Welcome to Balance Community!

Cutting a Main Highline Anchor

Introduction

Over the past several years I have been constantly optimizing my highline setups to make them lighter and easier to rig, while maintaing the redundancy and strength required to stay safe on a highline. Recently I have begun using static rope as my main highline anchor in combination with short lengths of tubular webbing tied between each bolt individually to prevent any sort of extension in the anchor if the static rope were to fail. Since adopting this method, I have been curious as to what would happen if the static rope were to fail and the tubular webbing strips were to be loaded. So, at a recent highline at CRG, we tested this thought.

 

Methods

First I will explain the type of highline anchor we were using. Then I will describe the line we tested this on. Lastly, I will describe the process of cutting the anchor.

 

THE ANCHOR

On either side of the highline there are 4 glue-in bolts from Fixe Hardware. These glue-in bolts have eyes which are large enough to accommodate a static rope passing through them with additional room. Because I used static rope for my anchor, this allowed me to avoid the use of any hardware for the anchor (which decreased the weight significantly). To build the anchor, I took a piece of 10.5mm static rope that was about 30 feet long and threaded one end through all 4 glue-in bolts. I then proceeded to tie the ends of the rope together with a triple fisherman's knot. I then proceeded to build a Sliding-X Anchor making sure that the triple fisherman's knot was located on an outside leg (likely to have the least force on it). At the master point of this anchor I connected both my mainline and backup line using offset Alpine WebLocks, as seen in the picture below.

Highline Anchor Failure Simulation Highline Anchor Failure Simulation

Having just the static rope sliding-x would not give me full redundancy because if any one of the strands of the rope were to be cut or fail, the entire anchor would fail. To avoid this situation, I have decided to go with individually backing up each bolt separately. To do this, I take a 20 foot length of 11/16" Slack-Spec Tubular and wrap it between the master point and a single bolt 2 times, then connect the ends with a waterknot. I do this for all 4 bolts. This sort of backup serves two purposes: as a backup for the main anchor and as extension prevention if a single bolt were to fail. In the case that a bolt were to fail, these backup slings would prevent the master point from extending out, thus shockloading the remaining bolts. It's my favorite anchoring solution I have found so far and would highly recommend it to other highliners out there.

Static Anchor Setup Static Anchor Setup
THE LINE

The highline that we had rigged was 240 feet long and about 80 feet high located at Cosumnes River Gorge. The mainline was a heavyweight nylon prototype webbing and the backup was a lightweight, high elongation nylon webbing. On the tensioning side we had a high frequency dynamometer in the system on the mainline only. With this we were able to log peak loads of the event. The standing tension throughout the day fluctuated from 500 lbf to 1,200 lbf. At the time of testing, the line was sitting at 830 lbf.

 

THE CUTTING

We did 3 cuts to test various methods and to ensure we were getting accurate results. The first cut was to a center leg on the 4 bolt anchor where all the backup slings were rigged tightly to limit extension as much as possible. The second cut was done with much looser backup slings to see how violent the extension would be in such a scenario. The third cut was to one of the backup slings that was loaded first to simulate a multifaceted failure.

 

Results

The results are somewhat amusing. On the first cut we started with 830 lbf on the mainline before the cut. We cut the rope anchor and virtually nothing happened. The anchor did not move even a centimeter and the tension dropped 2 lbf. All the slings were loaded perfectly and the two rope ends where the cut was made basically did not even move.

The second cut was a bit more eventful. We cut the same leg on this anchor, but since the backup slings were rigged much looser, there was quite a bit of extension. The tightest of the legs was loaded first, but it was a seemingly small shockload to this sling. Since there were two wraps of webbing on every bolt, the strength is far beyond what is needed. The strength of the double-wrapped slings has been tested at 6,000 lbf with 9/16" climb-spec tubular. With 11/16" Slack-Spec Tubular, I am sure the strength would be somewhere near 7,000 lbf for each sling.

The third cut was made to this sling that was loaded on the second cut to simulate if that first loaded anchor were to fail during the extension. When it was cut, there was further extension as the other slings were not yet tight, but the severity of the shockload was very minimal. Only 2 of the remaining 3 slings were loaded as the third was rigged much looser.

On inspection of the dyno after the second 2 cuts, we settled at around 430 lbf with a peak force of 830 lbf (standing tension from the beginning). Even though the total extension was less than 2 feet, we lost almost 50% of the tension. I suspect that this tension would rise slowly back up to around 550 - 600 lbf as the fibers regained their elasticity.

 

Discussion

This experiment has made me much more confident in this anchor style. I was always a bit unsure of what would happen if the main anchor were to fail on such an anchor, but now I know that even if the backup slings are rigged very loose, there would not be enough force to cause any sort of harm to the slings. However, even though the shockload was minimal when the slings were rigged loosely, I would still recommend always making them as tight as possible. I wouldn't make them tight enough that they are loaded when the line is tensioned, but just enough to make them not loose.

