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TeamPlayer25 wrote:Hey everyone,

I am competing for Towers this SO season. My last structure had an efficiency of 1583. However, I am looking for a design that can break 2200. Can anyone suggest a good design?

So what u gotta do is, get a piece of wood, get some glue, and glue some sticks togheter, it'll get you to 2200.

Not exactly helpful.

Try looking through previous discussions on buckling strength. First off, you need to know the wood you are working with so that you can calculate which ones will be sufficient to carry the 15kg load.

I know, short answer but the wood you pick is really important. Bracing- we've discussed butt vs. lap joints previous as well, lap being the more superior one (just learned this myself) but basically most people are also going with X bracing and ladders (It sounds like so) which are basically just X's going all the way down. I myself am bracing at a 1/5th interval meaning every 1/5th of the way down my leg, I put a ladder. There's a LOT of information back in the thread, so feel free to luck. It will help you a lot.

Since it’s all inter-related, some thoughts/responses to last 4 posts above, and the posts in the recently created thread, ‘Glue and attaching trusses-tower.’

Overview/general comments on bracing:
In an attachment to one of my posts, on pg 10, I discuss bracing, and describe one approach/configuration, “ladders and Xs.” In that I note, and I’ll say it again here, that while there are aspects of design I’ve developed a….deep/solid understanding of, where I understand the underlying science, and how to reliably apply it, and have compiled data, I can’t yet say that in terms of ‘the most structurally efficient’ design for bracing.

By the term “most structurally efficient”, I mean, for the lightest weight (wood + glue), it provides the bracing needed to brace a long thin column against buckling failure; to effectively turn it into a set of shorter ‘stacked columns’ with much higher buckling strength than the un-braced long column. In addition to column bracing (of each of the legs) against buckling failure, a bracing system provides overall structural integrity, preventing flexing, twisting, etc., of the overall structure.

It’s an aspect I’m still learning about and trying to figure out …..conclusively. I’ve presented/described the ladders and Xs approach that I, and the teams I’ve coached over the years, have come to use because a) it works (and we understand why and how it works), b) it’s pretty darn light (it may well not be the very lightest way to go, but it is certainly….well into in the competitive range) , and c) its pretty straightforward to do, to construct ( involves a few specific techniques, a couple of tools, but nothing beyond the ability, with a bit of practice, of a junior high/middle school student). If/as I develop new understandings/insights this year into how to do effective bracing that is lighter, I will post updated info. I will say, from discussions with a couple other coaches, and reports on tower weights/scores, I think there’s a….reasonable chance there is a lighter way, or lighter ways to provide effective bracing.

So, with that said, comments on recent posts about bracing, in the context of the ‘ladders and Xs’ approach.
First, a couple of nomenclature things- “ladders” = horizontal pieces of wood joining the “bracing points” on the legs. “Bracing points” = those points on (each of) the legs that divide the legs into the bracing interval you’ve selected in your design (e.g., ½, 1/3, ¼, 1/5, etc. “Diagonal bracing” = pieces of wood joining a bracing point on one leg, to the bracing point on an adjacent leg that is either above or below that bracing point- they run diagonally. If, looking at one pair of ‘ladders’ between two legs, you have one diagonal bracing piece, this is referred to as “Z bracing.” If you have two that cross over, this is referred to as “X bracing.”

Ladders positioned/glued (butt-jointed) between the legs (vs positioned/glued (lap-jointed) onto the faces of the legs): In photos/videos of past S.O. towers (and bridges, bracing compression members), you will see many examples of ladders butt-jointed between legs/members. I’d intended to refer readers to the image gallery to see this, but (unless I’m blind), with the new forum…layout, I can’t find a way to get to the gallery. As discussed in my “ladders and Xs” attachment, ladders installed this way block/brace against the legs attempting/starting to buckle inward- toward the adjacent leg. The butt-joint needs very little glue; the glue only needs to hold the ends of the ladder in-place. As discussed, this joint is VERY weak in tension; it/the ladder will not work to prevent outward bowing of a leg away from the ladder ends, away from the adjacent leg. But it doesn’t have to, because the X braces do that. The reason for placing ladders between the legs (instead of on the sides/faces) is because when in between the legs, when one of the legs starts/tries to buckle inward toward the adjacent leg, the force put on the ladder is “axial”- straight along its long axis. In a ladder lap-jointed on the leg faces, the force put on it will not be axial- because the force is not being applied straight along the central axis, it will have a component acting to bend/bow the ladder; to start it buckling as soon as there’s any force. Also, because the ladders can/should be of quite low density wood (like 0.8 to 1.0 gr/36” for 1/8 balsa), the ‘shearing’ force the lap joint will see (whether the leg is trying to bow outward, or inward) may well exceed the ‘shear strength’ of the ladder wood- a thin layer of the wood will get ripped off, and the joint will fail.

Yes, it takes a bit of ….technique to get the ends of each of the ladders at the right angle, and get the ladders the exact right length, but….its not rocket science. Because the legs all slope in at the same angle, from base to top, the end angles on all the ladders are the same. A simple tool will allow you to make end cuts at this angle, and sand/file at that angle to do final adjustment to the exact right length. On a piece of flat wood, or poster board would work, draw the angle- one horizontal line for the ladder(s), and intersecting that line, draw the sloped line of the leg. Take a piece of 1/8 x 1/8” wood (you can use balsa, but I’d recommend bass for durability), oh, 2-3” long- your ‘ladder line guide piece’. Glue it down so one edge is carefully aligned with the horizontal line. Have the right end positioned along the (ladder) line so that the end is right at the leg line. Then glue down another piece (your ‘leg line guide piece’), an inch or two long, along the leg line, so that a) the upper end just touches the bottom side of the ladder line guide piece, b) the right side of the leg line guide piece is carefully aligned with the ladder line you drew. This will give you a ‘cutting jig.’

