Arboriculture & Urban Forestry 37(3): May 2011 Arboriculture & Urban Forestry 2011. 37(3): 139–145 139 Withdrawal Resistance of J-Lags from Three Hardwood Species Brian Kane Abstract. Cables are commonly installed to reduce the risk of failure of branches or co-dominants, but there are few empirical data to describe their performance. In contrast, the withdrawal resistance of lag screws in timber connections has been studied more carefully and is re- lated to the shank diameter and threaded length of the lag, as well as the specific gravity of the wood. J-lags of three diameters were in- stalled in three species and withdrawal resistance was measured immediately and up to four years after installation. Since only three ful- ly installed J-lags were withdrawn, J-lags were also partially installed in two species. Growth of trees around the “J” of J-lags increased their withdrawal resistance, and the withdrawal resistance of partially installed lags generally followed prediction equations developed for lag screws used in timber connections. Withdrawal resistance of J-lags did not exceed cable tensions previously measured and simulated. Key Words. Cabling; Decay; J-lag; Tree Support Systems; Withdrawal Resistance. Cables are commonly installed to reduce the risk of failure of branches or co-dominant stems, but there are few empirical data to describe their performance. For example, the ANSI A300 Stan- dard Part 3 Supplemental Support Systems (Anonymous 2006) and its accompanying Best Management Practices guide (Smiley and Lilly 2007) provides appropriate sizes of hardware (J-lags, eyebolts, cable) for various branch diameters, but these guide- lines were derived from Thompson (1935; 1936), who extracted J-lags immediately after installation in logs approximately 30 cm in diameter. Although he tested a range of species and shank di- ameters, Thompson’s (1935; 1936) work involved only three rep- licates of each test. Presumably in recognition of the small sample size, Thompson (1936) cautioned that, “Considerably more work is necessary [to] know the most efficient length and diameter of a lag screw for maximum efficiency.” Despite this admonition, there appear to be no robust studies on the withdrawal resis- tance of J-lags in living trees published since Thompson’s work. In contrast, the withdrawal resistance (P, in N) of fasten- ers (nails, screws, lags, etc.) used in seasoned timber has been studied more carefully (Soltis 1999). For lag screws, which have a hexagonal or square head and are not bent into a “J,” Soltis (1999) presented the following equation: [1] P = 125.4G1.5D0.75 L where G is oven-dry specific gravity of the wood, and D and L are the shank diameter (mm) and thread length (mm) of the lag, respectively. McLain (1997) proposed an alternative prediction equation based on archived data from the USDA FS Forest Products Lab (Newlin and Gahagan 1938), as well as more recently collected data (McLain and Carroll 1990): [2] P = 185G1.35D0.61 L Previous work on lag screws has demonstrated that the principal component of the screw’s withdrawal resis- tance is its thread bearing perpendicular to wood grain; fric- tion between wood and the unthreaded portion of the shank contributes little resistance (McLain 1996). For J-lags in- stalled in trees, however, growth around the unthreaded “J” of the lag may increase withdrawal resistance because the tip of the “J” may be rigidly anchored and resist bending. Both of these equations assume withdrawal of the lag screw, as opposed to failure of the screw itself, and are based on em- pirical evidence; there are no theoretical approaches to predict withdrawal resistance (McLain 1996). It is unclear whether tests conducted on timber are relevant to trees, especially considering that most of the empirical work has been conducted on lag screws inserted into samples of clear, straight-grained, seasoned wood. It is also unclear if and how withdrawal resistance of J-lags in- stalled in trees may change over time. As the tree grows around the J-lag, the wood into which the lag was originally installed may simultaneously begin to decay. Predicting this relationship presents a challenge because data to describe discoloration and decay associated with the installation of hardware in trees are anecdotal (Shigo and Felix 1980; Kane and Ryan 2002), with the exception of Dujesiefken et al. (1989) and Smiley (1998). Considering the potential for liability associated with failure of a cabling system and the dearth of data that quantify with- drawal resistance of J-lags, additional scrutiny is clearly justi- fied. The objectives of this project were to determine the with- drawal resistance with respect to time of J-lags installed in trees and to simulate the effect of wood decay (i.e., wood that offers no withdrawal resistance) by partially installing J-lags in trees. METHODS AND MATERIALS In July 2005, 19 J-lags 9.5 mm in diameter were installed ap- proximately one meter above ground in red oaks (Quercus rubra ©2011 International Society of Arboriculture
May 2011
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