70 Smiley: Dead-end Stop Terminated Tree Support Cable Systems Figure 5. Galvanized EHS steel cable that has widened the hole in the bark and terminated with a Wire Stop fastener. significantly increased the strength of the system from a mean of 2.1 kN with the standard screw eye to 4.5 kN with the welded eye. With dead-end swage stop terminated cables, a single swage stop with no washer pulled through the wood of all of the pines with a mean force of 2.7 kN. This force level was not significantly different from either screw eye. In oaks, the majority of failures occurred when the swage stop failed at slightly less than 4 kN. With oak, this was a significantly higher peak force than the screw eye anchored cable systems, but not the welded screw eye system. When a washer was added to the single swage stop termi- nated cable, the swage stop consistently pulled off the cable in oak with a mean peak force of 4.8 kN. In pine, the major- ity of failures were also when the swage stop failed. However, failures involving the swage stop pulling through and the branch breaking also occurred. With pine, the mean peak force in- creased 47% to near 4 kN, with the addition of a washer. The differences with and without washers were not significant. When two swage stops and a washer were installed on the cable, the failures occurred near the listed breaking strength of the cable (7.6 kN) with oaks. This level of force was not sig- nificantly different from the peak force associated with eyebolt terminations. The majority of these failures occurred when the cable broke (19 of 25 tests). For the remaining six samples, the entire anchor system pulled through the branch. Branch pull-through occurred on branches from 38 to 48 mm in cali- per, and the cable broke on branches from 45 to 64 mm in caliper. With pine, the failures occurred at a lower mean force (5.5 kN), with about half of the terminations pulling through the wood and the other half when the cable broke. The pull- throughs in pine were in the branch caliper range of 31 to 56 mm and cable breakage was in the caliper range of 76 to 86 mm. With pine, the washered double swage stop pulled through the branch slightly more often than the cable failed. So with pine, the larger diameter ‘fender washer’ was tested in place of the standard washer. There was no increase in system strength with the larger washer. However, the failure pattern changed from the swage stop pulling through the branch with the standard washer to the branch breaking with the fender washer. If the ‘branch broke’ samples were removed from the analysis, the mean strength increased from 5.4 to 7.0 kN, which indicates that the fender washer adds considerable strength as compared to a standard washer. Since cable failure was typical with willow oak with the washered double swage stops, fender washers were not needed or tested. There was no oak branch breakage if the caliper of the branch was over 33 mm, or in pine if the branch was over 52 mm in caliper, even when forces up to 8.9 kN were applied to the cable. DISCUSSION Figure 6. Galvanized EHS steel cable that has grown tightly against the cable with Wire Stop terminations. Tests with exactly the same eye lags, except for the eye por- tion being welded closed, showed a significant increase in system strength in oak but not in pine. With both oak and pine, the failure mode changed from the opening of the lag eye, with the standard lag eye to pulling out of the wood, with the welded eye. Since oak wood is more resistant to fastener removal, welding the eye closed ©2011 International Society of Arboriculture The supplemental support cable system commonly used in North America consists of EHS cable, manufactured dead- end grip, thimble, and eyebolt, seldom fail when installed and maintained in accordance with the ANSI A300 Stan- dards (2006). Where failures were observed, they were typi- cally with J-lags opening, screw eyes pulling out of the tree, or failure at the apex of the manufactured grip cable termina- tion due to lack of a thimble (author’s unpublished data 2010). Failures associated with cables that had been overgrown from radial branch growth are apparently more related to ca- ble corrosion rather than other reasons. This indirectly indi-
March 2011
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