Towers B/C

[Complexity]

What does it mean to pre-tension strips, and how does it help?

[BananaPirate]

What type of wood thickness is best for bracings?

The lightest way to carry (only) tensile loading (the Xs in a ladders and Xs system) is thin strips. The very lightest way is 1/16” to 3/32” (width) x 1/64” (thickness) strips, cut from fairly high density 1/64” sheet (like 7-8gr 3” x 36” sheet). A slightly heavier alternative (if you don’t have access to 1/64” sheet), is cutting these strips from 1/32” sheet (like 8-9gr 3” x 36” sheet).

I understand that you prefer to use the 1/64 strips, so your mass for those sheets is probably from many years of experience but what about the 1/32 sheet (which you say should be a little heavier)?
Is there a reason for the mass of a 1/32 sheet to be 8/9 grams(like from experience as well, physical standpoint etc)? I ask this because I can't really come up with a reason why the 1/32 sheet is around 15% heavier.

Also, how important is trying to account for variation in the sheet? For example, calculating the theoretical mass of each strip and then massing the actual strips cut out, throwing out the ones that are too light etc.

My first time stripping wood, hope it goes well.

[Balsa Man]

What does it mean to pre-tension strips, and how does it help?

In my post that you're referring to, a couple of posts above, in the first paragraph, I say "...and if you go back to page 10, there’s a post with an attachment that discusses how bracing works, and how to implement a ladders and Xs system."
That attachment explains both how to do it and why to do it, in some detail.

[Balsa Man]

The lightest way to carry (only) tensile loading (the Xs in a ladders and Xs system) is thin strips. The very lightest way is 1/16” to 3/32” (width) x 1/64” (thickness) strips, cut from fairly high density 1/64” sheet (like 7-8gr 3” x 36” sheet). A slightly heavier alternative (if you don’t have access to 1/64” sheet), is cutting these strips from 1/32” sheet (like 8-9gr 3” x 36” sheet).

I understand that you prefer to use the 1/64 strips, so your mass for those sheets is probably from many years of experience but what about the 1/32 sheet (which you say should be a little heavier)?
Is there a reason for the mass of a 1/32 sheet to be 8/9 grams(like from experience as well, physical standpoint etc)? I ask this because I can't really come up with a reason why the 1/32 sheet is around 15% heavier.

Also, how important is trying to account for variation in the sheet? For example, calculating the theoretical mass of each strip and then massing the actual strips cut out, throwing out the ones that are too light etc.

My first time stripping wood, hope it goes well.

As I've discussed a number of times, the X strips in an Xs and ladders approach need a tensile strength on the order of 1kg. By pulling on strips you've cut, you can roughly measure that- if you pull on a strip with a pull pressure of 2-3 pounds and it doesn't break, it should be good to use.

Yes, there is variation within a sheet. In 1/64th sheets, its easy to see the areas where its sort of translucent, and grain structure is not apparent. These areas are significantly weaker, and should be avoided/not used. Harder to....read a 1/32 sheet. That's why pull testing is the reliable way to tell if strips are good to use. Individual strip weights are low enough, you're not going to see the weight variation, unless your scale is precise to less than 0.01gr.

1/32 sheet is, of course twice the thickness of a 1/64th sheet, If, to keep weights about the same you'd get with 1/64th sheet, you cut the density of the 1/32 sheet in half (i.e., a 1/32 sheet weighing the same as a 1/64 sheet), you have enough lower density that you start to run into problems with shear strength in the glue joint, because the wood's much softer That bump in density (the 15% you note) is/should be enough to avoid that problem. Also, the tensile strength, while a function of density, is not....one to one. The tensile strength of a piece at twice the density of another piece is not 2x; its a bit more than 2x.
Make sense? That's why I say high density 1/64th strips are the must structurally efficient/best for pure tension members/use.

[BananaPirate]

I understand that you prefer to use the 1/64 strips, so your mass for those sheets is probably from many years of experience but what about the 1/32 sheet (which you say should be a little heavier)?
Is there a reason for the mass of a 1/32 sheet to be 8/9 grams(like from experience as well, physical standpoint etc)? I ask this because I can't really come up with a reason why the 1/32 sheet is around 15% heavier.

Also, how important is trying to account for variation in the sheet? For example, calculating the theoretical mass of each strip and then massing the actual strips cut out, throwing out the ones that are too light etc.

My first time stripping wood, hope it goes well.

As I've discussed a number of times, the X strips in an Xs and ladders approach need a tensile strength on the order of 1kg. By pulling on strips you've cut, you can roughly measure that- if you pull on a strip with a pull pressure of 2-3 pounds and it doesn't break, it should be good to use.

Yes, there is variation within a sheet. In 1/64th sheets, its easy to see the areas where its sort of translucent, and grain structure is not apparent. These areas are significantly weaker, and should be avoided/not used. Harder to....read a 1/32 sheet. That's why pull testing is the reliable way to tell if strips are good to use. Individual strip weights are low enough, you're not going to see the weight variation, unless your scale is precise to less than 0.01gr.

