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Slack line anchors !?! 9-May-2012 At 11:02:07 AM Olbert

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On 8/05/2012 Cool Hand Lock wrote:
>On 14/03/2012 sliamese wrote:
>>Do people get that an american death triangle will put about 140% of
>the
>>load onto EACH anchor?
>
>It dosn't put 140% on each anchor. Only 50%. And 50% of body weight on
>a dynamic rope = fudge all.

Ummmm...as pointed out by tris (at 3am) this is wrong. Either your understanding of the American Death Triangle doesn't match the community or your maths is wrong. Not that an American Death Triangle definitely puts 140% of the load onto each anchor every time it's used, but it certainly can. Check out the wikipedia link for an adequate explanation.

>Heres some maths on a slack line anchor. Using a tight line with a 10degree
>deflection. An 80kg climber falls from it 2m untill his back up catches
>him. Gravity(10ms/s)

Seems fairly reasonable approximations - I won't point out that 10ms/s = 10m. ^{Heh heh heh}

>Aparent weight of someone falling would be 400kg roughly.
>400kg relative weight because you'll be falling at about 5m/s when the
>line catches you.

Ok, I did some calculations and this part wasn't as daft as I first thought. The 5m/s part is a bit of an approximation, if you use the numbers above, you would actually be falling at ~6.3m/s but close enough. The 400kg apparent weight would be the equivalent to the actual downward force put on the slackline with the following assumptions: the deceleration was uniform from when the slackline was straight (no additional force), to the slackline being bent at a 10 degree angle (slackliner at rest); the slackline deformed no more or less than 10 degrees; the slackliner was in the middle of the line at the time of the fall; and the apparent weight includes the static weight of the slackliner.

These all seem reasonable assumptions. It should be noted that this setup means that the slackline 'catches' the slackliner in a distance of 0.5m which also seems reasonable.

It should be noted that if the slackliner was nearing either end of the line the forces on those anchors might be different (the maths is more complicated so I'm not gonna bother).

Anyways, yes, 400kg is a reasonable approximation. I would rather it in newtons though as this is what climbing gear is rated in. With gravity approximately at 10ms^-2 it comes to 4000N or 4kN.

So we now have the downwards force on a slackline from a slackliner falling is 4kN.

>Load angle calculation. Sin(10) = .17 then 1/.17= 6ish multiplier.
>
>Multiply your relative falling load (400) by 6 = 2400kg Is the load on
>the anchors.

Your maths is correct but your answer is kinda misleading. Let me break it down properly. The anchors in total pull approximately 6 times the downwards force, so together the anchors are holding approx 2400kg or 24kN. However each the anchor on each end only does half the work, so the load on each end is approximately 12kN.

Then we come to the American Death Triangle. If they used an ADT then they it is reasonable to surmise that they used two anchor bolts at each end of the slackline. To calculate the forces on each individual anchor bolt the angles of the triangle must be known or estimated.

A reasonable assumption is that the ADT they used was an equilateral triangle, in which case they anchor load on each anchor is 100% of the total load. So each bolt is now copping 12kN.

Note that the original unloaded tension in the line has been ignored. A quick google got me an approximation of 2kN, so add that each anchor to make each anchor take 14kN.

This is an approximation of the actual load on the bolts. For industrial safety (using the numbers below) you would want a safety factor of 5. This means you would have to rate each bolt at a SWL of 70kN. (Industry safety is ridiculously conservative). I'm not quite sure why you mentioned industry here.

>Load ratings of anchors.
>
>1. Rigging equipment such a crane slings, chains, strops and shackles
>has a SWL (safe workign load) with a safety factor. X5 for steel, X7 for
>fabric.2ton working = 10/14ton breaking.
>
>2. Climbing anchors have a MBS (minimum breaking strain). ie what they
>have written on them is what they will break at.

Most slings and lockers are rated at approximately 20kN (MBS), I'm not sure what bolts are rated at but I'm thinking that 20kN would not be a bad guess for reasonable rock.

With the above calculations up you are getting 70% of the rated load. This is not from 'conservative' calculations, if they were conservative calculations I would wager you would get something much closer to 20kN.

This also is the load when in the middle of the slackline, when nearing either side the loads on each anchor are not necessarily the same, they may be larger or smaller, you would have to do some more diagrams to work that one out.

I'm not sure about destructive limits and pulling directions on bolts but with some of the discussions on Chockstone between knowledgeable people (thanks Mikl, Macca and probs a couple of others) I would be dubious about using those bolts again.

>So when you put your 2 ton shackle into a climbing 2 ton anchor. Your
>really putting a 10 ton link in a 2 ton anchor.

Huh? I'm not sure what you are trying to say here, it seems as if you are saying that when you put a 2 ton link onto a 2 ton anchor you actually get a 10 ton setup - which is wrong. The weakest link is still the anchor. Putting a 10ton shackle onto a 2 ton climbing anchor doesn't mean the whole setup is magically able to take 10 tons.

>If you have questions on rigging loads, feel free to ask, or read John
>Longs, Climbing anchors or More climbing anchors. Or go do a rigging course.
>Personally I've all of those things, out of greed, because it pays well.

Conclusion: I would be scared using the American Death Triangle to set up a slackline.