Rigging and the Likelihood of Failure

Figure 1. In drop tests, the lead of the rope (True Blue in the image) was secured to a load shackle using a figure-eight loop. The load shackle also was secured to a webbing sling supporting three steel barbell plates – the drop weight. All photos courtesy of the author.

Rigging is probably the most dangerous thing climbing arborists do. In addition to working aloft, which is dangerous in itself, rigging often involves cutting with a chain saw and, sometimes, large pieces of wood moving very quickly.

When butt-hitching or blocking wood (which also is known as negative rigging), the anchor point of the rigging is below the rigged piece. If the piece has to be stopped quickly or “snubbed off ,” shock loading occurs, and the forces that are generated can be very great. A preferred alternative is to let pieces run, which means gradually slowing them down before they reach the ground. Letting pieces run results in much smaller forces than shock loading. Minimizing force is important when trying to reduce the likelihood of failure of the rigging system, and it’s especially important when removing structurally deficient trees, which might not be able to withstand shock loading.

A simple rigging system consists of a rigging rope, an anchor (either a branch union or a block attached to the tree by a sling) and the tree itself. A climber ties off the piece to be cut and runs the rope through a block or over a branch union, and a ground worker holds the rope to control the descent of the cut piece. The fall is the part of the rope between the anchor and the ground; the lead of the rope is the part between the anchor and the rigged piece.

It’s also possible to use a crane or another tree for the anchor, but for this article, we’ll assume the anchor is in the tree being removed. When larger rigging forces are expected, another component of the rigging system is a friction device, which is usually attached near the base of the tree. This could be a Port-A-Wrap or GRCS (Good Rigging Control System), or simply taking a wrap(s) with the rigging rope around the trunk. Friction opposes motion of the rope, so the ground worker does not need to exert as much force to control the descent of the piece. When using a branch union as an anchor, running the rope over a branch also provides friction, which means the tension in the lead of the rope will be greater than the tension in the fall of the rope.

Using a block instead of a branch union as an anchor point makes it more convenient to set an anchor in the tree – you do not need a branch union to anchor it. Using a block also allows the use of double-braid rigging ropes. Another presumed advantage of using blocks is the low friction in the rotating sheave of the block.

Conventional wisdom previously was that less friction in the block allowed a greater length of the rigging rope to carry the rigging load, so there was a smaller chance that the lead of the rigging rope would fail. If friction in the block was small, a greater length of rope would stretch as it was loaded, decreasing the shock load. This is analogous to a bungee cord stretching after a person jumps off a tall bridge – the greater the stretch, the less force exerted on the person as they slow to a stop before rebounding upward. One study by Peter Donzelli showed the differences in friction in arborist blocks, but the study measured friction by slowly raising and lowering weights, not under actual rigging conditions and shock loading.

The downside to increasing the length of rope that carries the rigging load is that, as long as the lead and fall are close to parallel, the force at the anchor is nearly double the rope tension. This means that while the rigging rope may be less likely to fail, the anchor may be more likely to fail, so choosing an anchor point with enough load-bearing capacity is critical.

Recently, alternatives to arborist rigging blocks have been developed to increase friction in the “block.” Examples include X-Rings, SafeBloc and Triple Thimble. The presumed advantage of such devices is that greater friction means the force experienced by the anchor won’t be double the rope tension, it will be less, and less force at the anchor means less likelihood of anchor failure.

Despite the risk and uncertainty associated with arborist rigging, very little rigorous research has examined forces in different parts of rigging systems. Colleagues and I recently conducted tests to determine whether friction forces differed between conventional arborist rigging blocks and alternatives such as X-Rings. We also conducted tests to determine whether the test method affected the findings. In particular, we wanted to find out whether simple raising and lowering tests (like the ones Peter Donzelli conducted) reliably predicted forces compared to drop tests intended to recreate actual rigging situations.

Methods

We conducted drop tests and raising/lowering tests. Drop tests involved dropping a weight tied to a test rope that ran through a block hung from a tree and then back to an anchor at the base of the tree. We tied the test rope to a load shackle (Straightpoint, LLC) and connected the load shackle to a webbing sling that supported the weight (Figure 1). The load shackle measured tension in the lead of the rope.

