Arboriculture & Urban Forestry 37(3): May 2011 31.8 mm, as well as full thread length (59.0 mm and 69.0 mm for lags 6.4 mm and 9.5 mm in diameter, respectively) and im- mediately tested as previously described. Since ultimate tensile stress of lag screws is achieved when screws are inserted to a thread length 7 times the shank diameter for wood with specific gravity greater than 0.61 (Soltis 1999), an upper bound of par- tial thread length less than the predicted value was chosen. J-lags were not partially installed in red oaks. Partial installations pro- vided data to compare withdrawal resistance to that predicted from Equations 1 and 2; they also indirectly simulated the ef- fect of decay on withdrawal resistance. For J-lags 12.7 mm in diameter, tests involving full and partial installations in fresh logs of sugar maple and paper birch were repeated, but the thread lengths were 25.0 mm, 50.0 mm, 75.0 mm, and 100 mm (the lat- ter value corresponded to fully-installed J-lags) to accommodate the greater overall thread length of J-lags 12.7 mm in diameter. After testing each lag, a disc (approximately 40–50 mm thick) that included the cross-section of the bolt in which the lag had been installed was cut from the log. Including a ruler for scale, a digital image of each disc was taken. If the amount of discolor- ation appeared to vary between the two cross-sectional surfaces of the disc, the digital image was taken of the surface with a greater amount of discoloration, estimated visually. Unless an area of dis- coloration was visually distinct from the installation of the J-lag, all discoloration was assumed to be due to installation of the J- lag. Several images of poor quality were not analyzed. The Raster Design extension in AutoCAD 2010 (AutoDesk, Inc., San Rafael, CA, U.S.) was used to scale images of each cross-section (saved as .tiff files) and to delineate the extent of discoloration associated with installation of the J-lag from hand-digitized polylines along the edge of the discolored area. For each delineated polygon, the “measuregeom” command was used to calculate its area and pe- rimeter. The difference between the measured area of discolor- ation and the area of the hole into which the J-lag was inserted was divided by the latter to calculate the percent increase in area of discoloration beyond the discoloration associated with drilling the hole. While most cross-sections exhibited discoloration cor- responding to the hole drilled to install J-lags, probing the cross- sections with an awl did not reveal decay in any cross-section. For fully-installed J-lags 6.4 and 9.5 mm in diameter, a three- way analysis of variance (ANOVA) was used to investigate the effects of time (in years after installation), shank diameter, and species on the strength of the lags. Shank diameter and species were nested within year because no J-lags were tested in sugar maple in year three, and only one J-lag 6.4 mm in diameter was tested in year four. Since J-lags installed in red oaks came from a different site, they were not included in the ANOVA, but descrip- tive statistics for qualitative comparison were presented where ap- propriate. A similar approach was taken for fully-installed J-lags 12.7 mm in diameter, since they were only extracted immediately after installation. For all partially-installed J-lags, withdrawal resistance was divided by G1.35 (from Equation 2) to normalize comparisons. A three-way ANOVA was used to compare normal- ized withdrawal resistance (NWR) among species, shank diam- eter, and thread length (which was nested within shank diameter because J-lags 12.7 mm in diameter were partially-installed to different thread lengths than those 6.4 mm and 9.5 mm in diam- eter). Tukey’s Honestly Significant Difference (HSD) test (and the Tukey-Kramer adjustment for uneven sample sizes) was used for multiple comparisons of significant (P < 0.05) treatments. For 141 significant interactions, the ‘slice’ statement in the GLM proce- dure was used to test for differences between levels of one main effect within another. A t-test was used to compare specific grav- ity between sugar maple and paper birch. Where significance tests included disparate sample sizes and/or nested effects, least squares means were presented. Predictions of withdrawal resis- tance of partially-installed lags from Equation 1 and Equation 2 were compared with measured values by calculating the percent difference: (predicted – measured) / predicted. Three J-lags 12.7 mm in diameter installed to 75 mm thread length in sugar maples failed rather than withdrawing; three J-lags 12.7 mm in diam- eter fully installed in paper birch withdrew rather than failed. All of these J-lags were removed from quantitative analyses. All analyses were conducted in SAS (ver. 9.2, Cary, NC, U.S.). RESULTS Mean moisture content (± standard error) of sugar maple (64% ± 0.6%, n = 91) was less than (p < 0.0001) paper birch (79% ± 1.0%, n = 95), but moisture content for all bolts [includ- ing red oak (67% ± 0.8%, n = 19)] remained well above fi- ber saturation point (25%–30% moisture content) throughout the tests. Mean oven-dry specific gravity (± standard error) of sugar maple (0.67 ± 0.004, n = 91) was greater than (p < 0.0001) paper birch (0.55 ± 0.004, n = 95). Mean moisture con- tent (± standard error) of red oak (n = 19) was 0.72 ± 0.012. Fully Installed J-lags For J-lags 6.4 and 9.5 mm in diameter, no fully-installed lags were withdrawn, irrespective of species and the number of years after installation. Instead, all J-lags straightened or, in four cases, failed at the point where the lag protruded from the trunk, just beneath the bark (the same point at which the thread began on the shank). The load at which J-lags 6.4 and 9.5 mm in diam- eter failed was greater one or more years after installation than J-lags tested immediately after installation, but that was the only difference (Table 1a). For J-lags 9.5 mm in diameter withdrawn one year after installation, mean strength of J-lags in red oaks (8.04 kN) was similar to that of J-lags in sugar maple (7.90 kN) and paper birch (7.78 kN). As expected, J-lags 9.5 mm in diameter consistently failed at greater loads than lags 6.4 mm in diameter (Table 1b). J-lags 6.4 and 9.5 mm in diameter in- stalled in sugar maple and paper birch failed at similar loads when tested immediately, as well as one or two years after instal- lation (Table 1c). There were insufficient tests of sugar maple in year three and paper birch in year four for valid comparison between species (Table 1c). For J-lags tested immediately after installation, the least squares mean load (± standard error) at which J-lags 12.7 mm in diameter failed (20.3 ± 0.41 kN) was greater than (p < 0.0001) that of J-lags both 9.5 mm in diam- eter (7.84 ± 0.41 kN) and 6.4 mm in diameter (3.73 ± 0.36 kN). Partially Installed J-lags Equation 1 and Equation 2 over-predicted withdrawal resis- tance of partially-installed J-lags by an average of 51% and 58%, respectively. NWR increased with increasing shank diam- eter (Table 2a), and with increasing thread length within each shank diameter (Table 2b). Least squares mean NWR (± stan- dard error) was also greater (p = 0.0020) for sugar maple (11.1 ©2011 International Society of Arboriculture
May 2011
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