Towers B/C

[BananaPirate]

In confirmation to what you said about one leg being possibly weaker, I discovered that one of my legs was about .2 grams lighter than the rest. This was a huge diff for 3/32 wood, and probably resulted in the breaking. I remembered that I did not have a very good selection of 3/32 at the store, as I did not have an accurate scale at that time and there were not many options. Have a more accurate scale now, and hopefully will not repeat this mistake when I go out to buy wood tomorrow. Will keep in mind to make selection based on BS. Dont know yet which density to buy, have to see what they have at the store. I called, and they had received a selection of a couple hundred pieces of wood, so hopefully I can be selective.

What store do you go to? None of the stores I frequent get shipments of such large quantity. I'm jealous >.>. Had to go to 3 different stores and ended up buying 11 sticks.

I am considering buying specialized wood online, but for now I go to Nankins Hobby. They have much more selection, and I get my glue from there as well. I feel your pain, last invite I used low end wood as the shelves were all empty. This shipment came the day after my invite When I did bridges last year, I used to go to random places like Hobby Lobby, and it was like you said- low stock and inconsistency

It's unfortunate as that store seems to only have 3 locations in Michigan...I hope I can find a better supply of wood soon.

[Complexity]

Some information in a previous thread:
viewtopic.php?t=8449

Don't know if it will help you or not, but worth a look

[BananaPirate]

Some information in a previous thread:
viewtopic.php?t=8449

Don't know if it will help you or not, but worth a look

Thanks. I've already ordered from Specialized Balsa this year for making the tower legs but didn't get any for bracing...I haven't heard of the other ones though.

[candle 1586]

What type of wood thickness is best for bracings?

[bernard]

What type of wood thickness is best for bracings?

Best for all bracings? Can't say. Generally sufficient without adding too much mass? Probably 1/16" x 1/16", 1/16" x 1/8", and sometimes 1/16" x 1/4".

[Neharaval]

Can anyone please suggest what is the difference between Eye Protection: B for building event and Eye Protection : C for lab event ? Thank you so much






/

[Adi1008]

Can anyone please suggest what is the difference between Eye Protection: B for building event and Eye Protection : C for lab event ? Thank you so much

https://www.soinc.org/eye-protection

CATEGORY B

Description: Impact protection. They provide protection from a high inertia particle hazard (high mass or velocity)
Corresponding ANSI designation/required marking: Z87+
Example: High impact safety goggles

CATEGORY C

Description: Indirect vent chemical/splash protection goggles. These seal tightly to the face completely around the eyes and have indirect vents constructed so that liquids do not have a direct path into the eye (or no vents at all). If you are able to see through the vent holes from one side to the other, they are NOT indirect vents
Corresponding ANSI designation/required marking: Z87 (followed by D3 is the most modern designation but, it is not a requirement)
Example: Indirect vent chemical/splash protection goggles

[Balsa Man]

What type of wood thickness is best for bracings?

Best for all bracings? Can't say. Generally sufficient without adding too much mass? Probably 1/16" x 1/16", 1/16" x 1/8", and sometimes 1/16" x 1/4".

First, there is no size/thickness that is best for all situations . It depends on 1) size of legs (as in cross section dimension; e.g., 1/8”, 3/32”), 2) the bracing system/configuration you’re using; ladders and Xs, ladders and Z bracing, and Xs only. If you go back to page 23, there’s a long post discussing bracing options, 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. There are many other posts discussing the issue of bracing. It would be really worth your time to go back to the beginning of the various threads in this forum, and digest all the information that’s been posted.

Second, to be clear on terminology, let’s define “best.” Scoring is based on structural efficiency; mass carried divided by mass of tower. That means (for all pieces in the tower) the “best” is the lightest that will carry the design load. So, there is no size that is ‘generally sufficient without adding too much mass.’ Depending on the bracing system, bracing system pieces will, when the tower is loaded, come under compression loading, or tension loading, or both. So, the problem is figuring out both the size and density of the pieces used in the bracing system that will get the needed strength at the lowest weight. The question of which bracing system is best – the lightest way to do it – is an open question; there’s been no definitive answer, in this year’s tower discussions, or in past years discussion of the issue in towers, bridges, and booms.

