Pictures, Videos, and Scores

[ashmmohan]

Oh sorry... in my last post I said our score was 200, I meant 2000 whoops! We're headed to the Wright State invitational this weekend.
I know I asked last time, but what is a good score for Div. B. We have not attempted the bonus. Our highest score was a tower of 7-8g and holding the whole weight (15 kilos). We have not attempted the bones and have a rather simple design: straight compression pieces from the middle of the edges of the hole to a 5cm x 5cm square at the top. Our cross bracing is the simple "x" design.
Another question- it the bonus worth it? We recently built a tower using the bonus that held the whole weight but was rather heavy, about 10 grams. However, we did utilize a design in which the compression pieces are straight and not the "2-part" design (you know, the one with a triangular base and a narrow top part). So, is the bonus worth it and if so should we use the two part design?
Thanks, and wishing everyone luck!

2000 is a good score in either division; you'll probably end up somewhere between 4th and 6th at Wright State. Based on my reading of previous posts, most people are in agreement that the bonus is worth it in C, and probably also in B. I'm pretty sure a 2-part design would be detrimental in all cases; if you're using the upper part to change your angle, either the tower's breaking point will be dependent on the lower part, or the lower part will need significantly stronger (and, assuming you've mostly optimized everything else, heavier) wood.

I'd say with a score of 2000, you might end up between 1st and 3rd...at MIT, the highest placing tower wasn't far from that.

[Unome]

Oh sorry... in my last post I said our score was 200, I meant 2000 whoops! We're headed to the Wright State invitational this weekend.
I know I asked last time, but what is a good score for Div. B. We have not attempted the bonus. Our highest score was a tower of 7-8g and holding the whole weight (15 kilos). We have not attempted the bones and have a rather simple design: straight compression pieces from the middle of the edges of the hole to a 5cm x 5cm square at the top. Our cross bracing is the simple "x" design.
Another question- it the bonus worth it? We recently built a tower using the bonus that held the whole weight but was rather heavy, about 10 grams. However, we did utilize a design in which the compression pieces are straight and not the "2-part" design (you know, the one with a triangular base and a narrow top part). So, is the bonus worth it and if so should we use the two part design?
Thanks, and wishing everyone luck!

2000 is a good score in either division; you'll probably end up somewhere between 4th and 6th at Wright State. Based on my reading of previous posts, most people are in agreement that the bonus is worth it in C, and probably also in B. I'm pretty sure a 2-part design would be detrimental in all cases; if you're using the upper part to change your angle, either the tower's breaking point will be dependent on the lower part, or the lower part will need significantly stronger (and, assuming you've mostly optimized everything else, heavier) wood.

I'd say with a score of 2000, you might end up between 1st and 3rd...at MIT, the highest placing tower wasn't far from that.

At MIT, the 4th place tower was around 2200, and the top tower was around 2600; this was without Stevenson in attendance (whereas Daniel Wright is going to Wright State never mind, they apparently didn't go).

[parvatipatel]

Thank you all for responding to me, love the feedback. yeah, when I tested my towers, they all broke by "exploding", pieces flying everywhere.I use one universal sized balsa, but recently ordered some smaller balsa from specializedbalsa. should arrive soon.

Just fyi, I notice I didn't state size on legs and ladders in the stick weights I provided above- 1/8" x 1/8"

Nice plan! I am planning on using it- thanks! Do you know how much that should carry?

[Balsa Man]

Thank you all for responding to me, love the feedback. yeah, when I tested my towers, they all broke by "exploding", pieces flying everywhere.I use one universal sized balsa, but recently ordered some smaller balsa from specializedbalsa. should arrive soon.

Just fyi, I notice I didn't state size on legs and ladders in the stick weights I provided above- 1/8" x 1/8"

Nice plan! I am planning on using it- thanks! Do you know how much that should carry?

Designed to carry full load w/ a 20% safety factor

[Random Human]

Just fyi, I notice I didn't state size on legs and ladders in the stick weights I provided above- 1/8" x 1/8"

Nice plan! I am planning on using it- thanks! Do you know how much that should carry?

