficiency at about 650ish (under Div. B rules). I still have the tower and was wondering how I could improve on the design to cut down the weight. Back then, I had no idea what I was doing so I used random wood with no calculations. Can I use this similar design in Div. C?
https://drive.google.com/file/d/0B1BYt2 ... sp=sharing

Thanks,
Musical

Hullo!
There's (like we always say) amazing advice on this specific issue out in last year's forum- maybe we need that as an announcement Bernard *COUGH COUGH*

But to address your question, most teams use 1/8 inch square wood with either 1/16 bracing square or 1/16 by 1/32 or even the rare 1/16 by 1/64. Bracing wood handles the tension force so the wood does not need to be as thick (no compression). Most teams also do not use "ladders" especially in division C- they add unneeded weight most of the time (up to the builder though). Your building and gluing technique look slightly sloppy (don't take it to heart, just improve!) but the idea is there! Best of luck in your future endeavors!

Yes, indeed, best of luck!
So, a few thoughts/comments that might help....

First on a couple things Raleway said:

Bracing wood has to, to some extent, handle both tension and compression forces,
The leg segments, when they're braced, form a set of 'stacked columns' - the shorter column sections between bracing intervals are much stronger than the full leg segment, un-braced (inverse square relationship between length and buckling strength). At the braced points, the bracing needs to prevent/block 'incipient buckling' - the buckling process trying to start, causing legs to start pushing or pulling on the bracing. As a leg comes under load, at some load it will try/start to buckle. If the braced points are held in position, it won't buckle. With a square leg cross section, buckling will happen/try to start toward one of the 4 faces of the leg. Which way it chooses to bend/buckle depends on the imperfections in the leg (slight bends), and imperfections in alignment. So, looking at any given adjacent leg pair, it will start/try to bend toward or away from an adjacent leg. If its trying to bend away, the brace will come under a tensile force - it will be pulled on. If its trying bend toward an adjacent leg, the brace will come under a compression force.

In an all Xs bracing approach, each X strip/piece has to carry (some amount of) both tensile and compression force - enough to keep the braced point from moving in 3-d space. As long as none of the braced points move, the buckling strength of the leg is the buckling strength of those short, braced segments. With 1/32 x 1/16 (all) Xs, the tensile strength is significantly more than the buckling strength, but there is some buckling strength, and by having the crossover point of each X set glued together is much higher than it would be without that glueing. There were many really competitive towers last year running 1/32 x 1/16 all Xs bracing; with the right bracing interval, it works. With 1/16 x 1/16 Xs, buckling strength is significantly higher (but so is bracing weight). Also, with 1/16 x 1/16 Xs, they have to be bowed more than 1/32 thick Xs do, which reduces buckling strength.

1/64" thick X strips won't work in an all Xs setup. They have virtually no buckling strength. Where they work is as the tension component in a ladders and Xs system.
As explained last year, in a ladders and Xs approach, the (very thin) X strips act/work only under tensile load- they keep braced points from moving outward/away from an adjacent leg. They are not glued at the crossover point. The ladders work/act to prevent/block movement of the braced point inward, toward an adjacent leg. The bracing interval for all Xs has to be significantly tighter/closer than what will work for ladders and Xs. However, results last year showed that the all Xs approach has a slight, but significant weight/score advantage.

One other comment on ladders; with this year's two part configuration, you absolutely do want/need a ladder set at the top of the base section. Because of the lean-in of the lower leg segments, there is a significant force trying to move the tops of the legs toward each other; about 2.15kg in a C-tower configured to get the 29cm circle bonus (about 1.7kg in a B-tower). Xs alone will almost certainly not be able to resist that force. Also, it is prudent to run a light ladder set at the top of the tower.

OK, with those clarifications, on to the tower pic you provided.

Sure, you can use the design, if, as you say, it meets the dimensional rules. But, if you want it to do well, you're going to have to make some adjustments.

First, it looks like the base is not wide enough to get the 29cm circle bonus.
Assuming that's a correct observation, the first change you want to make is spreading the bottom ends of the base segment legs to span the 29cm circle. Won't save you weight, but will boost your score BIG TIME. Last year, with a 2kg weight carried credit for the circle bonus, it was close, whether the bonus was "worth it." With this year's 5kg credit, it is a no-brainer. Say you get down to a 6gr non-circle bonus tower that carries full load; scores 2500; pretty respectable. But a 7.99gr circle bonus tower (carrying full load) will beat you- with 2503. I can tell you for certain, the wood weight you'd need to add to the base section of that non-circle bonus tower to get it carrying full load and meeting the circle bonus is going to be a lot less than 1.99gr.