Third, the strength of wood/balsa (both buckling strength and tensile strength) depends on its density, so size alone doesn’t drive what wood to use- it’s a question of size and density. So to….have control of your design, to really be ‘engineering’ your tower, you need to be tracking both weight of pieces and buckling strength of pieces under compression loading.

In a ladders and Xs system, the ladders are/should be designed to carry compression loading. The ends of the ladders are being pushed in toward each other; at some load/force level they will buckle and fail. The ladders need to have sufficient buckling strength to not buckle under these forces. The Xs are/should be designed to carry tension/tensile loading. The ends are being pulled apart; at some load/force, they will tear apart and fail. They need to have sufficient tensile strength not to tear apart under these forces. In an Xs only system, the Xs need to carry both tensile and compression loading. In all three cases (as has also been discussed), the forces involved (when the tower is under a full 15kg load) are on the order of 1kg. These forces are generated by the legs attempting to buckle (bow inward or outward) at the braced points; the bracing acts to prevent this buckling. Note, as has been discussed, the bracing interval for an Xs only bracing system needs to be significantly tighter (smaller interval between bracing points) than it does for a ladders and Xs system.

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).

The lightest way to carry only compression loading (the ladders in a ladders and Xs system) is with square cross section pieces. The size (cross sectional dimension) of these pieces should, I believe, be the same as that of the legs. Why square cross section? Buckling strength depends on, is a function of 1) the inherent stiffness, 2) the cross section, and 3) the length.
The inherent stiffness (technically the ‘modulus of elasticity’, ‘E’) depends on, is a function of density, In sticks of the same size/cross section, the heavier the stick, the stiffer it will be- the higher its buckling strength will be.

For sticks of the same density, sticks with a larger cross section will be stiffer- have a higher buckling strength. A stick put under compression loading will buckle in its weakest direction. In a square cross section piece, it will buckle toward one of the faces/sides. Which side it buckles to will depend on….small imperfections, like does it have a slight bow/bend, or is one dimension slightly smaller than the other. It will not buckle toward one of the cross sectional corners, because the dimension between two parallel faces is shorter than the diagonal dimension between two corners. In a piece with a rectangular cross section, it will always buckle in the direction of the short cross sectional dimension, because that dimension is shorter. Comparing, as an example (for sticks with the same density), a 1/8” x 1/8” stick with a 1/8” x ¼” stick, the 1/8” x ¼” will buckle across the 1/8” dimension, and the buckling strength in that plane/direction will be very close to the buckling strength of the 1/8” square piece, and the piece weighs twice what the 1/8” square stick weighs. That additional weight gives you nothing- its just dead weight.

So, the ‘best’ sticks for ladders will be same cross section as your legs, at the lightest density that will give you a buckling strength of about 1kg, at the length of the ladder(s). The density of lower/longer ladders will need to be higher than that of higher, shorter ladders. There is extensive discussion in this, and other threads here in the towers forum, about how to measure buckling strength of a stick, and how to calculate the buckling strength at shorter lengths (i.e., the lengths of your ladders).

Now for an Xs only system. Over the years, I’ve developed a very good understanding of how to design and implement the ladders and Xs approach. Can’t say that for an Xs only approach. What I can say, is I know there are folk out there successfully using 1/16” wood (with 1/8” square legs).

[CrypticBuilder23]

Regarding rule 3hi, could I use sawdust and glue in the strengthening of joints? Could this be used as a pseudo gusset or does it qualify as a "member"? Thanks!

[Unome]

Regarding rule 3hi, could I use sawdust and glue in the strengthening of joints? Could this be used as a pseudo gusset or does it qualify as a "member"? Thanks!

The usual disclaimer

I'm thinking probably not, since 3.h.i specifically lists sawdust as not being wood, and the rule says that the tower must be constructed of wood, not of wooden members. I don't know whether a gusset is generally considered a member, but I'd think it would be irrelevant in this case.