Arboriculture & Urban Forestry 37(2): March 2011 6) Double swage stop, with washer: Aluminum swage stops were fastened to the dead end of the galvanized steel cable after it was inserted through a 0.40 cm (5/32 in) diameter hole; a 1.12 cm O.D., 0.45 cm I.D., 0.9 mm thick (number 8) round, flat, stainless steel washer. The two swage stops were installed adjacent to each other. 7) Double swage stop with fender washer: Copper or aluminum swage stop were fastened to the dead end of the cable after it was in- serted through a 0.40 cm (5/32 in) diameter hole; a 22.5 mm (7/8 in) O.D., 4.7 mm (3/16 in) I.D., 1.4 mm thick round, flat, steel “fender washer.” The two swage stops were installed adjacent to each other. A galvanized or stainless steel 7×7 strand, 3.1 mm (1/8 in) diameter cable with a manufacturer rated breaking strength of 7.56 kN [working load limit (WLL) = 1.5 kN] was used for all attachments. This size cable is recommended for branches be- tween 2.5 cm and 12.5 cm in diameter (Smiley and Lilly 2007). 69 Figure 4. Representation of the devise used to test cable attach- ments to small branches. Not to scale. RESULTS Figure 3. Close up of a crimped single swage stop terminated cable with washer. Branches were positioned longitudinally across two steel pins that were 14 cm apart, with the cable attachment point near the center (Figure 4). Approximately 40 cm from the branch attachment, the cable was formed into a 5 cm eye fas- tened with an oval swage connector. The cable eye was con- nected with a steel carabiner to a peak reading dynamometer (Dillon ED 2000 plus, Kansas City, MO, U.S.), which was connected with a larger cable to a mechanical winch (Fulton, Mosinee, WI, U.S.). The winch was hand cranked until one part of the system failed. However, for reasons of operator safety, the winch was not intentionally operated in excess of 8.9 kN. Four to 25 tests were conducted per termination type per spe- cies. Fewer eye lags were tested for each species due to the consistency of failure pattern that was irrespective of species. Peak force reading, branch diameter, and failure type were re- corded for each test. However, tests were terminated for safety rea- sons when the peak load was near 8.9 kN; ‘No failure’ was recorded. The data analysis was conducted separately for each species. The overall effect of the seven treatments (seven cable system categories) on the response (peak force at point of breakage) for each species was determined with an analysis of variance (ANOVA). If the ANOVA indicated an overall effect of the treatment on the response, specif- ic comparisons of the mean responses among the eight treatments were determined using Tukey’s Honestly Significant Difference Test (HSD). All analyses were conducted in SPSS (Chicago, IL, U.S.) and all statistical tests were performed using an alpha level of 0.05. Field Study After a nearly five-year period, visual inspection of the cables, trees, and terminations found no failures of any termination, cable, cabled trees, or noncabled trees. Most trees had partially or fully overgrown the washer, nut, eyebolt or the Wire Stop fastener. None of the trees had enveloped the thimble and grip portion of the tradi- tional cable system. None of the cables had visually apparent rust. There were no visually apparent changes in the size of the hole associated with the eyebolt anchored cables. With Wire Stop terminations, the tree either grew into contact with the cable (11 of 18 terminations), or the cable enlarged the hole in the tree (7 of 18 terminations) (Figure 5; Figure 6). Hole enlargement oc- curred on five white oak and two red oak stems. When the hole was enlarged, the average degree of enlargement was 6 mm. Static Breaking Tests In static breaking tests of cable systems installed in small di- ameter branches, it was found that eyebolt anchored cables were the strongest with a mean peak force of 8.5 kN (Table 1; Figure 7; Figure 8). With oak, most tests were terminated when the force level approached 8.9 kN. However, four of the 13 tests broke the branch in which the eyebolt was installed. This resulted in a lower peak force (8.3 kN) than if the bro- ken branch force measurements were removed from the analy- sis. Without the broken branches, the mean peak force was 8.8 kN, which is comparable to the value (8.9 kN) found in pine. At the other end of the strength spectrum was the bent eye lag screw which failed with the lowest mean force, approximately 2.1 kN. With screw hooks, the hook was straightened by the pulling force, allowing release of the cable. Tree species made no difference since there was no pull-out or branch breakage. ©2011 International Society of Arboriculture
March 2011
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