Designs B/C

someusername
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Re: Designs B/C

Postby someusername » February 2nd, 2015, 8:52 pm

Personally for bridges, I like to stay away from the square beams for the most part. Since a majority of the force is going down, having beams that are thicker from side to side don't seem to make much sense to me, as the cross-bracings already cover that motion. Its like trying to bend a piece of paper in half by bending the edges of it and not the faces, if that makes any sense. the thicker something is in some direction, the harder it is to break in that direction.
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Re: Designs B/C

Postby iwonder » February 2nd, 2015, 9:04 pm

actually, both are correct... Cylinders provide thew best overall efficiency, but they are unpredictable as to which direction they will buckle. Squares are not quite as efficient, but you only need to brace them in 2 planes, so what is lost by going to the square may be gained back in the bracing... depends on the individual situation.
Well I meant it mostly as a joke, but since you brought it up, any cross section of a member can only buckle in two dimensions, both perpendicular to the compressive load. That makes up one plane, and as long as I brace something with two braces that are in the plane and perpendicular (say, vertically and horizontally) that should stop all motion in that plane. (I guess I'm pretty bad at describing geometric things in words.... Sorry)
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Re: Designs B/C

Postby lukas » February 3rd, 2015, 3:44 pm

actually, both are correct... Cylinders provide thew best overall efficiency, but they are unpredictable as to which direction they will buckle. Squares are not quite as efficient, but you only need to brace them in 2 planes, so what is lost by going to the square may be gained back in the bracing... depends on the individual situation.
Well I meant it mostly as a joke, but since you brought it up, any cross section of a member can only buckle in two dimensions, both perpendicular to the compressive load. That makes up one plane, and as long as I brace something with two braces that are in the plane and perpendicular (say, vertically and horizontally) that should stop all motion in that plane. (I guess I'm pretty bad at describing geometric things in words.... Sorry)

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Re: Designs B/C

Postby dholdgreve » February 4th, 2015, 9:30 am

actually, both are correct... Cylinders provide thew best overall efficiency, but they are unpredictable as to which direction they will buckle. Squares are not quite as efficient, but you only need to brace them in 2 planes, so what is lost by going to the square may be gained back in the bracing... depends on the individual situation.
Well I meant it mostly as a joke, but since you brought it up, any cross section of a member can only buckle in two dimensions, both perpendicular to the compressive load. That makes up one plane, and as long as I brace something with two braces that are in the plane and perpendicular (say, vertically and horizontally) that should stop all motion in that plane. (I guess I'm pretty bad at describing geometric things in words.... Sorry)
Ummm... ok... I'll go there with ya... A piece can actually only buckle in one plane, which would be perpendicular to the length of the piece. With a square piece, it is most likely to buckle in either this direction, or one 90 degrees to this direction... With a round piece, it could just as likely buckle in any of the 360 degrees of that plane... When bracing is applied, it is most effective when placed in the direction of, or 180 degrees to the vector of buckling. So in the case of the square, you need to account for 4 possible failure vectors, each 90 degrees to each other, but in the case of a round member, it could fail in any direction, so certainly more thought needs to be placed on the potential vectors of failure, and possibly adjust the mass of the brace to compensate for those vectors not 90 degrees to either brace.
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Re: Designs B/C

Postby iwonder » February 4th, 2015, 10:24 am

Ummm... ok... I'll go there with ya... A piece can actually only buckle in one plane, which would be perpendicular to the length of the piece. With a square piece, it is most likely to buckle in either this direction, or one 90 degrees to this direction... With a round piece, it could just as likely buckle in any of the 360 degrees of that plane... When bracing is applied, it is most effective when placed in the direction of, or 180 degrees to the vector of buckling. So in the case of the square, you need to account for 4 possible failure vectors, each 90 degrees to each other, but in the case of a round member, it could fail in any direction, so certainly more thought needs to be placed on the potential vectors of failure, and possibly adjust the mass of the brace to compensate for those vectors not 90 degrees to either brace.
Yeah, it certainly wouldn't be as effective if the failure isn't parallel with the brace, but it's not that it wouldn't do _anything_.

