matthew8817 wrote:thank you very much for this information. now if only i had discovered this site sooner. this should still help for next year though. i already built the nebraska nationals bridge and it is on its way to georgia by means of school van and teachers driving.
You're quite welcome- that's what this place is for... lots of stuff to think about and work with next year; lots of good info from a number of people besides me. Hopefully, more good pics and info after Nationals from various folk; I'll get some up.
Balsa Man wrote:
that's what the colored markings in the photos are about- an easy way to know which piece is which.
Does making markings on the bridge (with pen/pencil/sharpie) 'hurt' the wood, for a lack of a better term. Does it degrade the wood, or make it weaker?
Balsa Man wrote:
that's what the colored markings in the photos are about- an easy way to know which piece is which.
Does making markings on the bridge (with pen/pencil/sharpie) 'hurt' the wood, for a lack of a better term. Does it degrade the wood, or make it weaker?
when i was doing towers we founds that the places that we had marked with sharpie broke more often then the other wood
Edit the Wiki. Upload to theImage Gallery
[medals]Get Medals[/medals]
[chat][/chat]
If you use a pencil just make sure you don't break the wood grain. It's very easy to do so. I'd recommend a felt-tipped marker (Sharpie or water-based) that won't damage the wood.
ok cause on balsa man's pictures the wood pieces were marked all over the place with differnt colored markers and i was wondering if this had any negative effect on the bridge. My old coach used to say that writing on the bridge degrades the wood so i try to write as light as possible.
andrewwski wrote:If you use a pencil just make sure you don't break the wood grain. It's very easy to do so. I'd recommend a felt-tipped marker (Sharpie or water-based) that won't damage the wood.
Does that mean that a ultra fine point sharpie is preferable over a pencil?
andrewwski wrote:If you use a pencil just make sure you don't break the wood grain. It's very easy to do so. I'd recommend a felt-tipped marker (Sharpie or water-based) that won't damage the wood.
Andrewwski has it right, I believe. If you physically damage the wood, you......damage the wood;it will be weaker. With a pen, like a ball point pen, you will damage the wood, with a pencil, you're probably going to damage the wood; with a felt tip, and a light touch, you won't. That's all it takes is a very light touch.
I respect what Robotman says about observing more breaks near marks; however, we've done testing on, oh, I don't know, 75 or a hundred pieces (marked gently) and have seen no such pattern.
For labeling, I use a .2mm felt pen. You can pick them up at a craft store, and they work excellent for marking even the smallest pieces of balsa. As long as you don't press hard when you write, it shouldn't make any sort of indent on the wood.
I would really like any thoughts on this recent, rather significant bridge problem:
I opened my bridge for the National competition today to verify that no pieces had been broken during transport. After weighing the structure, I discovered the bridge had gained .2g- I'm attributing this mass gain to the high humidity in Augusta. I recognized that I would likely see a mass gain, but .2g on a light structure seems excessive.
While mass gain wouldn't be that big of an issue, the secondary effects of the moisture have wreaked havoc on my bridge. My entire bridge has now become torqued. When I left home, the whole thing was perfectly level, all the legs sitting perfectly on the table, etc. Now, the legs diagonally across from each other teeter back and forth, about 2mm from the surface of the table.
With a tiny bit of pressure, the bridge compresses back into what I believe was the original position.
Can anyone offer any thoughts on this? I have not sanded the bridge at all, fearful that any attempt to compensate for the torque would create a new torque. Argh- it's so frustrating! Does anyone know the effects of high humidity on wood?
Aia,
I’m not all that surprised to hear of what you’re running into. Weight gain is about what I’d expected, actually a bit less-based on experiments with drying. Unfortunately, no experimenting/work with hydrating (increasing moisture). Literature on the swelling/shrinking of wood due to higher/lower moisture content indicates that the most change takes place in the tangential and radial directions (cross-sectional change), and very little in the longitudinal direction (length). Also, the lower the density, the smaller the effect. Putting those two bits of info together – and I need to say I’m guessing here – the teeter-totter distortion you’re seeing is likely not a matter of the legs getting differentially longer, nor of other members getting longer; its cross-sectional changes inducing distortional forces at joints. If you have any lamination- particularly with a higher density layer on a lower density piece, I’d expect some distortion to be going on.
I don’t think there is anything you can do at this point to adjust; certainly not to un-do distortions from cross-sectional changes, and I don’t think you should change leg lengths to get the bridge to sit perfectly level in its…..hydrated state. If there’s someone else with a better understanding out there, I hope they chime in, because I’m not sure on this. As you note, with very little pressure, things “come back to shape.” 2mm over 15cm is only 1.3%. I think if you were to trim legs, when it settled in under load, you’d be inducing distortional forces because of the change of/difference in key member length, and that would have worse effect than the moisture induced torqueing. Just my 2 cents worth. I would suggest looking at all joints very carefully ; the distortional forces could be stressing, even partially un-doing joints. If, by very careful checking with a straight edge you spot a piece/pieces that are detectably distorted coming into a joint, you may want to consider a glue touch-up….yeah, added weight; balance that decision against likelihood of joint failure. Wish I could say something more certain; best of luck.
Its something a number of folk are going to be facing; I wouldn’t be surprised to get a call from the Jr High team I helped for Nationals with a similar concern, since its so much dryer here in Colorado than there.
2008 NY BLG Champ
