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  1. #1

    Compression force on speader bars for bridge

    At the risk of triggering TeeDee and Griz into a flurry of formulas, I have what I hope is a simple question about compression force on spreader bars. I have no hope of working out the math myself because I've spent way too much of my post-college life as a right brain creative geek and to be honest I spent most of my actual college life either recovering from or planning for a party.

    Can I use Smee's chart from Hammock Physics 101 to figure out the compression force on a spreader bar? It looks to me as though I can use his formula for shear force (S) to easily figure out compression force on the bar.

    http://whiteblaze.net/forum/vbg/show...imageuser=1908

    Using Smee's formula, the total compression force on a spreader bar would be 2*S, correct? Or is it just S?

    Be gentle.

  2. #2
    Senior Member Walking Bear's Avatar
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    I'll wait for Griz to do the calculations for you on the compression forces. He developed the formula back in the bridge thread. I used some trekking poles that had shock springs in them and measured as accurate as I could the compression on the poles. The two methods were very close. If I remember correctly factors that entered into the equation were the angle of the suspension, the length of the spreader, the length of the spreader triangle and weight in the hammock. Change one and the compression will change.

  3. #3
    krugd's Avatar
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    Quote Originally Posted by Boris Losdindawoods View Post
    At the risk of triggering TeeDee and Griz into a flurry of formulas, I have what I hope is a simple question about compression force on spreader bars. I have no hope of working out the math myself because I've spent way too much of my post-college life as a right brain creative geek and to be honest I spent most of my actual college life either recovering from or planning for a party.

    Can I use Smee's chart from Hammock Physics 101 to figure out the compression force on a spreader bar? It looks to me as though I can use his formula for shear force (S) to easily figure out compression force on the bar.

    http://whiteblaze.net/forum/vbg/show...imageuser=1908

    Using Smee's formula, the total compression force on a spreader bar would be 2*S, correct? Or is it just S?

    Be gentle.
    I don't think its either. If T is as in the drawing - the tension on a single rope, then using a little trigonometry the total compression on the bar is T /sin(angle), where angle is the angle between the spreader bar and one of the two support ropes. (Generally, this means you want to keep the angle large - ie a large side length to the support triangle. Not very practical.)

    In particular, as the angle becomes very short, the force on the bars becomes...infinite.
    --Don---

    May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view. May your mountains rise into and above the clouds. - Ed Abbey

  4. #4
    How sure are you about that? I can't contradict you, but T/sinΘ would be (W/2sinΘ)/sinΘ which generates some pretty whacky values for me.

  5. #5
    GrizzlyAdams's Avatar
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    Tee has the first, middle, and last word on compression forces. See the tail of this post, and the beginning of this one.

    The fast answer is one that WalkingBear already gave...there are a number of factors that contribute to compression force that Smee's shear force does not contain.

    have fun!
    Grizz
    (alias ProfessorHammock on youtube)

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