Designs

[LKN]

Thanks for the post balsa man, that was well thought out and explained.

Chimney Question:
The chimney is a 3.9 by 3.9cm chute (measured from the inside).
I am currently using 1/16 by 3/32 bass for legs on my chimney. I use 1/16 by 1/16 balsa for X bracings, with a density of .2g or .3g per 36" stick. My chimney length runs about 36.5cm (my bases that I make run shorter than 15cm), so I have braced 9 X's every 4cm on each side and use the remaining .5cm on the legs as a margin for error. This design seems simple enough, and the chimney runs just over 4g.

I was wondering what seems to be working for everyone else on bracings for your chimneys? This seems to be working, but I was wondering if anyone has any other suggestions or ideas?

[lllazar]

So what does everyone think of chimney column sizes? I really don't think 1/8 squared (balsa) is necessary, i downsized to a bit less than 3/32 x 1/8 today and cut .4 grams off my total tower mass. I actual was able to use slightly stiffer (denser too) wood, but still come out with a lot less mass. After testing numerous towers i've never had one break in the chimney so it just got me thinking...anyway, what sizes are you all using?

[hpfananu]

Thanks for all the help SLM and BalsaMan! I think that information correlated with exactly what was happening and I appreciate it a lot!

[Balsa Man]

Glad to hear it was helpful, hpfananu.

Thanks for the post balsa man, that was well thought out and explained.

Chimney Question:
The chimney is a 3.9 by 3.9cm chute (measured from the inside).
I am currently using 1/16 by 3/32 bass for legs on my chimney. I use 1/16 by 1/16 balsa for X bracings, with a density of .2g or .3g per 36" stick. My chimney length runs about 36.5cm (my bases that I make run shorter than 15cm), so I have braced 9 X's every 4cm on each side and use the remaining .5cm on the legs as a margin for error. This design seems simple enough, and the chimney runs just over 4g.

I was wondering what seems to be working for everyone else on bracings for your chimneys? This seems to be working, but I was wondering if anyone has any other suggestions or ideas?

Couple comments-
Weight of chimney is a slight bit over ours- a couple tenths. We only have compression data on 3/32nd square. The 1/16th dimension will mean less than 3/32nd square. The bracing/effective column length you're using is awfully short, though; I think shorter than it needs to be. We're running 8.8cm ladder spacing; 3 ladders in the chimney. 1.37g/24" leg material. Column failure in testing at 8.5 cm length at 7kg- so (so, using the inverse square relationship of buckling strength to effective column length in Euler's Buckling Theorem, column strength at 8.8 cm is around 6.5 kg. At the 4cm you're using, would be around 32. So, even if the column strength of your 1/16x3/32 is one quarter of 3/32 square (and I think its stronger than that) you're at 8kg; sounds like overkill.

Other comment- at that low density, your Xs aren't going to be able to hold much tension- wood's too soft, subject to shear failure at the joint. Tensile strength of the wood is around 2kg, but unless you use a lot of glue, joint probably woin't carry that much.

[T-B]

Has anyone looked at using slightly larger columns on the top part than the bottom section? It seems that it would be worthwhile to go a little larger on the bottom because it would allow you to lengthen the exposed column sections. The trade off between column thickness and bracing weight favors that. But on the top, there is less advantage to increasing the exposed column length because the bracing saved is pretty minimal.

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?

If this has been covered before, I apologize, but I couldn't find that discussion.

Thanks in advance for your help.

[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]

Both

[SLM]

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.



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.

[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!

[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.

[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?