At the base of the tree, we tied an Impact Block (which has a built-in load cell) and then tied the other end of the test rope to the Impact Block (Figure 2) so it could measure tension in the fall of the rope. The test rope ran through a block attached to the tree. We tested four different blocks: an Impact Block (identical to the one at the base of the tree) and three combinations of X-Rings: a pair of extra-large rings, a single extra-large ring and a single large ring (Figure 3) . On each block, we tested four different ropes commonly used for rigging: Atlas (Sterling Rope, 9/16-inch), Polydyne (Yale Cordage, 1/2-inch), Stable Braid (Samson Rope, 1/2-inch) and True Blue (Samson Rope, 1/2-inch). From the tension measurements, we calculated a ratio (tension in the lead of the rope:tension in the fall of the rope) that reflected the effect of friction in the block. Higher ratios indicated greater friction.

The other thing we measured during the drop tests was the time it took the tension in both parts of the rope to reach its maximum amount – the duration of loading. This is important because if the load duration is similar to the sway motion of the anchor, the likelihood of failure increases even if the force isn’t greater.

Figure 2. In drop trials, the Impact Block anchored at the base of the tree measured tension in the fall of the rope. A short piece of throwline suspended the Impact Block, since the rigging rope (True Blue) was not under tension before the trial. The rigging rope was secured to the top of the Impact Block with a figure-eight loop; the slack part of the rope curving up and to the right ran through the block [X-Ring(s) or a second Impact Block] at the anchor point in the tree.

We used the same experimental setup to measure friction in raising/lowering tests for each combination of block and rigging rope. In the tests, we slowly raised and lowered the weight using a GRCS and then calculated the same ratio of tension in the lead:tension in the fall. This way, we could compare not just the effects of different ropes and blocks on the ratio, but also different types of tests – drop tests vs. static tests like those Peter Donzelli conducted. For all tests, we used a different section of rope so abrasion wouldn’t build up in one spot. We also tested ropes that were new, except for the Stable Braid, which was used. And during tests using X-Rigging Rings, we rotated the rings so they wouldn’t be abraded in the same spot over and over.

Figure 3. Rigging rope (True Blue) running through two extra-large X-Rings in a drop trial. The sling into which the X-Rings were spliced is anchored in the tree.

Results

The results of the experiment were informative. First, there were obvious differences in the ratio of tension in the lead:tension in the fall between different test methods. For drop tests, the ratio was 1.2:1, which means that there was more tension in the lead of the rope than the fall, which is what you’d expect if there was friction in the block. When we raised the weight in the static tests like Peter Donzelli conducted, the ratio was 1.5:1, which means there was more friction during the static raising test than the drop test. When we lowered the weight, the ratio was 0.7:1. The reason the ratio was less than 1:1 when lowering the weight but greater than 1:1 when dropping or raising it is because friction opposes motion. Imagine running a rope over a branch, attaching a weight to one end and pulling on the other end to lift the weight. To lift the weight, you have to apply a force that’s large enough to lift the weight plus an additional force to overcome the friction force that opposes motion of the rope relative to the branch. But when you lower the weight, friction again opposes motion of the rope relative to the branch, so you have to apply a force that equals the weight minus the amount of the friction force.

It’s worth noting that the ratio of tension in the lead:tension in the fall when dropping the weight was less than when raising the weight. This suggests that the raising/lowering tests are not the best way to measure friction in a block. But it’s also important to point out that the drop tests don’t exactly mimic real-world rigging, so it would be helpful to have additional tests to confirm the results.

The ratio of tension in the lead:tension in the fall also differed for different ropes and blocks. In drop tests, the ratio was about 10% greater using Atlas and Polydyne than True Blue and used Stable Braid. This means that tension in the lead was about 10% greater than in the fall for Atlas and Polydyne, and that the friction force was greater for those ropes. But the differences between blocks were greater; the ratio when using two extra-large X-Rings was 29% greater than when using a single extra-large X-Ring. And it was 22% greater when using two extra-large X-Rings than the Impact Block. But the ratio was nearly the same when using a single large X-Ring and the Impact Block, and it was greater for both of those than when using one extra-large X-Ring.

