Towers B/C

[dhdarren]

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

Stevenson won with a score of 3856.

[Random Human]

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.

I'm still a little confused. So, a 1/16*1/16*36 inch (.5gram ) stick would have a density of 14.8997 pounds per cubic foot. So I go to the store and buy a 46.9(ish) gram sheet?

[bernard]

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.

I'm still a little confused. So, a 1/16*1/16*36 inch (.5gram ) stick would have a density of 14.8997 pounds per cubic foot. So I go to the store and buy a 46.9(ish) gram sheet?

Depends on the size of the sheet you're buying.

[Dohnnovan]

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

Stevenson won with a score of 3856.

what!! how light was that bridge?

[Unome]

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

Stevenson won with a score of 3856.

This was Div C by the way, Div B was Daniel Wright with a bit over 5000.

[Balsa Man]

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.

Remember, while buckling strength increases with density, at any given density (stick weight), you'll see variation, so selection for use should be based on the buckling strength you measure.. In 1/8" 1.2gr/36" sticks, the data I have says median one finger push down buckling strength at about 28gr, and for 1.5gr/36", about 35gr. Doing inverse square calcs, and using effective length multiplier of 2.3 (for ladders and Xs bracing) and using a 20% safety factor, for 1/5 interval bracing, you'd want a scale reading about 36gr, for 1/6 interval about 25gr, and 1/7 interval, about 18gr..

[Complexity]

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.

Remember, while buckling strength increases with density, at any given density (stick weight), you'll see variation, so selection for use should be based on the buckling strength you measure.. In 1/8" 1.2gr/36" sticks, the data I have says median one finger push down buckling strength at about 28gr, and for 1.5gr/36", about 35gr. Doing inverse square calcs, and using effective length multiplier of 2.3 (for ladders and Xs bracing) and using a 20% safety factor, for 1/5 interval bracing, you'd want a scale reading about 36gr, for 1/6 interval about 25gr, and 1/7 interval, about 18gr..

In confirmation to what you said about one leg being possibly weaker, I discovered that one of my legs was about .2 grams lighter than the rest. This was a huge diff for 3/32 wood, and probably resulted in the breaking. I remembered that I did not have a very good selection of 3/32 at the store, as I did not have an accurate scale at that time and there were not many options. Have a more accurate scale now, and hopefully will not repeat this mistake when I go out to buy wood tomorrow. Will keep in mind to make selection based on BS. Dont know yet which density to buy, have to see what they have at the store. I called, and they had received a selection of a couple hundred pieces of wood, so hopefully I can be selective.

[Balsa Man]

Good confirmation data. The difference in buckling strength you'd expect with one 0.2gr lighter works out pretty darn close to the difference in load carried and full load.

Catching a fresh delivery of wood is cool- that'll give you a good chance of finding some choice sticks; stiffer than average for their weight. Happy hunting!

[BananaPirate]

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.

Remember, while buckling strength increases with density, at any given density (stick weight), you'll see variation, so selection for use should be based on the buckling strength you measure.. In 1/8" 1.2gr/36" sticks, the data I have says median one finger push down buckling strength at about 28gr, and for 1.5gr/36", about 35gr. Doing inverse square calcs, and using effective length multiplier of 2.3 (for ladders and Xs bracing) and using a 20% safety factor, for 1/5 interval bracing, you'd want a scale reading about 36gr, for 1/6 interval about 25gr, and 1/7 interval, about 18gr..

In confirmation to what you said about one leg being possibly weaker, I discovered that one of my legs was about .2 grams lighter than the rest. This was a huge diff for 3/32 wood, and probably resulted in the breaking. I remembered that I did not have a very good selection of 3/32 at the store, as I did not have an accurate scale at that time and there were not many options. Have a more accurate scale now, and hopefully will not repeat this mistake when I go out to buy wood tomorrow. Will keep in mind to make selection based on BS. Dont know yet which density to buy, have to see what they have at the store. I called, and they had received a selection of a couple hundred pieces of wood, so hopefully I can be selective.

What store do you go to? None of the stores I frequent get shipments of such large quantity. I'm jealous >.>. Had to go to 3 different stores and ended up buying 11 sticks.

[Complexity]

Remember, while buckling strength increases with density, at any given density (stick weight), you'll see variation, so selection for use should be based on the buckling strength you measure.. In 1/8" 1.2gr/36" sticks, the data I have says median one finger push down buckling strength at about 28gr, and for 1.5gr/36", about 35gr. Doing inverse square calcs, and using effective length multiplier of 2.3 (for ladders and Xs bracing) and using a 20% safety factor, for 1/5 interval bracing, you'd want a scale reading about 36gr, for 1/6 interval about 25gr, and 1/7 interval, about 18gr..

In confirmation to what you said about one leg being possibly weaker, I discovered that one of my legs was about .2 grams lighter than the rest. This was a huge diff for 3/32 wood, and probably resulted in the breaking. I remembered that I did not have a very good selection of 3/32 at the store, as I did not have an accurate scale at that time and there were not many options. Have a more accurate scale now, and hopefully will not repeat this mistake when I go out to buy wood tomorrow. Will keep in mind to make selection based on BS. Dont know yet which density to buy, have to see what they have at the store. I called, and they had received a selection of a couple hundred pieces of wood, so hopefully I can be selective.

What store do you go to? None of the stores I frequent get shipments of such large quantity. I'm jealous >.>. Had to go to 3 different stores and ended up buying 11 sticks.

I am considering buying specialized wood online, but for now I go to Nankins Hobby. They have much more selection, and I get my glue from there as well. I feel your pain, last invite I used low end wood as the shelves were all empty. This shipment came the day after my invite When I did bridges last year, I used to go to random places like Hobby Lobby, and it was like you said- low stock and inconsistency