Testing Equipment and Techniques to Reduce Loads on the Stem

Photo 1. Project 1 site, subject tree. All photos and diagrams are courtesy of the author.

Tree-removal operations in urban and suburban settings often employ extensive use of rope-based rigging to control and manage the dismantling of large trees. These scenarios typically conclude in what is commonly referred to as a negative-rigging situation, where the load the rigging must bear “falls into the rope” from above the anchor point (such as when we work our way down a spar). This results in dynamic forces being placed on the tree and equipment that far exceed the static load of the mass of the cut log. Any opportunity to mitigate these forces while still working efficiently is a bonus for us – the working arborists – and our equipment.

Recently, our team investigated two different rigging scenarios to better understand how the selection of specific equipment and the use of a strategic limb-removal sequence can affect the load (and subsequent stress) on the stem of the tree being removed. The following outlines some results from these studies. We hope you will find this information both informative and thought provoking, and perhaps influential in your work during these types of operations, as we all continue to strive for both a safe and efficient workplace.

Project 1: Rigging rings vs. conventional arborist blocks

For many years, the traditional upper anchor point in a rigging scenario has utilized a revolving sheave block (a pulley). Due to the inherent nature of this system, to slow or stop the falling load on one side of the system, a similar force must be exerted on the opposite side. The result is a magnification of the falling load to approximately 2x for what the anchor point must bear, and this can lead to the anchor point being subject to considerably large forces.

Photo 2: Project 1 tree, craning piece back in place.

A recent alternative to the block has been the evolution of rigging rings and thimbles. These fixed devices are meant to maintain a consistent friction point, which in theory would reduce the magnification of forces that occurs with a rotating sheave (dissipation of energy as heat through friction). While a variety of devices and techniques have been introduced, there has been little empirical research on the efficacy of these systems. In a negative-rigging scenario, Kane (2019) showed little difference between rigging rings and a block for peak loads. However, in this test, the load was dropped into the rigging system from above and stopped abruptly – ideal to simulate the “worst-case scenario.” Since the intent of the rigging rings is to dissipate energy as heat over time, more work was needed to examine these systems under running-rope conditions.

Figure 1: An example of the stem strain and Portawrap data displayed over time.

General experimental design

We set out to test an assortment of anchor points under a running-rope scenario, where the falling load would be repeatedly and consistently brought to rest over time, comparing stationary anchor devices to a traditional rotating block. The subject tree was an 18-inch (45-cm) DBH (diameter at breast height) green ash, (Fraxinus pennsylvanica). The notch for the rigged section was 24 feet, 7 inches (7.5 meters) above ground level, and the piece had a mass of 425 pounds (93 kilograms, kg).

To decelerate the falling piece with a consistent rate and distance, a “robot groundie” rising-rate inclined ramp was devised to mimic the role of the ground person in “letting the rope run.” A four-wheeled cart ran along the ramp with an attachment point for the rigging rope (see Photo 1). As the cart climbed the parabolic curve, the resistance increased, mimicking the increasing grip of the ground person.

Scratch-built “strain gauges” (resolution of 0.001 mm) were used to measure fiber elongation of the trunk, a tension load cell was installed between the Portawrap and the sling and an accelerometer was installed on the falling log. All devices sampled at 50 Hertz (Hz, cycles per second).

  • Four rigging anchors were compared:
  • DMM Impact Block Small (IMB-S, DMM International Ltd.; working load limit [WLL] 40 kN [kilonewtons]);
  • Elevation Canada Single Rigging Thimble – Size 3 (Elevation Canada; maximum break strength [MBS] 110 kN);
  • Notch Double Rigging Thimble – Size 2 (Notch Equipment; MBS 15,000 pounds); and
  • X-Rigging SafeBloc (SherrillTree; WLL 2,700 pounds).

The rigging rope was Samson Rope’s 9/16 Stable Braid (Samson Rope; WLL 1,200 kg) with minimal use (fewer than 10 previous rigs). The friction device was a Notch Large Portawrap (stainless steel, WLL 2,000 pounds).

A total of three drop tests were conducted for each anchor device. After each test, the piece was hoisted back into position with a crane and the event repeated.  (Photo 2)

A summary of results

An example of the strain-gauge and load-cell data can be seen in Figure 1. Due to large stem oscillations when the falling piece impacted the stem, we focused only on peak strain during the initial loading of the system. This can be noted in Figure 1 as well, indicated by the red circle. Note that the graphs display the relevant data synchronously over time; observe that the initial peak load in the Portawrap corresponds with the initial peak in stem strain. We isolated the peak rope loads, stem strains and acceleration data for all events and compared them using a one-way analysis of variance (ANOVA).

Figure 2: Peak stem strain for each tested device. Note the strain is reduced by approximately one-third when using the rigging-ring-type devices; however, it must be noted that this is a running-rope, “best case scenario.”

The force measured at the Portawrap when using the stationary rings routinely saw approximately half the load compared to the block at the Portawrap. This matched our expectations, as experience says we typically remove a few wraps from the Portawrap when we are using a ring or thimble. The initial strain on the stem can be seen in Figure 2; note that there is an approximate one-third reduction in measured stem stress for the three stationary anchors as compared to the rotating block. By adding the measured load from the Portawrap and the calculated load from the accelerometer mounted in the falling piece, we can get an estimation of the load on the upper anchor point. Here we saw a decrease in anchor load (Figure 3), which is similar to the decrease in stem strain seen in Figure 1.

Figure 3: By combining the accelerometer data with the Portawrap load-cell data, we can arrive at an approximate load at the upper anchor point.

