Towers B/C

[BananaPirate]

I have some more questions as well

--Why have I seen a lot of X bracing instead of Z bracing for each face of most towers? It's probably a balance/torque? issue but I can't put it into words how X is better. In previous years of bridge building, Z bracing was a lot more common.
--more specifically to balsa man: can you explain why the ladder's are in compression but not the Xs? To me it seems like they would do the same job, except that one is horizontal and one is diagonal. Why do the compression forces always go to the ladders?

[Balsa Man]

Balsa Man, thanks for your data. What glue are you using? We've been using Titebond II yellow wood glue. Students apply it with a tooth pick to both pieces to be joined and remove excess. It really soaks into the fiber of the wood and when there is a failure it rips the wood. But I wonder if it might be a little too heavy. I always felt that CA glue makes wood hard and brittle. I know it's a son of a gun to sand for final adjustments.

You know one thing I don't think we have ever addressed here... Final Adjustments. After Tower build we place a loading block with about 50 cm of chain on top of the tower and check how level the tower is.
- Does the block sit flat on top of the tower with no rocking. Figure out which leg is slightly longer and lightly sand with a flat sanding block. Do this a little at a time so as not to do too much because then you'll have to sand 3 legs.
- Does the Tower sit flat on the table without rocking. Here we use the chain to help to see if the center line is straight down the center. Again with the level on top of the loading block, make the necessary adjustment to make the tower level and firm to the table.
- This slight sanding on the top and bottom helps to make sure that the full surface of each leg is making full contact to the block and to the table.

You're welcome on the data, Baker, - its all about... helping everyone up their game.

Glue; lots of discussion over the years on this topic, lots of opinions, not much real data to work from. I'm pushing on the students I'm working with this year to capture glue weight data (weigh each piece before it goes into place). For Xs, weigh a bundle (which are at a length greater than they'll be when installed); get a weight- get a grams per centimeter (total bundle weight divided by total length of all pieces), then apply that to the installed X-pieces length. Then when you have wood weight, you subtract it from tower weight to get glue weight. From past year's (limited) data, using 1.2gr in our current estimating sheet (0.4gr for ladder installation, 0.8gr for the Xs). Just for fun, and to develop good building notes, I did a B-Div tower with bracing at 1/4 intervals, and total glue weight came out at about 1gr, so I think the 1.2 for a C-tower is...pretty good, and with care probably can be bettered (and work).

We're using/I'm personally recommending thin CA for ladders to legs. Work the ladder ends so they just barely hold themselves in-place on their alignment marks- you don't want them tight, because that starts to bow/buckle them. You definitely want ladder alignment marks on the legs, and its important they be correct and consistent on all 4 legs- as in same height/same separation (so the braced intervals are the same.) We use a ‘go-by stick’- a 1/8 bass stick with precise markings. Using a micro-tip, drop one little drop on the top of the joint where ladder meets leg. The thin CA will wick into the joint. All this glueing needs to do is hold the ladder in-place between the legs; all the load it sees is compression onto the ends. Actually, more correctly, all the forces it has to resist are compression on to the ends of the ladder. Those forces develop as the tower is loaded, and the legs try/start to buckle. That buckling will be toward one of the four faces of the leg stick. If it is inward, toward one of the faces of the leg to which a ladder is attached, the ladder blocks/braces against that force/movement, preventing the leg from starting to bow inward at that point, keeping the leg straight, maintaining the “stacked columns” you’ve created by bracing at intervals along the legs. The joint where ladder joins the leg is VERY weak in tension (end grain of pretty low density wood. If you only have ladder braces, and leg tries/starts to bow outward, the joint would pull apart, even with more glue. With X-strips in-place, if the leg tries to bow outward at a braced point, the Xs block/brace against that force/movement. For the Xs, recommending/using medium CA. It stays in-place when you put on...a bit (again, with micro-tip); also provides a way to....address the fact that as you overlap the ends of X-pieces, as you move up the intervals between ladders, you don't always have....tight/flat contact (strip to leg, or strip on top of the end of an underlying strip) when you keep each X-strip taught and straight; a little bit of a....glue puddle allows the needed straight and taught alignment.

Yes, CA is itself rather brittle (more so than Tightbond), but in towers and bridges, with a reasonably well constructed/glued joint, I have never seen a...glue failure. The only situation in which I’ve seen glue failure has been in boomilevers, with a single tension member, under a BIG tension load. Like with Tightbond (and other glue options, like Gorilla Glue, epoxy), the glue is much stronger than the wood, failure is always wood shearing (a thin layer of wood pulling away), not the glue itself cracking/breaking. Brittleness… isn’t really a problem, because if you get enough deformation in the area of a joint (or wood into which glue has soaked) to crack/break the glue, the structure is already breaking. There is an option for CA that’s less brittle- the black CA with rubber nano-particles added; Hobbytown sells as “tire glue.” One of the big advantages of CA is time- in less than a minute, a few seconds with thin CA, the glue has gone off; the glued pieces are firmly in-place.

