3 Sided Tower?

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3 Sided Tower?

Post by TheGeekyScientist » April 26th, 2017, 7:32 pm

I have had this idea on having a 3 sided tower. All the 4 sided towers have been over 5 grams. I was thinking that if I get rid of 1 side... then I might be able to reduce the weight by possibly around 1-2 grams From what I'm imagining, it might not be as good as a 4 sided tower. Has anyone tried making a 3 sided tower or might know if this is a good idea?
Thanks.

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Re: 3 Sided Tower?

Post by Unome » April 26th, 2017, 8:06 pm

It is definitely possible to make a successful 3-sided tower. I have personally witnessed several. However, if you can reach 3000 (or even 2000 for that matter), there's probably no reason for you to be doing so, as the benefit of weight loss is probably outweighed (pun not intended) by the structural instability (from what I can remember of Balsa Man's posts on the subject, though I easily might be wrong here). I would not recommend attempting this right before your state tournament; fine-tuning a jig for a 3-sided tower sufficiently to make it competitive will likely be difficult even for very skilled builders.
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Re: 3 Sided Tower?

Post by a boy » April 26th, 2017, 8:35 pm

A 3 sided tower's legs will be longer, and will need to be stronger than 4-legged tower's legs to support the same mass. One benefit of 3-legged towers is that the 3 legs will always be in contact with the surface so less fine sanding needed. It is definitely harder to build though since you probably need an actual jig, while 4-legged designs can get away with being built on 2D plans.
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Re: 3 Sided Tower?

Post by Crtomir » April 27th, 2017, 5:54 am

We tried a 3-sided tower with only limited success. We build a 3-sided jig, which was pretty difficult to begin with. Also, aligning the mounting block at the top so that the center of the block is at the center of the triangle formed by the three columns is difficult. More importantly, the legs (or columns) are even less vertical than a 4-sided one, meaning that the loading forces are not just straight down the legs, but there is a significant transverse force as well. The legs have square cross sections, so when you put them in a 3-legged design, the angle of the faces of the legs is not flat across the side of the tower. If you do your bracing with lap joints (recommended), you will see that the braces have to bend significantly at the legs to attach to the faces of the legs. We overcame this by filing or sanding the faces of legs to the correct angle that matched the side of the 3-legged tower at each of the bracing attachment points. This took a long time for our kids to build. In the end, the 3-legged towers still broke too soon and never reached the kind of scores we were able to get with a 4-legged tower. Now, we know the reason is because for a 3-legged tower, each leg has to support 15kg/3 = 5kg, but for a 4-legged tower, each leg has to support only 15kg/4 = 3.75kg. Yes, for a 3-legged tower, you save on the weight of one leg, but you would do better to cut the density down of the four legs of a 4-legged tower. Whatever weight you save by cutting down the density of the wood for the legs is multiplied by a factor of 4. That becomes pretty significant. Also, for a 3-legged tower, the length of the X-braces becomes considerably longer making them weaker. That's the reason ALL of the towers scoring over 3000 points have not attempted the bonus. They are using such low density wood for the X-braces that they need them to be short in length. Consequently, they have to have the legs of the tower closer together. Also, by putting the legs of the tower closer together (not going for the bonus), the legs are more vertical and the loading forces are mostly directly down (parallel to) the legs.

That being said, has anyone seen a tower score over 3000 points this year with the bonus?

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Re: 3 Sided Tower?

Post by TheGeekyScientist » April 27th, 2017, 1:30 pm

Crtomir wrote:We tried a 3-sided tower with only limited success. We build a 3-sided jig, which was pretty difficult to begin with. Also, aligning the mounting block at the top so that the center of the block is at the center of the triangle formed by the three columns is difficult. More importantly, the legs (or columns) are even less vertical than a 4-sided one, meaning that the loading forces are not just straight down the legs, but there is a significant transverse force as well. The legs have square cross sections, so when you put them in a 3-legged design, the angle of the faces of the legs is not flat across the side of the tower. If you do your bracing with lap joints (recommended), you will see that the braces have to bend significantly at the legs to attach to the faces of the legs. We overcame this by filing or sanding the faces of legs to the correct angle that matched the side of the 3-legged tower at each of the bracing attachment points. This took a long time for our kids to build. In the end, the 3-legged towers still broke too soon and never reached the kind of scores we were able to get with a 4-legged tower. Now, we know the reason is because for a 3-legged tower, each leg has to support 15kg/3 = 5kg, but for a 4-legged tower, each leg has to support only 15kg/4 = 3.75kg. Yes, for a 3-legged tower, you save on the weight of one leg, but you would do better to cut the density down of the four legs of a 4-legged tower. Whatever weight you save by cutting down the density of the wood for the legs is multiplied by a factor of 4. That becomes pretty significant. Also, for a 3-legged tower, the length of the X-braces becomes considerably longer making them weaker. That's the reason ALL of the towers scoring over 3000 points have not attempted the bonus. They are using such low density wood for the X-braces that they need them to be short in length. Consequently, they have to have the legs of the tower closer together. Also, by putting the legs of the tower closer together (not going for the bonus), the legs are more vertical and the loading forces are mostly directly down (parallel to) the legs.

