Shrinking Balsa

This discussion area is for basic structure building concepts. Not intended for advanced concepts.
*Good Example: Balsa can bend with the addition of boiling water.
*Good Example: Triangle is the strongest shape for most applications of bracing.
*Bad Example: Balsa can hold X pounds under these conditions.
*Bad Example: Use this shape for this application.
Basically I want you to think a design through with the basics instead of someone else thinking through your design for you.
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How much does the average structure shrink with weight on it?

.01 inches
0
No votes
.02 inches
4
20%
.03 inches
1
5%
.06 inches
3
15%
.09 inches
2
10%
.13 inches
2
10%
.19 inches
3
15%
.25 inches
4
20%
.38 inches
0
No votes
.50 inches
0
No votes
.75 inches
0
No votes
1 inch
0
No votes
1.5 inches
1
5%
 
Total votes: 20

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admin
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Shrinking Balsa

Post by admin » Mon Feb 06, 2006 10:52 am

I have heard of balsa expanding and contracting in width -- that was evident in the It's a Snap problem in OotM but what about the length in compression? I had a tower today at 7.275 inches tall and after the weights were removed it measured 7.150 inches. During testing it shrank to 7.042 inches. I thought I measured wrong for the first measurement, but I have the sticks that were cut at the same time as the ones I used and they are 7.275 -- the same also. Has anyone else ever seen this before? By the way, I think a 110 ratio is attainable after what I just saw. :shock:
Jake Zimmer
jake@specializedbalsa.com
Specialized Balsa Wood, LLC
405 8th Street SE Unit #2
Loveland, CO 80537-6491
(970) 461-WOOD (9663) Phone
(970) 461-9662 Fax
http://www.specializedbalsa.com

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ClarkInMI
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Post by ClarkInMI » Mon Feb 06, 2006 11:29 am

Yes, We've seen it before during It's A Snap. The kids' structure that year consisted of three, square, four-legged, outer structure stacking "rings" made of three laminated vertical sticks glued as an "L" at each corner, with a fourth full height inner structure made of a single light weight stick at each corner that lined up all the pieces and held them together through friction. Final fit made it a standard square tower with four vertical weight bearing sticks at each corner. However, in loading the resulting structure it always broke at a much earlier weight than expected for a similar structure that was made of one piece (all four verticals laminated).

After ruling out sloppiness, leveling, and wood quality, it became apparent that while the outer rings were compressing and shrinking evenly due to their cross bracing and laminating, the inner single sticks were bowing and producing greater stress on the outer rings by pushing against their sides. This was because the inner single stick structure was not braced the same way. It wasn't intended to be a true weight bearing portion of the structure, just an alignment mechanism.

The kids' solution was to intentionally cut the inner structure 1/8" shorter than the outer rings all stacked up so as not to induce bowing when weight was placed on the structure. Instead, the outer rings compressed around the inner structure which didn't bear any weight due to its shorter height. Must have worked because they nearly doubled their weight held between State and World competition from just this one modification.

So, yes, structures do shrink under weight. How much is probably a result of many factors including density of the wood being used, elasticity and malleability of the wood, the amount of weight being held, and length of time the weight is held. And, if the structure does not break, it would not surprise me to see that compression partially rebound once the load has been removed. You're dealing with standard mechanical deformation characteristics of solid materials where some stress is translated into permanent strain, while other stress can be recovered from. This is dependent upon the elasticity and rebound characteristics of the material being compressed which can vary widely. Consider rubber versus putty. Both compress easily, but only rubber springs back to its former shape, it's elastic. The malleable putty, on the other hand, tends to stay in the shape it was squeezed into.
ClarkInMI

GAOM
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Post by GAOM » Thu Feb 09, 2006 9:34 pm

Its funny you post this.... A few Fridays ago I did some compression testing here at school. My test subjects shrank as you described and this is a common occurence... I knew it would happen and wondered how much.. Last year we had to account for that because of the OM ruling on 8" throughout weight placement.


Balsa wood compresses as any other material will under great loads. The higher compression strength a material has, the more load it will encounter before failure. At the same time, more ductile materials will yield (as yours did Jake showing the permanent deformation), and they will continue to hold alot more until they hit the ultimate strength where fracture occurs.

