Abstract
Background: Arboricultural tree climbing is inherently dangerous, in part because of the possibility of failure of the tie-in point (TIP). To help climbing arborists choose TIPs wisely, we have conducted several studies to quantify the magnitude and frequency of loading associated with arboricultural tree climbing. One parameter that has not been previously studied is whether the choice of climbing line influences loads experienced by a TIP as a climbing arborist ascends. Methods: The lead author conducted trials in which he ascended to 3 TIPs (in different trees) using 2 ascent techniques and 3 different climbing lines. During each trial, we measured loads at the TIP, and from the resulting time histories analyzed the magnitude and frequency of loading. We compared the effect of ascent technique, climbing line, and their interaction on the magnitude and frequency of loading. Results: During trials, the magnitude of loading varied between 1.1 and 1.5 times the lead author’s weight and did not differ between ascent techniques, climbing lines, or their interaction. Loading frequency varied among ascent techniques, but not climbing lines. Footlocking induced loads at a wide range of frequencies, but 2 distinct frequencies were associated with ropewalking. Conclusion: Climbing arborists can use the results of this and our previous studies to help select a suitable TIP. It is important for climbing arborists to understand the magnitude of forces associated with ascending into and working in a tree. Future studies should investigate the load-bearing capacity of a TIP from the ground.
INTRODUCTION
Climbing trees is often necessary during arboricultural operations, but it can be especially dangerous. From 1992 to 2017, falls from height have consistently represented approximately one-third of fatal occupational injuries to tree workers (Wiatrowski 2005; Buckley et al. 2008; Castillo and Menéndez 2009; Ball et al. 2020). Marshall et al. (2018) reported that 104 of 698 tree-related injuries in the 3 years after Hurricane Sandy in New Jersey, USA, were falls. In a thorough review of arboricultural incidents (including falls), Ball et al. (2020) noted that approximately one-third of 56 fatal and severe nonfatal falls occurred as a climbing arborist ascended into a tree.
To address the risk associated with arboricultural tree climbing, safety standards have been developed in many countries—either by industry consensus, government regulation, social insurance, or a collaboration between them (Lim et al. 2020). All or nearly all standards address some aspects of climbing safety, but fewer standards provide explicit and detailed instructions on how to avoid incidents and injuries (Lim et al. 2020). In the United States, the industry consensus safety standard Z133 (American National Standards Institute 2017, §8.1.11) explicitly addresses pretesting a tie-in point (TIP) prior to ascending by loading it with a weight approximately equal to twice the climbing arborist’s weight. But the magnitude of loading is only one aspect that can affect the likelihood of TIP failure.
The likelihood of TIP failure also depends on (1) the frequency and duration of loading, and (2) the sway frequency and load-bearing capacity of the TIP itself (Cetrangolo et al. 2018). In previous experiments, we investigated the magnitude and frequency of loads associated with arboricultural tree climbing (Kane 2018, 2020a, 2020b; Kane et al. 2020). The initial part of arboricultural tree climbing is the ascent from the ground to the TIP. At the International Tree Climbing Championship (ITCC) in 2017, we measured tension in the climbing line as climbing arborists competed and demonstrated the correlation between a climbing arborist’s weight and the magnitude of TIP loading (Kane 2018). Magnitude and frequency of loading were also influenced by the technique climbing arborists used to ascend to a TIP (Kane 2018). In subsequent experiments, we investigated the effects of basal and crown anchors (Kane 2020b) and the presence of leaves (Kane et al. 2020) on the magnitude and frequency of TIP loading. On crown-anchored stationary rope systems (SRS), maximum loads during an ascent ranged from 1.4-times body weight (Kane et al. 2020) to 1.8-times body weight (Kane 2018). Ascent technique and the presence or absence of leaves also affected the loading frequency (Kane 2018; Kane et al. 2020), but measured sway frequencies of typical TIPs were outside of the range of forcing frequencies, reducing the dynamic amplification of stress (Kane 2018).
