Towers B/C

[Cow481]

Anyone know if 1/32x1/16x36 medium density wood it okay for competitive towers. The wood weighs 0.1 grams. Thanks

[cheese]

Anyone know if 1/32x1/16x36 medium density wood it okay for competitive towers. The wood weighs 0.1 grams. Thanks

Step 1: read this years forums
Step 2: read last years forums

There are many different factors involved than just that.

[Random Human]

Anyone know if 1/32x1/16x36 medium density wood it okay for competitive towers. The wood weighs 0.1 grams. Thanks

I don't know if this question is specifically addressed in the previous years. But some digging will find. I've personally tried this, and yes its competitive. As in all other wood is. I specifically find this nearly infeasible to hold weight, unless there is a rather stronger section. What's key is when you get down to those densities, buying is the best way to ensure consistency throughout. I often find wood boards inconsistent, in terms of density, throughout. If you do strip wood, it will be rather difficult to ensure consistency, and as one side may be denser than the other,(balancing out to meet 0.1 grams), that one side that's less dense will cause premature failure. What's a even a bigger issue with wood boards is that your strip often causes the wood to be warped a certain way because of the inherent inaccuracy of strippers. Still, I recommend you buying, and if that's not possible, then stripping may be a possibility, but opens the gates for risk of both premature failure in competition, or inaccurate test results.

[Balsa Man]

Anyone know if 1/32x1/16x36 medium density wood it okay for competitive towers. The wood weighs 0.1 grams. Thanks

I don't know if this question is specifically addressed in the previous years. But some digging will find. I've personally tried this, and yes its competitive. As in all other wood is. I specifically find this nearly infeasible to hold weight, unless there is a rather stronger section. What's key is when you get down to those densities, buying is the best way to ensure consistency throughout. I often find wood boards inconsistent, in terms of density, throughout. If you do strip wood, it will be rather difficult to ensure consistency, and as one side may be denser than the other,(balancing out to meet 0.1 grams), that one side that's less dense will cause premature failure. What's a even a bigger issue with wood boards is that your strip often causes the wood to be warped a certain way because of the inherent inaccuracy of strippers. Still, I recommend you buying, and if that's not possible, then stripping may be a possibility, but opens the gates for risk of both premature failure in competition, or inaccurate test results.

Random Human is right-on in understanding the variation in density as you,,, strip through a sheet of wood. It’s also important to understand, as I’ve discussed about leg wood, density is only an approximate guide; just a way to ‘get in the ballpark’ on the STRENGTH needed. Two pieces with the same weight/density can have substantially different strength. It is the strength that matters. This applies to all pieces- legs and braces. It applies to ‘bought pieces’ (e.g. 1/16 x 1/32 sticks), and ‘stripped pieces.’

To be seriously competitive, you have to be using/tracking measured strength, and applying some level of safety factor. When, through testing, you establish strength(s) for various pieces that ‘work’ (allow the tower to carry design/full load), to optimize performance you then can ‘play the statistical game’ of finding the lightest pieces that have the needed strength. The more pieces you have to sort through, the higher your odds of having a set, or sets that are… toward the light end; significantly lighter than the average weight/density that exhibits a given strength.

When I say strength, I’m referring to buckling strength. Legs only see axial compression (at least until a tower starts to deform); it is only their buckling strength that determines if they hold. X braces, in an “all Xs” bracing approach, have to be able to carry both compressive and tensile loading. As a practical matter, I can say from experience if an X brace has sufficient buckling strength, it will also have sufficient tensile strength, so if you pick/use X pieces based on minimum buckling strengths (discovered/defined through tower testing) you’ll be ‘good to go.’

Key thing to keep in mind regarding buckling strength of X braces is that inverse square relationship of buckling strength to length; a piece half the length of another will have 4 times the buckling strength. The Xs at the bottom of the base section (particularly in a tower meeting the 29cm circle bonus) are a lot longer than the last Xs at the top of the base section, and the Xs in the chimney. This variation in length means the needed buckling strength is different- the lower Xs need a much higher buckling strength than upper Xs, and chimney Xs can have lower buckling strength than upper base section ones (assuming the bracing interval you’re using in the chimney is fairly tight, i.e., the length of chimney braces is less than the length of upper base Xs).