Finishing/final adjustments- a good thing to bring up. Yes, its really important. The techniques and issues you note are….right on. In a competitive structure, you’ll be pushing the limits of the wood- wood that’s been selected to be the lightest that will just handle the forces at full load. You’re assuming the forces from the load will be evenly distributed down the legs. If you don’t have four points in full contact, both on top where the load block sits, and at the bottom where the legs sit on the base, one leg is going to get more than it’s share of the load. Failure always happens at the weakest point. If the tower isn’t vertical, you get load/force shifting to the legs on the side the tower is leaning toward. We first make sure the tower’s vertical, and all four legs are in even/full contact at the base. We use a loading block with a plumb bob with a thread suspending the plumb bob from the center of the load block, so it is just above the base. We then use a sanding block (a bit bigger than the load block) at the top to get the top level, and all four leg tops into the plane of the bottom side of the load block. Level checking is done by putting a ruler (we use an 18” one) centered on top of the load block. Hold a vertical stick up at one end of the ruler, mark the distance the tip is above the base plane. Then move the stick to the other end of the ruler. The ruler should be very close to the height/mark from the opposite side. Turn the ruler 90 degrees, same check. Sand leg tops until the distance from ruler ends to the base is the same on all 4 sides. If you get, oh, within a couple millimeters, all the way around, out 9 inches from the centerline, you have a sufficiently level top.

[Balsa Man]

I have some more questions as well

--Why have I seen a lot of X bracing instead of Z bracing for each face of most towers? It's probably a balance/torque? issue but I can't put it into words how X is better. In previous years of bridge building, Z bracing was a lot more common.
--more specifically to balsa man: can you explain why the ladder's are in compression but not the Xs? To me it seems like they would do the same job, except that one is horizontal and one is diagonal. Why do the compression forces always go to the ladders?

See post immediately above. Also, go back to page 10 of this thread, where I attached a detailed discussion (with drawings) on bracing, and how a ladders and Xs configuration works.. An important aspect/function of the X-pieces/tension strips, beyond bracing against one component of... buckling initiation forces is overall structural rigidity/integrity- the dreaded/often seen 'death by twisting.' If you (carefully) take a tower off your construction jiig with only the ladders in-place, and gently...play with it, twisting/moving top relative to the bottom, you'll see/feel its pretty....loosey-goosey; things move around with not much force applied. With the tension strips/Xs in-place, there is a magical transformation to a very stiff structure; things don't/can't move around.

This help?

[BananaPirate]

We're using/I'm personally recommending thin CA for ladders to legs. Work the ladder ends so they just barely hold themselves in-place on their alignment marks- you don't want them tight, because that starts to bow/buckle them. You definitely want ladder alignment marks on the legs, and its important they be correct and consistent on all 4 legs- as in same height/same separation (so the braced intervals are the same.) We use a ‘go-by stick’- a 1/8 bass stick with precise markings. Using a micro-tip, drop one little drop on the top of the joint where ladder meets leg. The thin CA will wick into the joint. All this glueing needs to do is hold the ladder in-place between the legs; all the load it sees is compression onto the ends. Actually, more correctly, all the forces it has to resist are compression on to the ends of the ladder. Those forces develop as the tower is loaded, and the legs try/start to buckle. That buckling will be toward one of the four faces of the leg stick. If it is inward, toward one of the faces of the leg to which a ladder is attached, the ladder blocks/braces against that force/movement, preventing the leg from starting to bow inward at that point, keeping the leg straight, maintaining the “stacked columns” you’ve created by bracing at intervals along the legs. The joint where ladder joins the leg is VERY weak in tension (end grain of pretty low density wood. If you only have ladder braces, and leg tries/starts to bow outward, the joint would pull apart, even with more glue. With X-strips in-place, if the leg tries to bow outward at a braced point, the Xs block/brace against that force/movement. For the Xs, recommending/using medium CA. It stays in-place when you put on...a bit (again, with micro-tip); also provides a way to....address the fact that as you overlap the ends of X-pieces, as you move up the intervals between ladders, you don't always have....tight/flat contact (strip to leg, or strip on top of the end of an underlying strip) when you keep each X-strip taught and straight; a little bit of a....glue puddle allows the needed straight and taught alignment.

Quick question again...From this explanation I assume that you are overlapping your strips so that even though the X's have good contact with the legs, in the middle where the two strips intersect, there is a slight bend? How much of a negative effects on the strips do you think it will have? I was considering putting one strip of each X on the reverse side so that the two wouldn't come into contact.

[tayhan_23]

I'm wondering about the bracing spacing for like a regular 51 cm tower, can someone post how long each gap should be?

[dholdgreve]

I'm wondering about the bracing spacing for like a regular 51 cm tower, can someone post how long each gap should be?

You have way oversimplified this. The bracing spacing is a function of the column strength and stiffness. The larger more stiff columns will require fewer braces... The thinner, lighter, less rigid columns will require more braces. I believe we will see anywhere from a low of 2 or 3 tiers per side to as many as 15 or 16 tiers per side.

