Wood

[Random Human]

makes sense....

What would be better, the 3/32 .6 or the 1/16 .3 (densities)

What do you mean, "better"?

Which would be more efficient?? Would you say the weight matters more or the additional surface area

It really depends on your design and am frankly confused. Buckling strength, as balsa man said, is key. Buckling is what it all comes down too. Then, all you have to do is apply these concepts on your tower initially. Take close analysis footage and watch the wood warp. Look where the points on your towers are weakest and do research to further develop these weak points.

[dholdgreve]

This is getting a little off this specific topic, but a really helpful tool is a safety cage... a heavy cage that can be built from shop scraps that is roughly 1 cm shorter than the tower (I made mine adjustable, using the leg adjusters that thread in and out at the bottom of table legs). The tower sits inside the cage on the testing table. The 5 cm x 5 cm loading block gets switched out to 5 cm x whatever length is needed to span the safety cage. Load the tower as normal. When the tow collapses, the loading block is not able to fall completely through the tower doing secondary damage, so you are able to clearly see where the primary failure was... column?, bracing?. bottom tension band?, compression piece at the top of the bottom section?

This truly allows you to upsize only the pieces you know for a fact are undersized.

[Balsa Man]

This is getting a little off this specific topic, but a really helpful tool is a safety cage... a heavy cage that can be built from shop scraps that is roughly 1 cm shorter than the tower (I made mine adjustable, using the leg adjusters that thread in and out at the bottom of table legs). The tower sits inside the cage on the testing table. The 5 cm x 5 cm loading block gets switched out to 5 cm x whatever length is needed to span the safety cage. Load the tower as normal. When the tow collapses, the loading block is not able to fall completely through the tower doing secondary damage, so you are able to clearly see where the primary failure was... column?, bracing?. bottom tension band?, compression piece at the top of the bottom section?

This truly allows you to upsize only the pieces you know for a fact are undersized.

Right-on! This is a very important tool if you're really serious; fighting for those last few points, last few tenths of a gram. The safety cage Dan describes is a variant of a 'safety tower' I described/provided a picture of some years ago. By limiting the amount the load block can fall, you can clearly see what the initial failure mode is; where it is, exactly what kind of failure it is. . Assuming the failure is in a piece of wood (as opposed to glue/joint), you have to increase the strength of that piece of wood- you can do that by up-sizing, or you can do it by increasing density (bearing in mind the variability of strength vs density we've discussed before). BTW, from my experience, a full centimeter of load block fall is too much; the initial failure will have progressed to secondary, even tertiary failure modes. Except under.....very unusual conditions, about 1/8" of block drop is all you need/want. If something has happened in the tower to allow the top/the block to move down by 1/8", something has broken.

Safety cage or safety tower has important advantages over video. With 'regular' video (as opposed to high speed-high frame rate video), you may miss the actual initial failure. With even high speed video (in addition to the issue of massive file size if you start when loading starts, and failure isn't until very near full load), unless the camera position is just right, you won't be able to see the point/area of failure clearly enough to know for sure what happened. Multiple high speed video cams from different angles can help the visibility problem, but a safety cage/tower... freezes everything in time- you can look at things from any angle while it's still under load, you can take the load off, and examine as closely as you want.

[pjspol]

Balsa Man,

When you say "The effective length factor for all Xs is significantly less; about 0.55.", are you referring to the K value in Euler's Buckling theorem explicitly or to some derivation? If explicitly, then X braces should be much better than X s and ladders because the value is squared in the denominator. Thank you for your help!

[Balsa Man]

Balsa Man,

When you say "The effective length factor for all Xs is significantly less; about 0.55.", are you referring to the K value in Euler's Buckling theorem explicitly or to some derivation? If explicitly, then X braces should be much better than X s and ladders because the value is squared in the denominator. Thank you for your help!

That's a very good question, pjspol!

