Towers B/C

[LiveMas]

Hi guys, thanks for always providing such useful information.

I have read many of the posts, and I'm not sure if this has been discussed already, but I have been building my towers with rectangular bases, so the towers have a trapezoidal base with a rectangular prism on top. This way, the cuts for the diagonal frame pieces (on the trapezoidal prism) are straight and easy, unlike those for towers with square bases.
I was wondering how this type of tower compares to those square base towers, assuming both of those towers are built perfectly.

[DarthBuilder]

Can you post a picture of iyour tower on this forum?

[Balsa Man]

Hello,

Looking for some specific input on how to improve our tower. We recently had invitationals. Attached is an image of the tower before testing. It weighed 13.43 g and held only 4.1 kg but got the bonus. We used a plexiglass jig (based on suggestions in the forum) so it didn't break at the connection between the base and the chimney, but it broke in the middle of the chimney. I think it was because the 1/16th x 1/32 X-braces we used in the chimney didn't hold, but if we use denser pieces, the tower will weigh much more. The brace is made completely of 1/8 x 1/8 and so are the corner pieces of the chimney. Should I change the density of the X-braces in the chimney, or could it be our gluing/building technique that caused it to break so quickly?

https://photos.app.goo.gl/LvwuoiWzoiKLLJpz1

Thanks for your input!

First modification is definitely for those X bracings in the bottom; they should definitely get thinner (1/16 X 1/32) would work for that. Also, denser wood does not mean your chimney will do as well. Based on the formulas (if you read through the forums), adding more intervals will actually help instead of getting denser wood. Of course, this will add weight but denser wood will not help you with bracing.

I'm very curious as to why your tower weighs so much. Most towers with that design, while using a jig and doing math, will get 4.5-6g towers (giving end spectrum numbers). Maybe check your leg weights but should not be that heavy. Good luck!

Well, there are a number of possibilities that may be at play…… and insufficient information to know.
A few thoughts/comments:

Assuming jig is nice and symmetrical, and glue joints are solid, and given the … substantial weight, implying pretty high density leg sticks, failure at only 4.1kg…. something’s wrong.

Looking at bracing intervals; 1/3 in the base, 1/5 in the chimney, and at inverse square table; legs that should work at those intervals (both in base and chimney) would need SFPD BS around 54gr. This is assuming 1/16x1/32 Xs. That gets you into the range of 1.9 to 2.0 gr/36” sticks. But at 2gr/36 leg wood, total leg weight would be around 5gr.
That leaves 8.4gr for bracing and glue. That is a LOT.

Rough calcs using 9gr 3x36x1/32 for Xs suggests around 2gr for Xs and glue; double that and we’re still below half of that 8.4gr. Chimney Xs (1/16x1/32) at 1/5, from 9gr sheet would be about 1gr. That would mean base Xs at ~7.4gr. Base Xs at 1/16 x 1/32 from 9gr sheet would be a bit less than a gram, say 0.8-0.9. There are eight 1/32 x 1/16 in a 1/8”x1/8” cross section. So going with 1/16x1/32 Xs same density as the current 1/8 x 1/8 would take base Xs weight to a hair over 0.9gr. That would reduce tower weight by over 6gr....... Can you confirm leg sticks from something close to 2gr/36”? If so, with 1/16x1/32 bracing in the base, the base should be strong enough to carry 15kg.

Now, looking at the chimney. With 1/5 interval, and 1/32 x 1/16 Xs, legs with a 36” SFPD BS of about 16.5gr should theoretically hold ~4.1kg, and sticks showing this BS should be around 0.85gr/36” If the chimney legs are more on the order of 2gr/36”, failure at 4.1kg is very premature. If the Xs in the chimney are from really light 1/32 sheet (<5 gr 3x36x1/32), failure of the Xs might be possible, but it doesn’t sound like you’re using such light Xs.

