Designs B/C

[noobforce]

Maybe I'm skipping over some valuable posts, but I looked in the forums and couldn't find anything about the bracings on the top and the bottom of the bridge.

Because the top layer is undergoing compression, would some Z or X bracings work to connect the top two members? And would the bottom layer just not have anything because it is tension?

[chinesesushi]

Maybe I'm skipping over some valuable posts, but I looked in the forums and couldn't find anything about the bracings on the top and the bottom of the bridge.

Because the top layer is undergoing compression, would some Z or X bracings work to connect the top two members? And would the bottom layer just not have anything because it is tension?

Yes. Yes. The principles are the same as in boomilever and any other structural engineering event, because tensile force does not rely on the length of the member, there is no need for bracings. However, compression => buckling and buckling strength does rely on the length of the member, so there should be enough bracings on the compression members of bridges. Good luck!

[dholdgreve]

It's important to brace the ends and across the top... not so much on the bottom for reasons stated, but depending on your design, it may also require a couple of X or at least Z braces in the Z plane near, but not directly at the middle of the bridge to keep it from torquing. Another thing to consider... if you are going with a design where the loading block sits on top the bridge, find a different way to brace that area, so the loading block does not sit directly on the braces and not the top chords of the bridge... This creates point loads at those spots, when you want to distribute that load as consistently as possible.

[someusername]

I was wondering on a bridge what would make a bigger impact. The "height" of the top beams or the way the cross bracing is laid out? I know that as a general rule the thicker something is, the stronger it is but it would also cause it to weigh more.

[chinesesushi]

I was wondering on a bridge what would make a bigger impact. The "height" of the top beams or the way the cross bracing is laid out? I know that as a general rule the thicker something is, the stronger it is but it would also cause it to weigh more.

Your question seems to be extremely vague. But from what I gather from it, the answer is both, and only experimentation may reveal the balance between the two that reveals the highest possible efficiency. Good luck!

[noobforce]

Maybe I'm skipping over some valuable posts, but I looked in the forums and couldn't find anything about the bracings on the top and the bottom of the bridge.

Because the top layer is undergoing compression, would some Z or X bracings work to connect the top two members? And would the bottom layer just not have anything because it is tension?

Yes. Yes. The principles are the same as in boomilever and any other structural engineering event, because tensile force does not rely on the length of the member, there is no need for bracings. However, compression => buckling and buckling strength does rely on the length of the member, so there should be enough bracings on the compression members of bridges. Good luck!

It's important to brace the ends and across the top... not so much on the bottom for reasons stated, but depending on your design, it may also require a couple of X or at least Z braces in the Z plane near, but not directly at the middle of the bridge to keep it from torquing. Another thing to consider... if you are going with a design where the loading block sits on top the bridge, find a different way to brace that area, so the loading block does not sit directly on the braces and not the top chords of the bridge... This creates point loads at those spots, when you want to distribute that load as consistently as possible.

Thanks for the insight. I'll keep that in mind.

Also, I have another question. How would you go about making a jig for the bridge? I don't have to tools necessary to use wood. What should I make it out of and how should I construct it?

[retired1]

Where is your coach for this event??
You do not need a formal jig, but it helps. Put your drawing on a board. Use clear packaging tape over the joints to keep the glue from sticking to the drawing. If you have access to insulation board, use it and pins. If you are using hard boards, hold the balsa in position with bolts or nuts or any small reasonable weight. You might recycle some old science fair presentation boards or some really strong cardboard like some appliances come in. A local supply company that sells cardboard and paper may give you a piece if you ask very nicely. You would prefer 3 ply.
If you do use pins, do not stick them in the balsa for the bridge. BAD!! Use them to block movement or use them with scraps that will hold the balsa down and in position like a clamp.
I coach both middle school and high school. We use 3/4" building insulation .Students have sections of 1/16 x 1/2 aluminum that has small holes drilled in it and this is held in position with push pins. The balsa is held up against this with grip pins. Any pin will do, but these are great and have been used for 4 years now.
My opinion, you will be better off making your bridge like a funny tower with the weight block on the top. The taller it is, the more weight it will hold (to a point) because of the angles.
A computer program shows that a height of 5 to 10 centimeters will give the best results.
build one with no bracing and break it. A joint will probably break, so put a thin scab patch on the upper joints. This will tell you if you need bigger wood or cross bracing. Then build another one.

[bernard]

Also, I have another question. How would you go about making a jig for the bridge? I don't have to tools necessary to use wood. What should I make it out of and how should I construct it?

If you plan on making the jig for testing your own bridges, I would recommend saving having to go through the trouble and cost of making a jig and using this setup. I just placed two tables 45.0 cm (Div. C) apart. However, If you don't mind going through the process of making a jig, let me know and I can show you some of the pictures of jigs I have saved to my computer.

[retired1]

A boo boo in my last post. By no bracing, I meant no vertical bracing. You obviously need bracing to tie the two sections of the bridge together.
Bernard, a Jig is for construction of the bridge. The testing device is just that- a testing device. No jig required.

[noobforce]

Where is your coach for this event??
You do not need a formal jig, but it helps. Put your drawing on a board. Use clear packaging tape over the joints to keep the glue from sticking to the drawing. If you have access to insulation board, use it and pins. If you are using hard boards, hold the balsa in position with bolts or nuts or any small reasonable weight. You might recycle some old science fair presentation boards or some really strong cardboard like some appliances come in. A local supply company that sells cardboard and paper may give you a piece if you ask very nicely. You would prefer 3 ply.
If you do use pins, do not stick them in the balsa for the bridge. BAD!! Use them to block movement or use them with scraps that will hold the balsa down and in position like a clamp.
I coach both middle school and high school. We use 3/4" building insulation .Students have sections of 1/16 x 1/2 aluminum that has small holes drilled in it and this is held in position with push pins. The balsa is held up against this with grip pins. Any pin will do, but these are great and have been used for 4 years now.
My opinion, you will be better off making your bridge like a funny tower with the weight block on the top. The taller it is, the more weight it will hold (to a point) because of the angles.
A computer program shows that a height of 5 to 10 centimeters will give the best results.
build one with no bracing and break it. A joint will probably break, so put a thin scab patch on the upper joints. This will tell you if you need bigger wood or cross bracing. Then build another one.

Haha.. I don't have a coach.. the teacher supervisor is a biology teacher so he only knows how to help the more-sciency events.

Do you recommend building the sides and then connecting them, or building the top and working around? Also, to solve the problem of the bracings interfering with the loading block on the compression members, would there be any noticeable impact on the performance if the bracings were on the bottom side of the compression members rather than on the top, and having an even number of bracings so the chain goes through the bracings?