Jon and all,
This is a question which is actually important. My daughter is in a team competition to build a weight bearing structure in Balsa wood. The amount of wood allowed is limited, making “design” the competition’s focus. Some of the gang are toying with steam bending as a method of making arches. I probably understand enough of the mechanics of steaming, at least enough to ask them questions which could steer them in the right direction to accomplish the steaming and forming process.
The structure will not be large, so the radii of any arches would be fairly tight. The structure will be approx. 8″ tall, with an outside measurement of 6″ at the top and 8″ or greater at the bottom. The Balsa is 1/8 x 1/8, of any length to suite the design. The overall weight limit of the structure is 18 gr. (I think) The team whose structure supports the most weight wins.
Is there anything about Balsa, as a medium, which would caution against steam bending? (brittleness, structure, components?) I’m told Balsa is specified in the rules, so other woods are not options. I think the question they have is whether bending this timber is worth pursuing, or if it is a recipe for disaster, making other design options more desirable.
As an afterthought, is there a particular glue which is desirable? Any model builders out there?
As always, thanks very much to all for this indulgence.
Cheers,
Greg
Replies
Greg,
Is this a structure to span a certain distance, or will it be in tension or compression? How is it to be loaded? If you're making a truss, your tension connections are more likely to fail than a compression connection. Also, you may have trouble with slender column failure.
Try bending some 1/8" dry balsa by hand before you think about steam bending anymore. You don't specify the radius, but some of this stuff bends pretty easily without steam. All balsa is not equal. It varies in weight, strength and flexibility. If you have a hobby shop in your area that deals to model airplane enthusists, you probably will find that they have a good selection. Play with the stuff.
CA (cynoacrylate sp?) - super glue is good for balsa. Maybe try medium or heavy bodied, rather than the thin stuff. Also epoxy, but epoxy is heavy.
jdg
Greg, there's no body of literature that I'm aware of on the art of steam bending balsa...since it falls into the "Who on earth would want to do that" category...So, all I can suggest is you could give it a try.
I think selection of stock will be something you'll want to consider closely. Reject any pieces that show signs of grain run off. Unfortunately, with a weight limitation of 18gr, selecting for denser material is self defeating, but it would add strength...So, if there is variation in the density of the materials, use the denser stock in locations that will experience the most stress perpendicular to the grain.
We have a lot better engineering minds on this board than me, but I do think design is the key to success here...more so than materials. I think truss techniques would be superior to convex steam bent beams. In other words, I think you'll get better strength to weight performance by putting the lower horizontal member of a truss under tension stress than relying on beam elasticity to carry the load.
Also, I'd make sure the vertical bearing points transfer the load directly into the end grain of the vertical supports that carry the trusses...In other words, the supports should be splayed out or buttressed slightly to put less stress on the joint and transfer the load more directly into the end grain. Mother Nature has designed the trunk of a tree to support an incredible downward load parallel to the grain...so, take advantage of Her good judgement and direct as much of the load into the end grain as you can, wherever you can. Hope this helps.
Edited 1/24/2003 1:04:55 PM ET by Jon Arno
http://bridgecontest.usma.edu/
I don't know if this link will help or not, but it may be worth looking at as far as bridge design is concerned.
Good luck and let us know how things turned out. I think balsa would bend alright with the proper steaming. As stated earlier, getting nice straight-grained wood is paramount to your success.
Tom S
http://www.pre-engineering.com/modelsmart/ms.html
This is another link that may be helpful.
Edited 1/24/2003 5:03:46 PM ET by Tom
No one seems to have answered your glue question. For wooden model planes I always used Ambroid cement. Should be available at any hobby shop.