Designed to carry full load w/ a 20% safety factor

Balsa Man, when you say "20% safety factor" what exactly do you mean?
In all technicality, when you test structures, you do understand if this design or whatever works or not. In competitions, you can be the unlucky one, and somehow have a "hole" or something, wrong with your wood, you never know.
Is that what you mean by safety factor. I was always told that in a competition, there is a +- 10% level of uncertainty of how your tower will perform.

[Balsa Man]

Nice plan! I am planning on using it- thanks! Do you know how much that should carry?

Designed to carry full load w/ a 20% safety factor

Balsa Man, when you say "20% safety factor" what exactly do you mean?
In all technicality, when you test structures, you do understand if this design or whatever works or not. In competitions, you can be the unlucky one, and somehow have a "hole" or something, wrong with your wood, you never know.
Is that what you mean by safety factor. I was always told that in a competition, there is a +- 10% level of uncertainty of how your tower will perform.

I’ve discussed/explained this in a number of past posts, but its worth going over.

From Wikipedia- “Factors of safety (FoS), also known as (and used interchangeably with) safety factor (SF), is a term describing the load carrying capacity of a system beyond the expected or actual loads. Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects, such as bridges and buildings, but the structure's ability to carry load must be determined to a reasonable accuracy.
Many systems are purposefully built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation (reliability).

So, when I say designed to carry full load with a 20% safety factor” I mean exactly that.

“Design” encompasses two things- the configuration, the ‘geometry’; dimensions and shape; this is driven a) by the rules; base ends of the legs far enough apart to span the base opening (or 29cm circle); top ends of the legs close enough to support the load block, and b) by your choice of the bracing interval (and system/pattern) to use. The choice of bracing interval (heavier, stiffer legs with/requiring less bracing (wider spacing of braced points), or lighter less stiff legs with more/closer bracing interval can be made by comparing estimated tower weight, using estimated densities (stick weights) of components. The same approach can be done to evaluate bracing system weights.This has all been discussed in some detail in many past posts, with factors to do these calculations discussed.

The calculations to make the legs and bracing design decision are based on your “design load”- 15kg. In a 4 legged tower (built precisely/symmetrically, which a jig allows you to do), each leg will be carrying ¼ of the design load. (15/4= 3.75kg). However, because the legs are slanted (slope in toward the top), the force each leg sees with a 15kg tower load is a bit more than 3.75kg; for a B tower (meeting the 29cm circle bonus) leg force is 3840kg, for a C tower 3810kg. Again, as discussed in depth in past posts, the wood to use is selected based on measuring the buckling strength of sticks from which you cut your legs , then calculating the increased buckling strength of the leg sections between bracing points (using ‘inverse square’ calculations/table. You’re looking for the sticks that will give you the buckling strength you need/want at the lightest weight. Applying a 20% safety means that instead of using 3840, or 3810 as your design load, you use a design load that incorporates your safety factor; for a C tower, with a 20% safety factor, 3810gr x 1.2 = 4572gr, and for a B tower, 3840gr x 1.2 = 4608gr. As discussed before, calculating forces on bracing system members is…beyond my current understanding/knowledge, but from years of experience, I know if bracing system components can handle a force on the order of 1kg, they generally will work. With a 20% SF, that would be 1.2kg.

And why should safety factors be used? The short answer is to control variables. The biggest variable (but one that is straightforward to deal with, and control is shape/symmetry. If your legs are exactly the same length, at exactly the same angle, oriented symmetrically, and the bracing intervals are all the same (e.g., 1/3, ¼, 1/5, etc., then the force the legs will see at design load, and the strength of legs between bracing points will be equal. If alignment is off, forces and strengths quickly will become unequal. The more precise your construction is, the less of a safety factor you need to account for/deal with unequal loading. The more difficult variable to control is the inherent variability in wood- its grown, not manufactured to some precise physical properties specifications. Each stick is different. However, there is a relationship between density and strength.

