182 Brian Kane: Compatibility of Toothed Ascenders with Arborist Climbing Ropes test apparatus, bending stress (s) at the axial mid-point of the strain gauge (550 mm from the branch attachment) was calculat- ed assuming a circular cross section (Lardner and Archer 1994): [1] s = 32Pl / (pd3 ) where P is the load (kN), l is the perpendicular distance from the eye bolt to the mid-point of the strain gauge (mm), and d is branch diameter (mm). Young’s modulus (E) of the branch was calculated from s and strain (e) measured during static loading using Hooke's Law (E = s/e); the value was 3.4 MPa. This was a conservative estimate because branch diameter was measured outside the bark, which has smaller values of E than the wood itself. Determining the deflection of tapered, can- tilevered beams loaded at their free end is complex. Instead, branch deflection (d) in mm at the point of loading was conser- vatively estimated assuming a constant branch diameter (265 mm) and the following equation (Lardner and Archer 1994): [2] d = PL3 /(3EI) where L is the perpendicular distance from the eye bolt to the branch attachment, and [3] I = pd4 /64 for a circular cross section. For the geometry and mea- sured E of the branch, Equations 1, 2, and 3 simplify to: [4] d = 0.5P Impact loading will increase the deflection depending on the magnitude of the impact and stiffness of the branch. Maxi- mum deflection during impact can be estimated from static deflection using the energy approximation method, but this method assumes that the mass of the branch is negligible rela- tive to the drop mass (Werner 1998), which was clearly not true. Substituting the maximum impact load of 8.5 kN (during a test with a Prusik loop) into Equation 4 yields d = 3.9 mm; substituting the mean impact load from tests with ascenders of 5.1 kN yields d = 2.3 mm. Impact factors in bridge design are typically 1.2 to 1.3 (Kim and Novak 1997). Doubling deflections predicted from Equation 4 still results in minimal values that were considered unlikely to have confounded the test method. The ascender was attached to the rope according to manu- facturer's instructions (Petzl 2010). In accordance with the EN 12841-2006 Standard (Anonymous 2006b), the ascender was statically loaded with the drop mass for 60 seconds prior to re- leasing the mass (by cutting the throwline), which fell freely until it loaded the ascender, which was 1000 mm below the dy- namometer. This test represented a fall factor (the ratio of fall distance to length of rope in the system) of 1.04. To investigate the effect of adding rope to the climbing system and the in- creased absorption of energy by the greater length of rope, drop tests were repeated after placing the ascender 2000 mm below the dynamometer; a fall factor of 0.52. Limited tests were con- ducted with the following variations: placing the ascender 4000 mm below the dynamometer (a fall factor of 0.26); increasing the drop mass to 110 kg; tying a backup friction hitch, a Valdo- tain Tresse (VT), with a Tenex spliced eye-and-eye Prusik 610 mm long to the rope above the ascender; and replacing the as- cender with a 3-wrap, 6-coil Prusik knot and a polyester kern- ©2011 International Society of Arboriculture RESULTS After several tests on most ascenders, signs of fatigue (typically, bending of the frame that held the rope next to the cam) were evident and the ascenders were retired from testing. For 33 tests with rope length set at 1000 mm, the arrest distance was ≤2 m only three times (all occurred when testing Velocity). For 34 tests with rope length set at 2000 mm, the arrest distance was ≤2 m seven times: four for Velocity and three for Blue Streak. The arrest distance was least for Velocity and greatest for Tachyon, which was the only rope for which arrest distance was greater with rope length of 1000 mm than 2000 mm (Table 2). Arrest distance did not exceed 2 m for any test with Super Static (Table 3). Although the mean arrest distance for Blue Streak tested with 4000 mm the rope length did not exceed 2 m (Table 3), arrest distance did exceed 2 m for two of the five tests. The mean arrest distance for Velocity tested with a VT exceeded 2 m (Table 3), but for three of the 10 tests, the mean arrest distance was 0.14 m. The arrest distance for single and doubled ropes of Blue Streak tested with a 6-wrap Prusik was quite small (Table 3). Both tests of Blue Streak with 110 kg completely severed the rope, and the arrest distances for Tachyon and Velocity (Table 3) were 0.72 m and 1.53 m greater, respectively, compared to tests with 86 kg. Across all ropes and rope lengths, impact load did not vary (Table 2). Curiously, impact load was greater with 2000 mm of rope length than 1000 mm for Blue Streak, while the opposite was true for Safety Blue (Table 2). Impact load did not differ between rope lengths for Tachyon and Velocity (Table 2). Impact load for Super Static (Table 3) appeared to be similar to impact loads on other ropes tested with 1000 mm of rope length. Im- pact load for Blue Streak tested with 4000 mm of rope length (Table 3) fit between impact loads for Blue Streak tested with 1000 and 2000 mm of rope length. Impact load for Velocity tested with a backup VT (Table 3) appeared to be greater than impact loads for Velocity tested without a VT. Tests with a Prusik instead of an ascender resulted in the greatest impact loads (Table 3). While impact loads of tests on Velocity backed up with a VT ap- peared to be greater than tests without the VT, the arrest distance was essentially the same with and without the VT, and the same number of tests (3) resulted in an arrest distance less than 2 m. mantle Prusik loop 8 mm in diameter and 1000 mm (including connecting carabiners) long tied around a single or doubled rope. These tests were not included in the quantitative analysis because of the complex nesting of treatments that would have resulted. Results of these tests were presented for qualitative compari- son. Impact load and arrest distance (the distance traveled by the ascender—or Prusik knot—prior to stopping) were measured for each test. To comply with the EN 12841-2006 Standard for Type B rope grabs, arrest distance must not exceed two meters. An analysis of variance (ANOVA) was used to compare impact load and arrest distance between ropes, the length of rope between the dynamometer and ascender (rope length), and their interaction. Each test was replicated a minimum of five times and a maximum of ten times, depending on the amount of rope that was available. Because of the unbalanced design, a general linear model and least squares means were used for the ANOVA, which was conducted in SAS ver. 9.2 (SAS Institute, Cary, North Carolina, U.S.). Arithmetic means were calculated for tests involving qualitative comparisons.
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