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1. Yes
2. No, but you may have to dissemble it, if you have a tube boom

Hey guys, what exactly is the purpose of cross-bracing? I know it sounds naïve but I get that it is to shorten the length of a piece and in a sense have it segmented since shorter pieces have better strength both in tension and compression (at least I think) but I would like to know this: if I have 2 pieces of balsa connecting my tension and compression members at a certain point (on either side of the boomilever) and I would only brace them with one "x" is bracing really necessary since it isn't "segmenting" the piece at all? Or could it be necessary to prevent the pieces from bowing outward? The pieces are 1/8 by 1/8 and roughly 4 cm long, would an x really be required there? And if I have 2 that are roughly 8 cm long would I need to brace them with 2 "x's" or 1 "x" since technically with the 1 "x" I am still cutting the length in half... Thanks!

thsom wrote:Hey guys, what exactly is the purpose of cross-bracing? I know it sounds naïve but I get that it is to shorten the length of a piece and in a sense have it segmented since shorter pieces have better strength both in tension and compression (at least I think) but I would like to know this: if I have 2 pieces of balsa connecting my tension and compression members at a certain point (on either side of the boomilever) and I would only brace them with one "x" is bracing really necessary since it isn't "segmenting" the piece at all? Or could it be necessary to prevent the pieces from bowing outward? The pieces are 1/8 by 1/8 and roughly 4 cm long, would an x really be required there? And if I have 2 that are roughly 8 cm long would I need to brace them with 2 "x's" or 1 "x" since technically with the 1 "x" I am still cutting the length in half... Thanks!

You're correct that shortening members increases compressive strength, however interestingly tension strength actually is not dependent on the length of a member.

If i'm reading this correctly, you're asking why not use only one x instead of two, since it shortens the member in half anyway. Here's how I would answer the question. You were on to something when you asked if the pieces were necessary to prevent bowing. While cross bracing is necessary for "shortening" the compression members, it is also important for increasing the "mode of buckling" that your compression member is experiencing. (More intervals of bracing means a higher mode of buckling, this image sums it up pretty well: http://1.bp.blogspot.com/-vuPu5rsMkPo/T ... shapes.jpg ). In the diagram, the pinned points that separate the columns are sort-of imaginary. The way I see it, in your boom with two compression beams, the members are in a way pinned to each other. Leaving any untrussed open space introduces an essentially unsupported area. Because of this I would say that having only one x where there should be two really wouldn't be separating the members, and that you're much better off completing the truss. But correct me if i'm not reading into your initial question the right way.

What kind of efficiencies are people seeing at Regionals? At our's last week the winning B boom was over 1500 (and didn't break.)

if you look back a bit, someone posted that at their regionals, the top score was ~1800.

Azn wrote:

thsom wrote:Hey guys, what exactly is the purpose of cross-bracing? I know it sounds naïve but I get that it is to shorten the length of a piece and in a sense have it segmented since shorter pieces have better strength both in tension and compression (at least I think) but I would like to know this: if I have 2 pieces of balsa connecting my tension and compression members at a certain point (on either side of the boomilever) and I would only brace them with one "x" is bracing really necessary since it isn't "segmenting" the piece at all? Or could it be necessary to prevent the pieces from bowing outward? The pieces are 1/8 by 1/8 and roughly 4 cm long, would an x really be required there? And if I have 2 that are roughly 8 cm long would I need to brace them with 2 "x's" or 1 "x" since technically with the 1 "x" I am still cutting the length in half... Thanks!

You're correct that shortening members increases compressive strength, however interestingly tension strength actually is not dependent on the length of a member.

If i'm reading this correctly, you're asking why not use only one x instead of two, since it shortens the member in half anyway. Here's how I would answer the question. You were on to something when you asked if the pieces were necessary to prevent bowing. While cross bracing is necessary for "shortening" the compression members, it is also important for increasing the "mode of buckling" that your compression member is experiencing. (More intervals of bracing means a higher mode of buckling, this image sums it up pretty well: http://1.bp.blogspot.com/-vuPu5rsMkPo/T ... shapes.jpg ). In the diagram, the pinned points that separate the columns are sort-of imaginary. The way I see it, in your boom with two compression beams, the members are in a way pinned to each other. Leaving any untrussed open space introduces an essentially unsupported area. Because of this I would say that having only one x where there should be two really wouldn't be separating the members, and that you're much better off completing the truss. But correct me if i'm not reading into your initial question the right way.

Thanks so much (you got exactly what I meant!) Also, I had a question guys. If you were doing a tube boom how could you prevent it from crushing and compressing against itself (I guess I mean flattening) because of the loading block and the force directly in that location (since it is hollow) or would that not happen? And would you use thin sheets (i.e. 1/32) and soak them and roll them or would that retain too much water so you would have to steam them? Thanks!

You can soak them, a few hours and they'll only slightly moist, a few days and they'll be back to normal.

Has anyone gotten decent efficiency with a tube boom with this years rules. I know another school hit 1800 without using a compression box, that's amazing.

Compression boxes aren't necessary for a good score. I have had ok success without them (won't release too much but not MORE than 1600) However, is it in general consensus that compression boxes are better (as it's consensus that tension boomis are better than compression ones)?

EDIT: OH, and thanks iwonder!!!

EDIT: and with the new hook requirement, could you really do a single tension member design?

thsom wrote:Compression boxes aren't necessary for a good score. I have had ok success without them (won't release too much but not MORE than 1600) However, is it in general consensus that compression boxes are better (as it's consensus that tension boomis are better than compression ones)?

EDIT: OH, and thanks iwonder!!!

EDIT: and with the new hook requirement, could you really do a single tension member design?

Yes. No problem at all with our single tension arm and mount.

We did break a tube at Regionals today. The weight of our tube boom was 9.88grams.