 

Future Research

In the future I would like to see what happens where there is a load on the line when such a failure occurs. I would also like to capture peak loads of an extending anchor during a leash fall.

 

Thanks for reading the article! As always, feel free to leave any comments or questions below.



← Previous Post Next Post →

22 thoughts on “Cutting a Main Highline Anchor”

  • Why not tie a Big-Friendly-Knot (BFK) to prevent extension?

    • That is one method that is often used. However, I do not like BFK's due to the fact that they will never equalize as well as a sliding-x. Proper equalization is a very important point for properly rigged highlines as it limits the force on each equalized point as much as possible.

  • Sweet, thanks Jerry! I've always been a big fan of just a straight up redundant sliding x anchor system... basically exactly what you did with your main static rope anchor, but then with a secondary rope threaded through the entire thing identically (albeit not fully loaded - mainly just serving as a backup sliding x system). If you use two static ropes, there can be plenty of room for it, but not necesarrily so with spansets. I've gotten it to work with double purples, but you definitely need larger hardware to accommodate that, and it seems way overkill.

    Similar to my idea, you could tie a big knot into the backup sliding x's master point (making it no longer a sliding x) in order to get full isolation of each strand leading to each bolt, which would produce essentially the same backup system that you have outlined, sort of covering the best of both worlds. Is that what you're getting at when you choose to just individually backup the masterpoint to each bolt with the 11/16"? That definitely makes more sense weight wise, but no matter what, if your primary sliding x anchor gets catastrophically cut, you should count on all of the force being instantly transitioned entirely to your backup anchor... in which case I would feel safest with that being another entire sliding x setup, so everything is optimally equalized (avoiding potentially loading a large portion of it onto one singe bolt unintentionally, due to poor pre-equalization of the tied of individual strands). As you mentioned, your 11/16" wraps are most likely strong enough to hold the entire line even if one strand were to bear the entire load of the line... but I've seen a lot of people backing up lines similarly,with individual backup strands that would NOT support the entire system if shock loaded all at once, not to mention the undesirable aspect of loading only one bolt. If you'r sliding x blows, and you have unequalized backup protection that's insufficient, one strand could get shock loaded, snap, and then the same thing could happen again on the next strand, etc, etc... clearly not a likely scenario, but I think more people need to be aware of that outcome. Whatcha think?

    • That method also works quite well Jon. One possible thing that I could see with that method is the BFK not being very tight in combination with the sliding-x anchor. Meaning, if the main anchor were to fail, the entire force would extend until being caught by the BFK anchor. This could potentially shockload this anchoring causing some unwanted peak forces.

      The individual backups provide a number of benefits outside of the basic extension prevention. For one, you can rig them quite tight so that the anchor virtually does not move if the main anchor fails (as in test one). This is fantastic because you have zero shock loading and you still maintain full equalization. Because of the high stretch of the nylon webbing, we found that the 4 slings equalized perfectly on test one. The looseness of the slings in test two were very different from each other, which is why there wasn't equalization in that test.

      Another benefit to the slings is their ease of rigging. Wrapping the pieces of tubular between each bolt and the master point takes very little time compared to trying to optimize the BFK position. I also find that you can reduce the hardware size significantly with this method. I was using 7/16" Bow shackles at the master point (and 1/2" Van Beests) and still had quite a bit of room within the bow of the shackle after everything was rigged.

      • I agree with all of your points! Tying off the backup with a "BFK" isn't something I consider to be practical in an actual rig... and I think that all the aspects of your backup system you've described are spot-on. Great post Jerry, thanks on behalf of the rest of us!

  • you just use one piece of webbing for all the bolts? what knot do you tie? how long of a piece of webbing are you using and how far back are the bolts? very interesting :D

    • I use a single piece of static rope about 25 - 30 feet long to equalize all the bolts. The rope is fed through all the bolts and the ends are connected with a triple fishermans knot. I then tension the line and tie individual backups for each bolt with a piece of tubular webbing (one piece per bolt - about 20 feet long). The tubular webbing is wrapped between a single bolt and the masterpoint 2 times and then tied together with a waterknot. The slings are made as tight as I can get them using this type of knot. This is repeated for each bolt. So in this case I have 4 webbing slings on both anchors backing up 4 bolts on both sides.

      I hope this answers your questions, let me know if you need any additional help.

  • Nice stuff mate!!!
    Also you cud back it up instead of each bolt to the master point, 2 bolts and 2 bolts with a force triangule (name in english??) in order to load both bolts at the same time.
    the outter bolts together and the inner bolts together, so if somehgin goes wrong you wud never be shockloading one bolt in particular...
    Thoughts??

    • This method also is a good way to go. However, instead of using the inner two and the outer two, you should scatter your bolt selection. For example, one triangle can use the far left bolt and the third bolt from the left. The other triangle can use the second bolt from the left and the far right bolt. This will ensure a much more even loading between the two slings in the event that they are loaded.