1/32 sheet is, of course twice the thickness of a 1/64th sheet, If, to keep weights about the same you'd get with 1/64th sheet, you cut the density of the 1/32 sheet in half (i.e., a 1/32 sheet weighing the same as a 1/64 sheet), you have enough lower density that you start to run into problems with shear strength in the glue joint, because the wood's much softer That bump in density (the 15% you note) is/should be enough to avoid that problem. Also, the tensile strength, while a function of density, is not....one to one. The tensile strength of a piece at twice the density of another piece is not 2x; its a bit more than 2x.
Make sense? That's why I say high density 1/64th strips are the must structurally efficient/best for pure tension members/use.

Yes that makes sense, exactly the answer I was looking for. Thank you!
About pulling to test tensile strength...How should I attempt to pull at 2-3 pounds? I don't really have any relative idea of what that feels like. Also I just tried doing a pull test on a scrap piece of 1/16 stick. By the time I was done with it, the ends were smooshed. Too much?

[Balsa Man]

Excellent. You're welcome.

How to get a feel for....a kilogram of pull? This is a good example of an important concept- that is important to doing well; that is a technique/concept applied in "real engineering" all the time, Using what you have to to solve a problem/answer a question that is....a step in a larger problem/project.

So, what weighs a kg?....hmmm, a liter of water. What if I attached a strip/piece to the top of a liter bottle, and lifted the bottle? Hmmm... how could attach the piece to the bottle.... Hmmm, with a piece of string, or wire, I could make a loop around the top, and attach a....small piece of wood....like a couple inch long piece of scrap from legs (1/8 x 1/8). Then I could glue the end of a test strip to it, and grab the upper end between two fingers, and lift it up. The fingers/hand doing the lifting will feel a 1kg pull...ah, that's what it feels like. Then take a strip, hold both ends, pull to what it felt like lifting the bottle.

The problem you ran into pulling a low density (1/16) stick a) is another reason I like high density, thin strips for tension control/use. If the density is low enough to easily crush, that's telling you the shear strength in a glued joint is going to be....quite low. You're not going to get (when in-place, in a loaded tower) the full tensile strength of the stick piece; its strength will only be the shear strength of a thin portion of the glued face.....

[jgrischow1]

So at Wright State the towers supervisors (from Boyceville I think) seemed to be a little consumed with their own sense of power… I know the discussion of "should a team be allowed to take pictures of other teams' devices" has been hashed out elsewhere on this site in the past but these folks would not even allow spectators' cell phones out in the room because they "couldn't tell whether people were texting or taking pictures". (What if I was using the soinc rules mobile app to see if there was such a rule in the manual and I was neither texting nor taking pictures?)

A student waiting to test his tower was having trouble with his goggles and his eyeglasses on at the same time so he went to hand his eyeglasses to a parent and they wouldn't allow that due to the "no outside interference" rule.

Other spectators, perhaps non-competing team members had some back up towers sitting on desks in the spectators area and the supervisors made them take them down.

All seemed a little overkill to me.

[sciolycoach]

Hello,

Just want to address some of the concerns as it was a group of parent volunteers from Boyceville and at least one other school. On the photos this was emphasized in tournament mailings and notices as a tournament rule and there was an issue early in the day with cell phone video and dishonesty. As the old saying goes, sometimes one bad apple spoils the bunch.

I am not aware of the goggle issue but I do know there were issues with parents coming into the competition area despite tape and signs and in a few instances refusal to listen to requests to leave.

Lastly and probably most importantly there was a a student tower that was broken before it could be tested at least partially due to concerns listed above. It was very crowded in the room and we cannot control that. These rules were in place in an effort to minimize instances like this, as these can be devastating to the kids and teams involved.

I want to give a huge thank you to the Wright State folks and ALL of the volunteers for all of their hard work in making this tournament possible. Remember SO is always looking for more volunteers and I encourage you to consider giving back to the program in the future. I can only speak for the towers volunteers but they have been trained by the nationals folks over the past several years and strive to create a fair positive learning and competition environment for all. Please do not underestimate the difficulty of running an event of this magnitude (around 120 towers) and, by and large, things went well and, as is often in life, rules have a reason and are based on past experiences at tournaments from small to large.

In fact many folks thanked the event supervisors for their attention to detail and keeping photography to a minimum. Regardless of whether it is a national rule or not ( a debate for another day) it was clearly a tournament rule.

[baker]

I was trying to post a picture, but no longer see the image tab any more. I only get an options tab at the bottom of the page. Is this still available or do I need some special permission?

[Unome]

I was trying to post a picture, but no longer see the image tab any more. I only get an options tab at the bottom of the page. Is this still available or do I need some special permission?

I'm getting the Attachments tab, but when I hit Submit the image isn't showing up (Edit: this should have a picture of an Ophiuroidea attached, but it doesn't seem to even save the file when I'm editing the post)