To do end angles, cut a ladder piece, oh, a half of an inch longer than the length needed. Put it on your cutting jig, with the right end sticking out past the leg line by just a bit, maybe 1/8”. To cut the end at the correct angle, take your cutting tool of choice (I’d recommend a fine-toothed razor saw, but a fresh, sharp single edged razor blade will also work). Use the leg line guide piece to align your cutting tool- gently press the cutting tool along the right edge of the leg line guide piece, and make your cut. Try to make it vertical, but its not critical. To finish the cut end, use a small, fine file, or sanding block/strip, Use the leg line guide piece to hold the file/sanding in correct alignment. Very gently sand/file the end. Put the piece up against your tower, with the angled end in position (centered on your alignment mark on one keg). With the side of a pencil (not the point, you don’t want to dent the ladder) mark just a hair longer than it needs to be (where it crosses/intersects the other leg (at the alignment mark) its going to be joined to. Put it back onto your cutting jig. Make the cut (as before), but be careful to make the cut just a bit long. Then, sand/file, as before, until you’ve taken it down to exact, correct length. Be careful at first (after you’ve done a few correctly, you’ll develop a good feel); take the end down a bit, move it to the tower, coming up from below your alignment marks. As you take a bit off, where it will fit will move up toward the alignment marks, a little more off, it will fit further up/closer to the marks. What you are working toward is when it will just barely hold itself in place, with both ends centered on your alignment marks. To glue, use a fine micro-tip applicator to drip a small drop of thin CA right where the top side of the ladder meets the side of the leg. It will wick into the joint. Then do the other end.

Has anyone ever seen a 2-legged tower? We're thinking about this but worrying about it tipping...

geedee wrote:Has anyone ever seen a 2-legged tower? We're thinking about this but worrying about it tipping...

Getting the balance would definitely be tough. Depending on the common tilting amounts of test bases, that might be rendered impossible just by the variation in the test base (Balsa Man - any data on this?)

Balsa Man wrote: Ladders positioned/glued (butt-jointed) between the legs (vs positioned/glued (lap-jointed) onto the faces of the legs): In photos/videos of past S.O. towers (and bridges, bracing compression members), you will see many examples of ladders butt-jointed between legs/members. I’d intended to refer readers to the image gallery to see this, but (unless I’m blind), with the new forum…layout, I can’t find a way to get to the gallery. As discussed in my “ladders and Xs” attachment, ladders installed this way block/brace against the legs attempting/starting to buckle inward- toward the adjacent leg. The butt-joint needs very little glue; the glue only needs to hold the ends of the ladder in-place. As discussed, this joint is VERY weak in tension; it/the ladder will not work to prevent outward bowing of a leg away from the ladder ends, away from the adjacent leg. But it doesn’t have to, because the X braces do that. The reason for placing ladders between the legs (instead of on the sides/faces) is because when in between the legs, when one of the legs starts/tries to buckle inward toward the adjacent leg, the force put on the ladder is “axial”- straight along its long axis. In a ladder lap-jointed on the leg faces, the force put on it will not be axial- because the force is not being applied straight along the central axis, it will have a component acting to bend/bow the ladder; to start it buckling as soon as there’s any force. Also, because the ladders can/should be of quite low density wood (like 0.8 to 1.0 gr/36” for 1/8 balsa), the ‘shearing’ force the lap joint will see (whether the leg is trying to bow outward, or inward) may well exceed the ‘shear strength’ of the ladder wood- a thin layer of the wood will get ripped off, and the joint will fail.

I'm still having trouble understanding something about this bracing you're describing, even after I read the doc you put up on page 10 again. How do we know that the ladders will handle the compressive force and that the Xs will handle the tension forces? From the way they are positioned, I don't see much of a difference, other than that one is horizontal and one is diagonal. My interest in this topic has gone up a ton since the last invitational I went to, where a I saw quite a few teams successfully brace the legs with only 1/16*1/16 Xs and ignoring the thicker ladders.

Also on a different note, do you guys think it would be ok to cut wood from broken towers? Any wood that still looks good can probably be used again right? I'm mostly thinking about bracing here.

BananaPirate wrote: Also on a different note, do you guys think it would be ok to cut wood from broken towers? Any wood that still looks good can probably be used again right? I'm mostly thinking about bracing here.

You should probably feel along the stick to make sure it's not damaged anywhere, but pulling bracing off a damaged tower should be fine. I would cut off the ends of the stick though, since they are already soaked in glue.

This elevated bridge apparently got first place Div. B at the national tournament in 2010 according to the best of nats page. HOW DOES IT WORK, and do you think the same principles could be used for building high efficiency towers?

JZhang1 wrote:https://scioly.org/wiki/images/thumb/3/ ... BB_1.3.jpg
This elevated bridge apparently got first place Div. B at the national tournament in 2010 according to the best of nats page. HOW DOES IT WORK, and do you think the same principles could be used for building high efficiency towers?

Honestly, that's just cheap, people spens hours building a bridge, and someone wins with that?