Samantha wrote:Hello!
I've built many towers with my partner and we are at an impass where the 'cage' part of the tower that has to be 8cm always leans when we test it. We don't know if the connection from the cage to the 'bridge part'/ base or if it's something wrong with the cage itself. But the cage and the bridge don't break themselves. Only the connection. Would really appreciate any help!
Hi, welcome to the forum.
First, rest assured you are not the Lone Ranger – running into towers failing in the way you describe is, and is going to be, the typical failure mode with this year’s rules. The 8cm circle constraint, and the “2 part” configuration it sets up, makes it really hard to produce a nice light tower that carries close to full load. Once the upper portion of the tower starts to lean at all – for whatever reason, at whatever point in loading, failure will happen almost instantaneously. And it will look like/happen like you describe.
If you take the time to read through the various threads here in the towers forum, you will see a lot of discussion that will help you understand this design challenge, and ways you can manage and overcome it. Yeah, there are a lot of pages, it’ll take time, but there are a lot of answers……
Quick terminology check- just for easy understanding; what you refer to as the bridge part/base is most commonly referred to as the base or base section. The long, narrow section that goes up from the base, what has to fit inside an 8cm circle; what you refer to as the cage, is most commonly referred to as the chimney.
Very briefly, there are a number of factors that…..play together to create the type of failure you’re seeing. There are ways/techniques to control/minimize each of these factors. To get to a good performance level, you have to deal with all these factors. That’s not easy, and it takes work.
An overview/review of the key factors-
Symmetry of the tower – for good tower performance/scoring, you need to get to the lightest pieces of wood that will carry the forces they see when the tower is loaded close to full load. If the legs are perfectly and symmetrically aligned, the load put on top of the tower is equally distributed onto the 4 legs. If the legs are not symmetrically located in 3 dimensional space, one leg is going to see more force than the others; it will fail first. On page 6 of this thread (2nd post on the page) there is detailed info to figure out the forces on the legs and leg segments, the strength of leg segments needed, for a range of bracing intervals, the density/stick weights needed to get to needed strengths. In addition to introducing overloading of one leg, asymmetry will also introduce twisting and leaning as the tower is loaded. That will rapidly overload both legs and bracing.
The best way to produce a tower with high precision and symmetry is a jig. Trying to do this without a jig is somewhere between extremely difficult and impossible. It is very difficult to get a really symmetrical tower using separate jigs for base section and chimney section (and then trying to put the two segments together so the leg segment ends line up perfectly). A “one piece” jig that aligns base and chimney legs together is the best way to get really good symmetry. There’s lots of discussion and tips in the forum of how to put together a decent jig to do this.
The joint between the base and chimney segments of the legs – probably THE critical point in the tower. As discussed, you need full contact between the bottom of chimney and top od base leg ends – no gaps, no mis-alignment. You want the leg ends the same cross section, and to do that means both mating ends cut at the same angle, and that angle is ½ of the angle between the chimney and base leg segments.
Strength/stability of the base section – As discussed before, once the chimney starts to lean, things go downhill quickly. The base needs to provide a solid, stable platform. If you get any distortion in the base, so that one of the four points of support moves down, that’s going to allow the chimney to start tipping – one or two thousandths of an inch can start the process.
Vertical vs ‘leaned-in’ chimney legs. You really help things if you provide for some lean-in of the chimney legs- at the lower end of the chimney, leg ends out pretty darn close to just fitting inside the 8cm circle; at the top, a bit closer together than they need to be to just/fully fit underneath the 5cm square load block; maybe 1/8 inch in all the way around. If the chimney legs are vertical, a) that means the tops of the base legs are leaned in more than they need to be- they will be seeing higher loading forces than leg tops all the way out to just fit the 8cm circle (= need to be heavier/stronger), and b) the vertical legs will be hyper-sensitive to any off-center loading, and to even a very small amount the tower not being really vertical. With lean-in, you significantly reduce this sensitivity.
“Disproportional loading” of the legs. It is really important to make sure that the load on the chimney legs is equally distributed. If one leg is seeing higher load than the others, additional pressure/load is being applied to one corner of the top of the base. That will push that corner of the platform the chimney is sitting on down, that will start the chimney leaning. This starts with careful sanding of the ends at the top of tower- so that all four ends are in the same plane, and are flat so they fully contact the bottom of the load block. You also want the bottom ends of the base legs all in the same plane, with flat/full contact with the testing base surface, AND when you have full four point contact at top and bottom, you want/need the tower to be standing as vertical as you possibly can. How to do this takes careful thinking, measuring, and working. The better you do this, the more load the tower is going to carry before leaning sets in…..