However, if we isolate the calculated load from the accelerometer, we have what can be considered the tension load in the lead section of the rope, and here we do not see such a decrease (Figure 4). This, of course, makes sense, as we still have the same potential energy to dissipate; the only way to realize a reduction in the load here would be from a change in the time needed to slow the falling piece once on rope.

Figure 4: Here is the accelerometer data used to calculate an approximation of the load in the lead rope. Note it is not reduced by one-third, as in the case of the stem strain and anchor load. In fact, there was no significant difference for the SafeBloc as compared to the arborist block. The potential energy in the system has not changed, so the lead rope sees the same effective load. Only reducing the impulse (slowing the falling piece over a longer period of time) canreduce the peak load this portion of the rope experiences.

Some thoughts to consider

This specific finding and its implications are perhaps the primary results from this study for consideration as a climber using rigging rings. While the forces the stem and upper rigging anchor are subjected to have been reduced, as climbers we feel this reduction – and indeed it is a benefit. The same cannot be said for the rope attached to the falling piece. This may pass unnoticed, because our own experience – through our connection to the system (standing on the stem) – has improved, and perhaps we will now be inclined to “take a larger piece” than the equipment (the lead rope and associated attachment point) can bear.

Project 2: Utilizing damping concepts in rigging operations

Can we use limb-removal sequence to help mitigate stem stress? Can the effects of mass and aerodynamic damping be utilized to reduce the stress the stem experiences in a negative-rigging scenario? While extensive work has been done to improve our understanding of the concepts of mass and aerodynamic damping within the tree crown, the majority of this has been with a focus on tree/wind interactions. How this relates to tree-removal situations and the changing dynamics of the tree’s structure has seen far less investigation. This project set out to examine the impact of leaving limbs below the upper anchor point in a negative-
rigging situation.

Experimental setup

For this, we systematically dismantled an 11¾-inch (30-cm)-DBH, 46-foot (14-m)-tall green ash, Fraxinus pennsylvanica (Photo 3). Once again, we utilized strain gauges at the base of the tree, a load cell between the Portawrap and the tree and an accelerometer in the falling “top.” All sensors captured data at 50 Hz. To slow the descent of the falling piece and bring it to a controlled stop, the rising-rate ramp “robot groundie” described earlier was used again.

Photo 3: Subject tree and sequence of limb removal.

To eliminate their effect, the tree was stripped of small lateral limbs on the main stem, with no pruning to any of the remaining six subject limbs. (Photo 4) A pseudo top was used consisting of just a stem log. The top was dropped repeatedly (three times), then the lowest limb was removed and we dropped the top three times again. This continued as we worked our way up the tree until all the limbs were removed (as per Photo 3), and we conducted three final drops with just a standing spar. We systematically measured all removed components of the tree (length, diameter, mass, position of center of mass) to build an allometry model for further analysis.

Photo 4. Tree pruned of small lateral limbs and top removed. A pseudo top (just a log) was used for the falling mass.

Some results and points to consider

Figure 5 provides a sample of the changes in the raw data as we progressed through the experiment. Note the progressively increasing stem strain and the increase in the decay time (the number of oscillations in the stem). To analyze these data, we again selected peak strain and used one-way ANOVA. This is represented in Figure 6, where we see a significant increase in measured stem strain with the removal of the last two limbs. This suggests that leaving even one small limb (less than one-third stem diameter) near the upper anchor reduces the peak stem strain (in this case by approximately 20%). Furthermore, in this specific instance, leaving two limbs in close proximity reduced the strain by approximately 35%!

Figure 5: Examples of the strain and rope-load data for a selection of events. Note the increasing stem strain (upper graphs) and the increasing time to decay (number of stem oscillations).
Figure 6: This shows the increasing stem strains as progressive limbs were removed. Note the final limb was less than 2% of the total mass of the tree and had a diameter ratio of less than one-third the stem area of the section above, yet still reduced the strain by approximately 25%.

Obviously, it would be difficult if not impossible to retain limbs all the way to the bottom of the tree for a rigging operation, particularly in tight drop zones where we routinely utilize the upper anchors to manipulate large limbs into tight quarters on the ground. However, these results have the potential to adjust our habits when sequencing removal operations. While having a clear spar below makes the removal easy, in the case of tall, slender stems, perhaps consider leaving at least a few limbs below the “big ugly top” you want to throw. Go even further and leave some manageable limbs further down the spar to remove once you get close!

Climb safe!

Citations

Kane, B. (2019). Frictional properties of arborist rigging blocks. Urban
Forestry & Urban Greening, 42, 31–38. https://doi.org/10.1016/j.ufug.2019.05.00

A huge thank you to the TREE Fund, which supported this work through a Safe Arborist Techniques Fund (SATF) grant! Also, to all the teams and people who helped out, including the forestry staffs with the City of Dorval and Royal Tree Service.

Matt Follett is an ISA Certified Arborist, ISA TRAQ credentialed and a climbing arborist with a strong interest in preserving large trees in the urban environment. He is currently a Ph.D. student at Université du Québec à Montréal (UQAM), focusing on the effects of pruning on tree resilience in the urban environment, as well as climber safety. This article is a summary of his presentation at TCI EXPO ’21 in Indianapolis, Indiana.

Bastien Lecigne is a postdoctoral fellow in the Department of Biological Sciences, UQAM.

Andreas Detter is a consulting arborist and researcher with Brudi & Partner Tree Consult, Germany.

Lothar Göcke is a design engineer in Germany; he has worked extensively to develop tree-sensing apparatus.

Dr. Christian Messier is professor of forest ecology and urban forestry at UQAM and Université du Québec en Outaouais (UQO).

The authors are looking forward to presenting these results again, plus more, at TCI EXPO ’22 in Charlotte, North Carolina, this fall.

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