I could certainly be wrong, but I think (assuming there was not a weak glue joint in the chimney Xs) the premature failure came from disproportionate loading on one or two of the chimney legs. That can come from three things- a) load not perfectly centered, b) chimney not perfectly vertical, and c) test base not being level. It appears your chimney legs are designed to be vertical- no lean-in- leg end separation the same at the top end and bottom end of the chimney- am I seeing that correctly?

As I’ve said before, if they are, then the chimney legs are going to be hyper-sensitive to load not absolutely centered and/or chimney not absolutely vertical (whether from construction, or test base not level). A very small amount of load not centered, or of leaning of the chimney is going to produce a force on 1 or 2 legs that’s WAY over ¼ of the total load on the tower.

It doesn’t take much lean-in of the chimney legs to really help this situation. If the bottom of the chimney is out far enough to get close to fitting the 8cm circle, and the top is leaned in far enough so leg ends fit underneath the 5cm square loadblock, the sensitivity will be dramatically reduced. Adding a little bit of extra lean-in – bringing the top ends of the legs in so the loadblock overhangs the leg ends helps even more to reduce the sensitivity.

Did you check the levelness of the test base before loading? Do you know if the crew running the event did (initially, and/or periodically during the event)? The rules only say that a level test base surface should be provided; no quantification of how level, no words about checking periodically, no words about (or extra time allowed for) either students or event crew adjusting to level. That’s why I think it is so important to design in reduced sensitivity to less than perfectly level base (by putting in some lean-in of chimney legs).

Now getting into personal opinion, I think it is really important that folk running the event recognize the importance of a level test surface, and do everything they reasonably can to assure all competitors get a level surface – using a good quality level to check before competition- left to right & front to back; adjusting test apparatus legs/supports and/or using shim material to get it as level as you can, and re-checking (and adjusting if it has changed) periodically. The floor of the testing area should be a “hard” surface (as opposed to a carpeted floor). I understand that one of the important demands on the crew running the event is to keep things moving along so that all competitors have time to test; another important demand is that to the extent practical, all competitors get… equal test conditions.

While its just my opinion, I think that every competitor, if they want to, should be able to check levelness – putting a level on, left to right, back to front, and if the surface is not reading level, to ask for adjustments so it does read as level. While the rules do state a right to do this, they do not prohibit it, and they do say a level surface should be provided. I’m curious what event supervisors think about this, and how they’re dealing with it….

[Balsa Man]

Hi guys, thanks for always providing such useful information.

I have read many of the posts, and I'm not sure if this has been discussed already, but I have been building my towers with rectangular bases, so the towers have a trapezoidal base with a rectangular prism on top. This way, the cuts for the diagonal frame pieces (on the trapezoidal prism) are straight and easy, unlike those for towers with square bases.
I was wondering how this type of tower compares to those square base towers, assuming both of those towers are built perfectly.

There has, indeed, been some discussion on square vs rectangular base- a number of times. My take is there are three significant downsides to a rectangular base:

First is the need for substantially longer braces in the lower portion of the base section (on two sides). Given the inverse square relationship of buckling strength to length, a pretty small increase in length (compared to lengths needed in a square base) means a substantial increase in density/BS needed to hold. On the other two sides, of course, bracing lengths are significantly shorter, but when you run the numbers, total bracing weight in the base is higher in the rectangular configuration.

Second is the same issue discussed in my post responding to bombsci’s tower – the hyper-sensitivity to off-center loading and levelness of test base. In a square base tower, this issue… exists/is at work in the chimney. In a rectangular based tower, looking in the plane that’s perpendicular to the long dimension of the base rectangle, the issue exists/is at work all the way from top of tower down to the base – in a C tower, 40cm of height for square base, but 60cm – 50% more – in rectangular tower. That extra height means the moments generated by some given increment of either leaning (because of non-level base), or off-center loading will be significantly greater (than they would be at lower height). So, in two directions, the rectangular base tower would be significantly more vulnerable/sensitive to lean/off-center loading.

