In addition to the issues noted- which I totally agree with, there is another important one; its a compression loading vectors issue; the direction that the legs try/want to buckle under load. Buckling will occur in the weakest direction.Crtomir wrote:You're welcome. Let us know how your 3-sided tower performed. Most of the best scores this year, probably all the scores over 3000, were built with 4 sides, not going for the bonus, and 9-10 simple X-braces on each side. We did try 3-sided towers for a little bit, but never got much over 2000-2200 points with them.
With a square cross section, buckling will be from the center axis toward one of the 4 faces. With a ladders and Xs bracing configuration, the ladders brace against one leg trying to buckle inward, toward an adjacent leg, and the Xs brace against outward buckling of one leg away from the adjacent leg. The load put on the ladder is axial- along the axis of the ladder. In an Xs only configuration, with the Xs lap jointed, compression loading isn't perfectly axial, but its parallel to the axis of the brace. With three square cross section legs, the legs are going to try to buckle toward one of the faces. That's at a 30 degree angle to the axis of the brace. That means that any pressure developing as load comes on the tower is acting to bend the brace. To resist that bending, the brace has to be significantly stronger than it would need to be if it saw axial, or near axial loading.
The initially apparent solution to that is triangular cross section legs; with an equilateral triangle cross section, the faces of two adjacent legs are parallel to each other, so one would assume buckling would be toward one of the faces, that would axially (or near axially) load a brace resisting incipient buckling of one leg toward an adjacent leg. However, that's not what happens. If you do a little research on "I", the second moment (aka cross-sectional moment) of inertia, it has both a....value- a number that depends on cross-sectional dimension, and a direction; a vector. That vector is not perpendicular to the face; it's on the order of 10-15 degrees. So, the off-axis loading onto the brace is not as bad/severe as in the case of a square cross section, but it is....quite significant, so, again, the bracing needs to be stiffer/stronger/heavier than it has to be in a 4 leg configuration. Put all these factors together, that's why a 3 legger is never going to be as efficient as a 4 legger.....
One last comment on the direction of going to really low density 3/16. Yes, as discussed, bigger cross section gets you substantially increased "I", and the value of "I" goes up faster than the weight goes up (the reason we've gone from 1/8" to 5/32" legs). But as the density goes down (the way you can take advantage of the increased value of "I"), you start to run into a tension problem; the bracing needs to handle both compression loading (one leg trying to buckle toward the adjacent leg), and tension loading (one leg trying to buckle away from the adjacent leg). At really low density, shear failure- a thin layer of the leg, where the brace is glued to it pulling away....becomes a limiting factor, and the only solutions carry a weight penalty. We're hoping/trying a little bit of thin CA, which will soak into the leg wood a bit, in the areas where Xs cross the legs, but with the number of X joints, even a little bit at each joint adds up quickly. This issue is most severe at the bottom, because of the force pushing adjacent bottom leg ends apart, so we're hoping that ...treatment to strengthen X joint s will only be needed on the bottom X, maybe the two bottom Xs. We'll see soon.
really bad