Designs

Balsa Man
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Re: Designs

Post by Balsa Man »

[quote="T-B"]But here is my question: If you use a smaller cross-section column on the top part than the bottom, what happens at the joint when you are bringing all the force down in a smaller square than the square you are meeting up with? Does it transfer the force to the whole lower column or just the piece that matches up with the top?
quote]

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Len Joeris
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Re: Designs

Post by SLM »

T-B wrote:But here is my question: If you use a smaller cross-section column on the top part than the bottom, what happens at the joint when you are bringing all the force down in a smaller square than the square you are meeting up with? Does it transfer the force to the whole lower column or just the piece that matches up with the top?
It does make sense to use a smaller cross-section for the chimney and a bit larger one for the legs since the legs carry a larger force than the vertical chimney. Also, as you suggested, a stronger leg (with respect to buckling) would cut down on the amount of needed bracing. Our design uses 3mm-width section for the chimney and an 8mm-width (composite) section for the legs. Here is picture of the tower; it weighs less than 7 grams and holds the entire load.

Image

When the internal force in a structural member (with a large section) has to be transfered to another member through a smaller section, the fibers of the smaller section are going to be stressed more than the fibers of the larger section. This is analogous to forcing water to flow from a larger diameter pipe to a smaller diameter pipe causing water pressure to increase in the smaller pipe. As long as the fibers of the smaller section can handle the force (i.e., the wood has adequate density) then the transition should be okay.

At the very point that the two members meet, the part of the larger section than is not in direct contact with the smaller one does not participate in the load transfer. This should not be a problem if the two members have similar densities. Also, it is a good practice to use a "gusset plate" at such critical joints. In the tower shown above, the horizontally placed wide rectangular member at the base of the chimney serves as a truss member and as a gusset plate for the connections between the chimney and the legs.
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Re: Designs

Post by T-B »

BalsaMan and SLM,

Thanks for your help. Looks like we've got something else to look into this weekend. 7 g. and holding the entire weight is really impressive. We're too far down the path on our current design to try to implement yours, but it is something to look into for next year. I like building flat more than 3d on a form, so I'd probably like that approach better.

Thanks again!
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Re: Designs

Post by T-B »

SLM,

Are those L beams? On the base, yes? Top section too? We've got to learn how to do that next year.

Got any hints on techniques for that? Did you glue a 3/32 x 1/32 piece to a 1/8 x 1/32 (or thereabouts) in an end joint set up? Or did you try to cut the pieces at a 45 degree angle along the length to get a mitered joint along the length. I think that is what we need to do but I can't figure out how to do that.
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Re: Designs

Post by T-B »

SLM,

One other thing, it is obvious that you know a ton of technical stuff, and we need to learn that stuff to take it to the next level. Aside from reading every word on the forum, can you think of a book we could read on this topic with some of these formulas for buckling and tensile strength. I can build, and I can understand the basic stuff on compression and tension modeling, but a lot of that stuff I don't even know where to start. Any suggestions for summer reading? :lol:
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Re: Designs

Post by SLM »

T-B wrote:Are those L beams? On the base, yes? Top section too? We've got to learn how to do that next year.

Got any hints on techniques for that? Did you glue a 3/32 x 1/32 piece to a 1/8 x 1/32 (or thereabouts) in an end joint set up? Or did you try to cut the pieces at a 45 degree angle along the length to get a mitered joint along the length. I think that is what we need to do but I can't figure out how to do that.
The chimney consists of four long members with a rectangular section (1/8 x 1/16). The cross-section of the legs is in the shape of T, not L. The flange (the wider part that is visible in the image) is 1/32" thick and has a width of about 8 mm. The web is 1/8" x 1/16". The advantage of a T section is its ease of construction. We cut the flange and the web to length (about 18 cm), place the flange on a flat surface, put glue on the web (along its length) while holding it in one hand, then place the web on the flange along its centerline.
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Re: Designs

Post by SLM »

T-B wrote:SLM,

One other thing, it is obvious that you know a ton of technical stuff, and we need to learn that stuff to take it to the next level. Aside from reading every word on the forum, can you think of a book we could read on this topic with some of these formulas for buckling and tensile strength. I can build, and I can understand the basic stuff on compression and tension modeling, but a lot of that stuff I don't even know where to start. Any suggestions for summer reading? :lol:
I am not aware of any book that covers these topics in a manner that is accessible to middle/high school students. You can find various mathematical explanations for buckling on the web, but I am not sure how useful they are. A more comprehensive and accurate explanation for buckling, structural stability and strength can be found in most engineering textbooks on structural design. Although such books generally deal with steel and concrete structures, the underlying concepts and techniques are applicable to wood structures as well.
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Re: Designs

Post by LKN »

SLM,

Did you use all balsa for your tower? I have been using bass on the horizontal "gusset plates" on the base of my towers, and they have been working quite well. I am assuming that you used very dense balsa for tension members on your tower to get your weight that low? Thanks
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Re: Designs

Post by SLM »

LKN wrote:SLM,

Did you use all balsa for your tower? I have been using bass on the horizontal "gusset plates" on the base of my towers, and they have been working quite well. I am assuming that you used very dense balsa for tension members on your tower to get your weight that low? Thanks
Yes, the tower is entirely made of balsa. The density is not too low, it is around 200 kg/m^3 for the main members. The bracings are a bit lower in density. The legs (without bracing) are about 2.4 grams in total, the main chimney members weigh about 1.6 g, the rest goes to the bracings and the other tension/compression members.
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Re: Designs

Post by KBS »

SLM,

in your bracing posts, your analysis showed that a Z brace tendss to have relatively large lateral displacement. Your tower picture shows that you used Z braces on upper tower as well as on the base. I am not sure I understand your rationale for this. Why, for instnace, did you not use the P2 bracing desing which has very little lateral displacement? If I am misquoting P2, I mean the one that goes like:
\
-
/

Thanks!
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