Third is the difficulty creating a 3-dimensional jig holding both base section and chimney section leg segments in alignment. With a square base, a simple cruciform jig that does this is easy to construct with high precision.

As to the joint where base and chimney legs meet, can’t tell for sure from your description, but it sounds like you’re talking top of base legs cut in a plane parallel to the testing base, and bottom of chimney legs cut in that same plane (which with a vertical chimney- no lean-in of the chimney legs- would mean bottoms cut square- cut at 90 degrees to the stick. So, with 1/8” square leg wood, the bottom of chimney legs would have a 1/8” square cross section. The exposed cross section of the top of the base legs would, because of the angle of the base legs, be rectangular- 1/8” short dimension, significantly greater long dimension. That means tto pull off on all 4 legs.o avoid off-axis loading onto the base legs, the square end of the bottom of chimney legs would have to very precisely centered on the rectangular end of the top of base legs – very difficult pull off.
So, for what its worth, that’s my take.

[Raleway]

Regarding the question about level test bases... that has always been one subject to constant annoyance. I just wish the bases were not on carpet and were on level solid flooring to ensure consistent results when leveling and what not. Just my two cents after having some interesting experiences with the event but definitely I know some invitationals actively try to solve these issues and am very grateful for that.

[LiveMas]

Thanks for the input! Will definitely try in the future.

For our upcoming invitational, Centerville - which is this coming Saturday, I am going with the rectangle base approach as that was what we had planned.
For the wood:

3/32 x 3/32 x 36" - at 0.4 grams or 0.7 grams each (still deciding) - These are the main frame pieces for the tower
1/16 x 1/16 x 36" - at 0.3 grams each - These will be the x bracings for the trapezoidal prism base
1/32 x 1/16 x 36" - unspecified weight (Because it became to expensive to choose weight for these!) - These will be the x bracings for the rectangular prism chimney

Using these pieces, what should be the interval of the bracings? Also, should I use 0.4 gram 3/32 x 3/32 or 0.7?

The way I have been building the towers so far is I have been adding 2 horizontal 3/32 x 3/32 pieces at the bottom of either side of the tower to keep the tower from moving outwards - to a certain extent. Is this the right choice of piece for doing this? Would 3/16 x 1/16 would be a better choice?
Sorry for the load of questions!

[LiveMas]

Here is the link to the tower design:

https://docs.google.com/document/d/1Ev2 ... v3CXE/edit

[Balsa Man]

Thanks for the input! Will definitely try in the future.

For our upcoming invitational, Centerville - which is this coming Saturday, I am going with the rectangle base approach as that was what we had planned.
For the wood:

3/32 x 3/32 x 36" - at 0.4 grams or 0.7 grams each (still deciding) - These are the main frame pieces for the tower
1/16 x 1/16 x 36" - at 0.3 grams each - These will be the x bracings for the trapezoidal prism base
1/32 x 1/16 x 36" - unspecified weight (Because it became to expensive to choose weight for these!) - These will be the x bracings for the rectangular prism chimney

Using these pieces, what should be the interval of the bracings? Also, should I use 0.4 gram 3/32 x 3/32 or 0.7?

The way I have been building the towers so far is I have been adding 2 horizontal 3/32 x 3/32 pieces at the bottom of either side of the tower to keep the tower from moving outwards - to a certain extent. Is this the right choice of piece for doing this? Would 3/16 x 1/16 would be a better choice?
Sorry for the load of questions!

You said earlier you’d read many of the posts. Might want to go back and review. You seem to have missed some important understanding.

The answer to your first question is on page 6. There I provided an inverse square table – it shows what 36” “SFPD” buckling strength braced at what intervals gets legs braced sufficient to hold full load. It explains how it is measured BS you need to know to calculate bracing interval needed. Stick weight/density doesn’t tell you what you need to know, because at a given density/stick weight, there will be significant variation in BS. To make your decision on 0.4 or 0.7gr/36 3/32, you’re going to need to measure and use BS.