I'd suggest that you go back and review this entire thread, reading every single post. There are many, many suggestions that go into great detail answering your question.

[Balsa Man]

We're using/I'm personally recommending thin CA for ladders to legs. Work the ladder ends so they just barely hold themselves in-place on their alignment marks- you don't want them tight, because that starts to bow/buckle them. You definitely want ladder alignment marks on the legs, and its important they be correct and consistent on all 4 legs- as in same height/same separation (so the braced intervals are the same.) We use a ‘go-by stick’- a 1/8 bass stick with precise markings. Using a micro-tip, drop one little drop on the top of the joint where ladder meets leg. The thin CA will wick into the joint. All this glueing needs to do is hold the ladder in-place between the legs; all the load it sees is compression onto the ends. Actually, more correctly, all the forces it has to resist are compression on to the ends of the ladder. Those forces develop as the tower is loaded, and the legs try/start to buckle. That buckling will be toward one of the four faces of the leg stick. If it is inward, toward one of the faces of the leg to which a ladder is attached, the ladder blocks/braces against that force/movement, preventing the leg from starting to bow inward at that point, keeping the leg straight, maintaining the “stacked columns” you’ve created by bracing at intervals along the legs. The joint where ladder joins the leg is VERY weak in tension (end grain of pretty low density wood. If you only have ladder braces, and leg tries/starts to bow outward, the joint would pull apart, even with more glue. With X-strips in-place, if the leg tries to bow outward at a braced point, the Xs block/brace against that force/movement. For the Xs, recommending/using medium CA. It stays in-place when you put on...a bit (again, with micro-tip); also provides a way to....address the fact that as you overlap the ends of X-pieces, as you move up the intervals between ladders, you don't always have....tight/flat contact (strip to leg, or strip on top of the end of an underlying strip) when you keep each X-strip taught and straight; a little bit of a....glue puddle allows the needed straight and taught alignment.

Quick question again...From this explanation I assume that you are overlapping your strips so that even though the X's have good contact with the legs, in the middle where the two strips intersect, there is a slight bend? How much of a negative effects on the strips do you think it will have? I was considering putting one strip of each X on the reverse side so that the two wouldn't come into contact.

Yes, there is a very slight bend/deflection, but it has no 'negative effects.' In the ladders and Xs approach I've described, the strips are very thin (1/64th" thick x 1/16th" wide), and they're put on pre-tensioned- pulled out so they're tight. So each one is is deflected from a straight line by half their thickness (i.e., 1/128th"). They only act/come under some force in tension- when the point of the leg(s) they're attached to try/start to deflect/buckle outward. They're tight, there's no slack, so they allow no movement. The deflection has no effect on their ability to carry a tension load.

I'm wondering about the bracing spacing for like a regular 51 cm tower, can someone post how long each gap should be?

You have way oversimplified this. The bracing spacing is a function of the column strength and stiffness. The larger more stiff columns will require fewer braces... The thinner, lighter, less rigid columns will require more braces. I believe we will see anywhere from a low of 2 or 3 tiers per side to as many as 15 or 16 tiers per side.

I'd suggest that you go back and review this entire thread, reading every single post. There are many, many suggestions that go into great detail answering your question.

And I'd second this suggestion, strongly. If you go back to the beginning, it starts with important basic understandings you need to have, and progresses into great detail on to figure out and implement a good design. In addition, there is important, detailed info in other threads within the Towers section.

[Random Human]

I use 1/8*1/16 for some braces and 1/16*1/16 for some braces but I've changed to a new tower design with 1/16*1/16 that performed better than my 1/8*1/16. All the 1/8*1/16 will bassically go to waste and as I am low on 1/16*1/16 is it a bad idea to litterally strip off 1/16*1/16 from the 1/8*1/16 sticks? Also, what stripper would be suitable for these kind of strips?

[baker]

I use 1/8*1/16 for some braces and 1/16*1/16 for some braces but I've changed to a new tower design with 1/16*1/16 that performed better than my 1/8*1/16. All the 1/8*1/16 will bassically go to waste and as I am low on 1/16*1/16 is it a bad idea to litterally strip off 1/16*1/16 from the 1/8*1/16 sticks? Also, what stripper would be suitable for these kind of strips?

Check out page 12 for a stripper I built some years ago. The stripper tool is a Master Airscrew Balsa Stripper. Hobby stores or google it. Hand held video..
https://www.youtube.com/watch?v=uXYXJPkYPak

[embokim]

If I remember correctly many of last year's bridges were comprised of four main uber thick and strong compression members with very thin tension members on the bottom, do you think teams are doing a similar type thing this year? With three very strong members with very thin cross braces running along side of them? Also would you know what kind of wood they used for those compression members?
Also, I was wondering what density of balsa you would use for a compression member, I am confused, as to what density one should buy, I am looking for a really strong compression member as mentioned above. Also how much stronger do you think a dowel would be rather than sticks? Has anyone tries this before?

Thank you for enduring through this long post.