I agree with Ealeway, the precision looks ...yeah, kinda sloppy.
As I noted above, it is in the imperfections- the ways and places in the tower that are not symmetrical - that buckling of the legs starts. The more imperfect, the lower the force that will actually make buckling happen. By having symmetrically aligned legs, and symmmetrical bracing placement, in carefully equal bracing interval spacing, you can save a significant amount of weight - wood density (hence buckling strength) can be lower. Using a carefully made jig to hold legs in symmetrical alignment is the key/the way to get this weight savings.

The bracing intervals in the chimney look even (as it should be). You can save significant weight by going to all Xs (1/16 x 1/32).

In the base section, its hard to tell for sure, but it looks like there is something problematic/strange going on. It looks like the 'chevron' braces- the angled pairs forming a horizontal V, are not all pointing the same way. So, one leg has two chevron 'points' at 90 degrees to each other, and the 3 other ones have a single chevron point (at what I'll call the first bracing interval- coming up from the testing base); the one with the two chevron points is much more strongly braced than the other 3. Even with that fixed, I think I'm seeing a more fundamental issue; the unevenness of the bracing intervals. The braced interval above the ladder that's a bit over half way up the leg segments looks longer than the two braced intervals below that ladder. Failure is going to happen at the weakest point. If there's a braced interval that's longer than the others, it will be weaker, and will break prematurely. As with the chimney, going with all Xs at 1/32 thickness will save significant weight.

What bracing intervals will be optimal for all Xs? Looks like low to very low density legs with 1/9 or 1/8 intervals in the chimney, and low to very low density with 1/5 or 1/6 intervals in the base.

One other thing on switching to a base that meets the 29cm circle bonus; the pieces joining the legs you have right at the bottom. A brace piece there is definitely needed. Just as the tops of the base leg segments are pushing in toward each other, the bottoms are pushing away from each other (with the same about 2.15kg force). A 'tension strip' to hold them together/in place will only see tensile loading, so this is the perfect candidate for 1/64 x 1/16 strips

And, again hard to see the detail, but it looks like chimney section and base section were built separately, then the bottoms of the chimney legs glued onto the top of the base legs. Alignment is not very good. It is VERY difficult to get the leg ends precisely aligned when trying to put the two sections together. It is also VERY important (for high performance) that they be precisely aligned, and that the leg end cut angle be the same on both chimney and base legs. In other words, the cut angle should bisect the angle formed between the base legs and the chimney legs.

Last, I think/hope you understand now that to get tower weight down (and performance up), you really do have to work with/track/use stick weights and measured buckling strength. The biggest weight savings will come from using the lightest leg wood that has sufficient buckling strength for the bracing interval you're using, and to know that, to pick the best set(s) of legs from the lumber supply you have, you have to be weighing sticks, and tracking weight and buckling strength.

So, again, good luck; hope this helps a bit.

[Gr8tor]

Out of curiosity, how would you make a "carefully made jig". What design and material would you use?

[wzhang5460]

What is the difference between X's and ladder bracing types? Thanks.

I'm kinda new at this.

[Unome]

What is the difference between X's and ladder bracing types? Thanks.

I'm kinda new at this.

In this image, X's only are P5, while X's and ladders are P4. As Balsa Man explains a few posts back, with only X's, the X's must be thicker because they need to carry both tension and compression, whereas with both X's and ladders, the X's carry tension (and therefore can be very thin) while the ladders (horizontal pieces) are thicker and carry compression - since strength in compression decreases with length, whereas strength in tension stays nearly the same.

[Balsa Man]

Out of curiosity, how would you make a "carefully made jig". What design and material would you use?

Uh, very carefully

But seriously, I really believe that a well done jig is the most important thing/step to being able to be seriously competitive.

"Carefully made" encompasses two things. First, carefully thinking through the design and construction of the jig; what materials, tools, techniques do you have access to; what level of competitiveness are you willing to reach for, what ... basic design are you going to run with (leg cross section, bracing configuration, bracing intervals- upper and lower sections)?, what, exactly, are the dimensions (hence x, y, and z coordinates) you want the jig to be built to?, how much time and money are you able to invest? For purposes of this discussion, I'm assuming a) a 4 leg, square tower, b) a jig that positions both upper/chimney leg segments and lower base leg segments, where both segments are held in position and glued together on the jig, and c) the legs are oriented so that their diagonal cross section is oriented so that it points to the vertical centerline of the tower. You need this so that the outside faces of each pair of adjacent legs are parallel, in the same plane.

Second, carefully working through technique- how, with what you have to work with, do you build, with the best 3 dimensional precision you can achieve, something that holds the legs (both upper/chimney, and lower/base segments) in place, at your 'design dimensions'/'design configuration', in the three critical horizontal planes, (which are parallel to the test base surface) - 1) at the top of the tower where the load block sits, 2) at the angle break/8cm circle plane (a bit less than 20cm above the test base for a C tower, a bit less than 25cm above the test base for a B tower, and 3) at/on the test base surface.

There are three basic approaches to create a... decent to good jig that I'm aware of. The degree of precision you can achieve depends on materials used, and cutting/shaping techniques you have access to/can use. I'll discuss those in a minute.

