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Anchors

  • 05-04-2011 10:48pm
    #1
    Howlin
    Registered Users, Registered Users 2 Posts: 199 ✭✭


    Im doing some work on anchors and i have noticed that if the angle increases between the two anchors, the force also increases and after a point the force on the anchors are greater than the force applied? any ideas on who this is? Im a bit stumped


Comments

  • #2
    SOL
    Registered Users, Registered Users 2 Posts: 1,155 ✭✭✭SOL


    http://en.wikipedia.org/wiki/American_death_triangle

    This is an article from climbing... but it should explain what is going on...


  • #3
    Sev
    Registered Users, Registered Users 2 Posts: 1,328 ✭✭✭Sev


    It's basically a maths problem involving addition of vector forces. Understanding trigonometry also helps to quantitatively understand the problem.

    You break the forces down into their horizontal 'x' and vertical 'y' components (with the help of trigonometry). The sum of the y-components of the forces on all anchors is equal to the weight of the climber held by the anchor system. Therefore, the sum of the forces on all anchors is always at least as large as the weight of the climber.

    However, when the anchors are placed in different locations, creating an angle, there are now x-components to the forces acting on all the anchors. These x-components must sum to zero because the weight of the climber has no x-component, it only acts downwards. These x-direction forces all act against each other and cancel each other out. This is where the extra force comes from.

    I hope this makes sense.

    Wikipedia has an illustration here:
    http://en.wikipedia.org/wiki/Anchor_(climbing)

    When there is an angle in the system, the two trees are no longer just acting against the weight of the climber, they're also acting against each other because the anchor system is trying to squeeze the two trees together. When the angle is greater than 90 degrees, they pull more against each other than against the climber. When the angle is greater than 120 degrees, the force on any one tree is greater than the weight of the climber.

    In the most extreme case, where the anchor forms a T (instead of a Y), the forces on the anchor points go to infinity! This could never actually happen in reality, because there's always some elasticity in the rope.

    The force calculation in the american death triangle linked above are a little more involved than in the Y anchor set up. This particular anchor setup should never be used as it magnifies the effect of the angle on increasing the forces on the anchor points.


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