Greg,
I've seen this type of thing before. Basically it is "risk architecture". That means the challenge of building a particular project fitting a one time use sufficient of bearing a specific load for a limited time. Balsa is a challenging medium and not a good candidate for steam bending. Thus, a better approach is via joinery, that is the use of triangular loading (trusses) and careful dovetailing or finger jointing to maximize stiffness of balsa as opposed to flexibility of the material. As to glues, I think that any typical wood glue would be stronger than the Balsa itself.
sawick
To Everyone,
Thanks very much for the thoughtful replies. My impression is that bending introduces an unnecessary complexity/unpredictability into the process. The 5th graders are intrigued by all that stuff, so they may like to experiment with it on the side. They are a very fun group. The most helpful tip has to do with stock selection. We have already had discussions about compression (and distribution of compression forces downwards) and tension, so I think they are developing a good grasp of these. "Timber" selection, especially grain run off avoidance, may give them an advantage over less careful or aware teams. I have already suggested that trees have evolved in their way for a purpose and to take advantage of it (compression). It is indeed a game of "risk architecture", with practice sessions involving repeated "failure analysis". The youth of America. Not so bad....
Thanks also for glue input. I'll hit the yellow pages and look for model plane people rather than general craft houses which sell everything from doll houses to mocassin kits.
The competition is in a few months. I'll let you know.
Cheers,
Greg
Greg -
This reminds me of one of my first year structures project in the school of architecture at the U of W. We had to design a truss that would span a given distance and support the weight of the team members. In past years the truss was evaluated not only for its ability to support the design team but on a weight/load bearing ratio as well. In my class, all that was necessary was that it suppor the team members.
The first thing you should find out is how the structure your young budding designers are building will be loaded. Meaning, how will the design load be applied to the structure. Will it be suspended at mid span from below, set above, or ....
The simplest structure to design is what's called a determinant type post & beam or truss. In the case of a truss, all loads are imposed at panel points. That is, loads are only applied to the truss at points where top/bottom chords (the horizontal members) meet the diagonal (web) members. I'd shy away from arches, moment connections (where the connections between the members must resist bending and torsion), etc.
Knowing how the truss is supposed to support the load will give you an ensight on how to lay out the web members. But will also limit the truss to carrying the design load at one or more specific locations on the truss.
Are there any other design factors specified like maximum depth of the structure?
These design factors, along with the other suggestions on careful selection of stock and fastener technique (glue) should yield a winner.
An interesting experiment for your youngsters' fertile minds might be to take two pieces of balsa of eaqual depth, thickness and length. Set one piece between to block and load it gradually to failure. Take the second and carefully, using a very sharp knife, cut it into strips to fabricate a truss assembly. Load the truss (at a panel point) to failure and compare the weight carrying capacity of the two. A simple solid beam vs. a truss made out of the same amount of and same material.
...........
Dennis in Bellevue WA
[email protected]
Dennis,
I've been very neglectful in failing to mention the type of structure and the loading of it. I haven't yet read the rules (after last year's self propelled vehicle design/build, I've been putting off diving in beyond knee level...) but understand it to be kind of as follows: it will be on a flat surface, with a 2" vertical bar sticking up through what will be the center of the load, hence the center of the structure. The weights added will be kind of like bar bell weights. In other words, they will resemble doughnuts. Thus, it won't be a span at all, but will just beg for compression. It must be 6" OD at the top and 8" ID at the bottom, forcing a cone or pyramid shape right from the start. The points of contact with the flat surface are "pods", of which there must be at least three. I can't say more about these yet, having not done my own homework and actually read the rules! There may be up to nine pods, with additional points given for a design which uses the most pods, staying within the weight limit, etc..., which seems to invite buttresses.
The kids have so far been digesting concepts such as symmetry, compression and tension, along with moving their eyes and minds down structural members from a load to the base, trying to detect critical load/stress points. I repeat, they are 5th graders, and at all this sort of thing for the first time. Amazingly bright, though. I was an official advisor last year (my wife being the coach) but only a participant's parent this year. Hmmm, was that just mentioned as an excuse for not doing my homework? Feels like being back in school again!
Again, I very much appreciate the kind comments from the group. It is a delight to watch at this end.
Cheers,
Greg
Sounds like a very impressive academic environment for your kids. If you get a chance to photo the entries I'd be interested in seeing the results.
...........
