Forces in Highlines

by Damian
December 01, 2015
Contributors: Damian Jörren, Thomas Buckingham, Harald Höglinger, Igor Scotland, Philipp Gesing (Übersetzung), Marian Harbach (Übersetzung)
Material comparison and resulting peak forces in mainline, backup and leash in a 20m highline

Introduction

In this report, we attempt to shed light on important questions that haven’t been answered by neither simulation nor reproducible measurements: What are the loads in mainline, backup, anchor points and leash within a highline system?

The loads in a highline system are determined by numerous factors. Experience indicates that base tension, properties of the materials used (especially their stretch behaviour), and the length of the highline play an important role. Dangerous situations can arise, if these factors are combined in an unfavourable way.

Webbing and rope made from polyamide (PA) or polyester (PES) are most commonly used in highline systems today. Lately, high-tech materials like high-modulus polyethylene (e.g. Dyneema) or aromatic polyamide (e.g. Kevlar) have become more popular. Because of its high stretch, polyamide is described as dynamic and is used for climbing ropes to arrest and dampen falls. Webbing made from polyester and high-tech fibres, on the other hand, is less stretchy and is therefore described as static (Höglinger 2015). The importance of the dampening effect, related to the stretch and the length of the slackline, becomes apparent during a catch or leash fall. We want to evaluate what loads the webbing or rope, the anchor points, the leash, and the highliner are experiencing. We believe that only real experiments are currently able to provide dependable data on these aspects.

Until now, there have not been any systematic approaches to this subject. We analyzed and compiled all existing data for their experimental setup and results. (Mercier 2009, Hairer F. & Geyer D. 2009, Katlein 2010, Miszewski 2011, Jörren 2013, Jörren 2014). We found that preparing, executing, and double-checking the experiment is very time consuming. Given the resources available, we finally decided to focus this investigation on the most important aspects of the overall endeavour, meanwhile aiming to maintain as much reproducibility of the test method as possible. We hope that our description of the testing procedure below will enable the collection of complementary data in the future and want to encourage others to contribute to our collective knowledge. Therefore, in the following, we will detail the aim, the method, the results, as well as our conclusions from the available data.

Aim

As mentioned above, we decided to focus ourselves on evaluating and comparing the most commonly used slackline materials and a conventional static rope. This choice was made to enable us to provide detailed insights that can provide the greatest initial value to the community, even though it might not answer all questions exhaustively.

Our experiment is intended to provide insights into the behaviour of rope and webbing in relation to

  • base tension in comparison to working tension (and sag);

  • peak loads on the anchor points during a leash fall; and

  • peak loads on the leash during a leash fall.

 

A manufacturer of slacklining gear is interested in providing safe equipment and wants the experiment and measurements to be as close to a real scenario as possible. Furthermore, the contributing slacklining associations are also interested in comparing products of different companies.

Method

When designing our experimental setup, we quickly realized that there is an inherent trade-off between reproducibility of the setup as well as the results and environmental validity of the tests. The same situation can be observed in normative testing of, for example, climbing rope or carabiners, where environmental validity is rather abstract while providing perfect reproducibility. This allows for the comparison of products of the same type using a standardized setup and procedure. We will thus describe the limitations of the testing method we chose in order to provide maximized reproducibility.

Experimental Setup and Materials

We only used equipment with a minimum break strength of 30kN and more, which is the standard for highlines.

Nr

Typ

Material

Abbreviation


Model

Manufacturer

1

rope

Polyamid

PA

nylon 9mm

static rope, Typ B

Singing Rock

2

webbing

Polyamid

PA

nylon 25mm

Sonic 2

Landcruising

3

webbing

Polyester

PES

25mm

Core 2 HS

Landcruising

4

webbing

High-modulus polyethylene

HMPE

Dyneema 25mm

Aeon

Landcruising

Table 1: Overview of the materials used in this experiment and their properties.

Testmaterial

Figure 1: The materials used in our experiments (cf. Table 1).

Setup Specifications:

Length: anchor point to anchor point 20m

slackline about 18-19m

pulley system about 1-2m

leash 1.8m

Height: at half of the distance between anchors,
above ground ca. 9m

Base tension: varied depending on the webbing/trial

Working tension: fixed for each trial 2.2kN and 4.4kN

Drop weights:
person 82kg

tire 72kg

Tire specs: diameter 1.4m

sling for connection 0.1m

length of the load cell 0.2m


 

Befestigung Reifen zu Leash

Figure 2: drop weight, a 72kg tire, suspended statically from the leash of the slackline. The load cell is visible at the top, between the line and the leash (Rock Exotica Enforcer).

