102 Table 1. Mean (standard deviation) morphometric data for trees of each species.z Measure Slenderness (height/dbh) n AP n AR n AS Tree height (m) 7 16.8 (3.08) 6 18.5 (3.63) 10 16.7 (7.45) dbh (m) 7 0.45 (0.11) 7 0.71 (0.15) 10 0.71 (0.13) 7 38.0 (6.30) 6 28.6 (7.88) 10 24.1 (11.4) Crown height (m) 7 13.8 (2.26) 3 nmy Crown width (m) 5 12.6 (4.39) 2 nmy measured at breast height (1.4 m aboveground). y 6 17.2 (2.42) 6 15.7 (4.10) zAP Norway maple, AR red maple, AS sugar maple; dbh diameter nm indicates that there were too few samples to include in the analysis. uprooting, compromised root systems were deemed to be irrel- evant during testing. All trees were semimature to mature, but none was in declining health. Trees were chosen primarily ac- cording to practical considerations of the test procedure. Great care was needed when breaking large trees so that buildings, roads, aboveground utilities, and existing trees and shrubs on the property were not damaged. The availability of appropriate sites to conduct tests and the practical limitations of breaking large shade trees in situ naturally limited the scope of the study and the sample size. Before testing, tree height and diameter at breast height (dbh) were measured. Crown height and width were mea- sured for 16 and 13 trees, respectively. Table 1 presents mor- phometric data for each species. Field Tests Tests were conducted in the summers of 2002, 2003, and 2005; site constraints limited the number of days available to conduct tests each year. Trees were pulled to failure using a cable winch skidder (John Deere model 440D [Moline, IL], hydraulic winch capacity 90 kN); the cable was run from the spool of the winch through a large block attached to the main trunk and back to an anchor point on the skidder (see Figure 1). The height of the block ranged from 37% to 81% (mean 51%) of tree height. The variation was the result of the necessity of securing the cable to a substantial portion of the trunk and of variation among trees in both tree height and crown structure. The block was attached at a height that incorporated a codominant stem when one ex- isted in a tree. In such situations, the load was applied perpen- dicular to the attachment between the codominant stems. The distance from the skidder to the tree, the height of the winch spool and anchor point, and the horizontal distance between the spool and the anchor point were measured to determine the angle between the cable and the ground. The angle was necessary to resolve the applied load into components parallel and normal to the ground. Tension in the cable was measured by placing a load cell (111 kN capacity, accurate to 111 N; Futek Advanced Sensor Tech- nology, Inc., Irvine, CA) between the anchor point on the skidder and the cable after it was passed around the sheave of the block. Measured tension was doubled to determine the actual load on the tree because the cable was run through the block. To measure fiber extension on the trunk, a linear variable displacement trans- ducer (LVDT) (model TS50, accurate to 0.075 mm (0.003 in); Novotechnik U.S., Inc., Southborough, MA) was attached to the trunk at breast height on the side opposite the direction of the applied load (see Figure 2). Fiber extensions were converted to strains by dividing the fiber extension by the gauge length of the LVDT (50mm[2 in]). Loads and fiber extensions were collected ©2008 International Society of Arboriculture Kane and Clouston: Tree Pulling Tests of Large Shade Trees Figure 1. Diagram of tree-pulling setup. The dashed lines rep- resent the cable with arrowheads indicating the tension. DS is the distance between the skidder and the tree, HT is the height of the tree, and HB is the height of the block. The angle () between the cable and the ground was calculated as tan–1(HB/DS). The distance between the winch and the skid- der anchor was less than 2 m. at 2 Hz with a three-channel data logger (Mini-ModuLogger; Logic Beach, Inc., La Mesa, CA) that also recorded temperature and relative humidity. Trees were pulled until failure without stopping, which generally occurred within 15 sec of applying the load (the maximum time to failure was 30 sec). Tests during the summers of 2003 and 2005 were videotaped to quantify the amount of deflection of the crown during tests. Video images were scaled and the distance traveled by the block attached to the trunk was measured on the video image. The horizontal deflection of the crown adds a bending moment as a result of the offset mass of the crown. Crown mass was not measured, which means that the reported stress values underes- timate the actual breaking stress. Because the horizontal deflec- tion of the block did not exceed 2 m (6.6 ft) for any tree, the bending moment resulting from the offset mass of the crown was likely negligible relative to the applied load. After failure, a clean cross-section adjacent to the point of failure was cut with a chainsaw and the dimensions of the cross- section were measured parallel and normal to the direction of the applied load. The distance between the block and the point of failure was also measured. Decay and other defects on the trunk and cross-section were also noted and quantified with a digital image. Failure was categorized by its location either along the trunk (stem failures) or at the attachment between codominant stems (codominant failures). Stress Analysis Compressive stress () at the point of failure was calculated by adding the bending stress (the first fraction in Equation 1) and axial stress (the second fraction in Equation 1) resulting from the applied load: 32Pcosl/(ab2) + 4Psin/(ab) [1]
March 2008
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