If you take, say 100 1/8” x 1/8” 36” sticks, all weighing 1.5grams, most of them will have a buckling strength at 36” (measured by pushing down on the top with one finger when the stick is put vertically on a scale about 35 grams). Some will have more, or less; a few will test as high as 43, 44, a few will test as low as 28, 29 (approaching ~20% weaker). With a large enough sample size, you’ll see “a bell curve distribution” of buckling strength vs density. By increasing your design load by 20%, and picking sticks that display the buckling strength needed to carry that load, you “protect yourself” against sticks that are ‘to the weak end’ of the density vs strength spectrum. Understand that the inherent variability in wood is not only in/between sticks, but within each stick. If you were to measure buckling strength of a stick at 36” (in our example here, a 1.5gr stick testing at 35gr), and then cut it into 5 equal length pieces, you’ll see the same kind of distribution of buckling strength- weaker or stronger sections within the stick. If you’re really into looking for absolute maximum performance, you can reduce the unseen/undetected variability by cutting pieces just a bit longer than your finished legs will be, and doing buckling strength testing on them. If you do this, you can reduce the safety factor needed to be pretty sure the weakest still has the buckling strength needed to hold your design load.

The factors discussed above are the main objective/quantifiable factors to consider using/applying a safety factor to control the variables in play. There are a couple of subjective factors to think about, too.

Choosing what safety factor to use is a strategic decision; how close do you want to ‘push the limits; how important is it to have/take a shot at medaling, winning? Maybe you approach Regionals conservatively, 20%, or maybe even a bit more, and at State push the SF down to 10%, or even less.

And, as you noted, there is, to some extent, the…. in the heat of actual competition, under time pressure, everyone watching, factor. You could approach putting some control on this factor by making your tower a little stronger (hence a little heavier) by using design safety factors. I would suggest the better approach is a) quality control- building carefully, inspecting carefully, transporting carefully, and practice with the setup and testing process; doing this till you and your partner are comfortable doing everything right.

[dholdgreve]

Another great explanation Len...
In addition to inherent flaws within the wood, and construction flaws, there is also the potential for set up flaws... Allowing for the Event table to be .01 degrees from level... Allowing for the eyebolt to be .5 CM off center of the tower... Allowing for an inexperienced assistant to accidentally sway the bucket a bit... There are many, many, reasons for a safety factor... In real life structural engineering, many structures will have a 200% to 300% safety factor added to the base line calculations... Of course, in those cases human lives are on the line... here, we are just dealing with personal pride...

[Towerofterror]

Division B.

Main beams are 1/8 x 1/8

Horizontal ladders are 1/8 x 1/16

X's are 1/16 x 1/16.

Was too short so I added the top (I know, I know).

Too wide at bottom as well; approx. 31 cm (yikes)

Clocks in at 18.28 g. Testing tomorrow.

Obv. need to make it taller and narrower at bottom; other tips for making it lighter? thx

[Balsa Man]

Nice looking tower.

Tips for getting it lighter?
First one is to go back through the various threads of discussion and read. There are many specific tips, and detailed discussion on how to do it.

At that weight, one thing that can be said is that the density of the wood (in both legs and bracing) you used is much higher than it needs to be. That also says you have more bracing intervals than you need. There's detailed information waiting for you to read that will guide you to figuring out how to use much lighter wood.

[dholdgreve]

Any compression member is only as strong as its weakest axis... If your ladders are 1/8" x 1/16", they will be no stronger in compression that a 1/16 x 1/16 member... These should be square... either 1/16 x 1/16, 1/8 x 1/8 or split the diff and go for 3/32 x 3/32...

Imagine making your columns 1/8" x 1/4" and you'll see what I mean.

Tension members, on the other hand are just the opposite... Making them a little wider increases the gluing surface to the columns, making them thinner (as thin as 1/64") will reduce the weight while maintaining the strength. These do not need to be square.

And FWIW, lets get the terminology correct... "beams" are members that run horizontally... The Ladders, could actually be considered beams.
What I think you have referred to as Beams are actually the columns... vertical or semi-vertical members that transfer the specified load down to the table.

Overall, beautiful tower!... but to be competitive, we need to shed about 50% of the weight. It would help us analyze it, if we knew the column weights and / or bending strengths Then we could identify if the number of tiers of bracing could be reduced.