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Good question.
You are right, leg ends will be trying to slide out. So pieces connecting legs at the bottom would be under tension. As I mentioned (4th message above). you may want to put a tension strip around the very bottom (using same material you use for your Xs, and glued on the outside (not between the legs like a ladder)

Does anyone happen to know the national winning bridge score from last year?

All right Balsa man, my finished tower yesterday looked very sturdy, at 7.6 grams. Each leg was about 0.45 gram after cutting, and my bracings were about 1.3 grams per side. Took it to the competition, it failed at about 7kg. Near the top, one of the towers legs had bent. No bracings were lost from the breaking itself, but the loading block sheared an entire row of bracings as it fell. Have not had a tower before that broke like this. I feel like it could have held much more, but one of the legs must have been weaker than the rest. It happened on one leg, halfway between the first ladder bracing and the loading block base. Two of the tower sides are completely intact, no leg or bracing damage. This tower definitely did not reach its potential, not the usual explosion break.

Hey, thanks for the report.
That's a nice low weight.
It sure does sound like one leg weak. The fact it buckled in the middle of a braced interval (right where it should) says your bracing was sound. One other possibility that could cause what happened is if your top (top ends of legs) was not....perfect; one leg a hair longer than the others. That would put disproportional load on it. If it was a bit weak, and getting more than a quarter of the load, what you saw is what would happen.

One other thing to think about for your next tower; dealing with the inherent variation in wood. You can have two 36" sticks that weigh about the same, and show the same buckling strength at 36", and depending on where within the stick you take your leg, it can be lighter/weaker, or stronger/heavier. Using the same inverse square calculation (from above), you can check actual legs to make sure you didn't include a weak section- cut like a cm longer than final leg length, test, and compare to what it should be. So, if 36" (91.6cm) tests at 15, a 61cm piece should test at 2.255 x what it tested at 36" (1/0,666 squared); 2.255 x 15 = 33.8gr.

Hopefully, your next one will perform better!

Balsa Man wrote:Hey, thanks for the report.
That's a nice low weight.
It sure does sound like one leg weak. The fact it buckled in the middle of a braced interval (right where it should) says your bracing was sound. One other possibility that could cause what happened is if your top (top ends of legs) was not....perfect; one leg a hair longer than the others. That would put disproportional load on it. If it was a bit weak, and getting more than a quarter of the load, what you saw is what would happen.

One other thing to think about for your next tower; dealing with the inherent variation in wood. You can have two 36" sticks that weigh about the same, and show the same buckling strength at 36", and depending on where within the stick you take your leg, it can be lighter/weaker, or stronger/heavier. Using the same inverse square calculation (from above), you can check actual legs to make sure you didn't include a weak section- cut like a cm longer than final leg length, test, and compare to what it should be. So, if 36" (91.6cm) tests at 15, a 61cm piece should test at 2.255 x what it tested at 36" (1/0,666 squared); 2.255 x 15 = 33.8gr.

Hopefully, your next one will perform better!

Thanks Balsa Man! I have another invite next week, and hope to have 1/8 by then. The reason I had 3/32 for this invite was because I could not find 1/8. Probably looking for pieces of the same weight from 1.2-1.5 grams. Still plan on ladders and X's. Have to go to store with a scale... Have not figured out the bracing interval and stuff yet, maybe from 5-7.

How do you know how heavy a sheet would be if you want to strip balsa, for example can someone tell me the density of a balsa sheet if you do a 1/16*1/16 0.55 gram stick, I know the density, but at the store, they ain't have any label that says the density of the sticks.

Random Human wrote:How do you know how heavy a sheet would be if you want to strip balsa, for example can someone tell me the density of a balsa sheet if you do a 1/16*1/16 0.55 gram stick, I know the density, but at the store, they ain't have any label that says the density of the sticks.

Balsa density can be inconsistent throughout a sheet. You can bring a scale to the store. I have a balsa density calculator that allows any inputs: http://scioly.github.io/freeflight/#bal ... calculator. Entering 0.0625" x 0.0625" x 48" at 0.55 grams gives density 11.1748 lbs per cubic foot. Then entering 0.0625" x 4" x 48" at 11.1748 lbs per cubic foot gives a mass of 35.200 grams.

bernard wrote:

Random Human wrote:How do you know how heavy a sheet would be if you want to strip balsa, for example can someone tell me the density of a balsa sheet if you do a 1/16*1/16 0.55 gram stick, I know the density, but at the store, they ain't have any label that says the density of the sticks.

Balsa density can be inconsistent throughout a sheet. You can bring a scale to the store. I have a balsa density calculator that allows any inputs: http://scioly.github.io/freeflight/#bal ... calculator. Entering 0.0625" x 0.0625" x 48" at 0.55 grams gives density 11.1748 lbs per cubic foot. Then entering 0.0625" x 4" x 48" at 11.1748 lbs per cubic foot gives a mass of 35.200 grams.

So theoretically, the 4 inch x48 inch balsa sheet should weigh 35.200 grams, and is porportional in density to a 0.55 gram 1/16*1/16*36 inch balsa stick

Random Human wrote:

bernard wrote:

Random Human wrote:How do you know how heavy a sheet would be if you want to strip balsa, for example can someone tell me the density of a balsa sheet if you do a 1/16*1/16 0.55 gram stick, I know the density, but at the store, they ain't have any label that says the density of the sticks.

Balsa density can be inconsistent throughout a sheet. You can bring a scale to the store. I have a balsa density calculator that allows any inputs: http://scioly.github.io/freeflight/#bal ... calculator. Entering 0.0625" x 0.0625" x 48" at 0.55 grams gives density 11.1748 lbs per cubic foot. Then entering 0.0625" x 4" x 48" at 11.1748 lbs per cubic foot gives a mass of 35.200 grams.

So theoretically, the 4 inch x48 inch balsa sheet should weigh 35.200 grams, and is porportional in density to a 0.55 gram 1/16*1/16*36 inch balsa stick

No, I assumed your stick is 48 inches since you did not specify length. A 0.0625" x 0.0625" x 36" stick at 0.55 grams is 14.8997 pounds per cubic foot. Then a 0.0625" x 4" x 48" sheet at the same density would be 46.933 grams assuming consistent mass distribution.

Yup