 


Length of fall without dynamic deflection

(length of the leash + sag + height of the tire + connection sling + length of the Enforcer)


with 2.2kN base tension (1.6m sag) 5.1m static fall height

with 4.4kN base tension (0.8m sag) 4.3m static fall height

Dynamic deflection was not measured.

 

Testaufbau

Figure 3: Side view of the experimental setup.

The experimental setup consisted of a simulated highline between two trees. We tested four types of material (three slackline webbings and one rope; cf. Table 1 and Figure 1) in a single-strand configuration and without the rope or webbing backup usually found in highline setups. Additionally, a guide line was rigged about two meters above the simulated highline, in order to suspend the test weight. The test weight was connected to the guide line by a fast release that could be operated by pulling on a cord from the ground (cf. Figure 3). The main line anchors consisted of 4t WLL industrial roundslings, 2t WLL shackles and Zilla 2.0 (WLL 1.6t) weblocks. Two Ropelocks were used (a weblock for ropes with 96% break strength efficiency) during the trials on the 9mm static rope. Also, on one side, a 5:1 pulley system consisting of 3” PMP SMC Doubles, 10.5mm static rope and an Edelrid Eddy as a brake was installed. The pulley systems was used to adjust the base tension prior to each fall/trial. We used a 10.5mm dynamic rope threaded inside a 19mm tubular webbing, knotted on both sides with a figure of 8, as a leash.

Load Cells

The loads in the main line were recorded with a AST KAS 50kN in fastlogging-mode. For recording the loads in the leash, a Rock Exotica Enforcer load cell was threaded directly onto the mainline using its ring, while its other attachment point was knotted into the leash. Unfortunately, data collection on the leash load cell was suboptimal, as the threshold (the load at which the fast logging is initiated) was set to 4kN. This means that a lower sampling rate was used for loads lower than 4kN. Therefore no reliable values were recorded for forces between 1.5 and 4kN. Furthermore, the distance between the Enforcer and our smartphone was too great to prevent transmission errors.

Further Thoughts

Below is a list of factors which were chosen conservatively in our setup and therefore would generally lead to lower loads in a real highline system:

  • Distance between the anchors
    Longer distances would reduce the load as the line/rope can stretch more and therefore absorb more energy (cf. Jörren 2013).

  • Testing of lines without a second system (backup taped underneath)
    A backup can slightly dampen the impact force of a fall on the mainline, as it also bears part of the load  (cf. Jörren 2013, Miszewski 2011).

  • 1.8 m long leash (assuming a heavy person with the leash fixed to the side)
    Shorter leashs, as seen when the leash is placed between the legs for example, generate a lower impact force due to the lower fall height.

    • Fall in the middle
      Falls in the middle of the slackline generate higher loads on the anchors than falls near the edges (cf. Jörren 2014).

      • Lifeless Falling object
        A 72 kg tractor tire was used as our test weight to maximize repeatability, as characteristics of human test subjects can vary too widely. We ran some preliminary tests with a static (tree trunk) and a dynamic object (tire) and compared those to real falls of a human body (82kg). The tire mirrored the load characteristics of a falling human body more closely, however it is slightly lighter and can absorb less energy in a dynamic fall. A brief comparison between the tire and a human slackliner can be found in Table 2 as well as Figures 4 and 5.

      Table 2: Preliminary tests for the choice of the falling object.


       

      Figure 4: Comparison of loads generated by different falling objects on a polyamide highline.

       

      • Height of fall:

        • The leash was attached to the tire (diameter 1.4m) by means of a short sling, itself attached to the top of the tire. Thus, the point of attachment is higher in comparison to a human body, where a climbing harness is used to attach the leash close to center of mass. This mode of attachment was chosen for practical reasons.

        • The lower edge of the tire (opposite of the attachment point) was positioned level with the main line anchor points before initiating the fall. This was done in order to use the line as a height reference for the tire before performing the tests.

        • The greatest heights for a highlining leash fall can occur when bouncing or jumping (i.e. vertically deflecting the line upwards while using the line) and then accelerating from the highest point of this motion all the way down into the leash. Smaller falling heights can occur with greater sag (less base tension and thus working tension). To facilitate a comparison across the three types of webbing in our experiment (cf. Table 1 and Figure 1), we chose to not hold the base tension but the working tension (body or tire hanging from the middle of the line, respectively) constant for the experiment. This results in uniform sag across different materials and thus also uniform falling heights in our experiment, simplifying the setup.


