Elevated Bridge B/C

[gandhiji]

yay!!! i just got back from state competition and i got first place with a score of 1530 in elevated bridge!!

[andrewwski]

Good job!

[croman74]

That's a really good score! What state are you in?

[jander14indoor]

http://picasaweb.google.com/smartkid222 ... 0022392354

In this picture there are 4 pieces of wood 1/8 x 1/16. Notice how the top 2 and the bottom one look fairly the same, but the 3rd one from the top is ligher in color. Is this b/c of grain or density? I dont' know much about grain which is why i'm asking this. Also can someone tell me what grain are these pieces? How can you tell?

Difference in color seems to be one of those natural variations in balsa. Not sure what drives it, I suspect trace mineral content, but only suspect. Lighter color is typically, but not always less dense. Some sources of balsa seem to be naturally whiter while others are more tan. Also some sources seem to have a very fine grain like that third piece while others have a much coarser sturcture with prominent growth rings like the second and fourth pieces. Personally if I can find it, I find the fine grain stuff like that third piece tends to be better suited for my projects. More even properties across a sheet. The wood with distinct dark growth rings tends to vary more across a sheet.

Grain (as in A, B, or C) doesn't really seem to change color to my eyes, changes the look of the wood. A grain just looks like normal wood with growth rings. C grain looks all flecky and sparkly. Does that third piece change to match the other three if you flip it 90 degrees? If so, the effect is from looking at the A or C side. If not, just from different trees or different parts of the same tree.

As to which grain those pieces are, hard for ME to tell with pieces that small. If I HAD to guess, the second and fourth look more like A grain to me, while the first and third look kinda (but only kinda) like C grain. Though for the first one, I can't tell if the fleck I identify with C grain is grain or a cutting artifact. While the third may just be a very fine grain A.

PS, this is one reason why I buy only sheets. Easy to tell A from C grain then and I just orient the sticks cut from the sheet to have the grain the way I want.

PPS, everyone does realize a square stick has either two A and two C grain faces or all four B grain faces. A or C are just perpendicular or tangential to the growth rings. B is just in between.

Jeff Anderson
Livonia, MI

[croman74]

Jander14indoor, what school do you coach in Michigan?

[jander14indoor]

Jander14indoor, what school do you coach in Michigan?

Any school that invites me! Seriously.

I'm not a teacher, but an engineer at Ford. When my kids were still in school I was always available to their schools first, but not exclusively. Since I'm 50% successful as a parent (my oldest has a JOB, in another state, with medical coverage, Yaaayyy....) and my second is in college, that's not even a factor anymore. Plus, I'm a state board member, and an event supervisor at regional, state and national levels, so coaching one team exclusively would be tacky at best.

BTW, my real expertise is Wright Stuff, Balloon Launch Glider, and I wrote the new helicopter event (to be piloted at MI state contest this year). But balsa wood is CRITICAL to the flying events so I know lots about it, how to select it, test it, glue it, etc. So I monitor the other construction events and pipe in where I can contribute. Notice I don't say in much about structural design for bridges, other than basic engineering principles.

Now, how to get my coaching assistance. I run clinics all over the state. I was at the region 8 workshop this year for example. I ran a clinic in Grand Rapids in Dec. I attended several invitationals and ran clinics after the event. All I ask is for a team to provide suitable space at a mutually convenient time, and invite other local schools. If you are far from Livonia, gas money. Really far, some place to sleep the night before. REALLY far (ie the UP) air fare is nice.

Probably more than you wanted to know, but...

Jeff Anderson
Livonia, MI

[smartkid222]

http://picasaweb.google.com/smartkid222 ... 0022392354

In this picture there are 4 pieces of wood 1/8 x 1/16. Notice how the top 2 and the bottom one look fairly the same, but the 3rd one from the top is ligher in color. Is this b/c of grain or density? I dont' know much about grain which is why i'm asking this. Also can someone tell me what grain are these pieces? How can you tell?

Difference in color seems to be one of those natural variations in balsa. Not sure what drives it, I suspect trace mineral content, but only suspect. Lighter color is typically, but not always less dense. Some sources of balsa seem to be naturally whiter while others are more tan. Also some sources seem to have a very fine grain like that third piece while others have a much coarser sturcture with prominent growth rings like the second and fourth pieces. Personally if I can find it, I find the fine grain stuff like that third piece tends to be better suited for my projects. More even properties across a sheet. The wood with distinct dark growth rings tends to vary more across a sheet.

Grain (as in A, B, or C) doesn't really seem to change color to my eyes, changes the look of the wood. A grain just looks like normal wood with growth rings. C grain looks all flecky and sparkly. Does that third piece change to match the other three if you flip it 90 degrees? If so, the effect is from looking at the A or C side. If not, just from different trees or different parts of the same tree.

As to which grain those pieces are, hard for ME to tell with pieces that small. If I HAD to guess, the second and fourth look more like A grain to me, while the first and third look kinda (but only kinda) like C grain. Though for the first one, I can't tell if the fleck I identify with C grain is grain or a cutting artifact. While the third may just be a very fine grain A.

PS, this is one reason why I buy only sheets. Easy to tell A from C grain then and I just orient the sticks cut from the sheet to have the grain the way I want.

PPS, everyone does realize a square stick has either two A and two C grain faces or all four B grain faces. A or C are just perpendicular or tangential to the growth rings. B is just in between.

Jeff Anderson
Livonia, MI

Thanks, I took this picture months ago, so i don't know the densities or any other properties besides what can be derived from the picture. I probably actually used the pieces in one of the bridges i made.
A few more questions:
How important is grain when making bridges? Are you supposed to use specific grain for specific parts of the bridge depending on the forces (compression, tension)?

[jander14indoor]

<SNIP>
PPS, everyone does realize a square stick has either two A and two C grain faces or all four B grain faces. A or C are just perpendicular or tangential to the growth rings. B is just in between.

Jeff Anderson
Livonia, MI

Thanks, I took this picture months ago, so i don't know the densities or any other properties besides what can be derived from the picture. I probably actually used the pieces in one of the bridges i made.
A few more questions:
How important is grain when making bridges? Are you supposed to use specific grain for specific parts of the bridge depending on the forces (compression, tension)?

Go back to my last sentence. Every piece of wood has BOTH grains in it. Just depends on which way its turned. So, what MAY be useful is to orient the wood a particular way to take advantage of the different stiffness in the two orientations.

Balsa is typically stiffer when bent along the direction of the A face. So in wing spars I orient the stick so the A face is on front and back and C face is top and bottom. That makes the wing stiffer in vertical bending.

I suspect it can be useful in bridge building, but haven't played with which way to orient when. Though I'm sure it is important to be consistent side to side and in comparing one bridge to another. Its one of those variables in using wood which leads to balsa's reputation for inconsistency. Balsa isn't really inconsistent, there are just a number of factors to be aware of and control when using to ensure consistency. Subject discussed in much depth earlier in this string.

Jeff Anderson
Livonia, MI

[smartkid222]

ok thank you so much for your insight, it's always been helpfull.

[gandhiji]

That's a really good score! What state are you in?

oh im in delaware...