Abstract
Background Arborists are important in both the maintenance of urban trees and research on forest canopies. Tree climbing is hazardous work. This study investigated the causes of a branch-breaking accident involving a certified arborist.
Methods The branch was 11 cm in diameter. The density and mechanics of the branch wood were studied; the measurable stresses and deformations until the material ruptured were analyzed. The position of the tree climber and the applied forces were calculated, including the shear stresses, the bending moment, and the support reaction suffered at the point of rupture.
Results The shear stress grew exponentially from V = 25.78 MPa; a shifting of the angle of the lanyard by > 45° and moving one meter toward the tip of the branch caused a 64.67% increase in shear stress. The support distance of the arborist’s body on the one anchor point on the branch, combined with the angle of force on the same branch, caused the imminent rupture of the branch’s base. The study provides evidence that the arborists should avoid traveling over small horizontal branches using only one safety point with lanyards.
Introduction
Arboriculture and forestry are dangerous fields to work in, especially for tree climbers working for private arborists, landscaping companies, forestry and utility companies, or state agencies and municipalities who may require tree pruning on public roads. There are many occupational hazards and fatalities directly resulting from tree care operations, typically from falls, electrocutions, and being struck by objects such as trees or branches (Castillo and Menéndez 2009; Julius et al. 2014). Falls from height constituted approximately one-third of all occupational fatalities among tree climbers in the United States between 1992 and 2007 (Castillo and Menéndez 2009). Climber falls were also the leading incident for severe nonfatal injuries, followed by ground workers being struck by a falling branch, chainsaw incidents, and falls by aerial device operators (Ball et al. 2020).
Arborists falling from trees accounted for 14.9% (104) of the total (698) accidents in New Jersey, USA, in the 4 years following Hurricane Sandy (Marshall et al. 2018). The annual mortality of tree workers in the USA is very high—30 deaths per 100,000 people (Ball et al. 2020). This value is higher than the national average of 4 deaths per 100,000 people working in all other industries (Wiatrowski 2005; Ball and Vosberg 2010; Ball et al. 2020; Ball 2022). In the United Kingdom, the rate of tree climber accidents from 2005 to 2010 was 83 deaths per 1000 persons; 34 tree climber deaths were recorded from 2002 to 2012 (Robb and Cocking 2014).
Rope access methods are modern techniques used by professional arborists in the maintenance of trees (Lilly and Julius 2021; Staněk et al. 2022) and research conducted in forest canopies (i.e., studies on seed and genetic materials collected in large trees, ecological studies in plant and animal communities, crown dendrometry, etc.)(Lowman and Rinker 2004). These methods allow arborists to move within the tree crown and reach target branches, perform selective and targeted measures in the tree crowns, reach internal parts of the tree crown without damaging other trees, and reach trees in difficult-to-access areas (Lilly and Julius 2021). Tree climbers work at heights and use dangerous, sharp tools, such as handsaws, chainsaws, and pole saws, to conduct hazardous tasks, including pruning or removing tree branches. If arborists are not careful enough, these sharp tools can easily cut the tree climbing equipment or themselves (Kane 2021). Most climber falls relate to either tie-in-point/anchor failure, disconnecting from the climbing system, or severing the climbing line with a saw. Tie-in-point/anchor failures can occur with either moving-rope or stationery-rope systems; regardless of which system is used, any time a line is isolated to a single branch, it has better hold. Unfortunately, sometimes failures occur, often as the climber ascends, even when the climber first tests the anchor point with a load double their weight. Safe practice requires anchor points to be selected and inspected (Ball 2022).
Traditionally, indigenous Brazilians and amateur researchers climbed tree trunks up to 40 cm in diameter by using a loop of woven vines or cloth called a “peconha”; however, this method is dangerous and cannot be used on trees with a larger diameter. Safety is a high priority for researchers who must collect seeds or perform other canopy research; therefore, modern climbing techniques incorporating rigorous safety measures are used (Jepson 2000; Didham and Fagan 2004; Sillett and Antoine 2004; Lilly and Julius 2021). In Brazil, the number of arborists with ropes operating on trees is increasing. In general, these are highly specialized professionals using specific techniques and materials. Despite the use of these techniques in modern arboriculture, there has been scant research done on their use in tropical trees or on tree work in the tropical countries of South America. Even when using the best techniques, a non-fatal accident involving experienced arborists occurred while collecting seeds for a research project. The accident was caused by the breakage of an anchoring branch as the arborist moved in the canopy. The objective of this study was to identify the cause of the branch breakage, identify the condition of the wood, and understand the forces exerted on the branch by the arborist in a tropical tree species (Ocotea porosa).