      There are many many ways to accomplish the same thing as this anchoring method does. However, after much testing and tinkering with different setups, I find the method above to be the simplest, easiest to rig, and most redundant solution I've tried.

  • Hi Jerry,

    Thanks for always posting such relevant articles... And feeding the gear perverts we all are !!

    We were wondering why you use webbing for backing up each bolt and why not strands of ropes ?

    Then I have trouble understanding how you could tie the strands of backup webbings tight enough with a waterman knot so that nothing moves... Did I miss something ? It seems unreal... Maybe it's the fact that you make 2 wraps,it allows to "tension" a little bit...? Like a mini primitiv system sort of...?

    Thx

    • The reason I use webbing is because it fits inside the smaller shackles MUCH easier and also it's incredibly light. Having the 4 strands of the main anchor rope, plus an additional 8 strands of backup ropes would not fit in the standard shackles we use in slacklining. Not to mention, 11/16" tubular has favorable elongation characteristics that make it ideal for equalizing in the event of a main anchor failure.

      If you play with the waterknot a bit, it's quite easy to add a bit of tension. It does not need to be 'tight' per say, but definitely not loose. If you would rather have them a bit tighter, you can always tie the backups before you are finished tensioning. The rope anchor will elongate a bit more at higher tensions, thus tightening the backups slightly.

  • good, good stuff. thank you for this. my question:

    what were to happen to the rig in its entirety (with the main and backup both being on the same set of bolts) if a bolt were to fail? thanks.

    • We simulated this event on Test Three by cutting a backup sling (albeit, with very loose backup slings). What would happen is the remaining 3 backup slings would get loaded. If you tied the slings tight enough, you would see almost no movement in the masterpoint. If there was a person walking the line, it's very likely that they could continue walking without even noticing the bolt failed.

  • Awesome stuff! I was involved in rigging and attempting my first highline yesterday. We used 2 separate anchors for the main and back up line and used green 10' spansets equalized with sliding x on each. Coming from rock climbing, I prefer to use a limiting knot on my sliding x's, that way you have no need to worry of shockloading remaining bolts in the system. I like this anchor method used here very much, however how well does the triple fishermans come undone when you de-rig the line, or does this become a fixed loop? And more so, on the tries when it loaded the nylon strands, were you able to untie the water knots after they cinched down?

    • That's great to hear that you are getting more into highlines. That's a huge step into a crazy and amazing new area of slacklining. Welcome :D

      As for your questions, the knot we used on the rope was a double fishermans, which is extremely easy to undo after loading it on the highline. I usually try and keep the knot on the outside legs to ensure it will see the lowest loads as untying is sometimes an issue in other applications.

      The nylon slings had no problem coming undone once they were loaded. This is mainly because we double wrap the slings, as you should be doing anyway, ensure there is enough strength to adequately hold the transferred loads.

      I hope this answers your questions, let me know if you need any additional help.

  • Hey Jerry,
    really nice Anchor, I think I'll copy that style ;)
    One question though, what static rope do you use? Do you have any concerns considering breaking strength and especially abrasion, as it has no mantle?
    Cheers,
    Oli

    • We are using either 8mm or 9mm static kernmantle rope. The breaking strength is no issue at all, especially when using a 4 bolt anchor. You have to consider that there are 8 strands of rope in this 4 bolt configuration, which should be sharing the load quite evenly if the anchor is setup correctly.

      The abrasion should also be of no concern since it is a sheathed rope. Also, abrasion should not be happening with this type of anchor if it is built correctly (protect against extension!)

      • So, I'm a little bit confused now. You are now using 8mm or 9mm to rig your main high line anchors? In this article it says you are using 10.5mm? Out of curiosity are you using Sterlings Safety Pro?

  • Hi Jerry, thanks for sharing this very informative article.

    If I understand correctly, you connect both your main and backup weblocks to the master anchor point.
    1) Is there any reason to consider rather creating separate anchoring systems for each ?
    2) You seem to offset the backup weblock using a short purple spanset looped through the rope and webbing of the master anchor. Is there any abrasion concern regarding fabric-on-fabric here ?

    Thanks in advance !
    Jaco

    • Hey Jaco,

      Thanks for the comments, much appreciated! Please see the answers to your questions below:

      1.) I like the concept of a single anchor that is fully redundant. I am using the method described in the following article for my anchors: http://www.balancecommunity.com/slack-science/highline-anchor-analysis-4-bolt-anchor/ . This makes every point within the highline anchor act as it's own individual anchor. If something goes wrong, I am covered at every level within the anchor. This, to me, is much simpler than creating 2 anchor systems.

      2.) No concern at all for fabric-on-fabric abrasion. The reason for this is that there will be almost no movement between the spanset and the anchor. The pivot-point for the entire line is the bolts, so the spanset, being under tension, will remain motionless with respect to what it's attached to. Also, if something happens within the anchor, I won't see any abrasion either due to my bolts being individually backed up.

      I hope this answers your questions, let me know if you need any additional help.

Leave a Reply
Post your comment

Balance Community: Slackline Outfitters