Post
by **Balsa Man** » March 1st, 2011, 7:54 am

Haven’t had a chance to be on the board since Christmas (way busy at work).

As I’ve said before, there are many ways to skin a cat (and do bracing). A little review of the way we’ve chosen to do it (dig back to posts last fall discussing this):

We use (horizontal) ladders and X-bracing. Tower legs are 3/32nds. Ladders are also 3/32nds. Xs are 1/64th x 1/16th to 3/32nd. You can see what it looks like by looking at bridge pictures from 2009 – along the top of the bridge, joining the two top members. In a tower, those top members would be the legs.

Here’s why and how it works. SLM is pretty much right-on on the function of the bracing system- prevent buckling of the legs (the intervals you put ladders at divide the legs up into shorter exposed column length-shorter column lengths can take higher load before buckling) – and keeping legs “upright and aligned.” One important function beyond that; together, the bracing system makes the entire structure (tower) rigid.

Let’s visualize. Looking at just one side of the tower, and one “section”- three ladders- one at top, one in the middle, one at the bottom; laying on a flat surface. You may even want to cut a few pieces of wood and try/feel this. Between any two ladders, you have a trapezoid- parallel top and bottom, with the two sides somewhere between vertical and, oh, maybe 20 degrees sloped in on the base section.

If you “pin down” the four corners (i.e., if they can’t move), and push the middle ladder horizontally, what happens? It moves horizontally, and the legs bow. What happens if you join the corners where the ladders meet the legs with Xs? Let’s start with a brace from the lower left to the middle right, and from the upper left to the middle right. With them in place, what happens when you try to push the middle ladder to the right? Those brace pieces come under tension, and they keep the ladder from moving right. They would do exactly the same if they were string, instead of wood; the only way for the ladder to move right would be for the braces to stretch (lengthen); if they don’t stretch, the ladder doesn’t move. Now, let’s try moving the middle ladder to the left. What happens? It moves; the half-Xs bend (the points they’re attached to move closer together, shortening the distance between them). OK, now put the rest of Xs in place; pieces from the lower right corner to the middle left, and from the upper right to the middle left. Now, which ever way (left or right) you push the middle ladder, it can’t move.

If the legs try to bow inward, the ladder (acting in compression) resists that movement. If the legs try to bow out, the Xs (acting in tension) resist that movement. If now add another side (at 90 degrees in a 4-legged tower, at 60 degrees in a 3-legger), this same bracing works in 3 dimensions. Put all four (or 3) sides together, and you have a rigid structure.

The important thing to understand here is that the ladders work in compression, and the Xs in tension. The Xs (if the rules allowed) could be fine thread. The key is that they be tight. If they have any slack in them, then where they join has to move before they tighten up. That movement – buckling starting in the leg, starts the failure process. If the Xs are tight, and where they join can’t move, then that point of the leg stays in place, and everything is cool. In the upper portion of the tower, where the legs are near vertical, it doesn’t take much strength to hold the point where the Xs meet in place. Down in the base, the Xs going to the lower ends of the legs will be under significant tension load (the bottom ends of the legs trying to spread out). That force is on the order of a few kg. High density 1/64th balsa is amazingly strong in tension; pieces 1/16th wide can hold 2-3 kg (try it). 1/16th provides plenty of glue area (joint will be stronger than the wood).

So, for what its worth; it is one way that works.

Len Joeris

Fort Collins, CO