In testing X brace pieces for buckling strength (just as with leg segments), you want to cut them down to a length reasonably close to installed in the tower lengths. We’re using 2-3cm longer. For stripping from sheet wood, cut pieces from the sheet down to your cut lengths – much easier than trying to strip from full sheet length.

As you strip Xs (or from a bundle/pile of 1/16x1/32 sticks (cut to length), do SFPD testing, and sort by measured strength. At each ‘test length’ (and sheet weight or stick weight) the range of buckling strengths you’ll see will be different. We lay out a strip of paper marked into…6-8 sections, Example- 7,-8 8-9, 9-10, 10-11, 11-12, 12+. As you measure buckling strength, lay the sticks across the paper strip in bundles by measured strengths. Run a piece of masking tape over the bundles, on the paper strip. Track the buckling strengths used in different X sets for tower testing. If you’re stripping, weed out any that come out significantly twisted – and be sure to install on the tower nice and straight, without any slack. Because for a given X set, the ends will be in the same plane (the face of a tower side), but they cross at the center, there will be very slight bowing- by 1/64th- half of their 1/32 thickness.

As Random Human notes, you will see substantial variation as you strip through a sheet. This amount of variation will be greater with lower density sheet/sticks. In general, the weakest strips will come from… zones in the sheet where visible grain structure is absent/minimal, and the strongest will come from zones with apparent/significant… grain lines.

So, finally, to your ‘medium density/0.1gr 36” sticks. With 48 1/16” wide in a 3” wide sheet, at 0.1gr/stick, if a sheet was homogeneous, that would mean a 4.8gr (3”x36”) sheet. For… just a very rough sense, experience suggests that “good” (stronger than average) sticks from this density should work in the chimney, marginal in upper base Xs, but will not be strong enough in the lower end of the base. There appear to be two options for getting those long lower base Xs up to needed strength – going to higher density, and/or laminating center reinforcement strips (also from 1/16x1/32) on lower Xs- ?1, 2, 3, 4cm long.

Hope this helps.

[dholdgreve]

... Or possibly add a lower horizontal ladder of larger or heavier material that will resist the compression forces, allowing the lighter 1/16" x 1/32" X bracing at the bottom to deal with only the tensile loading... Just a thought.

[Cow481]

Thank you all for the feedback, I will add one or two horizontal braces to make up for the extra lenght on the bottom part of the tower.

[bombsci]

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!

For the bases, if we switch to 1/16 x 1/32 wood from 1/8 x 1/8, is it a good idea to increase the number of intervals from 3 to 5?

[Cow481]

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!

For the bases, if we switch to 1/16 x 1/32 wood from 1/8 x 1/8, is it a good idea to increase the number of intervals from 3 to 5?

Yes

[Balsa Man]

For the bases, if we switch to 1/16 x 1/32 wood from 1/8 x 1/8, is it a good idea to increase the number of intervals from 3 to 5?

Yes

Yes, depending.

The bracing interval needed depends on the buckling strength of the legs- higher strength (higher density) legs will work with more open bracing interval; lower strength (lower density) legs will require tighter bracing interval. With the tower only having carried about 1/3 of full load, you don't have a read on how strong the base section was. As I discussed earlier, looking at the inverse square table (and graph of buckling strength vs stick weight for 1/8 sticks) on pg 6, theoretically, using 1/16x1/32 Xs, a C-div base at 1/5 interval would need legs with a 36" SFPD BS of a bit over 20gr- sticks in the 0.85-0.95gr/36" range. At a 1/3 interval, the leg sticks would need a 36" SFPD BS of about 53.5gr - meaning sticks up around 1.8-1.9gr/36

[DarthBuilder]

Does it matter how fast you pour in the sand or no?