In that same post, I provided a graph of 36”stick weight vs 36” SFPD BS for 1/8” wood. I don’t have data for 3/32, so can’t provide any meaningful guidance on your 0.4/0.7 question. I need to note that this inverse square table is based on 1/8” legs, braced with all Xs, with the Xs being 1/16” x 1/32”. The effective length factor (approx. 0.55) is explained in this post, and discussed in more detail in other posts this season. It has been derived from data on 1/8” legs, braced with 1/16x1/32 Xs, and I don’t know how or how much going down to 3/32 leg wood (and using 1/16x1/16 Xs in your base) will change that factor; it may be in the ball park, it may be off significantly, I just don’t know.

As explained, to find the bracing interval needed for legs of a given 36” SFPD BS, you multiply the measured 36” SFPD BS by this effective length factor, and you use that value to do inverse square calcs on. You need to look at base section legs and chimney section legs separately, because the base section legs see significantly higher forces than the chimney legs do. That’s what the inverse square table does/shows. The graph then allows you to see what stick weight range is likely to get you some sticks that have a target/particular BS.

The primary reason the inverse square table is for 1/8” (and not 3/32”) is because 1/8” wood will be more structurally efficient than 3/32 wood. There are some posts discussing the relationship of density and cross section to BS that explain why this is. Also, 3/32 legs will mean you have only 75% of the glue area on your Xs to legs joints (compared to what you get with 1/8” legs); significantly weaker. You will not be able to get the performance out of 3/32 you could get with 1/8.

On no weight spec for 1/16x1/32 Xs in the chimney, understand that higher density ones will be stronger than lower density ones. Chimney braces are pretty short, so fairly light should work (ignoring the disproportionate loading issues I discussed in last post. Fairly light we’re talking 6-7gr for 3x36x1/32 sheet, and screen out the weak ones.

On 1/16 square Xs for the base – at 1/16x1/16, they’ll have a lot higher strength than 1/16x1/32 Xs would, but they’ll need to be considerably stronger than 1/16x1/32 would be in the lower Xs, because of the additional length required in a rectangle base (vs square base).

On the 3/32 strap at the bottom – assume you’re thinking 3/32 x 3/32- that’s overlkill; the only force these pieces see is tension – around 2.1-2.2kg at 15kg tower load. The most weight-efficient way to get needed strength would be 1/16 x 1/64th in fairly high density. Also, unclear from your drawing, these need to be set a little (like 1/16”) up from the very bottom. If they’re all the way down to the bottom (touching test base) you loose the 29cm circle bonus.

Last comment, your ladders at top of tower (1/16x1/16 bass), and at top of base section (“3/16”- which I assume is 3/16x3/16). Think you’ll find having ladders same size as legs will work better. Top one can be very light/low density; the one at top of base needs to be significantly denser.

Hope this helps; best of luck- let us know how it goes.

[bombsci]

Hello,

Looking for some specific input on how to improve our tower. We recently had invitationals. Attached is an image of the tower before testing. It weighed 13.43 g and held only 4.1 kg but got the bonus. We used a plexiglass jig (based on suggestions in the forum) so it didn't break at the connection between the base and the chimney, but it broke in the middle of the chimney. I think it was because the 1/16th x 1/32 X-braces we used in the chimney didn't hold, but if we use denser pieces, the tower will weigh much more. The brace is made completely of 1/8 x 1/8 and so are the corner pieces of the chimney. Should I change the density of the X-braces in the chimney, or could it be our gluing/building technique that caused it to break so quickly?

https://photos.app.goo.gl/LvwuoiWzoiKLLJpz1

Thanks for your input!

First modification is definitely for those X bracings in the bottom; they should definitely get thinner (1/16 X 1/32) would work for that. Also, denser wood does not mean your chimney will do as well. Based on the formulas (if you read through the forums), adding more intervals will actually help instead of getting denser wood. Of course, this will add weight but denser wood will not help you with bracing.