That being said, has anyone seen a tower score over 3000 points this year with the bonus?
Could I try making a 3 sided tower but with bass legs because if I could find some somewhat light bass, then each leg might be able to hold 5 grams. I do agree that building a 3 sided jig will be hard. Though it would be worth a try to have bass legs. If I could find some lightweight bass, and .3 or .2 gram balsa for the braces, then I could pull off a tower weighing about 5 grams. Though if it can hold all the weight is the real question.

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Re: 3 Sided Tower?

Post by TheGeekyScientist » April 27th, 2017, 1:34 pm

Unome wrote:It is definitely possible to make a successful 3-sided tower. I have personally witnessed several. However, if you can reach 3000 (or even 2000 for that matter), there's probably no reason for you to be doing so, as the benefit of weight loss is probably outweighed (pun not intended) by the structural instability (from what I can remember of Balsa Man's posts on the subject, though I easily might be wrong here). I would not recommend attempting this right before your state tournament; fine-tuning a jig for a 3-sided tower sufficiently to make it competitive will likely be difficult even for very skilled builders.
Do you know if the 3 sided towers that you have witnessed were successful have bass legs. Or were they balsa. I think I could probably pull off building a 3 sided tower that's pretty light but with bass legs and .2 or .3 gram braces. Though I do agree that building a jig for the 3 sided tower will be hard.

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Re: 3 Sided Tower?

Post by Crtomir » April 27th, 2017, 1:39 pm

TheGeekyScientist wrote:
Crtomir wrote:We tried a 3-sided tower with only limited success. We build a 3-sided jig, which was pretty difficult to begin with. Also, aligning the mounting block at the top so that the center of the block is at the center of the triangle formed by the three columns is difficult. More importantly, the legs (or columns) are even less vertical than a 4-sided one, meaning that the loading forces are not just straight down the legs, but there is a significant transverse force as well. The legs have square cross sections, so when you put them in a 3-legged design, the angle of the faces of the legs is not flat across the side of the tower. If you do your bracing with lap joints (recommended), you will see that the braces have to bend significantly at the legs to attach to the faces of the legs. We overcame this by filing or sanding the faces of legs to the correct angle that matched the side of the 3-legged tower at each of the bracing attachment points. This took a long time for our kids to build. In the end, the 3-legged towers still broke too soon and never reached the kind of scores we were able to get with a 4-legged tower. Now, we know the reason is because for a 3-legged tower, each leg has to support 15kg/3 = 5kg, but for a 4-legged tower, each leg has to support only 15kg/4 = 3.75kg. Yes, for a 3-legged tower, you save on the weight of one leg, but you would do better to cut the density down of the four legs of a 4-legged tower. Whatever weight you save by cutting down the density of the wood for the legs is multiplied by a factor of 4. That becomes pretty significant. Also, for a 3-legged tower, the length of the X-braces becomes considerably longer making them weaker. That's the reason ALL of the towers scoring over 3000 points have not attempted the bonus. They are using such low density wood for the X-braces that they need them to be short in length. Consequently, they have to have the legs of the tower closer together. Also, by putting the legs of the tower closer together (not going for the bonus), the legs are more vertical and the loading forces are mostly directly down (parallel to) the legs.

That being said, has anyone seen a tower score over 3000 points this year with the bonus?
Could I try making a 3 sided tower but with bass legs because if I could find some somewhat light bass, then each leg might be able to hold 5 grams. I do agree that building a 3 sided jig will be hard. Though it would be worth a try to have bass legs. If I could find some lightweight bass, and .3 or .2 gram balsa for the braces, then I could pull off a tower weighing about 5 grams. Though if it can hold all the weight is the real question.

