142 Brian Kane: Withdrawal Resistance of J-Lags from Three Hardwood Species (a) years after installation, (b) shank diameter (mm), and (c) species [sugar maple (AS) and paper birch (BP)]. In the ANOVA, shank diameter and species were nested within year. Table 1. Least squares (LS) means (followed by the standard error in parentheses) to induce failure of J-lags (PMAX (a) Years n 0 1 2 3 4 14 29 34 17 10 PMAX z (kN) 5.78 (0.09)a 6.24 (0.06)b 6.11 (0.06)b 6.15 (0.09)b 6.44 (0.20)b Shank Diameter 6.4 9.5 6.4 9.5 6.4 9.5 6.4 9.5 6.4 9.5 n 8 6 14 15 17 17 13 4 1 9 (b) PMAX y (kN) 3.73 (0.12)a 7.84 (0.13)b 4.08 (0.09)a 8.40 (0.09)b 3.93 (0.08)a 8.29 (0.08)b 4.05 (0.09)a 8.26 (0.17)b n/ax 8.62 (0.13) z For each year, LS means followed by the same letter are not significantly different (P > 0.05) by Tukey’s HSD test. y LS means for each level of shank diameter and species within each year followed by the same letter are not significantly different (P > 0.05) by Tukey’s HSD test. x n/a indicates that insufficient measurements were made for valid comparison. ± 0.23 kN, n = 46) than paper birch (10.3 ± 0.24 kN, n = 41). This difference was not apparent when NWR between species was compared within each shank diameter (data not presented). Area of Discoloration The percent increase in area of discoloration was greatest for J- lags installed four years prior to harvest (Table 3a). Surprisingly, the percent increase in area of discoloration was greater for J-lags installed three years prior to harvest compared to lags installed two years prior, but not compared to lags installed one year prior to harvest (Table 3a). Within each year for which valid compari- sons could be made, the percent increase in area of discoloration was similar for sugar maple and paper birch, with the exception of year four (Table 3b). Within each year for which valid compar- isons could be made, the percent increase in area of discoloration was similar for J-lags 6.4 and 9.5 mm in diameter (Table 3c). DISCUSSION In sugar maple and paper birch, even four years after installation, fully installed J-lags 6.4 and 9.5 mm in diameter failed rather than withdrew from the wood. Even though Thompson (1936) installed and immediately tested lags, his data for several species also re- vealed that J-lags 6.4 mm in diameter failed rather than withdrew. However, J-lags 9.5 mm in diameter withdrew from yellow poplar (Liriodendron tulipifera L.) (Thompson 1936), presumably be- cause of the low specific gravity of the wood of that species. For J-lags installed in lead holes that were 1.6 mm diameter smaller than shank diameter, Thompson (1936) extracted more J-lags when shank diameters were larger. This is consistent with the find- ing that fully-installed J-lags 12.7 mm in diameter withdrew from birch (which had a lower specific gravity) but not sugar maple. Growth of the tree over the blunt end of the bent section of the lag presumably accounted for the greater failure strength of lags tested one or more years following installation, because previous work has shown that withdrawal resistance of lag screws in timber was similar regardless of whether they were immediately extract- ed or extracted a year after installation, as long as moisture con- tent did not change (Cizek and Richolson 1957, cited in McLain 1997). J-lags that failed did so by bending (Figure 3), and trunk growth would obviously offer some resistance to deformation of the lag under load. Counter-intuitively, however, this effect did ©2011 International Society of Arboriculture not increase over time. Although growth was not quantified, J- lags tested four years after installation appeared to be more deeply covered by trunk growth than those in year one; for some, the steel bolt (Figure 1b) barely fit through the “J” of the lag during testing. This observation suggests that while an increment of growth sub- sequent to installation provides some resistance to bending of the J-lag, additional growth may not offer meaningfully more resis- tance. Once new growth fully covers a J-lag, it is unclear whether greater resistance to failure would accrue because of fatigue in the cable itself. Future work could investigate this by installing cables with J-lags (or eyebolts) and testing the entire system. The overestimation of withdrawal resistance of partially-in- stalled lags by Equations 1 and 2 can most likely be attributed to the effect of moisture content of the wood. Predicted with- drawal resistance of lag screws is based on oven-dry timber (12% moisture content), and the withdrawal resistance of lag screws in wood with moisture content greater than 19% must be reduced by 33% (Soltis and Ritter 1996). Cockrell (1933, cited in McLain 1996) observed a 50% decrease in withdrawal resistance of green wood compared to oven-dried wood. De- spite the overestimation, withdrawal resistance of partially- installed lags generally followed the predictions of Equations 1 and 2 (Figure 4): withdrawal resistance increased with shank diameter, thread length and specific gravity (for the latter, it was evident in the lack of difference of NWR between species). Considering the presumed effect of moisture content in reduc- ing withdrawal resistance as well as the fact that ultimate tensile stress in lag screws 12.7 mm in diameter would be obtained when thread length equals 88.9 mm for wood with specific gravity greater than 0.61 (Soltis 1999), it was not expected that any par- tially-installed lags would fail (as occurred with three J-lags 12.7 mm in diameter installed to a thread length of 75 mm in sugar maple). This discrepancy can be attributed to the effect of bend- ing stress on the J-lag as described earlier, and, perhaps, because mean specific gravity of sugar maple was 9.8% greater than 0.61. Partial installation of J-lags offers some insight into the ef- fect of decay on withdrawal resistance, with one important ca- veat: partially-installed J-lags of all diameters appeared to bend at the point of insertion into the wood more than fully-installed lags. This was expected because the load was applied offset to the threaded portion of the lag, and a longer distance between the point at which the load was applied and the point of insertion Species AS BP AS BP AS BP AS BP AS BP n 7 7 14 15 17 17 0 17 8 2 (c) PMAX y (kN) 5.80 (0.13)a 5.77 (0.13)a 6.29 (0.09)a 6.19 (0.09)a 6.09 (0.08)a 6.13 (0.08)a n/ax 6.15 (0.09) 6.89 (0.18) n/ax ) classified by
May 2011
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