144 Brian Kane: Withdrawal Resistance of J-Lags from Three Hardwood Species of the lag into the wood would cause a greater bending moment once the lag shifted its direction to align with the load. Although a) the offset loading mimics the load endured when the J-lag is under tension from a cable, and b) decay renders some thread length ineffective since the wood offers minimal bearing resis- tance, a fully-installed lag subject to decayed heartwood likely would not bend as much as the partially-installed lags at the point of insertion. Thus, the withdrawal resistance of partially- installed J-lags may be less than offered by fully-installed lags subject to decayed heartwood. In practice, it seems prudent to accept this probable underestimate as a safety factor. Future work should directly investigate the withdrawal resistance of hardware in decayed wood, as opposed to the surrogate data offered here. Relative to previously reported cable tension or drag on trees, lags provided a substantial safety factor. At wind speeds of ap- proximately 12 m/s, a load of 4.3 kN was measured in a cable in- stalled between two co-dominant stems in a 29 m tall sugar gum (Eucalyptus cladocaly × F. Muell.) (James et al. 2001). This load was considered an aberration; the next largest measured load was 1.2 kN (James et al. 2001). At the same wind speed, a computer simulation of wind-induced movement of a tree with stems 17.8 and 30.5 cm in diameter predicted a maximum cable tension of 6.4 kN (Greco et al. 2004). On a shingle oak (Quercus imbricaria Mi- chx.) 11 cm diameter above the root flare, at a wind speed of 22.4 m/s, the maximum recorded load was 1.2 kN (Kane et al. 2008). None of these loads exceeded the minimum failure load (7.18 kN) of a fully-installed J-lag 9.5 mm in diameter; and even the 6.4 kN load was less than half the minimum failure load (14.0 kN) of a fully-installed J-lag 12.7 mm in diameter. According to the A300 Standard (Anonymous 2006), J-lags 9.5 and 12.7 mm in diameter would be appropriate for the branch and trunk diameters in the studies (James et al. 2001; Greco et al. 2004; Kane et al. 2008). Even partially-installed J-lags of 9.5 and 12.7 mm in diameter would have neither withdrawn nor failed, except for withdrawal of lags 9.5 mm in diameter installed to 12.7 mm of thread length. Assuming branch diameter of 17.8 cm and concentric decay, the latter exception describes a branch for which 86% of the cross- section is decayed; an unlikely candidate for cabling, and not one in which J-lags would be used as anchors (Anonymous 2006). Although several studies have provided qualitative informa- tion supporting the notion that discoloration and decay may not be significant in certain species and under certain conditions (Shigo and Felix 1980; Kane and Ryan 2002), it appears that only Smiley (1998) has quantified discoloration associated with drill- ing to install support systems, and his work specifically addressed countersinking for the installation of eyebolts. The reported per- cent increase in area of discoloration may be an overestimate because it was assumed that discoloration was associated with installation of J-lags unless areas of discoloration were visibly separate from the lag. A more careful accounting of decay, for example by removing additional discs from each tree one meter above and below the point of installation of the J-lag, might have allowed a more accurate estimation of discoloration associated with the installation of the lag. It was unclear why the percent increase in the area of discoloration was greater in year three Figure 4. Measured withdrawal resistance (kN) plotted against withdrawal resistance (kN) predicted from equations 1 () and 2 (n). compared to year two, but not between years one and three. It may have been attributable to inherent differences between trees with respect to compartmentalization or confounded by the lack of sugar maples tested in year three. While discoloration and de- cay may be inevitable consequences of installation of hardware in trees, because many factors affect how rapidly and widely de- cay forms (Smith 2006), considerably more work is necessary. CONCLUSION The results of the present study offer some assurance that, in the absence of decay, lags provide substantial withdrawal resistance in species with moderately dense wood. They also suggest that for paper birch and sugar maple growing in Massachusetts, decay associated with installation of J-lags is minimal after four years. The results, however, should not be applied to other species with different specific gravity values or those predisposed to decay after wounding. Trees of less vigor, even of the same species, might also be more susceptible to decay, or less likely to grow around the J-lag. As Thompson (1936) suggested, considerably more work is necessary on many aspects of tree support systems. Acknowledgments. The author gratefully acknowledges the following students and staff at the University of Massachusetts-Amherst: Ed Car- penter and Andrew Putnam for installing lags; Dan Pepin for fabricating the testing apparatus; Mac Cloyes, Mollie Freilicher, and Joe Scharf for extracting lags; and Alex Sherman for analyzing images; as well as two anonymous reviewers for many helpful comments on earlier drafts of the manuscript. ©2011 International Society of Arboriculture
May 2011
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