Of course that's not considering the actual method of attaching a brace to a cylindrical member, which is a bit more complicated than putting some glue on the side and sticking another piece of wood to it :P

While we're on the topic, what do you make of this (aside from actually building it..)?

Take a square member under compression, looking at its cross section. There's one side of the square whose normal is north (we'll call north 0 degrees). There are two braces, one at 90 degrees and another at 180 degrees. Let's assume that the braces can only act under tension. Say the member is tending to buckle north, and exerts a tension on the brace at 180 degrees. We'll say it's under a tension of 1N.

Now repeat the same scenario with a square member that has a side whose normal is northeast (45 degrees), and still has the same 90 degree and 180 degree braces as before. This time it's tending to buckle to the northwest (315 degrees), putting tension on both of the braces, instead of just one. Since it's the same force, this time each brace is under a tension of 0.707N.

Aside from the fact that it would take a lot of notches and sanding and careful construction, wouldn't that mean that it'd be beneficial to use members that don't actually tend to fail in directions parallel to the bracing? Because by making the member fail parallel to the brace you're exerting all of that force on a single brace, and it's the maximum possible force, instead of distributing it across multiple braces and lessing the load on each brace.

It certainly wouldn't be of a great benefit, but holding 70% of the tension ideally means 70% of the cross sectional area, and 70% of the bracing mass. (For one of my lighter booms two years ago that's a savings of 0.12 grams or 1.4% of the structure weight), but it might be more useful for towers or bridge.
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Re: Designs B/C

Postby dholdgreve » February 5th, 2015, 7:27 am

Very interesting question. i see your point and agree in theory... I have never been very lucky at training my balsa detail exactly which angle it intends to buckle though... so most of the time it comes right toward my tension braces, putting them in compression...
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Re: Designs B/C

Postby tanjil2001 » February 5th, 2015, 4:17 pm

In class today i saw a video for science Olympiad about bridge building so in the video in the middle of the bridge there were six holes to place the loading block how come there are 6 holes not one can u please explain this thank u also in the video the guy placed the loading block on the last hole from the left why didnt he put the block on the middle i ma sorry for the question my school entered a long time ago and then we stopped know were entering as new school, so sorry for these questions :?: :?: :?: :?:

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Re: Designs B/C

Postby chalker » February 5th, 2015, 4:52 pm

In class today i saw a video for science Olympiad about bridge building so in the video in the middle of the bridge there were six holes to place the loading block how come there are 6 holes not one can u please explain this thank u also in the video the guy placed the loading block on the last hole from the left why didnt he put the block on the middle i ma sorry for the question my school entered a long time ago and then we stopped know were entering as new school, so sorry for these questions :?: :?: :?: :?:
That sounds like a video from a prior year when the rules were a bit different and proscribed certain spots on the bridge. If you haven't gotten a copy of the CURRENT rules from your coach you really should get them and read them closely.

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Re: Designs B/C

Postby lukas » February 5th, 2015, 6:21 pm

In class today i saw a video for science Olympiad about bridge building so in the video in the middle of the bridge there were six holes to place the loading block how come there are 6 holes not one can u please explain this thank u also in the video the guy placed the loading block on the last hole from the left why didnt he put the block on the middle i ma sorry for the question my school entered a long time ago and then we stopped know were entering as new school, so sorry for these questions :?: :?: :?: :?:
That sounds to me like the rules for the international bridge building competition, which has 6 different possible loading points.

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Re: Designs B/C

Postby bearasmith » February 6th, 2015, 10:58 am

Around 2004, bridges were built like that. The proctor could make you test any hole he wanted. The reason was, that when weight goes across a bridge it doesn't just press on the middle, but all points on the road have stress. Anyway, this year is different and you only need one area for a loading block, the center.
Last edited by bearasmith on February 7th, 2015, 7:24 am, edited 1 time in total.
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