Shade trees provide many benefits but can cause damage if they fail. Despite the potential for costly litigation that sometimes arises when damage occurs, there are no investigations of bending moments and stresses involved in failure of shade trees. Twenty-four shade trees of three species in the genus Acer were pulled to failure at a suburban property in Massachusetts, U.S. The maximum load and distance to failure were used to calculate maximum bending moment; stress at the point of failure was calculated from bending moment and stem cross-sectional dimensions. No trees uprooted, and failures were categorized as either stem at a lateral branch(es) or the attachment of codominant stems. Failures of codominant stems required one-half of the stress of stem failures. Similarly, failures of codominant stems occurred at only 45% of wood strength, whereas stem failures occurred at 79% of wood strength. Prediction of maximum bending moment from tree morphometric data was more reliable than prediction of maximum stress from tree morphometric data. Prediction of maximum bending moment and stress was more reliable for stem failures than codominant failures. Results are compared with similar tests on conifers. Implications of findings are discussed with respect to risk assessment of shade trees.
- Kane, B. and P. Clouston. 2008. Tree pulling tests of large shade trees in the genus Acer. Arboriculture and Urban Forestry 34:101-109
Trees and wind:
In light of the risk of litigation following damage related to tree failure in urban and suburban settings, more empirical data related to tree risk assessment are needed. We measured drag and drag-induced bending moment and calculated drag coefficient and trunk stress for three deciduous trees at wind speeds up to 22.4 m/s. We measured the modulus of rupture (MOR) of wood samples from trunks and calculated the factor of safety (MOR/stress) for each tree. We also investigated which tree morphometric variables best predicted drag and bending moment and whether simple two- and three-dimensional shapes accurately represented actual tree crowns. Drag, drag coefficient, bending moment, stress, and factor of safety differed among species in accordance with physical parameters. More massive trees experienced greater drag and bending moment, but stress was greater for trees with smaller diameter trunks. Tree mass reliably predicted drag and bending moments; crown dimensions, including crown area, were less reliable predictors. Crown reconfiguration varied only slightly among species, and drag coefficients were similar to previously reported values from trees of similar size. Our study has important applications for practitioners who manage tree risk, notably, the critical wind speeds and percentage of trunk cross-sectional area that could be decayed before trunk failure.
- Kane, B. M. Pavlis, J.R. Seiler, and J.R. Harris. In Press. Reconfiguration and stress in deciduous trees. Canadian Journal of Forest Research (expected August 2008)
Rigging is one of the most dangerous and technically-demanding practices undertaken by arborists, yet there are no robust data that describe the forces and stresses generated during rigging operations. We removed 13 red pines (Pinus resinosa Ait.) in pieces and measured forces at the block and in the rope, as well as stress in the trunk at breast height. Mass was the best predictor of force, but force increased more rapidly with increases in mass for branched sections of the trunk as opposed to unbranched sections. Arboricultural techniques such as angle and depth of the notch used in removed pieces did not affect forces relative to mass. We caution climbers to take smaller pieces when rigging, and to minimize the distance a piece falls, where practical.
- Kane, B. and S. Brena. In Review. Forces and stresses generated during rigging operations. Submitted to Arboriculture and Urban Forestry