One factor that may influence the magnitude and frequency of loading that has not been carefully studied is the line that a climbing arborist uses to ascend. Climbing arborists use a wide range of climbing lines, which vary in construction and relevant properties such as elongation. Some climbing lines are designed specifically for SRS, while others are suitable for moving rope systems (MRS). In previous studies, we considered the climbing line a random effect (Kane 2018) or conducted trials using a single climbing line (Kane et al. 2020). We investigated the effect of climbing lines during ascents on basal-anchored SRS (Kane 2020b) because friction between the line and the TIP affects the tension in each end of the line. We also investigated the effect of 2 climbing lines on loads during simulated work climbs (Kane 2020a). Our objective for the current study was to determine whether the magnitude and frequency of loading differed among climbing lines and ascent techniques used in SRS.
MATERIALS AND METHODS
In July 2018, the lead author, an experienced climbing arborist, selected a single TIP in each of 3 Quercus rubra L. growing along a forested edge in Sunderland, MA, USA (USDA hardiness zone 5a). Following guidance on TIP selection from Lilly and Julius (2021), he chose TIPs that (1) provided adequate load-bearing capacity, (2) were free of structural defects, (3) provided a clear ascent path into the crown, and (4) would be suitable for common ascent techniques for SRS. Table 1 includes relevant tree and TIP morphology. He conducted trials in July and August 2018 and July 2019 when trees were in leaf and temperatures exceeded 20 °C. He did not conduct trials when ambient wind speed exceeded 5 m/s.
Trunk diameter (cm) 1.37 m above ground (DBH) and the height (m) and diameters (cm) of 3 Quercus rubra L. tie-in points (TIPs).
Following the method of Kane et al. (2020), the lead author set up a crown-anchored SRS by installing a polyester webbing sling in each TIP. Then he connected the bow of a load shackle (StraightPoint LLC, Camarillo, CA, USA) to the sling and tied the climbing line to the bobbin on the screw pin of the shackle with a figure-8 loop (Figure 1). Before ascending, the lead author performed a load test of the TIP, as required in the Z133 (American National Standards Institute 2017, §8.1.11). The load test involved applying a load of approximately twice the author’s weight to the TIP; the load test removed slack from the system and tightened the figure-8 loop.
Tie-in point supporting, from top to bottom, polyester webbing sling, load shackle, and climbing line (knotted in a figure-8 loop).
On each day of testing, the lead author weighed himself with his climbing gear before conducting any trials. Before each trial, he zeroed the load shackle to exclude the weight of the climbing line. As he ascended, the load shackle measured tension in the climbing line at 100 Hz. The resulting load time history (Figure 2) was uploaded to a laptop through a wireless USB antenna.
Part of a load time history for ascents by secured footlocking (dashed line) and ropewalking (solid line); the y-axis represents the ratio of tension in the climbing line to the climbing arborist’s weight (including gear). The 10-second segments are from the middle of the respective time histories.
On each TIP, the lead author ascended using 2 common techniques for SRS—ropewalking and secured footlocking (Lilly and Julius 2021)—and 3 climbing lines (Table 2). Climbing lines were selected to represent typical ranges of elongation and diameter and to comply with the Z133 (American National Standards Institute 2017), which restricted both ranges. Two of the climbing lines (Escalator [Teufelberger Ropes, Fall River, MA, USA]; Mercury [Samson Rope, Ferndale, WA, USA]) were new at the start of the experiment, while the third (WorkPro [Sterling, Biddeford, ME, USA]) had been used in several previous studies and was noticeably fuzzy (Table 2). The lead author performed each of the 6 combinations of climbing line and ascent technique twice—12 trials in total—in a randomized design stratified by TIP. He completed all 12 trials for each TIP on 3 separate days, resting between trials to avoid the confounding effect of muscular fatigue. The lead author completed 36 trials overall: 12 trials on each of the 3 TIPs.
Properties of kernmantle climbing lines taken from manufacturers’ product literature; all lines made of polyester and nylon. Minimum breaking strength = MBS.