It’s worth looking a little more closely at the results of drop tests using different ropes and blocks, comparing the ratio of tension in the lead:tension in the fall for each rope separately within each block. This analysis showed that the general pattern was true for the new ropes (Atlas, Polydyne and True Blue). The greatest ratio (and therefore most friction) occurred for two extra-large X-Rings, and the smallest ratio (and therefore least friction) occurred with a single extra-large X-Ring, with the ratios (and friction) about the same for a single large X-Ring and the Impact Block. But the pattern was different for used Stable Braid. The ratio of tension in the lead:tension in the fall was similar for all of the X-Ring configurations (indicating they had the same friction), and they were all greater than the ratio for the Impact Block (indicating there was less friction in the Impact Block).

The reason for the different pattern wasn’t entirely clear. It could be that during the drop tests, new ropes didn’t cause the sheave of the Impact Block to rotate – instead, they just slid over the sheave, whereas the used rope had enough friction to cause the sheave to rotate. Another finding from the experiment – that the load duration was a tenth of a second longer for used Stable Braid than the other ropes – supports this idea, but more experiments with used ropes are needed to clarify what happened.

These findings are interesting, but it’s important to remember that greater friction in the block doesn’t necessarily mean a smaller likelihood of failure. Although greater friction means the anchor experiences less force (and therefore would have a smaller likelihood of failure), the likelihood of failure in the lead of the rope may increase. And remember, too, that if the load duration increases because of friction in the block, even if the force at the anchor is less, the longer duration might increase likelihood of failure if the duration is similar to the sway period of the anchor.

Assessing the likelihood of failure of rigging systems is not easy because there are many different factors that can influence how likely different parts of the rigging system are to fail. The research results help us understand some of these aspects, but more studies are needed to clarify the findings. Considering these limitations, the surest ways to reduce the likelihood of failure of the rigging system are to rig a smaller piece, avoid shock loading or, if neither of these is possible, find an alternative, such as rigging from another tree or using a crane.

Acknowledgements

I wouldn’t have been able to complete this experiment without financial support from a John Z. Duling grant from the TREE Fund, donation of rigging gear from Bartlett Tree Experts and North American Training Solutions and help collecting data from Richard Herfurth (Bartlett Tree Experts), Jacyln Lim (University of Massachusetts – Amherst), Prof. Jean-Claude Ruel (Université Laval) and Dr. Noel Watkins.

Further reading

Centrangolo, I.; S.R. Arwade; and B. Kane. 2018. An investigation of branch stresses induced by arboricultural operations. Urban Forestry & Urban Greening 30:124-131.

Donzelli, P.S. 1999. Comparison of the frictional properties of several popular arborist blocks. Journal of Arboriculture 25:61-68.

Detter, A.; Cowell, C.; McKeown, L.; Howard, P., 2008. Evaluation of Current Rigging and Dismantling Practices Used in Arboriculture. RR668 of the Health and Safety Executive of the Forestry Commission, UK 370 pp.

Donzelli, P.; Lilly, S., 2001. The Art and Science of Practical Rigging. International Society of Arboriculture, Champaign, IL 172 pp.

Kane, B. 2017. Forces generated in rigging trees with single and co-dominant stems. Urban Forestry & Urban Greening 24:14-18.

Kane, B.; Brena, S.; Autio, W., 2009. Forces and stresses generated during rigging operations. Arboriculture & Urban Forestry 35:68–74.

Brian Kane is the Massachusetts Arborists Association Professor in the department of environmental conservation at the University of Massachusetts in Amherst, Mass.

The material in this article originally appeared in “Frictional Properties of Arborist Rigging Blocks,” an article by Brian Kane that ran in the June 2019 issue of Urban Forestry & Urban Greening. See that publication for more detail on the experimental methods used (including pictures of the experimental setup) and the findings.

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