72 stalled by drilling into areas of decay, then the injury may predispose the wood to further decay development. Further research is needed to identify the factors associated with hole enlargement and means of preventing its development. Tests conducted on various, traditionally terminated and swage stop terminated small diameter cable systems found that the strength of the system varied with the strength of the wood and the system configuration. From a system strength perspective in oak: bent eye screw lags < welded eye screw lag = single swage stop = single swage stop with washer < double swage stop with washer = eyebolt. In pine, bent eye screw lags = welded eye screw lag = single swage stop < single swage stop with washer < double swage stop with washer = double swage stop with fender washer < eyebolt. The strength of the eyebolt anchored cables actually ex- ceeded the listed breaking strength of the cable. This may be due to the doubling of the cable in forming the loop which connected it to the eyebolt. Cable failures seen with other systems tested, and as reported by Thompson (1936), had a tendency to be near the surface of the branch, the area where a double cable was present with the eyebolt anchor. Bent eye lags failed at about 25% of the force of eye- bolts. This is somewhat less than Thompson (1936) re- ported for half-inch lag hooks, which failed at 33% of com- parably sized eyebolts. Strength of a washered swage stop terminated cable was nearly twice that of the bent eye lag anchored system that is commonly used in small trees. An eyebolt terminated system provides nearly double the strength of the washered single swage stop system in soft wooded pine. Results showed that adding washers on single swage stops did not significantly increase the strength of the system. However, the nearly 50% increase in strength with pine branches from the addition of a washer is well worth the additional cost of adding washers to the system when install- ing a swage stop cable in a small trees. Washers are most im- portant in branches of smaller diameter where ‘pull-through’ type failure were more commonly observed than in larger di- ameter branches. On soft wooded species, increasing the diam- eter of a washer will also increase the strength of the system. Cables in branches up to 12.7 cm (5 in) are intended to sup- port a working load of 1.33 kN (300 pounds) (Thompson 1935; Smiley and Lilly 2007), therefore the estimated 1.5 kN (340 lbs) WLL of 0.31 cm (nominal 1/8-inch) diameter steel cable can be appropriate for branches up to 12.7 cm (5 in) at the point of cable attachment. The 4 to 4.8 kN breaking strength (WWL approximated at 0.8 to 0.96 kN, 180 to 215 pound of Smiley: Dead-end Stop Terminated Tree Support Cable Systems force) of a washered single swage stop termination is appropri- ate for branches up to 7.6 to 8.9 cm (3 to 3.5 in) in diameter for soft and hard wooded species, respectively. For branches between 7.6 and 12.7 cm (3 and 5 in) in diameter the stronger washered double swage stop or eyebolt termination is necessary. In a yearlong monitoring of cable tension in a single 29 m tall Eucalyptus cladocalyx in Australia, the peak cable tension recorded was only 4.3 kN (James et al. 2002). This is less than the force required to break 3.1 mm (1/8 inch) cable with a wash- er and single swage stop. This tension data also indicates that the ANSI Standard (2006) and BMP (Smiley and Lilly 2007) may over estimate the forces applied to tree support cables. Additional research on dynamic cable tension would be valu- able in determining the strength of systems required in trees. Long-term, large tree trials are needed to fully understand the effects of dead-end stop terminated cables on the growth, health, and stability of trees in the landscape. With nearly 40% of the holes in this study being enlarged by the cable, the impact of this on the spread of decay becomes an important unanswered question. Dead-end stop terminated cables may be more appro- priate on small trees where the risk of limb failure can be toler- ated from a safety perspective. Lower side loading forces asso- ciated with smaller tree make the hole enlargement less likely. However, additional field testing of dead-end stop terminated small tree systems needs to be conducted to determine if there are weaknesses that were not apparent in the static test and to deter- mine if hole enlargement is a factor that needs to be considered. Acknowledgments. We would like to thank Robert Bartlett Jr. and the F.A. Bartlett Tree Expert Company for their support of this research, Bruce R. Fraedrich, Ph.D. Director of Research. Patrick Brewer, Bartlett District Manager for Texas for development of small swage stop cables; Liza Holmes Research Technician for small cable testing, data collec- tion, graph development and paper editing; Elden LeBrun, Research Technician for installation of the large tree cables; Patrick Anderson for large tree cable assessment assisted by Jarod Faas, Skyler Schaeffer, Craig Robinson, production arborists at the Charlotte NC Bartlett office; Lorraine Graney for paper editing; Liz Brewer for Excel graphs; Billy Bridges from Clemson University for assistance with statistical analysis, George Palmer, Technician, for art work and data collection; and Steve Tillitski. Arborists who contributed information to the cable failure survey were James Ackerman, David Anderson, Rex Bastian, Ph.D., Dane Buell, Jim Clark, Ph.D., Fred Fisher, Pat Flynn, Richard Herfurth, Brian Kane, Ph.D., Elden LeBrun, R.J. Leverne, Brian Maxson, Joe McNeil, Dennis Ryan, Ph.D. Rob Springer, Gareth Tudor-Jones, and Torrey Young. ©2011 International Society of Arboriculture
March 2011
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