It is a derived value. Figured it out (and posted a discussion) about a year ago:
viewtopic.php?f=243&t=10020

Its a relatively short thread, worth reading in its entirety. As I subsequently posted, it turned out that 2.3 is 'predictive' for ladders and Xs bracing- specifically, with 1/8" legs, ladders at 1/8" butt-jointed between adjacent legs, and Xs from 1/64" sheet (about 1/16" wide, put on under slight pre-tensioning, from high enough density to get a bit over 2kg tensile strength), ladder strength also at a bit over 2kg (doing inverse square calc just like done for legs). This post/description describes how/why this 2.3 factor is consistent with/comes from Euler's theorem- how 'end conditions' drive effective length. By "predictive" I mean it worked- in a tower with leg strength calculated at some bracing interval to be greater than force on leg at full tower load, tower held; with leg strength calculated at 90% of force at full load, held about 13.5kg. This was done using a 10% safety factor; design load at measured 36" single finger push-down (SFPD) x 2.3 at/greater than force on leg x 1.1. We also found that SF down to 5-6% worked when instead of testing sticks at full 36" length, you cut to installed length + a couple of centimeters. There is significant variation in BS within a 36" stick; by going to shorter piece, you get less of that variation.

I later described and posted about how we switched from ladders and Xs to 'all Xs' for Nationals, and got to a different factor (about 0.55) by reverse engineering using data I got from a couple of out of State, State-winning towers (data on measured leg wood BS and on (1/32") sheet density used for the Xs (at 1/16 x 1/32). This turned out to be predictive, in that both the C tower and B tower that went to Nationals from Colorado held full load at Nationals. The switch from ladders and Xs to all Xs did (as calculated) save us tower weight.

[MadCow2357]

Hi,
I am ordering balsa wood soon, but I am not sure which densities of balsa I should get. I know Balsa Man said some things about wood density, but I was a little overwhelmed at the amount of information in one of the posts (The math is too complicated for me at this point, but I will try to figure it out). I know I will be using 1/8s, 3/32, 1/16, and possibly 5/32. Any recommendations on Balsa Wood densities?

Thanks,
MadCow2357

[Balsa Man]

Hi,
I am ordering balsa wood soon, but I am not sure which densities of balsa I should get. I know Balsa Man said some things about wood density, but I was a little overwhelmed at the amount of information in one of the posts (The math is too complicated for me at this point, but I will try to figure it out). I know I will be using 1/8s, 3/32, 1/16, and possibly 5/32. Any recommendations on Balsa Wood densities?

Thanks,
MadCow2357

Hi, MadCow,
Welcome to the tower forums. The fact you’ve come here looking for info and understanding is a good thing. Ditto that you understand that wood density matters is a good thing, and a good first step.
Wish there was a simple answer to your question. Unfortunately there isn’t.
First, it depends on your goals – how “good” a tower do you want to get to? Good enough to not be in the bottom 10% at invitationals/regionals, good enough to be in the top 25%of the pack at invitationals/regionals, medaling/winning at State? Top 10 at Nationals?. How hard are you willing to study and work?
Any ‘recommendations’ on density would have to be based on what you’re trying to do- the higher your goal, the deeper you will have to get into, and come to understand that scary math. Before really getting into the math, it is important to get your head around the basic concepts, then dig deeper to understand in more detail, and understand how the math helps you figure things out.
You really shouldn’t be ordering wood at… whatever density specifications until you have at least to some extent nailed down basic design. The basic design decision is the tradeoff between stiffer/heavier legs needing less bracing (a wider bracing interval), and floppier/lighter legs with more bracing (a tighter bracing interval). It is the STRENGTH, specifically the buckling strength, of the leg wood that matters; higher density (and larger cross section) gets you more strength. Basically, you’re looking for the lightest wood that has enough strength for the tower to hold near full load. To be able to do that, you need to a) know the forces the legs will see at full tower load, b) be able to calculate the bracing interval needed (which will depend on the style/type of bracing you use) to brace a leg of a given strength into short enough intervals so that the leg has greater buckling strength than the load it has to carry.
Density/stick weight then comes into play. As I’ve said many times, there is a relation of density to buckling strength – higher density means/ will get you higher buckling strength, but there is significant variation within that ‘trend’ you can have a bunch of sticks at some given density/stick weight, and their buckling strength will vary considerably (+/- 10, even 20%). So, when you have figured out the buckling strength you need, then you know the density range within which you should be able to find sticks of sufficient strength. There is a graph posted that shows you buckling strength v stick weight (for 1/8” sticks) data. There is also an ‘inverse square’ table posted that shows you what bracing interval at what 36” stick buckling strength gets legs braced to sufficient strength to hold full load.
If you take the time to read, study, think about all the info that’s here in this forum, you will be able to figure this out. Feel free to ask questions as your understanding increases.

This said, if all this is…..just too much, if, for your base section you use “medium” density 1/8” for legs (1.5, 1.6, 1.7gr/36”), and brace it with all Xs (using medium density 1/16), at a bracing interval of ¼, and for the chimney use the same medium density braced at ¼ interval, it will probably carry full load (if you have used a reasonably decent jig to assemble it.

