Towers B/C

[dholdgreve]

I have a question.

When selecting wood for a tower, I usually try to get 4 legs that are about the same density as well as have the same BS. I know that having equal BS is extremely important as the tower will break at the weakest point, and if one leg has a lower BS, there is a very high chance that it will be where my tower breaks.

The thing is, does the density matter if the BS for all the sticks are the same?

e.g. leg 1,2,3 have a density of 108.29 kg/cm^3 and have a BS of 30. Leg 4 has a density of 121.29 kg/cm^3 but also has a BS of 30.

If you use all 4 of those legs, will the uneven weights in the legs cause the tower to break there? Maybe does an uneven weight play a factor?

First, let me say that I have nothing to support my opinion... No fact, data, or studies... so take it for what it's worth. I'd say that in the bending strengths are equal, the density is secondary... There is another measure of strength that no one talks about, because it is totally destructive... Once you test the piece, you are done with it... The actual point at which it breaks. The SFPD Test tests when the piece actually bows, and the resistance when it is in the bow, but the other test will test the amount of stress required to actually break the piece. I believe this test will be more proportionate to the density of the wood.

A few additional thoughts: I can't possibly stress enough how important it is for the loading block to be positioned so the bolt is perfectly placed in the exact center of the tower to give the best possible chances of distributing the load evenly to all columns. Next, it is every bit as critical to have all 4 columns exactly the same height so the weight is distributed evenly to all 4 columns. Next, make sure your tower is perfectly plumb... vertical... this to make sure that the ... you get the picture. Finally, make sure your table is perfectly level in all directions. This allows all 4 columns to carry the same load. If the table is not level, the column nearest the low corner will likely break first. To test this theory, place an inexpensive scale that you might weigh a tower with under each of the legs while in the testing position. Now begin loading... You only need to load 2 kg or so... Let the bucket settle, and check the readings on all the scales. Even when you think the top is level, the tower is plumb, and the bolt is centered, you will be surprised how different the loads are on the tower columns!

[Cow481]

I have never done the bucking strength test on my towers but I do a test where i get a piece of wood and let it hang to measure the bowing. Is there any difference between the two tests? I have heard of people doing the bucking strength test but never the hanging test.

[SPP SciO]

I have never done the bucking strength test on my towers but I do a test where i get a piece of wood and let it hang to measure the bowing. Is there any difference between the two tests? I have heard of people doing the bucking strength test but never the hanging test.

So as a matter of fact my students were doing this same thing last year - basically they'd hang a portion of a stick off the end of a table and hang a tiny hooked weight on the end, to see how much bending there was. I asked Balsa Man about this via email, and this is what he replied:

"In this thread viewtopic.php?f=243&t=10020, there’s good, detailed discussion of how to do testing (as in how I have come to do it). The discussion also goes on to cover how to use the data you develop from testing. What your students were doing (hanging weight, measuring/observing deflection – deflection testing) will get you rough, relative measurement (which is significantly better than nothing). This approach doesn’t directly measure buckling strength, but the deflection measured will (in the absence of other uncontrolled variables) be proportional to “E”, the modulus of elasticity, Young’s Modulus – the inherent stiffness in a given piece of wood (which, along with wood cross section, is what determines buckling strength). The biggest variable is the straightness of sticks being tested. Few sticks are really straight. If you test a stick (that has even a little bit of curve) when its oriented so its curving up, you’ll get some value. Rotate it 180 degrees, so it curves down, it will deflect more (under a given load). Rotate it 90 degrees, so the curve is left or right, you’ll get a different value.

Another variable is that with deflection testing as described, you’re only testing the portion of the stick hanging off the edge of the table; there could well be significant variation in stiffness compared to the portion on the table. So, with a fair amount of work, you could create an apparatus to ….reproducibly test deflection, and take four readings per stick, then switch ends being tested and get 4 more readings to get to a reasonably representative number that would allow you to assess the relative strengths of sticks. With a lot of testing, and a lot of calculation, you might be able (though I’m not sure exactly how) to get to a buckling strength value, in grams, you could then use with Euler’s Buckling equation to get to buckling strength when braced at a given interval "

[randomperson123]

Does nationals use auto loaders or hand pouring?

[waffletree]

Nationals is pretty competitive and they're usually really official and stuff. Unless something weird happens, they should be using auto loaders.

[MadCow2357]

How much does an auto-loader help, anyway. I have never used one.

[cheese]

it helps in it gives a consistent rate of sand, rather than having the tower being suddenly stressed after each scoop of manual loading.

[MadCow2357]

How do you conduct single finger push down tests? I have read through the forums already, but I may have missed some things.

I have been selecting my balsa based on mass per stick, but I have come to realize that the buckling strength can greatly vary even with balsa sticks of the same weight. I am using 1.2g 1/8"*1/8" sticks for the columns, and 0.3g 1/16"*1/16" for bracings.

[MadCow2357]

it helps in it gives a consistent rate of sand, rather than having the tower being suddenly stressed after each scoop of manual loading.

Ok that would have boosted all of my recent scores by about 200 then.

[stevepilot]

Does anyone know if 1/32 x 1/32 wood would be strong enough for the chimney X's (provided a fairly tight interval of 8-10 X)? Also, is it necessary to have a ladder at the top of the chimney or is it only needed at the top and bottom of the base?