From each load time history, we computed the load at the following quantiles: 75th, 90th, 95th, 99th, and 100th. We normalized the value of each quantile as a ratio with the lead author’s weight on the day the time history was collected; we designated the ratio as Pi, where i is the index of quantiles. In statistical analyses described below, we analyzed the mean of each Pi computed from the 2 trials the lead author conducted on each combination of climbing line, ascent technique, and TIP. For each Pi, there were 3 replicates (1 for each TIP) of each combination of climbing line and ascent technique.
We also counted the number of peak loads that equaled or exceeded the value of each quantile of the load time history. For example, if P75 equaled 1.1 in a load time history, we counted the number of peak loads greater than or equal to 1.1 in the time history. We averaged the 2 counts of peak loads that equaled or exceeded each quantile in the load time history from the 2 trials the lead author conducted on each combination of climbing line, ascent technique, and TIP, and analyzed the mean value in the statistical analyses described below.
We used a 2-way mixed model analysis of variance (ANOVA) to determine whether ascent technique, climbing line, and their interaction affected Pi and the number of peak loads that equaled or exceeded loads at each quantile. Random effects in the ANOVA were TIP and its interactions with climbing line and ascent technique. Tukey’s Honestly Significant Difference test was used for mean separation of significant (P < 0.05) effects in the ANOVA. We used JMP statistical software (v. 15, SAS Institute, Cary, NC, USA) for the ANOVA.
To determine loading frequency during ascents, we followed the methods of Kane et al. (2020) using MATLAB (MathWorks 2018). Briefly, we applied the Fast Fourier Transform (FFT) method to each load time history to calculate its power spectral density (PSD). Then we used the function “findpeaks” to determine peaks in the spectra. The output of the “findpeaks” function was a set of {frequency, PSD peak} pairs that indicated the peaks of the PSD of each trial.
To determine whether ascent technique and/or climbing line influenced loading frequency, we also used the approach of Kane et al. (2020), which is an alternative to direct averaging of the spectra across replications. The approach used here, and described briefly now, has the advantage of retaining information about the location and amplitude of the spectral peaks in individual replications, whereas in spectral averaging techniques all information regarding individual replications is lost. Scatter plots of PSD peaks vs. frequency were created for each combination of climbing line and ascent technique. To identify common loading frequencies that also had significant power spectral amplitude, a kernel density function was fitted to the {frequency, PSD peak} pairs in the scatter plot using the function “ksdensity.” Peaks in the kernel density fit were identified using the “findpeaks” function; the peaks were considered the characteristic frequencies for each combination of climbing line and ascent technique. To estimate variability of the loading frequencies, the width of the kernel density fit at 60% of the peak amplitude was evaluated. Sixty percent of the peak amplitude is equivalent to plus or minus one standard deviation for the Gaussian probability density function. The range of a loading frequency is the upper and lower frequencies on the kernel density fit intersected by a horizontal line (constant PSD peak value) at 60% of the kernel density peak.
RESULTS
The random effects of TIP and its interactions with climbing line and ascent technique were not significant in the ANOVAs for any Pi or the count of peak loads for each Pi. Values of Pi ranged from 1.1 for P75 to 1.5 for P100, but they did not vary between climbing lines or techniques for any Pi (Table 3).
Output of the analysis of variance of the effect of climbing line, ascent technique, and their interaction on the ratio of tension in the climbing line at the specified quantiles (75th, 90th, 95th, 99th, 100th) to the climbing arborist’s weight. Abbreviations of climbing lines are in Table 2. Footlocking = FL, ropewalking = RW, degrees of freedom = df, least squares means = LSM.
The count of peak loads at each quantile of the load time histories ranged from a single peak load for P100 to 31 peak loads for P75 (on WorkPro)(Table 4). The count of peak loads at each Pi except P100 was greater when ropewalking compared to footlocking; the difference was statistically significant for P75 and P99 and marginally significant for P90 and P95 (Table 4). There was some evidence (P = 0.0544) that the number of P75 peak loads was greater for WorkPro than Escalator (Table 4).
