Bridge Building Supplies

drcubbin
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Bridge Building Supplies

Postby drcubbin » Sun Jan 11, 2015 12:41 am

Is there any one company who has the best price/quality for bridge building wood? Also, the rules that specify "wood" to be the only substance allowed, but I see that most bridges are made of balsa - why not pine or some other non-particle wood? Is it because they want a low load/mass ratio when being scored? This is our first year in the competition after being out for many years (even before I arrived) so these will be my first seemingly ridiculous questions :? Thank you for not laughing ;)

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Re: Bridge Building Supplies

Postby DoctaDave » Sun Jan 11, 2015 4:29 am

If you're just starting the wood provided by your local hobby store should suffice, but I usually order wood in bulk from specializedbalsa.com, as they carry balsa in many different sizes and you can specify the density.

I mainly use balsa because it comes in many more dimensions than wood like bass or pine, and it's just a lot more common. Also because balsa is generally a lot lower in density, it means you get a larger surface area for the same weight, and thus joints will be stronger. I rarely use basswood in my builds unless I need a single component to be both very strong and very small, but I haven't encountered a situation like that this season yet for bridge building. But it also comes down to personal preference and what you prefer to work with, or what your specific needs are for your design. I've had good results with both bass, balsa, or a combination of both.

I've also never seen any bridges using pine. Perhaps the properties of pine aren't ideal for this event, but I haven't looked into it so I could be wrong about that.

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Re: Bridge Building Supplies

Postby bernard » Sun Jan 11, 2015 5:34 am

Another option is National Balsa. I believe they are cheaper than Specialized Balsa, but you have less choice over your wood densities and grains. I generally order 15-30 sticks of each type of wood I want and then pick from those. It's good for those who are just starting to build because you get a lot of variety to try and it's cheap.
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Re: Bridge Building Supplies

Postby sjwon3789 » Sun Jan 11, 2015 7:48 am

bernard wrote:Another option is National Balsa. I believe they are cheaper than Specialized Balsa, but you have less choice over your wood densities and grains. I generally order 15-30 sticks of each type of wood I want and then pick from those. It's good for those who are just starting to build because you get a lot of variety to try and it's cheap.


Doesn't the wood densities vary greatly though? Some may be extremely fragile.
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Re: Bridge Building Supplies

Postby bernard » Sun Jan 11, 2015 7:58 am

sjwon3789 wrote:
bernard wrote:Another option is National Balsa. I believe they are cheaper than Specialized Balsa, but you have less choice over your wood densities and grains. I generally order 15-30 sticks of each type of wood I want and then pick from those. It's good for those who are just starting to build because you get a lot of variety to try and it's cheap.


Doesn't the wood densities vary greatly though? Some may be extremely fragile.

Yes. If you know exactly what densities you want, then National Balsa probably won't be the best place to order your wood. However, it is good for you if you want to try a variety of densities at a low cost. I've make the same bridge designs several times with only densities changed, which has allowed me to determine which densities are ideal for which members.
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Re: Bridge Building Supplies

Postby iwonder » Sun Jan 11, 2015 5:41 pm

drcubbin wrote:Is there any one company who has the best price/quality for bridge building wood? Also, the rules that specify "wood" to be the only substance allowed, but I see that most bridges are made of balsa - why not pine or some other non-particle wood? Is it because they want a low load/mass ratio when being scored? This is our first year in the competition after being out for many years (even before I arrived) so these will be my first seemingly ridiculous questions :? Thank you for not laughing ;)


Probably your best path for wood is to build a bunch of designs out of cheap local balsa (craft stores, hobby shop, etc), or if that's not an option Balsa Wood Inc has really cheap wood. It's not consistent, and it won't yield an absolutely incredible bridge, but you'll learn how to assemble it, you can play around with densities and see where you need it to be light, heavy, etc, and it's just cheap. Building a bridge to try a new idea and breaking it doesn't cost you $10-$15 like it does with the fancy wood. When you do this, weigh and write down (I used excel) the weight and dimensions of all the pieces on your bridge.

When you feel comfortable with a design that you want to use for a contest, figure out what weights of pieces worked best and what sizes of wood you need. IMHO, buy only sheets of balsa, never buy sticks. You can get a tool called a balsa stripper for real cheap online, it holds a knife blade and has an adjustable width, it's used to cut sheets into sticks.