Materials and Methods
Accident Site
On 2023 February 27, when large Ocotea porosa trees were in fruit and daytime temperatures averaged 25 °C, seeds were collected for a research project supported by professional arborists in the state of Santa Catarina, Brazil. The goal was to collect 100 fruits on each tree. The study included the harvesting of 10 trees in 3 Araucaria Forest areas (Curitibanos, Fraiburgo, and Catanduvas). Fieldwork began in the Curitibanos area on 2023 February 27. Climbing commenced at 2 PM, and 2 trees were sampled (with each tree sampling lasting 40 to 50 minutes) before the arborist fell from a height of 19 m. Seconds before the accident, images were taken of workers measuring tree heights in the canopy (Figure 1). The tree was 23 m tall with a diameter at breast height (DBH) of 75 cm at 1.37 m.
Accident site: the arborist’s position in the canopy (a, b), broken branch in the canopy (c), and the anchoring equipment on the ground after the arborist fell (d).
Dendrometry
The branch that broke, causing the arborist to fall, was collected 2 days after the incident and taken to the Laboratory of Forest Resources at the Federal University of Santa Catarina, Florianópolis, Brazil (Figure 2). It remained immersed in water (approximately 10 days) to maintain the original characteristics of the green wood until samples could be taken to assess its strength and rigidity with a static bending mechanical test. A moisture percentage in the wood above 30% is considered the percentage corresponding to the fiber saturation point. According to de Mello et al. (2021), humidity above this percentage shows no significant variation in the strength and stiffness of the wood. The branch was cut in 3 transverse positions (Figure 2b), resulting in 2 branch segments. These were cut in half, resulting in 2 fragments each, for a total of 4 (Figure 2d). From fragments 1, 2, and 3, 3 adjacent samples (A, B, and C) were extracted, as shown in Figure 2d. From fragment 4, 2 samples were taken.
Details of the studied branch: (I) free-body diagram, (II) anchoring positions, (a) longitudinal view, (b) cross section, (c) basal longitudinal section, and (d) longitudinal section and extraction positions of the bodies of proof.
The samples were sized and evaluated as prescribed in Standard 555 of the Pan-American Commission for Technical Standards (Comisión Panamericana de Normas Técnicas 1973). In this test, material with a nominal size of 20 × 20 × 300 mm is placed on 2 supports and subjected to a load in the central portion with controlled speed to cause measurable stresses and deformations until the material ruptures. Before testing, the samples were weighed on a scale and the volume was measured with a digital caliper. After the test, the samples were dried in an oven at a temperature of 103 °C until constant mass was attained. These procedures allowed the evaluated material to be characterized; they had an average basic density of 420 kg/m3, green density of 790 kg/m3, and moisture content of 88.27% (de Mello et al. 2021).
Data Analysis
Analyzing the sample required the transformation of Figure 2I into a free-body diagram. This includes the branch division point (TB) and the break area. Figure 2II depicts the branch opening angle. We consider the opening angle (θ) proportional to all applied forces. This transforms the case into a linear bending moment problem. Thus, the free-body diagram in Figure 3 was produced. The support reactions were calculated to the breakpoint (BP), describing the stresses that caused the break.
Free-body diagram. BP: branch break area (red); A: rope support area; S1 and S2: lanyard support points (dark grey).
Considering the position of the tree climber and the applied forces, the shear stresses, bending moment, and the support reaction suffered at the breaking point were calculated. Shear stress was defined by:
where AS is the strap area and θi is the movement angle (between the strap position and applied force).
Considering that the sum of moments at point TB must be zero:
Thus, the shear stress at the breakpoint is defined as a function of the distances in the free-body diagram and the angles of movement of the tree climber’s hands. For the numerical analysis, we consider the mass of the tree climber, m = 75 kg, and the acceleration of the gravity constant, g = 9.81 m/s2. The equipment strap had the width L = 0.012 m and the branch diameter was D = 0.01070 m. The distances between the breakpoints and the beginning of the branch were D = 0.5 m, D1 = 1.5 m, D2 = 2 m, and D3 = 2.5 m.
Results
The values of green density, modulus of elasticity, and modulus of rupture were determined for the samples extracted from the branch are provided in Table 1. Figure 4a depicts the result of shear stress against the movement angle, considering that the movement was coordinated between the 2 sides of the lanyard, θ = θ1 = θ2. The shear stress grew exponentially starting from a V = 25.78 MPa. Exponential growth was expected, which demonstrates the danger of supporting oneself on the branch by shifting the angle of the lanyard. Figure 4b describes the shear stress as a function of lateral movement of the lanyard with θ1 = θ2= 45°. We suggest that the anchor point D2 is fixed, while D3 = D1 + 1 m. The shear stress growth was linear, but the involved magnitudes were a warning. A movement of 1 m toward the tip of the branch caused a 64.67% increase in shear stress.
Values of green density, modulus of elasticity, and modulus of rupture for the samples extracted from the damaged branch. MOE = Modulus of Elasticity; MOR = Modulus of Rupture.
(a) the shear stress as a function of the movement angle (= 1 = 2) of the mobile support straps (lanyard). (b) the shear stress as a function of D1, the distance between the breakpoint and the first mobile support straps.
