Abstract
When installing eyebolts and brace rods, the practice of countersinking into the xylem tissue using a chisel is common. This study was developed to assess an alternative method of cutting countersinks and to determine which method has the lower potential for introducing decay. It was determined that seating the washer on the bark rather than countersinking into the xylem produced the lowest levels of discolored wood and the lowest amounts of callus growth over washers. Data from this study support the recommendation that countersinking into the xylem be discontinued.
Bolts are installed in trees to strengthen weak crotches and to anchor cable systems (Thompson 1959; Mayne 1975; Harris 1983). It is standard practice to countersink through the bark and into the wood so that the washer beneath the nut is firmly seated on wood. Countersinking is usually done using a gouge and mallet to outline and remove the bark and some wood. Recommendations for countersinking specify cutting to a depth of 1/8 in. (3 mm) below the cambium (Thompson 1959; NAA 1970).
There are several potential problems with countersinking. First, drilling holes for J-hook lags causes discoloration of the wood, although decay is well compartmentalized (Shigo and Felix 1980). Cutting a larger hole for the washer is similar to turning a J-hook lag into the bark, a situation that Shigo and Felix warn may cause the spread of decay.
Second, countersinking can be a time-consuming procedure, often adding 10 minutes per bolt to an installation (Joseph Bones, F.A. Bartlett Tree Expert Co., personal communication). If this can be reduced, the client can save money on the installation. To potentially speed countersinking, a Forstner-type drill bit can be used, rather than chisel.
This study compares the potential for decay and wound closure for bolts installed with countersink holes that were either drilled or gouged and bolts installed without countersinking.
Materials and Methods
Three American beeches (Fagus grandifolia), 11 to 30 cm (4.5 to 12 in.) diameter at 4.5 ft, and 3 red oaks (Quercus rubra), 29 to 39 cm (11.5 to 15.5 in.) diameter were selected in a wooded area at the Bartlett Tree Research Laboratories in Charlotte, NC. The lower trunk of each tree was drilled 6 times in a spiral pattern. The holes were 0.95 cm (3/8 in.) diameter and were drilled 5 cm (2 in.) deep using a spade bit to accommodate a 5 cm (2 in.) long, 1.3 cm (1/2 in.) diameter lag screw. Lag screws with washers were used in place of bolts with washers to avoid drilling all the way through the trees and to simulate the effects of countersinking and not countersinking in bolt installation. Each hole was randomly assigned 1 of 3 treatments: 1) countersinking into xylem to a depth of 3 mm (1/8 in.) using a gouge, 2) countersinking to a depth of 3 mm (1/8 in.) using a Forstner drill bit, and 3) a control that was not countersunk. Forstner bits quickly cut a flat-bottomed hole with cleanly cut sides. Washers 3.5 cm (1-3/8 in.) diameter were installed with each lag. All lags were torqued to 54 Newton-meters (40 foot-pounds).
Lags were installed on April 25, 1995. Callus overgrowth of the washers was measured on June 11, 1996, and lags were removed on June 12, 1996.
Wood discoloration precedes wood decay in most cases and becomes evident much more quickly than decay. Therefore, discoloration of the wood around countersunk holes was used to compare the levels of damage to the tree. After lag removal, the bark and sapwood were removed tangentially from the area around the lag to reveal the area of wood discoloration. This area was traced onto a clear plastic sheet, cut out of the sheet, and weighed to determine the area of discoloration.
Results
The time required to drill countersink holes was considerably less than chiseling. However, even less time was consumed by not countersinking (data not shown).
Approximately half of the red oaks that were countersunk exhibited an oozing from the margin of the holes. There was also noticeable dieback of the cambium on the chiseled holes. Dieback was not observed on the drilled countersink holes.
With both oak and beech, discoloration of the wood was more than doubled by using either of the countersinking methods compared to the not-countersunk treatment (Table 1). Callus growth over the washer on the drilled countersunk oak was significantly greater than both the chiseled treatments and the control (not countersunk). With beech there was no difference between countersinking methods. However, both produced more callus than the not-countersunk control.
Callus growth over washers and area of wood discoloration one year after lag and washer installation using 2 methods of countersinking and not countersinking.
Conclusions
While countersinking is traditionally used when bolts are installed, it is apparent that this practice significantly increases the amount of discoloration. Therefore, it is recommended that the practice of countersinking into the wood of a trunk or limb when installing eyebolts or brace rods be discontinued as a routine practice. When installing brace rods on very thick-barked trees, it still may be desirable to countersink the outer bark so that there is no movement of the rod.
Callus growth over a washer is considered advantageous from an aesthetic point of view. However, it plays no part in the holding strength of the bolt. The benefits of countersinking in order to accelerate callus growth do not outweigh the increased potential for introducing decay.
Acknowledgements
Thanks to Bruce R. Fraedrich, Stephen W. Smith, Lynn Roberts, Joseph Bones, and Walt Dages for their contributions to this paper.
- © 1998, International Society of Arboriculture. All rights reserved.
In this issue
Jump to section
Related Articles
Cited By...
- No citing articles found.