Elevated Bridge B/C

[sewforlife]

Thanks for the help Balsa Man. In my opinion laminating would take too long. I guess I'm just lazy.
Allirog, did you see where it broke? Was it wood or at a joint? Maybe we could help.

did you test your bridge?

[Allirog24]

Well, I saw no bending whatsoever...I always see something, but this was a new design, and it just went PLOOF. I think the leg broke off, and since it was so light (7.7 grams), it took the wood with it and broke it. So, I think it broke at the join, so we are doing a lap joint instead of a butt joint. It only held 4 kg, so that small change will not make it hold all 15 obviously...but it may help. What else could I do? What is better, balsa, bass, or laminated? I would like it to hold more, but I want to keep it at less than 10 grams.

[sewforlife]

So far from my experience, I use laminated wood pieces for the main frame of the bridge, the main compression piece, and tension piece, and regular sticks of 1/8 balsa for the rest.

[croman74]

Tested the bridge. It held 5.8 kg at 15 grams, giving an efficiency of 387. The part that broke was the piece that I had reglued about 3 hours ago. I'm just going to reglue that, then wait a day then test it again. Hopefully it does better next time.
Balsa Man, I forgot to ask you this. You keep mentioning a safety tower. Could you maybe explain it? I know that it's supposed to show you where it broke, but would you mind describing it?

[croman74]

Well, I saw no bending whatsoever...I always see something, but this was a new design, and it just went PLOOF. I think the leg broke off, and since it was so light (7.7 grams), it took the wood with it and broke it. So, I think it broke at the join, so we are doing a lap joint instead of a butt joint. It only held 4 kg, so that small change will not make it hold all 15 obviously...but it may help. What else could I do? What is better, balsa, bass, or laminated? I would like it to hold more, but I want to keep it at less than 10 grams.

Using bass would increase weight, but bass doesn't rip at joints as easily. Another thing that you could do is to strengthen the joint using a gusset. (sorry to double post)

[sewforlife]

Balsa Man, I forgot to ask you this. You keep mentioning a safety tower. Could you maybe explain it? I know that it's supposed to show you where it broke, but would you mind describing it?

Yeah, actually that makes it two of us, I've heard you mention it a couple times, and I kind of still don't understand.

[Allirog24]

Sorry to sound stupid here, but what exactly are the compression pieces and the tension pieces? Also, how do you laminate the main frame without bringing the weight up too much? How do you do it anyway...won't other pieces get in the way, or do you do that first? Thanks for the help! By the way, double posts don't bother me at all!

Oh yeah, I can't exactly use gussets, that is because I am using lap joints, and there is nothing really to gusset. I will do it where ever I can though, thanks for the input though! I have found that lap joints (maybe one on each side of a piece of wood, but really thin on the inside) have helped alot, and it is less likely to rip at the joints (Though mine still do. That is just where it breaks. I don't know how to fix it...any ideas?) than butt joints.

[nejanimb]

Compression pieces and tension pieces are not inherently different pieces of wood, but depending on where they fit in your design, the forces that a piece (or "member") will undergo are different. Tension is what would happen if you grabbed two ends of a piece and pulled away from the center - it would make it more taut and try to stretch the wood. Compression is like if you grabbed both ends and pushed towards the center - "squishing" the wood upon itself.

These are not the only two possibilities - your members might undergo torque forces, bending moments, torsion... from my experience, however, it's best to minimize these other forces and design so that the wood deals with only straight tension and compression.

[croman74]

Sorry to sound stupid here, but what exactly are the compression pieces and the tension pieces? Also, how do you laminate the main frame without bringing the weight up too much? How do you do it anyway...won't other pieces get in the way, or do you do that first? Thanks for the help! By the way, double posts don't bother me at all!

Oh yeah, I can't exactly use gussets, that is because I am using lap joints, and there is nothing really to gusset. I will do it where ever I can though, thanks for the input though! I have found that lap joints (maybe one on each side of a piece of wood, but really thin on the inside) have helped alot, and it is less likely to rip at the joints (Though mine still do. That is just where it breaks. I don't know how to fix it...any ideas?) than butt joints.

I faced the problem of tearing at the joints constantly. But maybe you could consider using bass instead of the balsa. Bass wood doesn't break at the joints as much. And if you want to keep the weight down, then just use a smaller size of bass.

[Allirog24]

is lets say a 1/8 inch size of bass as strong as a 1/8 inch size of balsa or stronger? About what are the weights of a 1/8 inch piece of balsa and bass wood that is 36 inches long?