I'm very curious as to why your tower weighs so much. Most towers with that design, while using a jig and doing math, will get 4.5-6g towers (giving end spectrum numbers). Maybe check your leg weights but should not be that heavy. Good luck!

Well, there are a number of possibilities that may be at play…… and insufficient information to know.
A few thoughts/comments:

Assuming jig is nice and symmetrical, and glue joints are solid, and given the … substantial weight, implying pretty high density leg sticks, failure at only 4.1kg…. something’s wrong.

Looking at bracing intervals; 1/3 in the base, 1/5 in the chimney, and at inverse square table; legs that should work at those intervals (both in base and chimney) would need SFPD BS around 54gr. This is assuming 1/16x1/32 Xs. That gets you into the range of 1.9 to 2.0 gr/36” sticks. But at 2gr/36 leg wood, total leg weight would be around 5gr.
That leaves 8.4gr for bracing and glue. That is a LOT.

Rough calcs using 9gr 3x36x1/32 for Xs suggests around 2gr for Xs and glue; double that and we’re still below half of that 8.4gr. Chimney Xs (1/16x1/32) at 1/5, from 9gr sheet would be about 1gr. That would mean base Xs at ~7.4gr. Base Xs at 1/16 x 1/32 from 9gr sheet would be a bit less than a gram, say 0.8-0.9. There are eight 1/32 x 1/16 in a 1/8”x1/8” cross section. So going with 1/16x1/32 Xs same density as the current 1/8 x 1/8 would take base Xs weight to a hair over 0.9gr. That would reduce tower weight by over 6gr....... Can you confirm leg sticks from something close to 2gr/36”? If so, with 1/16x1/32 bracing in the base, the base should be strong enough to carry 15kg.

Now, looking at the chimney. With 1/5 interval, and 1/32 x 1/16 Xs, legs with a 36” SFPD BS of about 16.5gr should theoretically hold ~4.1kg, and sticks showing this BS should be around 0.85gr/36” If the chimney legs are more on the order of 2gr/36”, failure at 4.1kg is very premature. If the Xs in the chimney are from really light 1/32 sheet (<5 gr 3x36x1/32), failure of the Xs might be possible, but it doesn’t sound like you’re using such light Xs.

I could certainly be wrong, but I think (assuming there was not a weak glue joint in the chimney Xs) the premature failure came from disproportionate loading on one or two of the chimney legs. That can come from three things- a) load not perfectly centered, b) chimney not perfectly vertical, and c) test base not being level. It appears your chimney legs are designed to be vertical- no lean-in- leg end separation the same at the top end and bottom end of the chimney- am I seeing that correctly?

As I’ve said before, if they are, then the chimney legs are going to be hyper-sensitive to load not absolutely centered and/or chimney not absolutely vertical (whether from construction, or test base not level). A very small amount of load not centered, or of leaning of the chimney is going to produce a force on 1 or 2 legs that’s WAY over ¼ of the total load on the tower.

It doesn’t take much lean-in of the chimney legs to really help this situation. If the bottom of the chimney is out far enough to get close to fitting the 8cm circle, and the top is leaned in far enough so leg ends fit underneath the 5cm square loadblock, the sensitivity will be dramatically reduced. Adding a little bit of extra lean-in – bringing the top ends of the legs in so the loadblock overhangs the leg ends helps even more to reduce the sensitivity.

Did you check the levelness of the test base before loading? Do you know if the crew running the event did (initially, and/or periodically during the event)? The rules only say that a level test base surface should be provided; no quantification of how level, no words about checking periodically, no words about (or extra time allowed for) either students or event crew adjusting to level. That’s why I think it is so important to design in reduced sensitivity to less than perfectly level base (by putting in some lean-in of chimney legs).