Dennis in Bellevue WA
[email protected]
Dennis,
If you remember this thread, I'd like to let you (and all) know how it all worked out. The little 18 gr. structure supported 71 lbs.. (it just dawned on me that I don't know the top weight supported.... will have to ask around) It was made on a jig I helped the kids with. Bending was not a concept that stuck around long, especially since the structures were routinely "consumed" in practices. In the end, the structure was three legged. It kind of looked like a pyramid skeleton, with the top cut off, leaving a flat surface for weight stacking. The biggest issues turned out to be symmetry and repeatability, which the jig addressed nicely.
The jig simply consisted of a 3/8 sheet of mdf with some small strips of 1/4 plywood glued on to act as guides. The shape of the structure was drawn on the mdf and the strips were glued onto the outside of drawn lines. Just place the 1/8 x 1/8 balsa within the guide strips, cut to the lines, then glue with cyanoacrylate.
The process was to make a triangular top first, on one part of the jig. All corners were mitered, which was easy, as the jig had the pencil lines still on it from laying out the shape initially. Then they went to the other part of the jig and cut and glued three sticks together: a top and two legs. Once three of these minimal components were glued and cured, they could be glued to the underside of the triangular top. As this was done, each leg component found itself cheek to cheek with its neighbor. These were glued full length, creating a lamination at each leg. One more stick was glued in the channel created by this cheeky business, making each a three-piece lamination.
After that, they just cut and glued a bunch of cross members and braced right up to the 18 gr. limit. The final upper diameter was a bit wide, causing a 50 point deduction by the judges in a no-parent-or-coach closed room. My daughter snatched up an exacto and shaved the three corners, getting the 50 points back.
They all dressed up as island natives and called the structure a volcano, angry and about to erupt. The only answer was to place pretend ice blocks on top (the weights), until the volcano was cooled enough and its anger broken. They all cheered when it broke, as if this was a good thing. Kind of clever, that part.
There were other mind teasing events and the team got third place in the region. All pleased and proud.
No pictures except video tape by one of the parents. The structure failed at a lower part of one of the legs. Grain runout probably contributed. With so many structures gobbled up in practice, I think the timber selection got a little lax in the end.
I hope this light hearted note is amusing.
Cheers,
Greg
Greg -
Thanks for the follow up on the engineering project. Sounds like you hit, if not a home run, then a three-bagger to say the least. 71lbs on 18gr of structure. Impressive!
...........
Dennis in Bellevue WA
[email protected]
i've heard of structural engineering contests similar to this one.
71 punds over 18 grams is awesome. It would have won a national competition I competed (and was d/q'ed) in.
I've only seen two balsa wood bridges do better: a bridge built by me and a friend in 7th grade (210 lbs over 39 grams) and one built by two more of my friends (310 lbs over 50 grams). But then again, me and my friends had close to 15 hours of class time to build our bridges.
Dennis and Mark,
Thanks for the interest. These 4th and 5th graders are really terrific. So much enthusiasm and humor. It's fun to watch some of their wheels turn for the first time...and quickly too!
Thanks to all who replied above
Cheers,
Greg
Balsa wood.
just wet it with a wet wash cloth you will find it sucks up water.
It will not take much to steam it. a pot on the top of stove filled with water should be all you need to steam it.
By the way they make balsa wood cut into structural shapes, I beam, channel, angle and a lot more. A lot of the colleges engineering class use the structural shapes for models bridge. At least they use to.
Why not cut a series of pieces to the curve of the bridge, then add a piece of balsa overtop? Diagram follows.
Dennis,
Thanks. It is indeed a great school with terrific kids. I'll try and get a postable photo when available.
Chris,
It isn't a bridge they're building, but more of a doughnut, so to speak. It is placed down over a 2" central "pole", with the weights then added to the pole, much as one adds weights to a bar bell in the gym. The entire thing must be made of 1/8 x 1/8 inch balsa, so the only cutting will be to length. The 18 gr. weight limit of the structure also dictates it be very lean, with no extraneous members. Just balsa and glue.
Cheers,
Greg
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