Output of the analysis of variance of the effect of climbing line, ascent technique, and their interaction on the number of peak loads in the time history that exceeded the specified quantiles (75th, 90th, 95th, 99th, 100th). Abbreviations of climbing lines are in Table 2. Footlocking = FL, ropewalking = RW, degrees of freedom = df, least squares means = LSM.
Loading frequency did not differ between climbing lines, but there were differences between ascent techniques (Figure 3 and Table 5). When ropewalking, 2 dominant frequency peaks were present, but the frequencies at which those peaks occurred were independent of the climbing line (Figure 3A). Table 5 includes the range of loading frequencies for each climbing line and ascent technique; within each ascent technique, there is noticeable overlap of the frequency ranges of the climbing lines.
Kernel density plots for ropewalking (PBH, 3A) and footlocking (FL, 3B) ascents. Colored circles indicate each {frequency, PSD peak} pair from PSD plots of load time histories; colors identify the climbing lines shown in the legend (climbing line abbreviations are in Table 2). Colored lines indicate kernel density fits to the {frequency, PSD peak} pairs to identify common loading frequencies with significant power amplitude for each climbing line.
The results for footlocking trials showed a broader spectrum of frequencies in the load time histories (Figure 3B), so we selected the dominant peaks at approximately 1.25 Hz and 2.1 Hz for comparison with the peaks associated with ropewalking (Table 5). The broader frequency ranges of the footlocking data result in even wider peak frequency ranges and greater overlap among climbing lines.
DISCUSSION
This paper continues a series of experiments dedicated to investigating loads associated with arboricultural tree climbing. We are unaware of any other rigorous empirical data. We undertook the experiments because understanding the loads a climbing arborist imparts is 1 of 2 components necessary to assess likelihood of TIP failure during arboricultural climbing. Because a single climbing arborist conducted the trials in the current study, the results should be extrapolated with caution and may not apply to other climbing arborists. But the consistency of our results with those from the ITCC in 2017, which included 59 female and male climbing arborists (Kane 2018), suggests that accounting for the climbing arborist’s weight and ascent technique adequately reduced uncertainty associated with the climbing arborists themselves. Another limitation of the study is not measuring TIP motion during ascents. This omission prevented us from exploring dynamic interactions between the ascending climbing arborist and the TIP. Such interactions are important because (1) they could have distorted measurements of loads, and (2) they may have provided insights into the likelihood of dynamic amplification of stress. Regarding (1), the lead author did not perceive dynamic interactions during trials—they are sometimes observed as a distinctive “bouncing” motion during an ascent. It is also less likely that dynamic interactions distorted load measurements because TIP stiffness based on stem diameter below each TIP was approximately an order magnitude greater than climbing line stiffness, meaning that dynamic interactions were negligible. Regarding (2), sway frequency of the TIP must be known to determine the likelihood of dynamic stress amplification. In a previous study, sway frequency was not aligned with loading frequency (Kane 2018), but given the wide variety of TIPs, a more extensive knowledge of TIP sway frequencies is necessary to conclude that dynamic amplification is usually unlikely.
Values of Pi were consistent with previous studies involving a single climbing arborist and multiple TIPs (Kane et al. 2020) as well as multiple climbing arborists on a single, atypically large TIP (Kane 2018). The pattern that has emerged includes cyclically repeated loads at multiples of a climbing arborist’s weight. As the current study and previous work (Kane 2018; Kane et al. 2020) have shown, multiples of a climbing arborist’s weight tend to be consistent among ascent techniques and to vary between 1.1 for P75 and 1.8 for the maximum load in an ascent (P100). The number of times each Pi is repeated during an ascent obviously depends on the duration of the ascent; assuming a loading frequency of 2.1 Hz (Figure 3), the TIP would experience 21 peak loads for every 10 seconds of an ascent. The current study and previous work (Kane et al. 2020) have shown that the number of times each Pi is repeated also depends on ascent technique: excepting P100, which tends to occur only once regardless of the ascent technique, the number of peak loads at P75, P90, P95, and P99 was larger when a climbing arborist used the ropewalking technique. In the current study, ropewalking induced anywhere from 1.4 (for P99) to 1.8 (for P95) times as many peak loads as footlocking. In a previous study, ropewalking induced anywhere from 1.5 (for P99) to 1.9 (for P75 and P90) times as many peak loads as footlocking (Kane et al. 2020).
