Elevated Bridge B/C

[bob3443]

oh my bad, i thought he meant that he got an efficency of 2,000 at a competition

[cypressfalls Robert]

Oh i wish! I was just predicting efficiency for my next bridge that I built.

[jcms science boy]

I have state competition on this saturday.... after all u guys's sugggestions i decided not to bake my bridge (:
I think the ratio is going to be between 900 and 1000. Do you think that will be enough to win state(at regionals i won by a double score and my ratio was 640) or at least medal??? I don't think Indiana is like Michigan and Ohio, powerhouse science oympiad states. So i really can't get a feel of the competition... after reading other people's ratios in this, i am amazed at how good other people are..... and wonder if i could ever get a ratio like tat ):

[dholdgreve]

Due to the inherent unique properties of each piece of balsa, I'm not sure that baking a bridge absolutely weakens the wood. I believe we really are talking opinions... and we all know what they say about those!

My opinion is that baking, or "kiln drying" has little to no effect on the compressive and tensile strength of a piece of balsa, and undetermined effect on it's shear. For years, buildings were built using kiln dried spruce, dried to 15% moisture content, but 20 years ago, the Codes were modified to use air dried material, dried to 19%. Green lumber is still not permitted. I'm not sure why, but I can't believe that the Omnicient Code Gurus would require that structural members be dried to certain levels if it actually reduced its strength.

The glue on the other hand, is an entirely different story. Personally, I like Elmers wood glue, cut with 30% water. Yes... it dries slow, but it also soaks into the grain and stays reasonably pliable for up to 3 months, UNLESS BAKED! C/A on the other hand, will become brittle after just 3 weeks. In cases like towers, where shear is not an issue, glue going brittle may not be an issue, but with bridges, it's critical that shear joints remain a little pliable.

Any heat applied to the structure will speed the curing process of the glue... The challenge is to find the sweet spot, where the glue is still able to hold the structure together, while the wood is as dry as it can be.

I believe this can best be done by using a hot air hair blow dryer, not an oven. The moving hot air will also carry the moisture vapor off and away from the structure. After it's dried (30 minutes max), slip it into a sealed bread wrapper with several desicant packs (like silica gel). This will take care of any moisture in the bag.

Once removed from the bag, the bridge will be hygroscopic... it will reabsorb a certain amount of moisture over the first 2 to 3 hours, but probably will not reabsorb all of the moisture driven off with the hair dryer. The net weight will still be less than before drying.

Remember, this is only an opinion, and I have no empirical scientific tests to support it... just 10 years of experience with balsa wood structures. During these ten years, this philosophy has produced 4 Ohio State Division B champions and 3 Ohio State Division C Champions, from 4 different students, so I have to feel that this logic can't be too far off.

[andrewwski]

Due to the inherent unique properties of each piece of balsa, I'm not sure that baking a bridge absolutely weakens the wood. I believe we really are talking opinions... and we all know what they say about those!

My opinion is that baking, or "kiln drying" has little to no effect on the compressive and tensile strength of a piece of balsa, and undetermined effect on it's shear. For years, buildings were built using kiln dried spruce, dried to 15% moisture content, but 20 years ago, the Codes were modified to use air dried material, dried to 19%. Green lumber is still not permitted. I'm not sure why, but I can't believe that the Omnicient Code Gurus would require that structural members be dried to certain levels if it actually reduced its strength.

Simple, because it warps like crazy when it dries out. "Green" or fresh wood is has more water in it than the air, so it will eventually dry out, and when it does, it will warp. My grandpa had his cabin built by the Amish, and they used non-dried wood. A lot of extra work had to be done to get the walls to fit on straight, as the lumber warped.

[jander14indoor]

Search the internet with google, you will find many references where wood strength & stiffness (probably more important in this event), including balsa, is strongly affected by moisture content. Often with formulas derived from experimental data.

If you search this string back a few pages, you'll find drying has been discussed previously. The consensus of some VERY experienced folks (NOT me) was that these small pieces of wood remoisturized in minutes, not hours, so little to no useful effect in drying vs better design and building practices. In fact an example was given where at a national event drying cost a team first place!

Of course you are free to do as you wish, but do check that earlier discussion.

Jeff Anderson
Livonia, MI

[sewforlife]

You should only bake your bridge for 30 minutes at max. 200 degrees.
So far, my best bridge efficiency is 909. A 1000 is actually pretty easy to achieve, if you can think more strategically.

well my state is this saturday.... and my max. efficiency is 1000

1000 is good. the winning efficiency for our states was 770. which isnt that good.

[2win]

We don't know our efficiency yet, but we got second at galveston

[dholdgreve]

Search the internet with google, you will find many references where wood strength & stiffness (probably more important in this event), including balsa, is strongly affected by moisture content. Often with formulas derived from experimental data.

If you search this string back a few pages, you'll find drying has been discussed previously. The consensus of some VERY experienced folks (NOT me) was that these small pieces of wood remoisturized in minutes, not hours, so little to no useful effect in drying vs better design and building practices. In fact an example was given where at a national event drying cost a team first place!

Of course you are free to do as you wish, but do check that earlier discussion.

Jeff Anderson
Livonia, MI

Thanks Jeff... Taking your advice, I located the following article:

Balsa wood has a long history of being an excellent core material offering high compression properties, high shear strength, and great fatigue endurance. It has been used in a wide variety of applications ranging from recreational boats to high performance wind turbine blades.

As balsa wood is a natural product, moisture will inevitably have an effect its performance. The compression properties of balsa, for example, can significantly decrease as the result of a high moisture content. Processing problems, such as resin cure inhibition or delaminations, can also arise due to a high moisture content.

Due to growing concerns, there have been many questions as to what effect moisture has on balsa wood and how it specifically affects the balsa.

This technical bulletin will discuss two studies that were conducted in order to evaluate the effect of moisture, specifically relative humidity, on ProBalsa. The first study examined how the moisture content of balsa changes as the relative humidity fluctuates in an un-controlled environment. The second study analyzes this further and attempts to find a relationship between the density of balsa and the moisture content.

From http://www.globalspec.com/reference/725 ... n-ProBalsa

This certainly seems to be contraindicative of what has been discussed in this thread... If I understand this correctly, the compressive strength of Balsa actually INCREASES when dried... This, in addition to the lighter weight of the piece would certainly give reason to drying the structure... Just be very careful about drying the glue too much...

By the way, the example you noted was my example... and it was not Nats, but the Ohio State competition.

[dholdgreve]

Here is another chart that also indicates that drying will increase compressive strength.