Joshua Tree Anchor System

In the morning of November 15, 2012, waking up in my tent at the Hidden Valley Campground in the Joshua Tree National Park in California, I thought of the challenge of making a 3-point anchor system using 3-point sliding X.

The 3-point sliding X is self equalizing, but allows only one limiting knot.  When adding two limiting knots, it can no longer self equalize to evenly distribute force on 3 points (i.e. pros).  But with only one limiting knots on one of the three pros, when one of the other two pros fails, the extension in the system can be huge.

How to maintain the self equalizing advantage while minimizing the potential extension?  In my sleeping bag, I came up with the following configuration as shown in the following two immages.

On the left of the above configuration were two loops that can self-equalize.  This is achieved by a double loop figure 8 or double loop overhand.  The latter is used in the photos.  At the bottom is the regular sliding X with two limiting knots.  An extra overhand knot is used to tie off the two ends of the cord for added security.  This system reduces the extension significantly in the event of the failure of any of the three pros.

Since I have invented this system in Joshua Tree and the system looks like a Jashua tree, I name it the Joshua Tree Anchor System.

Xiaoping Li at Joshua Tree National Park in California in the United States on November 15, 2012.

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On Novemeber 16, 2012, I come up with the next generation of the Joshua Tree Anchor Systyem to double the strength of the single cord on the left side in the above images.  Here they are:

In the above image, a double loop figure 8 is used to create the two self equalizing loops on the left side.  In the following image a double loop overhand is used to create the two self equalizing loops on the left side, and on the right side, a clove hitch was used to tie to the biner to eliminate the weakness of the overhand knot that joins the two ends of the cord. (An overhand knot is easy to tie and untie, which makes it easier to transfer the cord from being used for anchor setup to being used for rescue operation.)

Xiaoping Li at Joshua National Park in California in the United States on November 16, 2012

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In the above image, in event that the pro connected to the right loop of the double loop knot  fails, the limiting knot below will move lower a little and the load of the belay will still be shared by the remaining two pros as the sliding X remains functioning.

In the first image at the top of this page, if the pro connected to the right loop of the double loop knot fails, because the extension caused by the collaps of the loop is greater than the distance between the limiting knot and the belay biner, the sliding X will stop functioning and the load of the belay will be on the pro on the far right side only.

To protect the function of the sliding X, the two pros connected to the double loops should be close together to limit the extension in the event one of them fails.  In the real world, these two pros are often not close to each other.  To address this challenge, I come up with the following design:

First, tier a clove hitch to a biner to connect to the top pro.  Then bring the cord down to the next pro to tie an overhand on a bite and clip a biner to it.  Now tie a double loop figure 8 or overhand knot to create the two self-equalizing loops with one loop clipped to the biner and the other clipped to the pro.  Use the remaining cord to create a sliding X with two limiting knots and clip in the pro on the far left side

Xiaoping Li at Joshua Tree National Park in California in the United States on November 17, 2012

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The weakest link of the Joshua Tree Anchor System is the single cord section between the double loop knot and the limiting knot in the 1st and 5th images above.  This cord is a Sterling 7mm with a tensile strength of 12.4kN. The limiting knot may weaken this single cord by 15% (according to www.divisionoftraining.com/.../Presentations/Rescue%20Knots.ppt).  If the double loop knot does not weaken the cord by more than 15%, the single cord strength will be reduced by 15% to 10.5kN.  In a scenario where the two arms of the sliding X forms a 60 degree angle, the combined strength of the two arms will be 10.5/0.66=16kN.

Xiaoping Li at Joshua Tree National Park in California in the United States on November 18, 2012

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To strengthen the anchor system where all 3 pros are not close to each other, I come up the following setup:

To set up this system, first tie an overhand knot of the cord with one of the two tails to be long enough to extend one pro connection to another.  Then use the long tail to tie a clove hitch to the highest pro and extend it to the next pro and tie a clove hitch with a biner.  Next use the cord next to the overhand knot to create a double loop knot (overhand or figure 8).  Clip the double loops to the two biners attachec to the two pros on the right side.  Then bring the next cord down to tie a limiting knot and then another about 1 foot down.  Clip the end loop of the cord to the 3rd pro.  Clip the belay device between the two limiting knots with a sliding X.  Now we have a completely self-equalizing anchor system connecting 3 pros with only one cord.

This system should double the force load it can bear to 16 x 2 = 32kN, which is more than enough to withstand the most severe factor 2 fall in a multi-pitch scenario.

Xiaoping Li at Joshua Tree National Park in California in the United States on November 20, 2012

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Conclusion

Once the double loop knot (overhand or figure 8) is tightened under load, when one of the two pros it is connected to fails, it is highly likely that no slippage will ocurr between the two loops.  Therefore, such failure will cause almost no or little extension at the sliding X.

Given that there is no or little extension in the double loop section when one of the pro fails, I prefer the setups in the 3rd and 4th images as they are easy and quick to set up and will maintain its self-equalizing ability and reduncancy.

The maximum force it can withstand when the angle between the two arms of the sliding X is 60 degrees can be closte to 32kN.

Xiaoping Li at Joshua Tree National Park in California in the United States on November 21, 2012

 

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