Since ascents are dynamic loads that induce a dynamic tree response (swaying of the TIP), loading frequency is another important factor to consider. Results from the current study align with previous work (Kane et al. 2020): ropewalking tends to induce loads at 2 distinct frequencies—one approximately twice the other. In contrast, footlocking tends to induce loads at a wider range of frequencies. We speculate that the greater overall motion of a climbing arborist footlocking is responsible for this occurrence. Anecdotally, the lead author observed that his hands exerted a downward force on the climbing line less frequently than his feet, but we did not quantify this. A careful analysis of the body motion of climbing arborists ascending using different techniques would clarify the biomechanics of various ascent techniques and may help explain why footlocking tends to induce loads at a wide range of frequencies. Similarly, it would be helpful in future studies to include a sample of climbing arborists of varying body types to investigate the effect of body type on loading frequency.
None of the loading aspects that we investigated in the current study (magnitude, number of peak loads, frequency) varied significantly among climbing lines, which supports the speculation (Kane 2018) that the use of different climbing lines among many different climbing arborists did not meaningfully affect the results. The lack of an effect of different climbing lines with a wide range of elongation properties, combined with the lack of an effect of different TIPs in the current and previous (Kane et al. 2020) studies, indicates that structural mechanics of the TIP-climbing line system does not influence the magnitude or frequency of loading during an ascent. While there are an infinite number of TIP geometries and many climbing lines and climbing arborists, the collective findings of the current and several previous (Kane 2018, 2020b; Kane et al. 2020) studies suggest that during ascents, (1) the magnitude of loading is primarily influenced by a climbing arborist’s weight, and (2) the frequency of loading is primarily influenced by ascent technique.
CONCLUSIONS
In combination with our previous studies, the current results provide a baseline understanding of loads associated with arboricultural tree climbing, including many of the relevant variables such as ascent technique and climbing line. In this and previous studies, maximum loads on a canopy-anchored climbing system have remained less than twice a climbing arborist’s weight; climbing arborists should conservatively assume that their chosen TIP must regularly bear twice their weight with climbing gear. But this is not an endorsement of §8.1.11 in the Z133 standard (American National Standards Institute 2017): a single proof load to assess the load-bearing capacity of a TIP grossly oversimplifies the dynamic structural mechanics of a climbing arborist ascending on a TIP.
Future studies should also explore the load-bearing capacity of a TIP, which is critical to assessing the likelihood of TIP failure during arboricultural tree climbing. Relevant mechanical parameters of TIPs such as sway frequency, stiffness, and wood strength are largely absent from the literature. Finite element analysis may be a useful approach to assessing the likelihood of TIP failure, but mechanical parameters are necessary to create robust finite element models (Cetrangolo et al. 2018). Although TIP load-bearing capacity remains uncertain, climbing arborists can reduce the likelihood of TIP failure by choosing one conservatively. Three factors that meaningfully influence a TIP’s load-bearing capacity and can be assessed from the ground are (1) the diameter of the stem below the TIP, (2) the degree to which the stem below the TIP diverges from vertical orientation, and (3) the presence of defects such as decay, cracks, or weakly attached branches on the stem below the TIP. To reduce the likelihood of TIP failure, climbing arborists should choose TIPs that are larger in diameter, closer to vertical, and free of defects.
ACKNOWLEDGMENTS
We thank Ed Carpenter (President, North American Training Solutions) for donating ropes and a saddle, and Emma Brigham (University of Massachusetts, Dept. of Civil and Environmental Engineering) for conducting spectral analysis of load time histories. This study was funded in part by a John Z. Duling grant from the TREE Fund (16-JD-03).
Footnotes
Conflicts of Interest:
The authors reported no conflicts of interest.
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