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Force on Slackline Anchors
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jt512


Jun 6, 2002, 8:25 PM
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Force on Slackline Anchors
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In the "kilonewton" thread teen_dawg asks how much force a slackline anchor feels. The topic deserves a thread of its own.

The following analysis assumes that the slackline is completely static and that it is not pretensioned. Neither of these assumptions are realistic, but I think that one would have to know the modulus of elasticity of the slackline to take stretch and pretensioning into account (though I could be wrong). It also assumes that the person walking the slackline isn't bouncing at all - again not realistic. Bouncing would increase the forces on the anchors.

I've calculated results for slacklines making angles of 1 to 10 degrees from the horizontal when the person is standing on the slackline at its midpoint, for persons weighing 200 and 250 lb. Results are in kilonewtons, rounded up to the next whole number.

Code
 
------------------------------
Force on anchor (kN) based on
angle between slackline and
horizontal and person's weight
------------------------------
Angle (degrees)
Weight -----------------------
(lb) 1 2 3 4 5 6 7 10
------ -----------------------
250 32 16 11 8 7 6 5 4
200 26 13 9 7 6 5 4 3
------------------------------
Force=(Weight/2)/sin(Angle)

Comments:

1.An angle of 1 degree exceeds the closed-gate strength of carabiners.

2.Angles less than 6 degrees exceed the open-gate strength of carabiners, mandating the use of locking carabiners.

3.This is just a first shot at the answer. If you can improve on the model, please do not hesitate to do so.

[ This Message was edited by: jt512 on 2002-06-06 20:30 ]


tenn_dawg


Jun 6, 2002, 9:21 PM
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That's very well worded Jay, That formula is exactly what I was looking for. As you said, there are several factors that are not taken into consideration here. Some are working to lessen the force, and some to increase it.

The elasticity of the material used is one factor that is going to work to lessen the force. It will do this in much the same way that a fishing pole allows a weight to be lifted that is well beyond the lb-test strength of the line. By streching, it will absorb some of the "shock" loading that is placed onto the system by way of bouncing, hard shaking steps, or falling onto the line in the most serious case. Imagine if a cable were used instead of webbing. The anchors would be much less tolerant of and shock introduced anywhere in the system.

Also, as a result of having a longer line, lets say around 75 feet, it will be impossible (or infeasable?) to put enough tension on the line to keep the theroretical person in the middle above the 10 degrees less than horizontal mark. This alone will help to keep the forces down, throw in the added elasticity of the extra line, and I believe it is feasable to say that a longer line will be more reliable, and stronger than a shorter one. Make sence?

The carabiner that I snapped was a 15kN nonlocking BD. The line was about 20' long. I'm going to roughly estimate the tension on the line as 900lbs. (I used a 3:1 ratchet system, and me and my buddy were tugging on it, this may be a conservative estimate) Because of the kind of system used, it's possible the gate was forced open resulting in the failure. It's damn sobering to see a crab break, gave me a little reality check about climbing gear.

Just a couple of my thoughts. And by the way it's TENN_dawg as in TENNESSEE the yeehaw state.
Travis

[ This Message was edited by: tenn_dawg on 2002-06-06 21:24 ]


biggernhell


Jun 14, 2002, 8:01 AM
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This is all with the assumption of a single strand of webbing. If you want to be easier on the gear and still stay at a reasonable angle in relation to the horizontal try using two or three strands layered together. I'm pretty sure that I remember Dean Potter saying that they used three strands on the 135' line in the Masters of Stone five video. Oh yeah, thanks for the chart, thats the most detailed info I've ever seen on the subject.

[ This Message was edited by: biggernhell on 2002-06-14 08:02 ]


kelownaclimber


Jun 14, 2002, 8:26 AM
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Ummm if you have 3 strands how does that lower the force on the biner??? Unless of course you are using seperate biners for each strand,which will make a nightmare out of the rigging.(of course on a high long line you would have to make a custom multi bolt anchor to build in a good safety factor) I have two strands on my slackline and both are clipped to one biner,a big beafy locker.My line is about 45' long and I'm using a come-a-long to tighten.I leave only one biner in the system once it's tensioned.It is most likely that if the line fails it will do so at a knot in the webbing.


biggernhell


Jun 14, 2002, 9:27 AM
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Good question. More strands=less stretch under body weight=less force needed to tighten the slackline to the point that its walkable. I've never had a reason to try this though. I can get any single strand line that I want to walk pretty tight without breaking biners. I could never get a line good and tight with a come-a-long. I think that I had a cheap come-a-long though. I use a hauling style pully system with an old climbing rope to tighten my lines now. It leaves me with no biners in the final pruduct, so I get a tight line with less equipment wear. I'll try to post a photo of the actual system sometime. Its actually a modified version of one that Potter demonstrated at Fosterfest a couple of years ago. Only he used actual wall pulleys and ascenders though. I don't wall climb so I don't have that stuff yet. I just use biners and butterfly knots tied in the haul line. This still gives me enough power to tighten a single strand 20m line plenty tight enough to be walked, if I have one other person help pull. Its funny that I can rig a haul system with my eyes closed now and I've never been on a wall. I guess I'll have the knowledge when I need it.

[ This Message was edited by: biggernhell on 2002-06-14 09:31 ]


estwing


Jun 16, 2002, 4:46 PM
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I use two biners, that way they don't get stressed as much. Just a thought.

sam


clymber


Jun 16, 2002, 6:42 PM
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Im wondering if someone canbreak that down into simple English. Dont know if i dont uderstand it because im tired or because math just isnt my subject.
Train A leaves Boston at 12pm
Train B leaves LA at 2 AM

they meet in they middle


beyond_gravity


Jun 16, 2002, 9:55 PM
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Boston and LA are 300 miles apart. Train A is traveling 5 times faster then train B. how far did train B travel untill it crossed paths with Train A?


biggernhell


Jun 17, 2002, 7:50 AM
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If their on the same track it doesn't matter. You still have one hell of a wreck. Seriously, this thread has just been rendered near useless by the link to Slackline Brothers that is now on the main page. It looks like they've got us all beat on technical BS.


punk


Jun 17, 2002, 8:12 AM
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I was wondering in the Tyrolean traverse if the angle is lower then vertical it make for a harder traverse I was wondering if I was to use 2 locking steel biners on each side with triple 1" webbing around each of the anchors and get the line as taut as possible is it still will fail?


Partner rrrADAM


Dec 16, 2002, 12:14 PM
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[small]This topic was moved to the Slacklining forum by rrradam[/small]


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