68 Kane et al.: Forces and Stresses Generated During Rigging Operations Arboriculture & Urban Forestry 2009. 35(2): 68–74 Forces and Stresses Generated During Rigging Operations Brian Kane, Sergio Brena, and Wesley Autio Abstract. Rigging is one of the most dangerous aspects of arboriculture, yet there are no robust studies of the forces and stresses generated during rigging. Compounding the inherent danger of rigging is the structurally-deficient condition of many trees that are removed using rigging. Red pines (Pinus resinosa Ait.) (n = 13) were removed using conventional techniques, and forces at the block and in the rope were measured as the top, and four subsequent pieces were rigged with a block and Port-A-Wrap. Stress in the trunk at breast height was calculated from strain measurements and each tree’s modulus of elasticity. Multiple regression was used to deter- mine which independent variables (mass of piece, fall distance and fall ratio, notch angle and depth) best predicted forces. Tops and pieces exhibited different relationships with mass, which was the best predictor of force at the block and tension in the rope. Other variables were not as important and exhibited counter-intuitive relationships with forces. There were few differences in stress gener- ated when removing tops and pieces, which appeared to be due to greater deflection higher in the trunk when tops were removed. Key Words. Rigging; Trunk Stress; Biomechanics. Rigging is one of the most dangerous aspects of arboriculture. Removing large branches and pieces of wood secured by ropes involves dynamic forces as pieces, either swing or fall some dis- tance before the rope decelerates and stops them. The tree re- sponds dynamically by oscillating because it is a flexible struc- ture. Rigging generates large stresses on the rigging gear and on the tree itself. Increasing this inherent danger is the structurally- deficient condition of many trees that are removed using rigging. Rigging operations have resulted in climber fatalities (Ball and Vosberg 2004). Excluding injuries resulting from improper rig- ging, there are two basic risks to the climber and ground crew during rigging operations: failure of the rigging gear (ropes, pulleys, friction devices) and failure of the tree or part thereof. Arborists use different types of rigging for different situ- ations (Donzelli and Lilly 2001), but the greatest stresses are generated when the rigging point (i.e., where the lowering rope is anchored to the tree) is below the piece being removed. For this situation, the rigging point is a block attached to the tree by a sling, and the lowering rope passes through the block and is tied off to the piece being removed (Figure 1). Commonly re- ferred to as, “butt-hitching” (Lilly 2001), this type of rigging is usually associated with tree removal, because once the branches have been removed, there are no more rigging points above the piece to be removed. Because the piece being removed free falls, its velocity is greater when the lowering rope decelerates and stops the piece. The increased velocity of the piece causes the stopping force exerted by the lowering rope on the piece to sub- stantially exceed the weight of the piece. Although the stopping force can be reduced by gradually decelerating the piece (this is known as “letting a piece run”), in some cases, there is no room to employ this alternative. When the stopping force acts quickly (e.g., an abrupt deceleration), the force required to decelerate the piece is, theoretically, much greater than in the former case. Advances in rigging gear and techniques may instill greater confidence in climbers to take bigger pieces of wood. Climb- ers relying on working load limits of rigging gear may not ap- preciate the potentially large forces involved in taking large ©2009 International Society of Arboriculture pieces. Furthermore, while it is comparatively easy to test the breaking strength of ropes, slings, and blocks (either working load limits or breaking strengths are printed on such equip- ment), there are very few studies that have measured the break- ing strength of large shade trees (Kane and Clouston 2008). Further complicating the issue is the difficulty in analyzing trees with engineering equations, because the simplifying as- sumptions upon which equations are derived are often violated. In spite of the great risk to life and property, there are no robust studies that have investigated the forces and stresses in- volved in rigging operations. Detter (2008) reported results from a laboratory study in which high speed photography was used Figure 1. Diagram of how pieces were removed by “butt-hitching” [from Lilly (2001), used with permission].
March 2009
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