I don't recommend using bass. The density is way too high for a super lightweight tower. To get above 3000 points you (mostly likely) have to have the main columns (legs) of your tower have a density of less than 6 # (6 lbs/ft^3). I doubt you will find that low density bass wood. If you do, then you still need to figure out the critical buckling length for the weight you need to support on each leg. That will tell you how many segments to divide up your legs using cross-braces. I'm re-posting this from earlier. The spreadsheet actually has an option for doing the calculations with a 3-legged tower. You need to specify the width at the base and width at the top.
I'm posting a link to a Google Drive folder with some (hopefully) useful Tower stuff: https://drive.google.com/open?id=0B-oLt ... lN5eUtQdUE

Towers.docx, Towers.pdf <--- Document describing some of the key points we learned about towers this season (mostly just before State)
Youngs Mouduli of Balsa 1956.pdf <--- Published document reporting measurements of the Modulus of Elasticity of balsa (different grains, different densities)
Tower_Jig.jpg <--- Photo of our last jig before State for a design not attempting the bonus
TowerDesignOptimizer.xlsx <--- Spreadsheet for estimating the weight/score of simple towers with only evenly-spaced X-braces on each side given the density and cross-sectional dimensions of wood. It also allows you to calculate the number of X-braces you need based on the Euler critical length (from the "Youngs Mouduli of Balsa 1956.pdf" data with a fudge factor to bring the numbers in-line with our own crude measurements)

Hope this can help some teams build some really good towers. The secret is science + hard work, but it helps to be brave enough to choose low density wood.

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Re: 3 Sided Tower?

Post by JZhang1 » April 27th, 2017, 3:03 pm

TheGeekyScientist wrote:I have had this idea on having a 3 sided tower. All the 4 sided towers have been over 5 grams. I was thinking that if I get rid of 1 side... then I might be able to reduce the weight by possibly around 1-2 grams From what I'm imagining, it might not be as good as a 4 sided tower. Has anyone tried making a 3 sided tower or might know if this is a good idea?
Thanks.
If you look at the smallest triangle inscribed in a 20x20 cm square, the amount of bracing needed is almost the same as the amount of bracing needed for a square tower. I am also pretty sure that triangular legs would need more cross sectional area to have the same strength as a square leg in addition to having to bear 5kg instead of 3.75kg.
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Re: 3 Sided Tower?

Post by Balsa Man » April 27th, 2017, 4:34 pm

Crtomir wrote:
TheGeekyScientist wrote:
Crtomir wrote:We tried a 3-sided tower with only limited success. We build a 3-sided jig, which was pretty difficult to begin with. Also, aligning the mounting block at the top so that the center of the block is at the center of the triangle formed by the three columns is difficult. More importantly, the legs (or columns) are even less vertical than a 4-sided one, meaning that the loading forces are not just straight down the legs, but there is a significant transverse force as well. The legs have square cross sections, so when you put them in a 3-legged design, the angle of the faces of the legs is not flat across the side of the tower. If you do your bracing with lap joints (recommended), you will see that the braces have to bend significantly at the legs to attach to the faces of the legs. We overcame this by filing or sanding the faces of legs to the correct angle that matched the side of the 3-legged tower at each of the bracing attachment points. This took a long time for our kids to build. In the end, the 3-legged towers still broke too soon and never reached the kind of scores we were able to get with a 4-legged tower. Now, we know the reason is because for a 3-legged tower, each leg has to support 15kg/3 = 5kg, but for a 4-legged tower, each leg has to support only 15kg/4 = 3.75kg. Yes, for a 3-legged tower, you save on the weight of one leg, but you would do better to cut the density down of the four legs of a 4-legged tower. Whatever weight you save by cutting down the density of the wood for the legs is multiplied by a factor of 4. That becomes pretty significant. Also, for a 3-legged tower, the length of the X-braces becomes considerably longer making them weaker. That's the reason ALL of the towers scoring over 3000 points have not attempted the bonus. They are using such low density wood for the X-braces that they need them to be short in length. Consequently, they have to have the legs of the tower closer together. Also, by putting the legs of the tower closer together (not going for the bonus), the legs are more vertical and the loading forces are mostly directly down (parallel to) the legs.