Once you know what you need, you have a few options, if you've got a good local place with a lot of selection, you can go there with a scale and weigh their sheets, trying to find ones that are the right density and obviously the thickness you need. Hold them up to the light and look for where light passes through more than other places, when more light gets through the sheet is less dense. Also look at the grain, it should run along the long axis of the sheet for most things you'll need. And feel the sheet, if it's powdery it's likely a little more brittle and not quite as strong.

What you're looking for is a pretty consistent (density) sheet of balsa, with straight grain (avoid curved grain, the pieces will break where the grain ends on the side of the sheet). If you can find that at a local shop that's fine. A lot of us order from specialized balsa for this stuff because you can buy specific densities of sheets and they're generally very consistent with a nice straight grain. No hassle. Plus they have about every thickness of sheet you could imagine.

When you go to cut sticks off the sheet, you want to look for the edge with the best grain and start from there. Anything over about 3/32" thick and you should probably make two passes with a stripper (one about halfway down and the other to finish it off). If you go for it all at once and the grain bends just a little bit, the blade will move with the bend and you'll have ruined that edge (you could take a straightedge and a knife to clean it up).

What I would do for boomilever, say I had a tension member (mine were 1/2" wide, 1/64" thick, and about 20" long), is cut maybe 4 or 5, even though I needed 2, and weight all of them to find one that was weight, and I would even go as far as to find ones that balanced near the middle, i.e. the most consistent. Pick the best two out of that and use it.

It's up to you if you want to put in that much work or be that picky about it, or even use up that much wood in the process, but that's the stuff I've gathered from the past many years of the forums (archives!) and a few years of doing it myself.
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Re: Bridge Building Supplies

Postby iwonder » Sun Jan 11, 2015 5:49 pm

drcubbin wrote:Is there any one company who has the best price/quality for bridge building wood? Also, the rules that specify "wood" to be the only substance allowed, but I see that most bridges are made of balsa - why not pine or some other non-particle wood? Is it because they want a low load/mass ratio when being scored? This is our first year in the competition after being out for many years (even before I arrived) so these will be my first seemingly ridiculous questions :? Thank you for not laughing ;)


I think it'll be more organized if I answer the other half here..

So most people use balsa because it's the standard for this sort of thing and it's a lot easier to buy than say, a 1/32" thick sheet of spruce. But it's also because balsa has very high strength to weight ratios, for it's density, it's about as strong as any wood (it's also not a 'particle' wood, that's a manufactured wood product). There are a few exceptions where I've seen people use things like red oak, spruce, or a hardwood in place of balsa, but only it very small quantities and in places where absolute strength is required, it's a rare occurrence. Softwoods like pine typically aren't used because when compared to hardwoods (yes, balsa is a hardwood), namely because when balsa or bass isn't sufficient, the case typically calls for some sort of other hardwood.

Basically unless you have pieces where very high density balsa keeps breaking (the wood, not the glue joint), stick with balsa (I don't personally care for bass, but some people use that too, yes I have used it). When you get to that point, you honestly probably won't be asking the question on scioly :P
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Re: Bridge Building Supplies

Postby sjwon3789 » Sun Jan 11, 2015 6:05 pm

iwonder wrote:So most people use balsa because it's the standard for this sort of thing and it's a lot easier to buy than say, a 1/32" thick sheet of spruce. But it's also because balsa has very high strength to weight ratios, for it's density, it's about as strong as any wood (it's also not a 'particle' wood, that's a manufactured wood product). There are a few exceptions where I've seen people use things like red oak, spruce, or a hardwood in place of balsa, but only it very small quantities and in places where absolute strength is required, it's a rare occurrence. Softwoods like pine typically aren't used because when compared to hardwoods (yes, balsa is a hardwood), namely because when balsa or bass isn't sufficient, the case typically calls for some sort of other hardwood.

Basically unless you have pieces where very high density balsa keeps breaking (the wood, not the glue joint), stick with balsa (I don't personally care for bass, but some people use that too, yes I have used it). When you get to that point, you honestly probably won't be asking the question on scioly :P


Last year, I saw in the aia's guide that she used bass wood for tension members. I still don't know when you would be using bass wood. Why was it better to use bass over balsa then? Is it because it's thinner, therefore easier to make the distal end? Should no one be using bass wood unless they have a unique design?
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Re: Bridge Building Supplies

Postby iwonder » Sun Jan 11, 2015 6:37 pm

sjwon3789 wrote:
iwonder wrote:So most people use balsa because it's the standard for this sort of thing and it's a lot easier to buy than say, a 1/32" thick sheet of spruce. But it's also because balsa has very high strength to weight ratios, for it's density, it's about as strong as any wood (it's also not a 'particle' wood, that's a manufactured wood product). There are a few exceptions where I've seen people use things like red oak, spruce, or a hardwood in place of balsa, but only it very small quantities and in places where absolute strength is required, it's a rare occurrence. Softwoods like pine typically aren't used because when compared to hardwoods (yes, balsa is a hardwood), namely because when balsa or bass isn't sufficient, the case typically calls for some sort of other hardwood.