Pertaining to balsa specifically, there is a reason Jake sells his testers for compression and glue. Once you have reached the critical length of balsa it will compress as described above. If you are anywhere close to this length you should experience some sort of deformation well before the structure breaks. This will start occuring instantly and will become noticeable once you have huge loads applied to it. Its only when you hit a load past where the balsa yields that you should experience permanent deformation. We know balsa's Modulus of Elasticity is quite high when comparing it to its density. But, at the same time Balsa is very ductile. How far in the elastic region is balsa capable of going?

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ClarkInMI
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The Amazing Shrinking Structure

Post by ClarkInMI » Tue Feb 14, 2006 12:26 pm

You're right about taking shrinkage into consideration last year with the constant 8" height requirement. Based on ther previous experience, the kids intentionally made the structure taller than usual to make sure it didn't dip below 8" before failure. Was only a 1/8" increase, but every little bit counts.

With regards your question about elasticity of balsa, I assume you mean ability to rebound from compressive stress parallel to the grain? I don't think there's much elasticity there as once the wood begins to buckle you've pretty much permanently deformed it. From compression tests the kids have run to verify critical length assumptions, once buckled in the vice the wood simply falls out as soon as the handle starts to get turned back. Would indicate complete destructive deformation under compression with no rebound. Otherwise, balsa bends pretty well with good recovery when stessed perpendicular to the grain. Not as good as ash or yew, but then no one's making archery bows out of balsa, are they? You'll also want to factor in moisture content as the drier the wood gets the more brittle it becomes.
ClarkInMI

Westilium
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Post by Westilium » Sun Mar 19, 2006 8:35 pm

Since Balsa wood has the ability to "bend". What would happen if a tower, built using SO rules, was designed to shrink as more and more weight was loaded on to it. Using the helical bracing tower picture somewhere on this site, as inspiration :idea: , imagine a tower with helical bracing running tightly woven throughout the height of the tower. Much like a wicker basket. The entire tower would compress therefore changing the vectors of load transfer. Causing a more direct disruption between the load on the table surface.

I’ve done some math and it shows it’s possible to build such a tower. But I am unable to predict the actual load transfers on the tower’s members because material just doesn't exist on how tapered helix balsa towers react.

Conceptually the design seems fine, but practically it may just be a huge waste of time.

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Solodex
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Post by Solodex » Mon Mar 20, 2006 7:36 am

It's an idea in theory, but still remember that you must manufacture it well in order to achieve those theoretical results. I think that it would be far more trouble than it's worth. I know that Jake spent about 3x more time on that circular one than he normally does. I've always liked the KISS principle, because the structures that you build usually end up holding closer to the theoretical max due to higher quality manufacture.

Just my $.02

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Twisting

Post by admin » Sun May 03, 2009 6:22 pm

I saw something for towers I had never seen before. I tested a column in a hydraulic tester that just presses straight down and no twisting or moving back and forth. The column twisted about 1/2 inch in the middle in a circular motion and did not break but was shorter. The press did not move other than straight down. Like I said I did not know that was possible.
Jake Zimmer
jake@specializedbalsa.com
Specialized Balsa Wood, LLC
405 8th Street SE Unit #2
Loveland, CO 80537-6491
(970) 461-WOOD (9663) Phone
(970) 461-9662 Fax
http://www.specializedbalsa.com

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Solodex
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Re: Twisting

Post by Solodex » Mon May 04, 2009 10:41 am

admin wrote:I saw something for towers I had never seen before. I tested a column in a hydraulic tester that just presses straight down and no twisting or moving back and forth. The column twisted about 1/2 inch in the middle in a circular motion and did not break but was shorter. The press did not move other than straight down. Like I said I did not know that was possible.
Only thing that comes to mind is that the 1/2 inch that twisted has a drastically different localized Young's Modulus value than the rest of the piece, resulting in a localized plastic deformation load. This column is uniform right? Not a different structure in the middle that would change the second moment of inertia?

Regards,

Nate

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UofI
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Post by UofI » Mon May 04, 2009 11:48 pm

Nate, you took the words out of my mouth.

That's why you would have to take the integral of the YM over the length of the stick to get the theoretical YM of the stick.

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