308 Kane et al.: Strength of Branch Attachments Arboriculture & Urban Forestry 2008. 34(5):308–316. Failure Mode and Prediction of the Strength of Branch Attachments Brian Kane, Robert Farrell, Shepard M. Zedaker, J.R. Loferski, and D.W. Smith Abstract. Predicting the strength of branch attachments is important for arborists and urban foresters because branch failure can cause damage and injuries. Previous studies have shown that the ratio of branch to trunk diameter is a better predictor of strength than the angle of attachment. Aside from these two factors, however, few other morphological measures of the attachment have been investigated with respect to predicting the strength of attachments. Many branch attachments from three species were broken on a testing machine and breaking stress was calculated. Prior to breaking, various morphological measures and ratios describing the attachment were made. Breaking stress varied by form of the attachment (u-shaped or v-shaped), failure mode, and the presence of included bark. The best predictor of breaking stress was the ratio of branch to trunk diameters. Results are discussed in the context of previous studies and with respect to tree risk assessment. Key Words. Branch attachment; breaking stress; included bark; tree risk assessment. The strength of branch attachments has been studied previously, and common themes have emerged. Of particular importance is the growing consensus that the strength of a branch attachment is best predicted by the relative size of the branch to its parent stem, which is referred to here as the “diameter ratio”. Contrary to common perceptions, several investigations have shown that the angle of attachment between the branch and trunk has less influence on the strength of an attachment (MacDaniels 1932; Miller 1959; Lilly and Sydnor 1995; Gilman 2003; Pfisterer 2003; Kane 2007). Included bark has also been shown to reduce the strength of attachments (MacDaniels 1932; Smiley 2003), and its presence is often associated with large diameter ratios and narrow branch angles. Aside from diameter ratio and angle of attachment, no other characteristics of branch attachments have been carefully measured with respect to how well they predict the strength of the attachment. This is especially true of included bark, which has only been tested as a binary variable, in other words, whether included bark is present or absent (Smiley 2003). In almost all previous investigations of the strength of branch attachments, a breaking load was applied to a branch either a short (MacDaniels 1923; Gilman 2003) or longer (Miller 1959; Lilly and Sydnor 1995; Smiley et al. 2000; Smiley 2003; Kane 2007) distance from the point of attachment. In all of these studies, either the attachment or the branch itself could fail. Consequently, the strength of attachments that did not fail (i.e., the branch failed) was only implicit. The objectives of this study were to 1) determine the breaking stress of branches of three species of shade trees; 2) qualitatively describe failure mode in greater detail; 3) identify and examine a broad range of morphologic measures that describe the branch attachment to determine which measure(s) best predict the strength of branch attachments; and 4) quantify the effect of included bark on the strength of branch attachments. MATERIALS AND METHODS During July 1996, branch attachments were removed from trees of three species (Acer rubrum L., Quercus acutissima Car- ©2008 International Society of Arboriculture ruthers, Pyrus calleryana Decne.) growing at Watkins Nursery in Midlothian, Virginia, U.S. (USDA Hardiness Zone 7b). When they were removed, trees were approximately 15 years old; trunk and branch diameter ranged, respectively, from 6.4 to 17.8 cm (2.6 to 7.1 in) and 1.8 to 8.4 cm (0.7 to 3.4 in); tree height ranged from 4.6 to 9.1 m (15.2 to 30 ft). All trees had grown narrow, upright crowns resulting from close spacing in the nursery. At- tachments were taken from many trees and classified as “trunk– branch,” in which a lateral branch smaller than and clearly sub- ordinate to the vertically oriented trunk was attached to the trunk; “branch–branch,” in which a second-order branch was attached to a first-order branch; or “codominants,” in which two vertically oriented trunks occurred on the same tree. A tree could have more than one pair of codominant stems as long as the stems were clearly extensions of a main trunk. Although branch– branch attachments were sometimes equal in diameter at the point of attachment, they were not classified as codominants because they arose from a lateral branch instead of being an extension of a trunk. Attachments were also classified according to their form, either “u-” or “v”-shaped. Form was determined by the presence or absence of a branch bark ridge: u-shaped attach- ments had a branch bark ridge and v-shaped attachments did not (Figure 1). Attachments were harvested from 75 trees (29 red maples, 22 callery pears, and 24 sawtooth oaks), and multiple attachments were taken from all but seven trees. Branches with decay, cracks, or other defects were not sampled. Cut ends of branches were coated with wax (Anchor-seal; U-C Coatings Corp., Buffalo, NY, U.S.) to prevent moisture loss because attachments were tested up to 45 days after removal from the nursery; the coating was reapplied as necessary to ensure that the ends remained sealed. To maintain sufficient moisture content of the wood, branch attachments were kept under a tarp, out of direct sun, and were sprayed daily with water. Subsequent analysis of moisture content revealed that all attachments had been tested in the green condition (i.e., moisture content remained above the fiber satu- ration point).
September 2008
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