      • Fall trajectory:
        The fall trajectory influences the loads experienced in the system. A sideways, pendulum-like fall will reduce the loads across all parts of the system (while, of course, introducing additional loads of smaller magnitude in different directions). To investigate a worst-case scenario, we released the testing weight from directly besides the line to produce a near vertical fall.

      • Stretch characteristics:
        After several leash falls, base and working tension were readjusted by using the tensioning system to increase tension. Stretching webbing will decrease the previously induced tension which was compensated for. This stretch will usually diminish the dampening properties of the webbing, as each consecutive fall will have less stretch “available”, causing potentially higher peak loads. Additionally, as the stretching process as well as the different stretching behaviors of the more dynamic materials causes the measured tension to fluctuate, we made sure to wait for the load readouts to settle after retensioning, in order to ensure the desired working tensions for all trials.

      • Pulley system:
        The pulley system remained in the experimental setup during testing for practical reasons (adjusting the tension). For the length of the pulley system, the 5 strands of 10.5 mm static rope have more static characteristics than the same length of webbing. This can influence our measurements.

      • Use of ropelocks (when testing the 9mm rope):
        The use of ropelocks provide a more conservative estimate of peak loads on the anchors. Real setups commonly use knots for attachment, which can provide additional dempening for at least the first fall.


      In the following, we also briefly discuss several factors that can result in higher loads during real highlining setups compared to our experimental setup.

      • Fall test weight:
        The weight we used for our fall tests (tractor tire) is below the 80 kg mass commonly used in such setups. Our brief comparison with a human slackliner shows, however, that the loads created due to the different rigidity, lower height, and attachment point are still similar. Yet, we cannot say which of these previously mentioned factors influences the observed loads most and how other test weights may perform.

      • Lack of a mainline backup:
        Having a backup under the main webbing can influence the peak load on the leash during a fall somewhat, as it influences the stretching behavior of the webbing.

      • Trees:
        The use of trees as anchor points can dampen peak loads as they move under high loads. We used two massive beeches, having diameters larger than 70 cm at the height of attachment. We monitored the trees during our simulated falls and did not notice significant movement.

      • Slippage:
        Webbing can slip through the weblocks used for attachment to the anchors if their designated working load is exceeded. This effectively further attenuates the measured peak loads. Similar attenuation of loads can be caused by a slipping brake in the pulley system or a slipping rope in the ropelock. All fixation elements (weblock, ropelock, pulley system brake) were monitored during the fall tests and slippage noted only in one case (test with dynema webbing as mainline - the Edelrid Eddy break used in the pulley system slipped).

      • Measurement errors:
        The loads measured by the load cells are constantly changing, due to the dynamic nature of the materials in the system (webbing stretch causing a reduction in measured load). We thus waited until the change of the measured value was negligible.

      • Fall position:
        Falling nearer to the anchor points generate less load on the anchors themselves, yet are a cause of higher loads on the leash. We only systematically tested falling in the middle of the line during this set of experiments.

        Base and Working Tension

        If we want to compare the behaviour of different webbings during a leash fall, we have to strive to keep as many parameters constant as possible. Intuitively, the base tension could be used as a baseline for each trial, since it is possible to adjust and monitor it easily. If we assume a fixed base tension and compare it to the working tension, we get great discrepancies between static (PES, HMPE) and dynamic (PA) materials. Not only in terms of force but also in the resulting sag.

        Therefore, one of our most important realizations was that, when comparing different materials (webbing and ropes), you have to use the working tension as baseline and NOT the base tension. This also because it gives us the same sag and therefore the same fall hight. Therefore we defined two working tensions (2.2kN and 4.4kN) which are representative of real highline scenarios.

        Results

        Wir conducted a total of 20 trials for this analysis. The first six trials were used to compare the characteristics of different test weights. As mentioned above, we chose a tractor tire as a human analog in these tests as the measured loads were most similar.


        Table 3: Results of the main trials, using the four materials mentioned in Table 1 and working load levels adjusted to 2.2 and 4.4 kN respectively.

        The figures below illustrate the relationship between working tension and base tension for the tested materials. Each figure also shows the ratio of working tension and base tension or peak load and base tension respectively for each of the materials at the bottom of the bars.

        As Figures 6 and 7 show, base tensions need to be much larger for more dynamic materials. The difference between the more static materials becomes more apparent at higher working loads (cf. Figure 7).

        Figure 6: Comparison of working tension and base tension across the tested materials. Working tension was kept constant at about 2.2 kN. The ratio between base and working tension is shown at the base of the blue bars.