[dholdgreve]

Hi,
I am ordering balsa wood soon, but I am not sure which densities of balsa I should get. I know Balsa Man said some things about wood density, but I was a little overwhelmed at the amount of information in one of the posts (The math is too complicated for me at this point, but I will try to figure it out). I know I will be using 1/8s, 3/32, 1/16, and possibly 5/32. Any recommendations on Balsa Wood densities?

Thanks,
MadCow2357

Rather than specifying exact densities (and spending as much as double what random densities cost), I'd suggest using the same budget and buying double the amount of wood, then carefully sorting what you have by mass, matching the higher density columns to fewer X braces, and the lighter density columns to more X braces. The key to this competition to find the magical ratio that will provide the highest possible efficiency. This can't be done if ordering just certain densities.

[Balsa Man]

Hi,
I am ordering balsa wood soon, but I am not sure which densities of balsa I should get. I know Balsa Man said some things about wood density, but I was a little overwhelmed at the amount of information in one of the posts (The math is too complicated for me at this point, but I will try to figure it out). I know I will be using 1/8s, 3/32, 1/16, and possibly 5/32. Any recommendations on Balsa Wood densities?

Thanks,
MadCow2357

Rather than specifying exact densities (and spending as much as double what random densities cost), I'd suggest using the same budget and buying double the amount of wood, then carefully sorting what you have by mass, matching the higher density columns to fewer X braces, and the lighter density columns to more X braces. The key to this competition to find the magical ratio that will provide the highest possible efficiency. This can't be done if ordering just certain densities.

I’m very reluctant to say someone who has (successfully) been coaching the ‘balsa building events’ for many years may be wrong on something. We each have developed our understandings, and ways/techniques that ‘work.’ Dan and I are…. ‘on the same page’ on so many things. I know very well I don’t know ‘everything’, and, like all of us, can certainly be wrong at times.

The reason I believe it is important/smart to incorporate some density… range/boundaries in wood ordering is because a) there is a relationship between density (stick weight) and buckling strength, and b) it is the buckling strength that matters. Sticks of the same weight/density can and will have significant variation in buckling strength. To get the best tower performance, you want to get/find the lightest set of sticks that have your ‘design’ buckling strength.

You can easily calculate – in fact you can just look up in the inverse square table I posted a link to, what buckling strength of legs, at what bracing intervals, will get you to a braced leg buckling strength that will carry the load. You can also look up, for 1/8” sticks, what stick weight range will get you sticks with the buckling strength you need.

If you’re really pushing/fighting for those last few grams, or tenths of grams of tower weight, you can buy/order (from a place like Specialized Balsa), sticks in a fairly narrow range, like some 1.1gr/36”, some 1.2gr/36”, some 1.3gr. These numbers are just “for example” numbers. If you’re not pushing as hard to be seriously competitive, you can order in a broader range, like “light”, “medium”, “heavy”, and by weighing/testing find leg sets that will “work” at a given buckling strength and bracing interval, and use the lightest in a competition tower. The…basic engineering information I’ve posted about is real; the approach of being able to calculate strengths needed works, and the data on the weight range of sticks that will have some sticks at a given strength is real. Just my perspective…

[dholdgreve]

Again, 2 different schools of thought... nothing wrong with that.
Another area where Len and I look at differently...
When we receive a shipment of column material, the first thing we do is to cut them from 36" lengths to 12" lengths. Then we separate by weight (to the nearest tenth of a gram), then further stratify by separating the individual weight piles by bending strength. We have all noticed how, if stripping your X braces from sheets, the weight can vary wildly from one side of the sheet to the other. The same holds true in bending strength. Testing a 36" long stick for bending strength assumes that the stick has those same properties throughout its length. In reality, the stick is more like a chain, and can bow at its weakest area, or more accurately, the weakest area can precipitate a bow in the middle of the stick. Also, if you were to take a 36" stick and cut it into (3) equal 12" lengths, it is unlikely that all 3 pieces will weigh exactly the same. With different weights will likely come different bending strengths. By blocking your potential columns into individual 12" lengths, you can analyze each column independently, without the influence of adjacent pieces in a 36" stick.

The downside to this might be that most super sensitive scales that can weight to .01 grams have a limit of 500 grams. This can be an issue on some 12" sticks in the .5 gram weight and up, as the BS may exceed the limits of the scale.