The previous result draws attention due to the percentage increase in shear stress. The study must consider the movement of the tree climber. We considered that only the left hand moved, moving the first strap of the lanyard (Figure 5). The result can be seen in the red line and compared with the previous case (black line), where both hands moved in coordination. A decrease in maximum stress occurred. Furthermore, considering the case of movement to gain vision range from the tip of the branch, the coordinated movement of both hands was more likely and natural, considering body balance.
Comparison of shear stress if both sides of the lanyard moved together (black line) or just the right side (red line), closer to the branch’s breaking point.
Figure 6 portrays a simulation where both quantities discussed are variables. Both the movement angle and the lanyard strap position can vary. For parts of the branch, the shear stress varied in the regions close to the rupture modulus obtained in the laboratory.
3-dimensional map of the shear stress variation as a function of the change in position of the lanyard and the movement angle (= 1 = 2).
Discussion
Arborists must recognize the most common symptoms of internal defects, decay, cankers, canker-decay, and root decay to identify hazardous trees (Tattar 1989; Jepson 2000; Shigo 2008). The branch anchor point must be inspected from the ground. Before starting to climb, the arborist should assess the tree by anchoring it with their and another’s body weight. Examining a potential anchor from 15 m (50 ft) high does not reveal all defects, particularly those that are not visible. Ball (2022) recommends against using the anchor if you cannot see it and cannot test it by loading the line from the ground with more than twice the weight of the climber; however, for large tropical trees with secure anchorage, it is impossible to check the visual condition of the branches, due to the presence of epiphytes.
The stationary rope technique (SRT) is the most energy-efficient means of tree climbing and, as such, is the most common method of accessing tall trees (Jepson 2000; Van Pelt et al. 2004; Van Pelt and Sillett 2008). SRT is a fixed-rope system, in which the rope is either cinched off around a limb in the tree or placed over a branch and tied off to a solid object near the ground. At the free end of the rope, the climber installs mechanical ascenders or friction knots. These devices hold the rope and slide only upwards with the climber’s weight. The rope is tensioned at one point, and another is free to ascend, alternating the support points to climb the tree (Jepson 2000; Anderson et al. 2015). Lilly and Julius (2021) list the safety criteria for anchoring to a live, healthy branch with no signs of decay; the minimum diameter is approximately 4 inches (10.16 cm). The branch anchorage weight test is a means of assessing the strength of the branch from the ground before climbing the tree. Before the arborist climbed, this test was conducted with the help of 2 people hanging from the rope. The result was that the branch (11 cm) resisted, and the arborist ascended to the crown.
After entering the tree, the climber must gradually transition from supporting their total weight on the first rope that was climbed and tested to supporting their weight on the second rope and an anchor point that has not been tested. At this stage, paying attention can prevent a catastrophic fall should the second anchor fail. Less noticeable are the nonlinear accelerating forces placed on both trees and ropes when a rope that is supporting a climber’s weight is tensioned horizontally (Dial et al. 2004). As a rope is tensioned from slack to horizontal, the physical forces exerted can exceed the strength of the rope and cause it or the anchor point to break (Harris 2010). It is important to recognize that branches are typically stronger when pulled downward than when pulled to the side (Anderson et al. 2015). Changing the anchor position for movement requires extra care. The anchorage inspection points, and the weight distributed over the anchorage generate changing forces due to the angle of movement in the position of the lanyard and the distance between the branch attachment point and the arborist’s position on the branch.
When collecting seeds, the arborist moved to the end of the branch. As the arborist returned, walking on a branch and supporting himself with the lanyards on another branch, he generated horizontal and vertical forces that caused the sudden break of the supporting branch. This was the tree’s climbing anchor. The ascension anchor points likely generated a false perception of security in the lateral return movement, causing carelessness in the distribution of weight and forces that caused the branch to break. Fortunately, the accident was not fatal. This accident provided valuable information. Even highly qualified certified arborists are prone to accidents. Fatalities can be avoided, and injuries reduced with the use of quality equipment and first-aid protocols with trained team members. Thus, we recommend that climbers obtain appropriate training that follows American National Standards Institute (ANSI) standards for equipment choices and best practices, as well as training and regular practice in aerial rescue methods that are essential for safe climbing (Anderson et al. 2015). Although there are still no specific standards for regulating activity at heights above trees in Brazil, only standards for risk assessment of trees (Associação Brasileira de Normas Técnicas 2019), the American National Standard for Arboricultural Operations—Safety Requirements ANSI Z133 (American National Standards Institute 2017) must be applied until local regulations are implemented; this also applies to other countries without regulation.
Conclusion
Arborists should avoid using horizontal branch as a lanyard anchor point when moving in the canopy and as primary rope anchor security, even though this branch was the point of rope ascension to the canopy and demonstrated support resistance at 90°. Changing the support angle of the lanyard to > 45° and the distance to the tip of the branch can significantly increase shear stress, regardless of whether the weight of the arborist is distributed under another branch.
Conflicts of Interest
The authors reported no conflicts of interest.
Acknowledgements
The authors thank the National Council for Scientific and Technological Development for the project grants: 405923/2021-0 and 406062/2023-4.
- © 2024 International Society of Arboriculture
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