        Figure 7: Comparison of working tension and base tension across the tested materials. Working tension was kept constant at about 4.4 kN. The ratio between base and working tension is shown at the base of the blue bars.

        Figures 8 and 9 show the comparison of peak anchor loads during a leash fall to the working tension across the tested materials. The effect observed for base tension is reversed here: The more static materials cause much larger peak forces on the anchor points during a leash fall. The difference becomes less pronounced for more dynamic materials at higher working tension (cf. Figure 9).


        Figure 8: Comparison of working tension and peak loads on the anchor during a leash fall across the tested materials. Working tension was kept constant at about 2.2 kN. The ratio between peak load and working tension is shown at the base of the green bars.

         

        Figure 9: Comparison of working tension and peak loads on the anchor during a leash fall across the tested materials. Working tension was kept constant at about 4.4 kN. The ratio between peak load and working tension is shown at the base of the green bars.

         

        Figure 10: A 4 second excerpt from the fast-logging data of the Rock Exotica Enforcer load cell, positioned between main line and leash. During this trial, the 72 kg tire was dropped onto the HMPE dyneema webbing.


        Figure 10 shows an exemplary result of the forces measured on the leash. The peak load exceeds 5 kN. Such large loads on the leash can become dangerous for the human body. The tubular webbing shielding the leash rope also got torn inside one of the attachment knots during one of the trials (cf. Figure 11).

         

        ripped leash mantle2.jpg

        Figure 11: The leash (T-Leash manufactured by Landcruising; 10.3 mm rope inside 19 mm tubular webbing). Peak loads exceeding 5 kN tore the webbing inside both attachment knots (Figure Eight knot).

         

        Figure 12: Comparison of working load (kept constant at about 2.2 kN) as measured at the anchor point and the corresponding peak loads on the leash during a leash fall across the tested materials. As mentioned in the text, the load cell was not configured appropriately for some trials and thus need to be interpreted with some caution. We therefore provide ranges for the two PA materials (PA Static rope: 3,6-4kN und PA Sonic 1.4-4kN).

        Figure 13: Comparison of working load (kept constant at about 4.4 kN) as measured at the anchor point and the corresponding peak loads on the leash during a leash fall across the tested materials. As mentioned in the text, the load cell was not configured appropriately for some trials and thus need to be interpreted with some caution. We therefore provide ranges for the PA material (PA Sonic 2.2-4kN). In one case the load cell did not perform (no result),

        Discussion and Conclusion

        We are aware that the limited number of trials we were able to conduct does not allow any scientifically valid conclusions. The figures we provided above are intended to coarsely show the different loads that occurred in our test setup using different materials (PA, PES, HMPE). We did not average results across several trials under the same conditions nor did we calculate error margins. Additionally, the misconfiguration of the load cell for the leash (no fine-grained data for loads smaller than 4 kN) limits the interpretability of this data.

        This report therefore primarily gives a preliminary overview of testing results and aims to provide an overview of the magnitude of the effects to be expected during further testing aas well as to initiate a discussion about a useful standardized test setup.

        While conducting the experiment, each of the authors provided a guess for the expected loads in each scenario before taking readings from the sensors. As our guesses were fairly close to the measured values, we believe that an experienced slackliner’s intuition can already go a long way.

        When setting out to collect these measurements, we planned to keep as many parameters of the setup constant as possible to be able to draw reliable and repeatable conclusions. Making such an endeavour difficult is the fact that parameters believed to be constant begin to vary over time. For example, the measured tension at the anchor point varied from one second to the next. This is a fundamental difference to existing drop tests conducted to test climbing ropes, for example, as these are conducted without applying a base tension. From our experience in conducting this set of tests, we propose to base comparisons between materials on working load instead of base load.

        We also made the conscious choice to investigate the two main components of a highline system -- main line and backup -- individually, in order to prevent the two types materials influencing each other. This makes experimentation easier and hopefully produces clearer results. However, as acknowledged above, the different stretch characteristics of a combined main line and backup system may cause higher peak loads on the anchor points.

        Based on the data we collected, the following conclusions can be drawn:


        • Given the same working load (or sag), the base tension is higher for more dynamic webbings when compared to more static alternatives. In turn, this means that PA ropes and webbings will have considerably more sag when rigged at the same base tension than PES or HMPE webbings or ropes.

        • More static webbing will usually cause higher peak loads than more dynamic materials.

        • Peak loads on the anchors during a leash fall will increase when a higher working tension is applied. Our measurements showed 5.7 to 15 kN anchor peak loads at 2 kN working tension and 9.0 to 17.9 kN at 4.4 kN.