That being said, has anyone seen a tower score over 3000 points this year with the bonus?
Could I try making a 3 sided tower but with bass legs because if I could find some somewhat light bass, then each leg might be able to hold 5 grams. I do agree that building a 3 sided jig will be hard. Though it would be worth a try to have bass legs. If I could find some lightweight bass, and .3 or .2 gram balsa for the braces, then I could pull off a tower weighing about 5 grams. Though if it can hold all the weight is the real question.

I don't recommend using bass. The density is way too high for a super lightweight tower. To get above 3000 points you (mostly likely) have to have the main columns (legs) of your tower have a density of less than 6 # (6 lbs/ft^3). I doubt you will find that low density bass wood. If you do, then you still need to figure out the critical buckling length for the weight you need to support on each leg. That will tell you how many segments to divide up your legs using cross-braces. I'm re-posting this from earlier. The spreadsheet actually has an option for doing the calculations with a 3-legged tower. You need to specify the width at the base and width at the top.
I'm posting a link to a Google Drive folder with some (hopefully) useful Tower stuff: https://drive.google.com/open?id=0B-oLt ... lN5eUtQdUE

Towers.docx, Towers.pdf <--- Document describing some of the key points we learned about towers this season (mostly just before State)
Youngs Mouduli of Balsa 1956.pdf <--- Published document reporting measurements of the Modulus of Elasticity of balsa (different grains, different densities)
Tower_Jig.jpg <--- Photo of our last jig before State for a design not attempting the bonus
TowerDesignOptimizer.xlsx <--- Spreadsheet for estimating the weight/score of simple towers with only evenly-spaced X-braces on each side given the density and cross-sectional dimensions of wood. It also allows you to calculate the number of X-braces you need based on the Euler critical length (from the "Youngs Mouduli of Balsa 1956.pdf" data with a fudge factor to bring the numbers in-line with our own crude measurements)

Hope this can help some teams build some really good towers. The secret is science + hard work, but it helps to be brave enough to choose low density wood.
Hey, I just wanted to say, its really great seeing someone else join the...brotherhood who believe its important to get real, detailed information and knowledge out there; to give everyone the opportunity to up their game. There's some very good stuff here. It always fascinates me how different people understand and communicate their understanding, and how different folk learn differently; same info said in a different way, and the lights go on. , Back at the beginning of the season, laid out the importance of understanding the important mathematical relationships you need to really design- engineer. I like your take on these things- nicely done.
First, hats off to have gotten to the scoring level you have; that's just plain awesome.
Second, great to see someone else picking up on the old Forest Service study; I think it was 3 years ago I first posted a link to it, and nobody had said anything about how useful that info is.
Third, one important thing that study ....reflects is the variability around the mean of E at any given density- if you look at both the graph and the data table, it jumps out at you. It is in/because of this variability, that ...high performance wood selection is another key to success- getting the lightest sticks in your....wood pile that have the design buckling strength. For instance, in this year's wood, in 1/8th at 1.4gr/36", we were seeing single finger push down buckling strength ranging from 26-40gr. This is an important variable to understand and work with.
One thing I'm still working on (as discussed in a number of posts) is how the Effective Length factor in Euler's equation works with different bracing configurations- understand how to use/apply for a ladders and Xs configuration; haven't gotten a good handle for Xs only- its very different between the two. I look forward to digging into your spreadsheet.
Gotta run, working w/ our two Ft Collins schools going to Nationals to see what we can come up with.
Thanks. Look forward to talking more when things slow down a bit.
Len Joeris
Fort Collins, CO

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Re: 3 Sided Tower?

Post by Juanyjose » April 27th, 2017, 6:50 pm

TheGeekyScientist wrote:I have had this idea on having a 3 sided tower. All the 4 sided towers have been over 5 grams. I was thinking that if I get rid of 1 side... then I might be able to reduce the weight by possibly around 1-2 grams From what I'm imagining, it might not be as good as a 4 sided tower. Has anyone tried making a 3 sided tower or might know if this is a good idea?
Thanks.
I'm gonna try one, just for kicks and post back how it goes. If I remember, I have terrible memory.

I have leftover wood that I don't want to use for competition, so: the 3 columns are 3/16" x 3/16" weighing 2 g/36" each. That should hold everything, with enough bracing. It will go for the bonus.

Isn't compressional strength very dependent on cross sectional area of wood? If someone built a 3sided tower with 3/16" wood that weighed maybe 1.5, that would be pretty competitive, right? After you get over the fact it's harder to build.

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