Now getting into personal opinion, I think it is really important that folk running the event recognize the importance of a level test surface, and do everything they reasonably can to assure all competitors get a level surface – using a good quality level to check before competition- left to right & front to back; adjusting test apparatus legs/supports and/or using shim material to get it as level as you can, and re-checking (and adjusting if it has changed) periodically. The floor of the testing area should be a “hard” surface (as opposed to a carpeted floor). I understand that one of the important demands on the crew running the event is to keep things moving along so that all competitors have time to test; another important demand is that to the extent practical, all competitors get… equal test conditions.

While its just my opinion, I think that every competitor, if they want to, should be able to check levelness – putting a level on, left to right, back to front, and if the surface is not reading level, to ask for adjustments so it does read as level. While the rules do state a right to do this, they do not prohibit it, and they do say a level surface should be provided. I’m curious what event supervisors think about this, and how they’re dealing with it….

Thank you so much for the tips! The chimney was intended to be vertical, but it was a little titled for the invitationals tower. I am a little confused about what you mean by the chimney lean in - do you mean to make it trapezoidal instead of rectangular? or to make the joints slant? If you could elaborate on that with an image or simpler description, that'd be great!

[Balsa Man]

Glad my comments were helpful.

On “lean-in” – yes, just as described, I’m talking about the legs in the chimney leaning in toward the vertical centerline of the tower- just as they do in the base section, but at a much lower angle. Yes, this means that, looking at a side of the chimney, it is trapezoidal- a long thin trapezoid. The square formed by the outsides of the legs at the bottom should just fit inside the 8cm circle (we use 1mm clearance all the way around. With the square formed by the outsides of the leg ends at the top of the tower at 5cm x 5cm- just fitting fully under the loadblock, the chimney legs have to lean in. You can get a bit more lean-in by ‘sizing’ the top smaller than 5cm x 5cm; 4.7..6..5. There is an additional advantage of having bottom of chimney out to close to 8cm circle fit- it means that the base section legs don’t have to lean in as far as they do to mate with chimney bottom set at 5cm x 5cm- that means they see less force under load due to the smaller lean-in angle.

As to ‘making the joints slant’, you’ll see I’ve mentioned/discussed this a few times. Worth reviewing.

It is about getting the cross-section size/shape (of bottom of chimney legs and top of base legs) the same. If using a vertical chimney, you cut the bottoms of the chimney legs square – cut at 90 degrees to vertical axis, the cross section at the cut end is a square. With cuts at this angle, the chimney by itself would sit with leg ends flush on a flat surface; if that surface is level, it would stand vertically.
To make these ends mate flush to the base, the base leg top ends need to be cut at an angle – parallel to the test base surface/at the same angle that the base legs lean in by. Looking at top ends of the base legs, you’ll see the cross section is a rhomboid – a diamond shape. The short diagonal axis of that diamond is the same as the diagonal axes of the square seen in the bottom of the chimney legs; the long axis of the diamond is significantly longer- the square fits inside of/is smaller than the diamond. If you line up the short axis of the base leg end with a diagonal axis of the chimney, the ends of the diamond’s longer axis will stick out beyond square of the chimney leg base. It would be a very tricky problem to dimension the plates of a cruciform jig so the chimney leg segments are ‘pushed out’ just the right amount to ‘center’ them on top of the rhomboidal cross section of the base legs.

To get the end cross sections the same size and shape, (so that with their inside diagonal edges against the jig plate, all four diagonal edges line up; they mate perfectly, the cut has to be made (on both base leg tops and chimney leg bottoms) at the angle that bisects the angle between the chimney and base legs. Play with this with a saw and some scrap wood, and you’ll see. Google how to bisect an angle. Set up a way/a tool/a little jig to hold leg pieces in place in the right orientation and hold/align saw blade to make a cut at the right angle.

Your mention/comment that your chimney was ‘a little bit tilted’ on your invitationals tower explains the very premature failure- if, with a ‘no lean-in’ (rectangular rather than trapezoidal) chimney configuration, you could see the chimney was not vertical/had a bit of lean to it, what happened was absolutely inevitable; just what would be expected.
Hope this helps.