1) A 'solid form jig'; creating a 3-d solid by cutting 4 as near to identical as you can side plates, and then putting them together (glueing/attaching edges) to form a 3-d solid. The legs are positioned along the 4 outside corners/edges of the solid. You will need something along the edges/corners of the solid to hold/position the legs in correct linear alignment (running straight along the edges of the jig), and rotational alignment (so that the diagonal cross section points to the vertical centerline of the tower). Viable solutions for this include glueing pieces of small angle iron along the edges, or glueing strips along the edges of the plates, forming a 90 degree "V-cut" or "V-groove" along the edges. The challenges/difficulties with this approach are that the side panels have to be VERY close to identical, and aligning/glueing them together has to be near perfect, or the chimney/tower will not turn out vertical (it will have a built-in lean).

2) A 'pole and plates jig' creating something to hold horizontal positioning plates in place in the 3 critical planes, where the plates are mounted to/attached to a vertical center pole/bar. You will need something at the corners of these plates to hold the legs in linear and rotational alignment. As with the solid jig, angle iron or constructed 'V-trough' can do this. The pieces need to be short, or aligned/positioned with a very straight bar/piece of wood, so that the correct vertical angle of the leg is supported. The challenges in this style of jig are a) getting the center post truly vertical. b) getting the holes in the plates accurately centered, and c) getting the plates mounted/glued to the center post at correct height above base, and so that the plate is really parallel to the test base plane. You also have to be careful putting bracing on between the three planes the plates are in - the legs are not supported as they are in the solid form and cruciform jig approaches.

3) a 'cruciform plate jig,' where you have 4 vertical plates at 90 degrees to each other (attached to a center post, and forming a cross when you look down from the top). These plates are glued/attached to a base plate. As with the other jig forms, something to hold/position/align the leg wood along the outside edge of the plates is needed; this leg holding...component can be small angle iron, as with the other two jig forms. The challenges of this approach are getting all of the 4 plates centered/aligned on the center post, and getting the 4 plates at 90 degree angles all the way around.

If you go back to the 2017 archives, I provided detailed instructions how to construct and put together a 4 panel cruciform jig, using 1/8" sheet plastic, with small plastic angle iron leg holders. The legs being straight and one piece, the jig plate edges were straight. With this year's rules dictating a '2-part tower configuration, the edges of the jig plates will need to have an angle in the outside edge- the near vertical chimney section, and the significantly angled base section. The descriptions include both hand cutting, and laser cutting.

Materials and cutting-
At the upper end of cost and precision, all three jig forms can be constructed using machined metal (i.e., aluminum), to a precision of a thousandth of an inch (or better). Cost to have a machine shop do this is in the hundreds of dollars. Thicknesses from 1/8" to 3/16" would be appropriate. The V-trough for holding the legs could be machined in.

Similar precision, using sheet plastic (plexiglass, acrylic) is possible using a laser cutter (with at least 24" bed/size capacity. Cast acrylic works best for laser cutting. Plate thicknesses of up to 1/4" can be cut. With the angle in the leg edge of the jig plates needed this year, hand cutting with decent precision becomes ...very difficult

Thin plywood/masonite, and poster board or even foam board (1/8" or 3/16") could also be used. I believe (but am not sure) laser cutting can be used. Because of the nature of these materials, precision (with laser cutting) will be a bit less than with cast acrylic. Using 'hand cutting', precision, even if you're very careful, will be less.

Improvements from last year-
Last year, we found two things that have prompted improvements for this year. Getting the small plastic angle iron we used for leg holding precisely aligned (along the centerline of the plate edges, and rotationally) proved to be a real challenge/pain. 3-D printing of 'edge strips' (with the width of the strips equal to the thickness of the jig plates), where the edge strips have a 90 degree V-cut in the outside face, and a flat inside face for glueing to the jig plate edges solves these alignment problems. We found that printing at the 1/8" jig plate thickness we used last year didn't provide enough edge width on the V cut faces to securely hold the legs, so we went up to 3/16" cast acrylic for the jig plates.

The other improvement was a switch from 4 "half width" panels, to 2 "full width" panels (which are the shape of two 'half width' panels w/ the center bar in place) with "slot cuts." These slot cuts are made along/centered along the vertical centerline. The width of the slot cuts is the thickness of the jig plates. In one plate, the slot cut is made up from the bottom, to just past the mid-height of the tower In the other, the slot cut is made from the top down, to just past the mid-height of the tower. To put them together, you put the panel with the slot cut in the bottom over the one with the slot cut in the top (at 90 degrees to each other, with centerlines aligned), and move the upper panel down. With laser cut squares, you can easily align the two panels to 90 degrees from each other, and glue together, and onto a base plate with very high 3 dimensional precision (well under 1/10mm, hopefully approaching 1/100mm)