        • The peak loads observed on the leash attached to the HMPE webbing (Aeon) were higher for lower working tensions (and thus sag) of the highline.

        • Thus, lower base tension will not cause larger peak loads on the anchors, but primarily on the leash. This is a particular risk for short low-tension highlines, as loads on the leash, the harness and most importantly the slackliner’s body increase. A shock load of 5kN transmitted through leash and harness can be painful for the slackliner and cause injuries for unfavourable falls. Towards the anchors of a highline, this problem likely gets worse as the loads on the leash are likely to increase further (Jörren, 2013-2). These observations encourage the use of more dynamic slackline webbing and backup ropes made from PA.


        Primarily, the presented results reinforce the recommendations we have previously given based on experience:

        • Webbing with very low stretch characteristics (e.g. made from HMPE) are not suitable for short highlines. Our experience suggests at least 60 m of length and appropriately high base tension when using webbing made from such materials. “Rodeohighlines”, i.e. highlines without tangible base tension, using webbing made from dyneema-based materials (including Vectran, Kevlar, etc.) are not recommended.

        • Webbing with low stretch characteristics (e.g. made from PES) are also less suitable for short Highlines. The large variety of stretch and dynamicity of different PES webbings, however, precludes a generalized warning. The suitability of a particular PES webbing should be assessed based on the particular conditions of a given highline (see also Höglinger 2015). A rule of thumb could be to only use PES webbings for highlines longer than 25 meters having a light base tension applied (min. 1.5 kN, not a “Rodeo highline”). Another problem is Webbing slippage in low-tension highlines (Gesing et al. 2015).

        • For short highlines up to a length of about 25 meters, we recommend PA-based webbings. However, we also recommend to apply a light base tension. Also keep the product-specific stretch in mind, as both PES webbing (approx. 2-10% stretch at 10 kN) and PA webbing (12-17% stretch at 10 kN) can exhibit similar stretch characteristics.

        • A 9 mm PA rope appears to behave similar to a 25 mm PA webbing. Highliners should thus plan for significant sag (several meters), if a leash fall engages the backup.

        • We recommend a strong, dynamic leash made from PA. Check your leash for damage on a regular basis, which should include untying all knots (leash ring, harness). Knots should only be hand tight, because an already loaded knot will absorb less energy.

        Future Testing

        An aspect that is missing from the presented experiment is the increase in leash and anchor (peak) loads due to increased height of a leash fall. The more dynamic PA webbings will usually cause higher falls than PES or HMPE webbings, as the greater stretch allows the weight to travel further and thus accelerate more. However, the additional stretch will also cause the energy of the fall to be absorbed continuously and over a longer period of time. Holding only the working load constant at 2.2 and 4.4 kN does thus not create a good comparison, as we did not adjust the height of the tire’s drop. Ideally, we would have lowered the drop height for PA webbings in order to also keep the distance traveled by the tire constant. In our setup, the increased length of the fall caused more load to be applied to the highline system. We are thus recreating a worst-case scenario for low working (and thus base) tension lines. In a more realistic setup, we would expect lower values for PA webbings.

        For future tests, we also recommend a more easily controlled testing environment (e.g. a storehouse or a drop shaft) and a standardized static drop weight, etc. Given this first set of tests we reported on, the community can work towards a common testing standard, that creates comparable and reliable results of worst-case scenarios between different products and types of materials. However, we also acknowledge that there still is considerable work to be done.

        On the other hand, it is also necessary to continue to conduct tests that a more closely related to real world conditions (at the cost of reproducibility), in order to obtain valuable data on peak loads using more human-like drop weights.

        Finally, future testing should strive to create at least five trials for each test condition (i.e. per chosen working tension and material, etc.) to provide more reliable data for the analysis (averaging, error estimation).

        Credits

        We would like to thank Landcruising for providing the materials we destroyed during our tests as well as the measurement equipment.

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        Gesing P., Bretagne L., Buckingham T.: Webbing slippage in low-tension highlines http://slacktuev.org/2015/11/05/webbing-slippage-in-low-tension-highlines/

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        Handlungsempfehlung mit speziellen Betrachtungen zu Fixpunkten im Fels und dem präventiven Baumschutz
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        Jörren (2013-2) Kraftmessung in Highlinesystemen, in Vorbereitungpress

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        http://www.balancecommunity.com/slack-science/gear-test-leash-fall-simulations/

        Pascal Fred, Melet Tanchrede (2010) Leashfalls into Highline backups
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        https://www.youtube.com/watch?v=1ovNZeBUPtU

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