Tesel wrote:fdf4 wrote:The numbers I gave were just measurements I'm most interested in using, but basically, how should I find what density of wood to use for bracing and the different lengths?
If you'll read back in the thread a little, you'll see that 1/16x1/16 wood has problems. 1/64x1/64 wood, meanwhile, would probably be too thin to effectively work. What's instead suggested is strips (1/16 or 3/32 wide) of 1/64" sheets.
Density will depend on the thickness, but what you need is a tensile strength of about 1 kg. I believe I was recommended to use 7-8g sheets (1/64x3x36), but if you have access to pre-cut sticks you'll have to calculate density yourself.
Density isn't really the number you should be looking for, you need to find pieces of the proper tensile strength, and density is only an approximate indicator.
Let me add a couple of things that may help and clarify.....
The strips (cut from 1/64th" thick sheet) are for the Xs in a ladders and Xs bracing system.
The density will depend on the density (i.e., weight) of the sheet, since, at whatever sheet weight, they're of the same thickness. Tensile strength is related to density, as is buckling strength. At 7-8gr/sheet, you're in the range where strips at 3/32" almost surely will have >1kg tensile, and at 1/16 probably will. Watch out for soft zones in the sheet, where the grain pattern... goes away. After you've cut a strip, pull from both ends; if you can pull at a force of a kilo or so, good to go; pre-tested. These Xs a) work in tension only, and b) have next to no buckling strength (because of the 1/64th" dimension. They act to prevent outward buckling of the legs at the braced points. Ladders at the braced points act to prevent inward buckling, coming under compression loading. Some are using 1/16th" sticks in bracing where they have to handle tensile load (Xs, Z, and V bracing configurations). The limits in this loading may come at very low density (0.2, 0.3, maybe 0.4gr/36" sticks) from shearing of a thin layer of the brace piece because of the low density.
I've been recommending 1/8" ladders, in really low density (0.7-0.8gr/36"). Some folk are apparently using 1/16" sticks for ladders (and Xs, Zs, and Vs) with success. I've described at length within the last two pages of this thread , (go back and find and digest the long post) how to test for, and apply in design, buckling strength. For pieces under compression, the longer it is, the weaker its buckling strength is, and a pretty small increase in length substantially reduces the buckling strength. So, that means that bracing pieces down toward the bottom of the tower that work in compression loading (ladders in a ladders and Xs system, Xs only, Zs, Vs) have to be stronger (in buckling strength) than they do toward the top, where they're shorter.
And last, remember that the key to design/successful performance is strength, be it tensile or buckling strength. Both are a function of density, but because we're dealing with wood, there is substantial variation around the statistical mean. You take 100 1/8" x 36" sticks weighing 1.7gr, you'll see a mean/average buckling strength around 50gr (single finger push down testing). But a few will show a buckling strength close to 60gr, and a few down toward 40gr.