Basically unless you have pieces where very high density balsa keeps breaking (the wood, not the glue joint), stick with balsa (I don't personally care for bass, but some people use that too, yes I have used it). When you get to that point, you honestly probably won't be asking the question on scioly :P


Last year, I saw in the aia's guide that she used bass wood for tension members. I still don't know when you would be using bass wood. Why was it better to use bass over balsa then? Is it because it's thinner, therefore easier to make the distal end? Should no one be using bass wood unless they have a unique design?


You really have to look at the properties of the wood and find out which one works best for your application. Bass won't bend as easily as balsa, but it's a lot heavier, if I recall correctly they're about equal in tension (but don't hold me to that). There're a lot of tradeoffs similar to that one. And of course you can always try and find out.
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Re: Bridge Building Supplies

Postby tanjil2001 » Fri Jan 16, 2015 11:27 pm

i am one of dr cubbins students so i really want to attend at this station because of previous experience with bridge building so i juts wanted to know if most of the bridges will they arch bridges or will they truss bridges or is it a possible to have a combination of the types of bridges because i was wondering if there is a restriction of how much would use depending of weight also the type of substance to attach the wooden pieces would it a be a problem of what type of glue to use
:geek:

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Re: Bridge Building Supplies

Postby JonB » Fri Jan 16, 2015 11:38 pm

tanjil2001 wrote:i am one of dr cubbins students so i really want to attend at this station because of previous experience with bridge building so i juts wanted to know if most of the bridges will they arch bridges or will they truss bridges or is it a possible to have a combination of the types of bridges because i was wondering if there is a restriction of how much would use depending of weight also the type of substance to attach the wooden pieces would it a be a problem of what type of glue to use
:geek:



Start here:

http://scioly.org/wiki/index.php/Bridge_Building

That addresses most of your questions.

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Re: Bridge Building Supplies

Postby drcubbin » Tue Jan 27, 2015 10:28 pm

Tanjil - How about this? Pretty cool, but I am not sure why we don't see more of this design in competition. Anyone?

https://www.youtube.com/watch?v=RRzqnvA ... J2&index=3

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Re: Bridge Building Supplies

Postby chinesesushi » Wed Jan 28, 2015 1:05 am

drcubbin wrote:Tanjil - How about this? Pretty cool, but I am not sure why we don't see more of this design in competition. Anyone?

https://www.youtube.com/watch?v=RRzqnvA ... J2&index=3


I have likewise seen that video on Youtube. Thing is, the record really isn't that high, as the "score" I believe it would get in a normal competition would only be 1650. And that 's if you were allowed to hold up to 419 pounds. You can try a design like that, but with the load limit of 15 kilograms, you would have to lighten down some of the pieces which would in turn decrease the strength of the bridge.
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Re: Bridge Building Supplies

Postby DoctaDave » Wed Jan 28, 2015 1:39 am

drcubbin wrote:Tanjil - How about this? Pretty cool, but I am not sure why we don't see more of this design in competition. Anyone?

https://www.youtube.com/watch?v=RRzqnvA ... J2&index=3


Although the bridge's score isn't too impressive, I have seen a few arch bridges do very well whose designs were somewhat similar to the one in the video. I don't think the design of the bridge in the video was very well optimized, because the builder braced the tension members, which only adds weight without giving any more strength to the structure.

If you're a beginner at building with balsa wood, I would just stick to a "normal" truss bridge as those are much easier to build. Both types of designs can get you great scores, but the extra time and experience needed to build and experiment with arch bridges would probably outweigh any advantages to using an arch.

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Re: Bridge Building Supplies

Postby dholdgreve » Wed Jan 28, 2015 2:20 pm

Agreed Dr. Dave... Last week at Magsig there were a couple of tall arched bridges... From an engineering standpoint (efficiency), they didn't score exceptionally well, but from an architectural standpoint, they were off the chart! (Very unique, symmetrical, and real pleasure to look at!)
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